Boca Raton London New York Washington, D.C.CRC Press

FORENSIC USES ofDIGITAL IMAGINGJohn C. Russ

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digital imaging / John C. Russ.

raphical references and index.03-4 (alk. paper)neering. 2. Image processDigital techniques. I. Title.

2001016190

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Introduction

The need for a book dimaging was made plainas an expert witness in adefendant hinged in larthe images on a poor qucamera. Interpretation ain general and digital expert witnesses on bothconfused and confusinreaching a decision. Thtion that can be found iadequately accessible to

Experiences as an ehave further emphasizetopic, which is growing and more confusing andcirculating in the populdoes not pretend to be aincluding my own

Ima

Press, Boca Raton, Flortechnical references andmatical underpinnings oto implement the algorijust what the advantagecomputer image processin images without comp

None of the methothat matter, particularaccepted and widely useapplied will survive

Frye

text to document the uand why such procedurreveal.ealing specifically with the forensic uses of digital to me a few years ago, after four days of testimony murder trial, in which the guilt or innocence of thege measure on whether or not he was the person inality and rather vague videotape from a surveillancend misinterpretation of information about imaging

image manipulation in computers in particular by sides, and by counsel who asked questions that were

g, may or may not have really helped the jury iney convinced me, as a participant, that the informa-n a variety of standard reference and textbooks is not witnesses and counsel.xpert witness in other trials, both criminal and civil,d to me the need to present a simple guide to thisin importance with more use of the new technology, sometimes misleading if not erroneous informationar press. This book attempts to address that need. It reference text for imaging professionals. Such books,ge Processing Handbook (third edition, 1998, CRC

ida, ISBN 0-8493-2532-3) and many others, present citations to the original literature, and the mathe-f the science. Others discuss computer programmingthms. This book tries to show in more simple termss and shortcomings of digital imaging are, and howing can be used to enhance the ability to access detailromising the truth of the images.

ds shown and described here is controversial, or forly state-of-the-art. All of them are based on welld algorithms and procedures, which if appropriately and Daubert challenges. It is not the purpose of this

nderlying science, but to explain, by example, whenes are appropriate, and what they can be expected to

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The examples showable computer programCA, http://www.adobe.ca consistent user interfacameras, scanners and oimage hardcopy. Fovea Graphics Inc., 20 Batthttp://reindeergraphics.cific algorithms for imato teach image analysisCarolina State Universitvide comprehensive too

I hope that this booabout their procedures have done, and why, in oing of the evidence. Forfor questioning to bringwho may confuse or miinformation or opinion

Figure 1

An unprocessed n here were produced with standard and widely avail-s. Adobe Photoshop (Adobe Systems Inc., San Jose,om) is an excellent platform for image analysis, withce, the ability to acquire images from many kinds ofther devices, to read standard format files and printPro and The Image Processing Tool Kit (Reindeer

ery Park Avenue, Suite 502, Asheville, NC 28801,com, ISBN 1-928808-00-X) provide the various spe-ge processing and enhancement, and have been used to thousands of students in workshops at Northy and elsewhere. Used together, these programs pro-ls for digital image processing and analysis.k will assist imaging professionals who must testifyand results to clearly and simply explain what theyrder to assist juries in reaching a proper understand- counsel, the book may suggest appropriate avenues forth such explanations, and to challenge witnessesslead juries (either intentionally or not) with wrongs.

John RussRaleigh, NC

digital mug-shot of the author.

2001 CRC Press LLC

The Author

Dr. John C. Russ

is an excourses and workshops trial clients, and providtrials.

A Caltech graduate,(Chicago, Illinois) and REngland). Since 1979, aDepartment at North Cavarious types of microscmore than 300 technicalthe Universitys Departmclients, have reached nea

(CRC Press, now in its reference text for imagedeveloped a widely usedbook on desktop compupert in image processing and analysis who has taughtaround the world, consulted for hundreds of indus-ed expert testimony in numerous civil and criminal

he was senior vice president of EDAX Internationalesearch Director of Rank Taylor Hobson (Leicester,

s Professor in the Materials Science and Engineeringrolina State University, he has used image analysis andopy in research projects, and authored 12 books and articles. Short courses and workshops taught throughent of Continuing Education, as well as for industrial

rly 5,000 students, and his Image Processing Handbookthird edition) is recognized as a worldwide standard processing and measurement. With his son, he has software package to implement the algorithms in theters. He can be reached at John_Russ@NCSU.edu.

2001 CRC Press LLC

Table of Con

1

Digital Cam

Introduction Why Go DigitaUses of Digital Film as a Light The Digital CamTonal and SpatiColor ResponseDigital PhotogrMaintaining ChDigital Video Scanners Taking Good PPresenting PictuSummary

2

Processing D

Noise in DigitaNoise ReductioMedian FilterinProcessing ColoNonrandom NoAdjusting ContAdjusting ColoAdjusting Size (Spatial DistortiOther Applicati

3

Enhancemen

Enhancement otents

eras and Forensic Imaging

l? Imagery Sensor era Sensor

al Resolution, and their Consequences aphy ain-of-Control with Digital Images

ictures res in the Courtroom

igital Images

l Images n Methods g r Images ise

rast, Brightness, and Gamma r Balance Magnification)

ons ons

t

f Detail

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Sharpening Sharpening ColOther EnhanceLess Common Color SeparatioFrequency SpacHigh and Low Periodic Noise Deconvolution,Measuring ImaResolution fromTonal (Grey ScaDetecting ComDetecting ForgeMaintaining Re

4

Identificatio

The Imaging CSurveillance VidRecording and Pixels and ResoNoise and TonaOther ShortcomEnhancement Image RestoratiRecognition anIdentification oIdentification bMisdirection an

5

The Expert

Working with C

or Images ment Techniques Processing Methods n and Filtering e (Fourier Processing) Pass Filtering, Bandpass Filtering, and

or Removal of System Defects ge Resolution the Fourier Transform le) Resolution

pression ries cords

n

hain eo Cameras

Playback Problems lution Elements l Resolution ings

on d Identification f Faces y DNA d Mistakes

Witness

ounsel, Judges, and Juries

2001 CRC Press LLC

Digital Cam

and Forensic

Introduction

For many people, the tpaste images of Tom Hmeet Presidents Kennedtionally intensive effortin Jurassic Park. These htechnology used to credisbelief when we enterseeing do not show actu

In fact, modern momovies where it generatof the effects that are explosions is very expenare digitally generated sso they can be superimpin front of a map. Whenperiod movies, it is oftendetails from the imagesand safety harnesses thaare logical extensions ofputers, but are far morenlarged to represent dphotographed to produ

Even more than mmanipulation into our

eras Imaging

erm digital imaging describes the process used toanks into old newsreels so that Forrest Gump couldy, Johnson, and Nixon, or the even more computa-

s such as creating dinosaurs to chase human actorsigh-profile movies generate a lot of publicity for theate them, and although we willingly suspend our the theater, the knowledge that the images we areal events is retained.

vie making uses digital imaging technology in manyes little if any publicity, and viewers may not be awareproduced. Building movie sets and creating hugesive. In many productions, the sets and explosions

cenes and the actors perform in front of blue screensosed afterwards, just as the TV weatherman is shown real neighborhoods are used as sets, particularly for necessary to remove power lines and other modern

. And in many movies the stunts require guy linest are later removed from the pictures. These methods techniques used in movie making long before com-e realistic than early movies in which lizards wereinosaurs, or model boats in a swimming pool werece war scenes.ovies, television advertising brings digital imageroutine daily experience. Except for the occasional

1

2001 CRC Press LLC

Gosh, how did they domidair while the camerainto another (or into aneffects that are introduimaging is that dramaaccepted as reality, so thof drama and excitemenopposite, danger is thaproduce false images thpeople distrust all imagAnyone with a home cduce or exclude informstill considerable, but expensive, and harder toSeeing is believing mu

Actually, the compurepresent only a tiny corone that has few conneceedings. Simulations ofto generate imagery thathese are used more inof the layout of buildiarchitects to allow walto help juries keep traphysical model.

The many other asroutine crime scene imacameras, discussed in thacteristics from film, ancan be applied to themhigh quality results. A rChapter 3, which use simreveal details that may applicable to pictures catized to permit this sortintroducing false informcedures are appropriateof images, including de

Chapter 4 discusseimpact in criminal casesof surveillance videotaptations and is discussed that? reaction when a dancer appears to freeze in moves, product packages dance, or one face morphs animal or car), we arent even aware of the graphicced. One danger in all of this exposure to digitaltic graphics produced by these methods becomeat real images of real events that lack the same levelt dont seem as real or compelling. The second, and

t awareness of the ability of computer methods toat cant be easily distinguished from reality will makees as representing truth. The widely held belief thatomputer and Photoshop can edit an image to intro-ation is far from true, because the skills needed arethe process is certainly more widely available, less detect than ever before, and the contrary belief thatst also be subjected to scrutiny.

ter graphics methods used in movies and advertisingner of the more general field of digital imaging, andctions to the use of related techniques in legal pro- accidents based on physical laws are sometimes usedt demonstrates the presumed sequence of events, but civil than criminal proceedings. Graphic renderingngs or rooms that use the same software used bykthroughs of proposed construction may be usefulck of crime scenes, as an alternative to building a

pects of digital imaging include the acquisition ofges using digital cameras rather than traditional filmis chapter. Digital images have some different char-d Chapter 2 covers the processing techniques that

to overcome some of their limitations and produceather fuzzy line separates these topics from those in

ilar methods to enhance the images and extract andnot be seen in the original. These methods are alsoptured with traditional film cameras and later digi-

of enhancement. This raises obvious concerns aboutation, so the chapter discusses in detail which pro-

and why. The closely related topic of authenticationtection of digital forgeries, is also covered.s an application of digital imaging that has a high: the identification of individuals. Digital processinges is becoming widely used in spite of serious limi-

in terms of the factors present in recognition and

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identification. Other idfaces, and automated repatterns. Identification separations also use im

Chapter 5 addressesa witness to present inattorneys and juries in from the point of viewsionals form one of the tgroup is the attorneys, wand pitfalls, in order toexperts. Some of these techniques for helping ttioned within the vario

Why Go Digital?

Digital cameras are takiphy marketplace. Only aeras with interchangeab(inexpensive automateding their own. All of thetrend and are major plcameras. It is thus naturimpact upon, the field osources of digitized imacomputer storage and cto digital form for prosuch as video cameras ufor computer manipulaand transmit their infotiny cassettes is also avquality, ability to dupliccording for accident an

The potential advafairly obvious, if often immediately without animage, not even the mithe photographer can captured. Second, the stduplicates can be made entification issues include digital aging of childrenscognition methods using facial dimensions and irismethods based on fingerprints and DNA protein

ages for recording, so these are discussed as well. the issues confronting the technical expert acting asformation on imaging methodology and assistingunderstanding image evidence. This presentation is of the technical expert, since these imaging profes-wo principal groups of readers of this text. The otherho must know about the technology, its possibilities

successfully use (or challenge) the evidence and theissues, including the admissibility of evidence andhe witnesses communicate well to the jury, are men-us chapters.

ng over a major segment of the consumer photogra-t the very high end (large format, professional cam-le and highly adjustable lenses) and very low end

snapshot cameras) are traditional film cameras hold- major camera companies, worldwide, recognize thisayers in the development and marketing of digitalal to consider the use of these cameras for, and theirf forensic photography. In addition, there are otherges. Scanners now routinely capture documents for

an also be used to convert conventional photographscessing. The signals from analog electronic devicessed in surveillance are easily converted to digital formtion. A new generation of video cameras that recordrmation digitally (DV, or digital video, format) onailable, which offer many advantages (size, imageate without loss of quality) over traditional videore-d fire scenes.ntages of digital imaging for forensic purposes areoverstated. First, the stored image can be examinedy need to wait for the chemical development of the

nute or so required for Polaroid instant prints, sobe assured that the desired information has beenored image can be transmitted via the internet, exact

for all interested parties, and the images can be filed

2001 CRC Press LLC

archivally with no degraformat such as CD-R diimages. Maintaining the

Finally, and most cputer manipulation to immediately clear whatthe area of improper maand the next chapters wfully documented and important detail from dmany of the same resultdarkroom professional

Certainly, the proceformed using computeneeded to run the softwappearance is misleadinof digital images is thatrecords of the steps perfor manipulated themsenew information or remintentionally or accidenpersons involved. DetChapter 3) is often muproduced by retouching

Finally, the use of thcourtroom, and overlayopens up opportunitiesnew ways that will challfortable to many jurors and computer screens.

Uses of Digital Im

Reserving until later chation of digital images, it iimage acquisition deviceand scientific fields. Somas recording the crime sdence, extracting informfingerprint and other mimagery during autopsiedation. Indeed, writing images in a tamper-proofsks is recommended to guarantee the integrity of the chain of control for evidence is thus simplified.

ontroversially, the digital image lends itself to com-enhance the visibility of details. It is not always constitutes enhancement and what may verge intonipulation that can distort or introduce information,ill discuss this in detail. With the proper procedures,well understood, it is often much easier to extractigital images than from conventional film, althoughs can be achieved from the latter by a knowledgeablegiven enough time.dures for image enhancement are more rapidly per-r-based processing on digitized images. The skillsare may appear to be easy to master, but in fact thisg. The great danger of computer-based manipulation because the software seems so easy to use, and theormed are not kept (or, if they are, can be easily lostlves), images may be altered in ways that introduceove original data. This may either be accomplishedtally, depending on the intent and skill level of theecting forgeries in digital images (discussed inch more difficult than when the same effects are or splicing film-based images together.e computer simply for presentation of images in theing labels and diagrams that can be easily removed, for communicating image information to juries inenge the courtroom infrastructure, but will be com-because of their increasing familiarity with television

agery

pters concerns over possible misuse or misinterpreta-s clear that the advantages of digital cameras and others make them an important tool for many professionale of these are directly involved in forensic work, suchcene and the exact location and surroundings of evi-ation from surveillance videotapes, enhancing latentarkings on documents and other surfaces, recording

s, acquiring videorecordings of fire scenes for arson

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investigation, etc. Therecivil trials, for example tliability cases. Images areas DNA identification byin the laboratory, autoppresentation advantagesapplied to these images

There are also peripthe images themselves inexample, most microscoscopes, and others) usedence, gunshot residuesrather than the film camthe advantages of immbased identification and

Such identification example, there are atlasenatural and man-made etc., that can be used tinvestigations. (There isuch data. See for insta

Institute, 2820 S. Mich

Figure 1

Identification oof particles: (a) dinitro-banother form of TNN.

Figure 2

Microscopic im

(a) are also many instances in which images are used ino show product failures (and consequent damage) in also used to bring into the courtroom evidence such electrophoresis, close-up images of evidence obtainedsy photographs, etc., and some of the processing and of digital imagery discussed in later chapters may beas well.heral uses of digital imaging that may never bringto a courtroom, but are nevertheless important. Forpes (this includes light microscopes, electron micro-d to examine particulates, tissue sections, fiber evi-, etc., now record the images using digital cameras,eras in widespread use only a decade ago. This has

ediacy and lower cost, but also permits computer- classification of objects.

was performed entirely by humans until recently. Fors of images for various types of particles (Figure 1),fibers (Figure 2), chemical residues from explosives,o match and identify features collected in forensics an enormous, but rather scattered, literature withnce the McCrone Particle Atlas, McCrone Researchigan Ave., Chicago, Illinois. A few other examples

f organic high explosives from microscope examinationis-oxamide (NENO); (b) trinitronaphthalene (TNN); (c)

(c)(b)ages of fibers: (a) cotton; (b) linen; (c) wool.

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include the

Atlas of Hu

and M. J. Fox, CRC Pres

tion Cards

, from the Foorado; and the

Atlas of F

Florida.) The procedureand only a comparativthem. Comprehensive cdence are presented reguThe use of these skilledis used (by either prosecin which it might poten

It seems reasonableatlases and available fromprograms to an ever incare automated programcharacteristic signature hands. Similarly, there aman-made fiber cross-setc. For the present, moof application, most oftewhere the classes of posas contrasted to millionbe extended to much information is put intoassemble and encode thmore rapid implementa

One area of successthe automatic fingerprito enable police to coma suspect with millionsnecessary to digitize theimental system for palmpolice departments. Thbase of shell casing and Databases of paints useders can be used to idendents. All of these systemcompositional data takevery rapid identification

The algorithms usednot understood by the This may create difficulman Hair Microscopic Characteristics, by R. R. Ogles, Boca Raton, Florida; the Forensic Insect Identifica-rensic Sciences Foundation, Colorado Springs, Col-ibre Fracture, J. Hearle et al., CRC Press, Boca Raton,s for accomplishing this are complex and exacting,

ely few laboratories and researchers have masteredourses in the microscopic examination of such evi-larly, for instance by the McCrone Research Institute.

experts is expensive and hence this type of evidenceution or defense) in only some fraction of those casestially be helpful. to expect that the knowledge contained in these experts will be incorporated in computer matching

reasing extent over the next few years. Already theres for scanning electron microscopes to search for theof gunshot residues on filters used to wipe suspectsre matching programs that can identify particulates,ections, etc. by size, shape, color, optical properties,st of these methods work only in rather limited areasn in scientific research and industrial quality controlsible matches of interest are rather small (hundredss of possible candidates), but in principle they can

wider arenas, as computers get faster and the raw suitable form. Indeed, it is the effort required toe databases that is the principal factor delaying thetion of these methods.ful application of automatic matching of images is

nt identification system (AFIS) that is now availablepare fingerprints taken from a crime scene or from of stored prints, within minutes. Of course, it is image for transmission and matching. A still exper-print identification has also been tested by several

ere are proposals to develop a unified national data-bullet toolmarks to identify handguns used in crimes. by all domestic and many foreign automobile mak-

tify paint flakes and smears from hit-and-run acci-s use images (and for the paint flakes, supplementary

n from the scanning electron microscope) to produce with high confidence levels. in these matching operations are complex and often

people using the instruments or computer systems.

ties for the use of such information in trial settings,

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as indeed similar probanalysis when the forentography-mass spectomequipment. Will someofication program be exprincipal components another of those approin another expert to expthe trial, add to its expethe jury?

Digital imaging wilfields, including forensicfor the courtroom. Judgtunities to be bewildereand overwhelmed by mof counsel to determinthem.

Film as a Light Sen

The light sensor in a filmlight photons trigger a opment process) turns idirectly or used as a negathat are monochrome respond to infrared or from X-ray photons, usment of film materials ahalf. The resolution of of silver halide, dyes, osensitivity to very low li

Generally it is reaso(the ability to capture vas many as 4000 measurlevels). This is usually Optical density is writtelight that is transmitted

or one part in 4000. Filpoints per inch (i.e., as width of a standard 35 lems have arisen in other fields such as chemicalsic researcher may know how to use a gas chroma-eter (GC-MS) but not be expert in the design of thene who knows how to run a particle or fiber identi-pected to understand the neural net, fuzzy logic, oralgorithms that it uses, and to explain why one oraches was proper in a given instance? Will bringinglain these functions help the jury or merely prolongnse, and raise the possibility of boring or confusing

l thus raise new opportunities for many professional science, and at the same time present new challengeses, counsel, and juries will be faced with more oppor-d by technical jargon, bulldozed by expert witnesses,asses of exhibits. It will be largely the responsibilitye how to use these new tools and how to challenge

sor

camera is a piece of photographic film. The incidentreaction that further chemical treatment (the devel-nto a variation in density or color that can be viewedtive to produce prints for viewing. Films are available(black and white) or color, and also ones thatultraviolet light. Film is also used to record imagesed in medical and industrial imaging. The develop-nd chemistry has been ongoing for a century and a

film images depends to some degree on the amountr other chemicals present. Usually, the greater theght levels, the poorer is the films resolution.nable to expect the best films to have a tonal responseariations in light intensity) of about 4000 steps (i.e.,able steps from the darkest to the brightest recordedspecified as the optical density of the negative.n as the negative of the logarithm of the fraction of

, hence an optical density of 3.6 corresponds to 103.6

m typically has a spatial resolution of 3000 to 4000many as 5000 discernible separate points across themm film negative).

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These values pertaiand white, and whetherpoorer in both tonal ansible to show in a print Polaroid instant filmrange, as little as 10 to 1for the use of both digitwhich will be discussed

The dynamic rangerespond to brightness c

from nearly single phota sunny day on the ski sall of these brightness lchanges of brightness oplace over a short laterworks out to the typicalvalues in a scene. Humaimmediate local compabrighter or darker thanThe visual ability to dehuman vision does not as film does.

In terms of spatial are about 150 million seof the color sensors cluwhich we use to look agiving the ability to dist

typically find that gooddetect visually in a scecommunicate all of thearchical selection and epieces of the image thatthat into familiar objecticularly for unexpecteup again.

The Digital Camer

While photographic filhuman eye, it is quite a The devices, first used inn to the photographic film, whether color or black positive or negative. Photographic prints are muchd spatial resolution, and it can be difficult or impos-all of the details that have been captured on the film.

produces positive prints with even less grey scale5 discernible levels. This has important consequencesal and photographic prints in a courtroom situation, later. of film is far poorer than the human eye, which canhanges over about nine orders of magnitude (109)

on detection on a starlit night to the illumination onlopes, but the eye cannot simultaneously distinguishevels. Human visual response is limited to detectingf about 2 to 3% (a factor of 1.02 to 1.03) that takeal distance. More gradual changes are ignored. This ability to distinguish only some 20 to 30 brightnessn vision does not measure brightness but relies onrisons to determine whether one region or object is another, and the same is true for color sensitivity.tect a percentage change in brightness means that

respond linearly to intensity, but logarithmically, just

resolution, the human eye is better than film. Therensors (rods and cones) in the human eye, with most

stered densely in the fovea (the portion of the retinat things). The spatial resolution there is very high,inguish marks as little as 100 m apart. And yet we photographs contain all of the information we canne. Part of this is because our eyes dont actually individual sensor outputs to the brain. A very hier-xtraction process is used to find just those bits and are likely to convey information, and then organizets. That is why people are such poor observers, par-d objects and situations, a topic that will come

a Sensor

m is inferior in tonal and spatial resolution to thebit better than the detectors used in digital cameras.

video cameras, are much newer, and still undergoing

2001 CRC Press LLC

rapid development. Thestate silicon-based devithe most common kind share much of the archisomewhat different man

The CCD chip is a twwafer. Each diode is simvolume of the device, wenergy there by raising eof the device. These elelower energy state. Givebut instead the charge reporting the light inten

The detectors are asurface of the device. Tydiagonal to about an inthere may be from a hathe upper end of this rathird that of film. Sensothe spatial resolution ofexperimentally, but appability (and will likely fi

There are other typsensor has the advantagas memory chips, whichthe future be possible tamplifiers, voltage meassavings. But for the prebecause they are much much more objectionab

In the CCD, the nfundamental statistical another). This means tmore noise there is. Bincreased noise is not pnoise appears more or design each detector is discharged by incomingso that dark areas, wherespecially noisy. CMOS surveillance video cameferencing cameras, in wy are a direct outgrowth of the developments in solid-ces used in modern computer technology, although(the CCD or charge coupled device) does not actuallytecture of the common computer chips and requiresufacturing technology.o-dimensional array of diodes fabricated on a silicon

ply a light bucket. Every photon that enters the activehich is a thin region near the surface, deposits its

lectrons from one band level to another in the atomsctrons are trapped and cant easily get back to theirn enough time and some heat energy, they would,is transferred out of the diode and measured, thussity at that location.rranged in a checkerboard pattern that covers the

pical devices in use now range from about 0.25 inchesch (a little smaller than 35 mm), and on this area

lf million to about 6 million individual detectors. Atnge, the spatial resolution is about one-half to one-rs with up to 16 million detectors, which can match

film (but not yet its tonal resolution) are being madeear to be a few years away from commercial avail-nd their way into high cost professional cameras).es of devices that can record images. The CMOS

e over the CCD that it is fabricated in the same way means that the cost is much lower. Also, it may in

o place some of the other circuitry (for addressing,urement, etc.) on the same chip for even more costsent these sensors are not used in high end camerasnoisier than the CCD, and the noise characteristic isle for viewing the pictures.oise is proportional to the signal (because of theproperties of moving the electrons from one state tohat the brighter a feature is in the photograph, theut because vision responds logarithmically, this

erceived as being greater and the overall result is thatless uniform across the entire scene. In the CMOSinitially charged up like a small capacitor and then light. The result is to reverse the noise characteristice vision is particularly sensitive to small changes, aredetectors may be encountered in a few very low costras. Their primary use at this time is for videocon-

hich a poor quality image is tolerated. However,

2001 CRC Press LLC

development of these dequality for both CCD a

Recent misleading higher resolution thasensors are smaller so This is irrelevant, since size detector is used; it area that is related to rthe image because the tincorporation of on-chconcerns about nonunitern noise in images.

One problem with hto read out the charge individual wires to eacneighbors so that the emuch like a bucket brig(Figure 3). These deviccameras, in which each hline in the broadcast imof detectors was simplya digital still camera dehigh speed measuring cthat can be stored in a c

It is a common mcorresponds to the inddifferent meanings in dthe chip, the number odisplayed points of light

Figure 3

Diagram of a dvices continues at a rapid pace and detector size andnd CMOS detectors are improving.claims from developers that CMOS devices offern comparable CCDs are based on the fact that thethat more can be packed into a given size device.the role of optics is to focus the image onto whateveris the total number of detectors in the entire imageesolution. The smaller detectors generally degradeonal range and efficiency of the device suffers. Theip circuitry to amplify signals in CMOS chips raisesform response of the detectors, which leads to pat-

aving an array of CCD detectors is figuring out howfrom each of them. There simply isnt space to runh detector, so instead they are connected to theirlectrons in entire rows or columns can be shifted,ade, to transfer the charge out to a measuring circuites were originally developed for use in televisionorizontal row of detectors corresponded to one scan

age. In that case, the charge from the horizontal row fed to an analog amplifier and became the signal. Insign, there is still an analog amplifier followed by aircuit that reduces the voltage to a series of numbersomputer-like memory.isconception that the number of measured valuesividual detectors. The word pixel takes on manyigital imagery, including the number of detectors onf stored values in the computer, and the number of on the cathode ray tube. Each of these may be quiteevice with address logic, readout register, and amplifier.

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different. The importanresolution elements in than the display pixels iresults when a photogrdetectors on the chip mseveral reasons, includinthe use of color filters toalso be less than the nuimply it is the camera rfication. This topic will

Transferring the chators creates several prosplashing of water inscanned at video rates, tand tonal range of the rlance video. In a digitalless noise (the analogy bucket brigade). In somraphy, the chip is coolportion of the noise duerate and is not generally

In a typical video caoccurs while light is stiladditional light to valuea digital still camera by end of the exposure. Idetectors on the chip, coring the charge from ththe signal out from themsmearing problems in vthe light sensitivity, sincto incoming light.

The arrays are not 1scatter from the detectotrons is not perfect, bubetween the individualdetector escaping to anothe smaller chips. Sinccameras that will only acceptable. For professiovide higher efficiency, aeach detector to collect t (but not often specified) value is the number ofthe image. Enlarging the image so that this is largers called empty magnification (the same thing thataphic negative is enlarged too far). The number ofay be greater than the number of resolved pixels forg the electronic characteristics of the amplifiers and obtain color information from a single chip. It maymber of stored pixels, which may be advertised to

esolution even though this represents empty magni-reappear several times in the following discussions.rge along the row of hundreds or thousands of detec-blems. One is electronic noise, equivalent to the

the buckets in a bucket brigade. When the chip ishis noise is quite severe and sets a limit on the qualityesulting signal, as discussed in Chapter 4 on surveil- still camera the readout is much slower and there iswould be to freezing the water in the buckets in thee laboratory cameras used for microscope photog-

ed to subzero temperatures to further reduce the to thermal effects, but this also reduces the readout practical (or necessary) for hand-held cameras.

mera, the transfer of charge along the row of detectorsl striking the chip, so there is also a contribution ofs as they are being transferred. This is prevented inthe use of a physical shutter to block the light at thet can also be avoided by doubling the number ofvering half of them with an opaque shield, transfer-

e active sensors to the hidden ones, and then reading. This in-line transfer method reduces some image

ideo cameras, but adds cost to the chip and reducese half of the detector area must be made insensitive

00% efficient in any case. Some light may reflect orrs, and the conversion of the photon energy to elec-t the main loss is due to the necessary separation

diodes. The ditches that prevent charge from onether take up as much as 50% of the area of some of

e these are usually used in consumer-grade videobe used in bright lighting situations, the tradeoff isnal-grade cameras, the active detectors typically pro-

nd this can be increased by placing a small lens over

light over practically 100% of the chip area.

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The output from this an important differenvision. Figure 4 shows alinear steps (equal increNote that if the smallelinear ramp, it would tathe entire range. This isgreater than for a logaritof an image. Some videa nonlinear amplifier (Chapter 2) to produce aeras, film, and human v

For color imaging, video cameras use threeso that the red, green, ainformation combined edelicate. Some digital catography in a studio) uand sequentially acquirquently combined in thcooled chip to gain greato hand-held or field ph

Many video cameraarray of detectors that hreceive red, some greenof colored filters are usetical lines of red, green

Figure 4

Brightness stepratios; (b) display of the v

Brightness

256

128

64

0

192

Step

EqualIncrements

R

(a)e solid-state detectors is linear with brightness. Thisce from the logarithmic response of film and human series of brightness steps. One graph shows a set ofments), while the second has steps with equal ratios.st step on the lower graph was used to construct ake more than twice as many discrete levels to cover why tonal resolution with a linear detector must behmic one, in order to show detail in the dark portionso cameras that use a solid-state detector incorporateusually referred to as gamma, which is discussed inn output similar to that from tube-type video cam-ision.there are several possible strategies (Figure 5). Some sensor arrays, splitting the incident light with prismsnd blue components are recorded separately and thelectronically. These cameras are costly and somewhatmeras (used for microscope attachment or still pho-se a single sensor array with a rotating filter wheel,e the red, green, and blue images which are subse-e computer. These are very slow, and often include ater dynamic range; they have little or no applicationotography.s and most digital cameras use a single chip with anave filters in front of them, so that some of the diodes and some blue light. Several different arrangementsd, the most common being the striped pattern (ver-

s: (a) plots of intensity with equal increments and equalalues from (a).

Equalatios

Equal Increments

Equal Ratios

(b), and blue) and the Bayer pattern, both shown in

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Figure 6. The latter has which gives the cameraeye is more sensitive to intensity at a location wis performed (and simibroad range of wavelenthe various color channwould be expected. Thiand about two thirds thof detectors would sugg

The color informatline is amplified and digbefore digitization. Somdirectly to produce the information, using filterlation of both signals frcamera with a Bayer pacolor information. This5 and 6, etc. for one intand 7, etc., for the othe

A 2.1 million pixelnumber of detectors budifferent meanings) may

pixel image, but in factresolution is less.

Figure 5

Schematic of t

(a)twice as many green-sensing detectors as red or blue, more sensitivity for green light (just as the humangreen than it is to red or blue). To determine the redhere there is no red-sensing detector, an interpolationlarly for blue and green). Because the filters cover agths and there is some overlap in sensitivity betweenels, the loss of resolution is actually slightly less thans effectively means that the camera has between halfe resolution in each direction that the total numberest.ion may be obtained from the detectors after eachitized, or by using analog combinations of the signalse video cameras even combine the analog signals

luminance and chrominance (brightness and color)s to smooth the signals and accomplish the interpo-

om the sparse array of detectors. When a single chipttern is used, two lines must be read out to obtain

is usually done by combining lines 1 and 2, 3 and 4,erlace field, and then using lines 2 and 3, 4 and 5, 6r field. digital camera (used in this instance to mean thet, as noted before, in other cases this word will have be advertised as being able to record an 1800 1200

hree-chip (a) and filter-wheel (b) color cameras. that is partially empty magnification and the real

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The tonal resolutiorather poor. A good studdetector rather than a sohave higher resolution,to that of human visionable to produce 100 distbe logarithmically spacand human vision are. devices. A solid-state vbrightness values, but aproduce that many visuhave to, because the huguish about 30 anyway, afor camcorder videotapquality). We will return4, when we discuss how

Figure 6

Diagram of Bayn of these devices, when read out at video rates, isio television camera (probably using a vacuum-tubelid-state chip, because these more expensive devices

logarithmic response, and a color sensitivity closer) should, according to broadcast video standards, beinguishable grey levels. Furthermore, these levels willed according to intensity, just as photographic filmSolid-state detectors, however, are inherently linear

ideo camera may produce about 60 distinguishables they are linearly spaced in intensity they do notally distinguishable brightness steps. But they dontman watching the television image can only distin-nd the result is good enough for television, especially

ing (where the tape recording causes a further loss of to the consequences of this performance in Chapter

er (a) and stripe (b) filters for single-chip color cameras. video signals are digitized for computer processing.

2001 CRC Press LLC

For digital still cameit can distinguish more and computers record.

bit image, convenient to8 bit bytes. Some digita

2

12

= 4096), but these being more suitable fomicroscope.

Tonal and Spatial R

We may take as a typicwith 2.0 to 3.5 million (256 discrete grey level sthese cameras acquire tactual image resolution

the word pixel means athe smallest feature or determining resolution photographic 35 mm fi

2400 pixels and, as we saresolution capability. W

The primary effect can be recorded and didistinguish objects thatobjects or distances of mfilm camera, the ratio depicted is about 100:1ones that are centimetesee the millimeter-sizedno longer show the lapermissible size ratio sh under good conditiothe need to take more c

When using a digitquently enlarge portions(Figure 7). The ability toand extract more informexist in real life. There is cand then record many clmarks, etc., and their plaras, the same chip is read out more slowly and hencegrey levels, usually the full 256 which most camerasSince 28 = 256 it is common to refer to this as an 8 store in computer memory, which is organized into

l cameras can achieve 10 or even 12 bits (210 = 1024,are generally not transportable hand-held cameras,r use in a studio setting, on a copystand, or on a

esolution, and their Consequences

al digital camera performance at this writing a chipindividual detectors, with a dynamic range of 8 bitsteps, linearly proportional to light intensity). Becausehe color image using filters as discussed above, the is likely to be about 1200 900 pixels. (In this case, resolution element in the image, corresponding todimension that can be distinguished from another;is discussed more fully in later chapters.) A typicallm negative will have a resolution of at least 3600 w above, the human vision system has an even higherhat are the consequences of this difference?of resolution is to limit the size range of objects thatstinguished in the image. Human vision can easily are millimeters in size in a scene in which other

eters are seen. This is a ratio of about 1000:1. In aof largest to smallest objects that are satisfactorily. This means that objects that are meters in size andrs in size can be recorded in the same photo, but to details it is necessary to take close-up pictures, whichrge meter-sized objects. With a digital camera therinks again, to about 10:1 (this is a trifle conservativens a factor of 20:1 may be achieved, but the point islose-up pictures).al camera, you cannot take a scene view and subse- of the image so that small details become recognizable endlessly enlarge or zoom the image magnificationation is sometimes shown in movies, but does not

onsequently a need to take a scene view for orientationose-up pictures showing the important details, objects,

cement, as shown in Figure 8.

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A similar restrictiondigital camera. As notedness levels). The digital brightness levels), and rather than logarithmicrange of 100 distinguishThis means that, while partially in shadow andto reveal details in eitheimage will not have as use image processing, as

Figure 7

Enlargement o

Figure 8

Example of theof the shell casing near th

(a) arises due to the more limited tonal range of the above, film can record a 12 bit image (4096 bright-

camera can typically record only an 8 bit image (256furthermore the fact that these are linearly spaced reduces the effective tonal range by about half. Aable brightness levels is a conservative figure to use.

a single photographic film negative of a scene that is partially brightly lit can be printed in the darkroomr the bright or dark portion of the scene, the digital

f a scene showing pixelation rather than detail.

need for a close-up image to show detail: (a) the positione table leg is documented; (b) the shell casing can be seen.

(b)much dynamic range. In some cases it is possible to shown in Figure 9, to reveal the details. It will often

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be necessary to change tto record separate imag

Most digital camerain a wide variety of situforensic work, it becomwhen photographing crof flame or floodlights necessary to use manuallit areas (which may conera preview and playbaately if satisfactory ima

Taking additional ppictures, means that in tures on scene than theshe must plan ahead to units), and plenty of stSmart Media memory mor can be connected dithere. As we will see latto squeeze more imagesalso makes it even moreidentify the images as twith pen and paper, or wis vital to establish the u

Color Response

Human color vision conm, corresponding to

Figure 9

Example of cooriginal; (b) expanded con

(a)he camera exposure (aperture and/or shutter speed)es of the bright and dark regions.s have automatic exposure capability and can be usedations to capture visually satisfactory results. But fores more important to have fill-in flash attachmentsime scenes. For events such as fires, the bright areaswill tend to control the exposure so that it will be settings to get satisfactory images showing the poorlytain vital information). Fortunately, the digital cam-

ck functions make it possible to determine immedi-ges have been recorded.ictures with different exposures, and more close-upgeneral the digital photographer will take more pic- film photographer. That in turn means that he orhave plenty of batteries (for the camera and the flashorage. Most digital cameras use Compact Flash orodules, while a few use floppy or hard disk storage

rectly to a portable computer to store their imageser on it is important not to use compression to try into limited space. The increased number of pictures imperative to use a good record keeping system tohey are taken. Whether the photographer does thisith a voice recorder, or in some other way, the recordtility of the images as evidence.

ntrast expansion to show details in a dark region: (a)trast, as described in Chapter 2.

(b)vers roughly the wavelength range from 400 to 700blue through red, with the greatest sensitivity (but

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least ability to detect smwavelengths (ultravioletgraphic film generally co(of course, this is by dessame coverage or responing far into the infraredend of the spectrum, orare generally insensitive special fabrication stepsof the chip. A digital cashowing the location ofnation. The wavelengthadjustment to make theing to human vision).

Most digital camerathe chip (a few, which don microscopes, may omown). Blocking the inffocus the long wavelengwould appear out-of-foabove, there will be othlengths for detection. Thbroad ranges of color tcamera design to anothein the eye, which are osensitivities that cover b

Figure 10

Wavelength rdetector.all changes in color) in the green (Figure 10). Shorter) and longer ones (infrared) are not visible. Photo-vers the same range, with about the same sensitivityign). Solid-state detectors do not naturally have thisse. Instead, they cover a much broader range, extend-

, and have their greatest sensitivity at the extreme red in some cases in the infrared. Solid-state detectorsat the blue and ultraviolet end of the spectrum, unless are used to permit light to enter from the back sidemera would thus be a poor choice to record images organic fluids revealed by black light (UV) illumi- sensitivity of solid-state detectors requires some

m useful for recording real color (i.e., correspond-

s incorporate an infrared blocking filter in front ofo not include lenses and are intended for mountingit that filter, expecting the user to install his or her

rared light is important because the optics cannotths at the same point as the visible light, and imagescus and hazy if the filter was missing. Then, as noteder filters used to separate red, green, and blue wave-ese are not precise wavelengths, of course, but rather

hat may overlap somewhat, and will vary from oner. Human vision also has three types of color sensors

ften referred to as red, green, and blue. These have

esponses of the human eye and a typical solid-stateroad ranges of wavelengths and overlap considerably.

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Human vision does may be quite a bit greeneach person tends to jupresent in the scene beination. People are quiteat remembering colors period of time. Indeedcomparisons rather thaquent chapters.

Color image recordcameras, has real difficuJust as with human visiof light that are detectereactions in the film, m(wavelengths of light) hand, the cameras are nowhen adjacent contrastithe pictures would allowas present at the originthe recorded image is mponent of human visionto correct the interpre

Even without consiimage, recording an imdifficult task, especially include light reflected frera, the development chto achieve fidelity. Digitthe red, green, and blueThis can be done (it is cof a colorless (i.e., whitand high end consumewhite balance, and tethation in the computer.

An even better meknown as accurately as with known colors on ired, green, blue, yellow,Macbeth Corp., New Wchart (as shown in Figurecorded in the image not measure color. What one person means by yellower than what someone else means, and furthermoredge colors differently according to the other colorsng viewed, and according to the color of the illumi- good at comparing two colors side by side, but notexactly or being able to make comparisons over a

, most of human vision is concerned with makingn measurement, which will be discussed in subse-

ing, whether it is accomplished using film or digitallties in capturing the color with absolute precision.

on, the broad range of sensitivity to the wavelengthsd by the diodes in the chip, or activate the chemical

eans that many different combinations of colorscan produce an identical final result. On the othert sensitive to the illusions that result in human vision

ng colors are present. One might expect that viewingw a human to sense the same color information as

al scene, but that doesnt quite work either, becauseuch smaller than the original scene, and one com- uses the surroundings and wide angle information

tation of color from the central portion of the scene.dering the difficulties of interpreting the color in anage with high fidelity color information is in itself aconsidering the effects of incident light (which mayom other nearby, colored objects). With a film cam-emistry and temperature must be carefully controlledal cameras are better in that regard, but require that sensitivities be balanced for the lighting situation.

alled making a white balance) by recording an imagee or grey) card with the camera. Most professionalr cameras include the circuitry for performing thisered cameras provide software to perform the oper-

thod when taking pictures in which color must bepossible is to include in the photograph a test cardt, such as a Macbeth color checker chart which has magenta, cyan, and several shades of grey (Gretag-indsor, New York). Using the known colors on there 11) with the measurements of the chart colors aspermits correcting the colors of any other objects

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present in the scene (prThis procedure will be

Digital Photograph

Taking pictures with a dfilm camera, and there iinformation on cameraassumed the forensic phby a combination of apeffective film speed thsame as most high qualsimilar to those used wispeed adequate to stop a slow shutter speed, wiaperture on depth of fie

One advantage of preview on the built-inwhich shows a low resoCCD chip. This can be uthat the exposure settin

Figure 11

Image of an eas Plate 1).ovided they are illuminated the same as the chart).illustrated in Chapter 2.

y

igital camera is very similar to using a conventionals no intent to duplicate here the general background functions, which is widely available and which it isotographer will already know. Exposure is controllederture and shutter speed. The CCD sensor has anat is typically in the ISO 50 to 200 range, about theity films, and the exposure settings will therefore beth a film camera. Of course, the need to use a shuttermotion, or a tripod to steady the camera when usingll be the same as for a film camera. And the effect ofld will also be the same.

the digital camera is the presentation of an image LCD display. Most digital cameras have this facility,lution preview of the actual image captured by thesed to verify that the image covers the expected view,

vidence scene with a color scale included (also showngs are correct, and that the image is in focus. Most

2001 CRC Press LLC

of the professional-graexposure and focus cirselect aperture or shuttethe desired image resulttography is an accuratcreation of artistic imagtools. A small aperture

Digital cameras are$100 to many thousandrange includes consumecations (or most other pcharacterized by poor, liing (darkening of the cosettings (shutter speed afor an off-camera flash iand to position the flashlighting), and often eve

In addition, the mosis a small amount of mstored in a compressed(Joint Photographers Eon a discrete cosine traare rarely used in camerasuch as wavelet compreniques has somewhat diin particular cases. The of the image informatioto recognize familiar ob

Figure 12

Example of iprofessional camera withformer (also note the losde cameras include both sophisticated automaticcuitry, plus the capability to adjust exposure or tor priority to choose an appropriate setting to achieve. Remember of course that the goal of forensic pho-e depiction of the evidence or scene and not thees in which defocus or motion blur may be importantfor high depth of field is usually preferred. currently available with prices ranging from abouts of dollars (Figure 12). Generally, the lower pricer models that are quite unsuitable for forensic appli-rofessional, serious uses). The low priced models aremited optics that cause image distortion and vignett-rners of the image), the inability to adjust exposure

nd aperture) and focus, only a built-in flash (supports important to enable the use of more powerful units, to reduce glare or sidelight surface detail for high-

n the lack of a tripod socket.t important single limitation that these cameras haveemory (which is expensive), so that the images are

format. This is almost always done using the JPEGxpert Group) compression method, which is basednsform. There are other compression schemes thats, but quite common in other computer applications,ssion and fractal compression. Each of these tech-fferent advantages for reducing the size of image filesgoal of all compression techniques is to discard somen while keeping enough cues to assist human viewers

mages taken with a consumer-grade camera (a) and a macro lens (b), showing the distortion produced by thes of focus and darkening at the edges).jects in familiar settings. In other words, you should

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be able to recognize snvacation sites like the Emuch of the image datmaintaining enough inf

It is possible to ach(or any data) by findingple, uses a lossless complike zipit or stuffit ttypically produces only achieve much higher rasion creates a serious prare not of familiar objecdetails, superficial markthe important evidencemation that is eliminateRecording the image informat is used as a computer platforms) reformat includes provisiopatterns of pixels, but thto 2 times in file size, ncompression. Only the heras include or allow thof Compact Flash, Smahigh-speed interface toallow the use of lossless

A compressed imagthe crime scene, to locaas the photographer yoanswer cross examinatio

Q: So you maintainthis case.

A: Yes.Q: Your camera sav

correct?A: Yes.Q: Isnt that what i

detail is discardeA: Yes, but only detQ: Dont you mean

objects?apshots of your family and pets, be reminded ofiffel tower, and so forth. It is in fact amazing how

a can be discarded often 99% or more whileormation for this rather limited purpose.ieve a modest amount of compression in an image repetitive patterns. Transmission of faxes, for exam-ression scheme, and most computers use programs

o reduce file size for transmission. For images, thisa factor of 2 or 3 in file size. Lossy compression cantios of 50:1, 100:1 or even more. But lossy compres-oblem for forensic photography, because the picturests in familiar settings. It is in many cases the surface

s, exact positioning of fragments, etc., that constitute to be recorded in the image. This is the very infor-d, modified, or shifted by the compression methods. a lossless format (often TIFF tagged image filestandard format recognized by many programs andquires much more memory for storage. The TIFFn for lossless compression based on finding repetitiveis rarely achieves more than a nominal factor of 1.5ot the factor of 50 to 100 times possible with lossyigher end consumer cameras and professional cam-e addition of enough storage memory (in the formrt Media, or a small hard disk) or provide a direct the computer (e.g., a SCSI or USB connection) to storage.e may seem to be useful to show a general view of

te the close-up photographs of specific evidence. Butu would not want to put yourself in the position ton questions like these:

that these photographs document evidence vital to

es these images in memory using the JPEG format,

s commonly called a lossy format because somed?ails that are not considered visually important.details that are not needed for recognition of familiar

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A: Yes.Q: But these crime

objects. Can youimages so that wcase was omitted

A: No, I cannot.

If you had used fracby reconstructing themthe questions would alsin the image reconstruc

At this point, of coevidence. The simple fawere omitted, moved, another and from one loimage may be useful forof an object whose appimage. But by far the sgether, because it is diffidetails may later prove the most common procreports from police depomy of using JPEG commake it easy to send thwise choice.

Many enforcement on the grounds that (a)finite resolution limit awith lossy compressionjurors are accustomed tthey are reliable. These

It is certainly true tFor film cameras the limand video cameras, whetors in the camera, the store and display the imdiscussed at length in imposed by the technoloin the same way. Also, equipment specificationcompression removes fand color of features, anscene photographs are not of familiar scenes and show us exactly what detail has been lost in thesee can be assured that nothing of importance to this?

tal compression, which introduces detail into images from many small copies of portions of the original,o have included, What details have been introducedtion process? and again there is no possible answer.urse, your pictures have been eliminated as usefulct is that you cannot demonstrate just what detailsor altered because that varies from one image tocation in the image to another. In fact, a compressed

some purposes, such as showing the general locationearance is documented in another, uncompressed

afest course of action is to avoid compression alto-cult to predict when the image is acquired just what

to be important. Storing uncompressed image is notedure with digital cameras at present: many of the

artments using digital cameras emphasize the econ-pression to store many images on a floppy disk, or

em via the internet. As noted above, this is not a

agencies defend the current use of JPEG compression it is widely done; (b) all recorded images have somend hence introduce artefacts, and so images stored are still acceptable for what they do show; and (c)o looking at images and can decide for themselves ifare incorrect and misleading arguments.hat all cameras have some definite resolution limit.it is usually the grain structure of the film. For digitalther resolution is controlled by the density of detec-digitization process, or the number of pixels used toage in the computer, there is a definite limit. This isother parts of this book. But the resolution limitgy is uniform across the image and affects all featuresit is possible to calculate that resolution based ons or to measure it from the actual image. However,eatures, relocates boundaries, alters the size, shape

d reduces resolution differently for different locations

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Figure 13

Three repredigital camera models: (consumer; (b) professionratory.

(b)sentativea) high end

(a)al; (c) labo- (c)

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in the same image. In pare preserved better thapixel values differently daries of the 8

8 pixel bartefacts and loss of deHence it is unrealistic toto determine what detvisibility of detail in onresolved elsewhere.

The argument thatmeaningless. It would beof cost (uncompressed ito transmit) and becauagainst such images. Sowill be challenged on threjected because of comhave to rapidly change

Some high end conand have optics that pezoom lenses that have linch to acquire close-upphotography applicatiocase (Figure 13). Profestype of chassis as a 35 mhensive adjustability. Thusually smaller than thsmaller than the 35 mviewfinder and increasething, since it is the edgproblems with distortio

There are also tethera direct connection to tthe laboratory (on a coused in the field. They ogood color stability, anThe digital images fromto supplement

in situ

phings of evidence whichbeing brought to the labredominantly smooth, uniform regions small detailsn in detail-rich areas. The JPEG method also altersepending on whether they lie at the center or bound-locks into which the image is divided. This producestail that are not consistent and cannot be predicted. expect observers to be able to judge the image qualityails and features can be confidently observed. Thee location does not imply that similar details will be

everyone does it is, of course, circular and hence more realistic to admit that everyone does it because

mages require more memory to store and take longerse thus far no significant challenge has been raisedoner or later images stored with lossy compressione basis outlined above, and the first time evidence ispression many agencies and police departments willtheir practice.sumer cameras which can save loss-free TIFF imagesrmit adjustment of exposure and focus also includeow distortion and can usually focus to less than one macros images. These are useful for some forensic

ns, and can serve as a useful backup camera in anysional digital cameras are usually built on the samem camera, with interchangeable lenses and compre-e CCD sensor is mounted in the film plane, but is

e size of 35 mm film. This makes the field of viewm camera, which requires some adjustment to thes the effective lens focal length. This is actually a goodes and corners of the optical field that have the mostn, loss of focus, and vignetting.ed cameras that have no internal storage, but require

he computer. These are generally intended for use inpy stand or a microscope), and will not normally beffer excellent resolution (both spatial and tonal) and

d allow the attachment of various lenses as needed. these cameras may also be used in forensic situationsotos taken to document the location and surround-

is subsequently photographed in more detail after.

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Maintaining Chain

Digital images are simpand can be stored withdata or programs, such enough space for the raimportant to use a med

The requirements fkeeping control of the prevents images from bimages, the equivalent sin a set to a permanenparticularly one that hasor add images to this seentirety. Magnetic storarewritable memory suchand it would in principan image from the set,

CD-R and CD-RWputers, and larger capacimportant to understanstorage as the pressed CThose disks use pressedinformation. Writable aspots change phase or rmay also be affected by ha 10 year useful life.

When image procesin images, it is necessarby-step procedures usedkeep a copy of each decreates something of a ndocument, it can be assnumber the sequentionto paper documents one to make sense of allin sequence, a separatechanges from one imag

For some cases, andbenefit. One strategy ofopposing side with a hfew that might actually -of-Control with Digital Images

ly arrays of numbers (the pixel brightness values), any of the storage devices used for other computeras tapes, disks, writable CDs, etc., provided they offerther large files. However, for evidence purposes it isium that can provide security from tampering.

or image evidence can be met for traditional film byoriginal negatives, preferably as an intact roll. Thiseing deleted, altered, or added to the set. For digitalecurity can be achieved by writing all of the imagest storage medium such as a writable CD-R disk, a serial number. It is not possible to modify, remove,t, which can easily and confidently be copied in itsge media, including tape and computer disks, and as Compact Flash, do not have this security aspect

le be possible to edit and replace an image, removeetc. recording is now a routine storage format for com-ity writable DVD disks are on the horizon, but it isd that these disks are not as suitable for truly archivalDs used to distribute programs, movies, and audio.

pits to encode the zeroes and ones that make up thend rewritable disks use lasers to heat materials so thateflectivity. These disks are more easily damaged andeat, humidity, or solvents. Most manufacturers claim

sing is performed to enhance the visibility of detailsy to keep a careful and complete record of the step-. The most straightforward way of doing this is to

rived version of the original image. Of course, thisumbering problem. If each image is thought of as aigned an identifying number (usually called a Batesal seven-digit number that is conventionally stampedand indexed). Of course, it is the index that enables this, and because the numbers will be chronological record of some kind will be needed to track thee to the next. for some attorneys, this offers an important fringe dealing with discovery proceedings is to flood the

uge barrage of documents, in which are buried the

matter. The expectation is that opposing counsel will

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not be able to discover ftant and which documprocessing, it is quite sirecorded image, in variwhich will never be useimportant ones that wientirely ethical to a sciethe adversarial mind-se

In any case, it is vitarecord (which will be primages and processing swill testify. I find it helpfmany threads that bransome final conclusions,best with their own stylyour head.

Digital Video

The important aspect othe solid-state detector, form that permits dupand facilitates processinvideo recording by usingvideotape recording wrias audio cassette tape ieach location on the tawriting, storing, and retortions in the informa

The most common(luminance) and color (requires reducing the baered an acceptable tradenot generally confused breproduced, particularlobjects in familiar scenvideo, the actual resolut

The light detector uis the same CCD array much faster, to achieve tper second in the Europrom the flood which strategies are considered impor-ents provide the key and relevant facts. With imagemple to apply every conceivable operation to everyous sequences, to produce a host of images most ofd again, but which hide in their profundity the fewll be relied upon to prove a case. (This doesnt feelntifically trained mind, but it does seem to appeal tot of some attorneys Ive worked with.)l that as the imaging expert you maintain your ownotected from discovery as your work product) of theteps employed to obtain the results about which youul to use a graphic aid for this, since there are typicallych and merge to lead from some original images to but each person will evolve a methodology that fitse. Dont expect to be able to keep the information in

f digital photography (as emphasized above) is notbut the storage of the image information in a digitallication, transmission, storage without degradation,g. Some of the same advantages can be achieved for digital video (DV) technology. Conventional analog

tes the signal to magnetic tape in much the same ways recorded. The saturation of the magnetic field atpe is proportional to the voltage of the signal, andading the tape introduces small but cumulative dis-tion.ly used video format (VHS) mixes the brightnesschrominance) information in the same signal, whichndwidth and resolution of the color. This is consid-off for video, based on the fact that human vision isy colors that bleed across edges, or are not perfectly

y when following motion or recognizing familiares. As will be discussed in Chapter 4 on surveillanceion of VHS analog video is quite poor.sed in most portable video cameras and camcordersas that used in digital cameras. The readout rate ishe 30 frames per second rate used in video (25 frames

ean video standards), and this introduces more noise

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to reduce the tonal resoeven numbered video land fills in the odd numintroduce artefacts thatmoving objects to be dithe greatest limitation t

The newest developvideo) which records thto audio recording is thsignal digitally. Audio Ccan become scratched osignal is still recorded mmagnetic saturation beiconverted to a series of bers is recorded as a sdirection or the other. Sithe digital recording is mthe transfer of the same the same advantages as

The density of recosettes. The camcorders ported (Figure 14). Somaccept interchangeable lshutter speed) controlsalso playback the recordfer the digital data at which is built into sominformation are not interecordings. Also, DV cavideo interlaced scan (ascan lines) which impmotion. The cameras cplays, in which case thelace, and the resolution

The downside to DVbasic compression step ito brightness (analog vidof human vision to colousing a JPEG-like algo(Moving Pictures Experdancies from one framelution of the data. The interlace method that scansines in one sixtieth of a second and then goes backbered video lines in the next sixtieth of a second can limit the vertical spatial resolution, and can causestorted. But it is the analog recording that representso quality.ment in video cameras and camcorders is DV (digitale video signal in digital form. A useful comparisonat CD (compact disk) audio technology records theD recordings do not degrade with time, and the disksr dirty without causing loss of information. The DVagnetically on oxide-coated tape, but instead of the

ng proportional to the voltage, the voltage has beennumbers and the binary representation of the num-eries of regions where the field is oriented in onence the magnitude of the saturation is not important,

uch more stable and robust. These recordings, andset of information to computer memory, can providedigital photography.rding is very high on DV tapes, which are tiny cas-that use this technology are small and easily trans-e can fit into a pocket, while the slightly larger onesenses. All have sophisticated exposure (aperture and, live displays using lower resolution LCDs that canings, and built-in computer interfaces that can trans-high speed (IEEE 1394, sometimes called Firewire,

e modern computers). The color and brightnessrmixed, as they are in VHS (but not in S-VHS) analogmeras use a progressive scan rather than the usuallternate 1/60th second fields for the even and oddroves image quality, particularly when recordingan usually be connected to conventional video dis- internal progressive scan data is converted to inter- is lowered to match that of conventional video. recording is that the data are compressed. The most

s to reduce the sampling of color information relativeeo does this as well, based on the relative insensitivityr boundaries). Then each video frame is compressedrithm. Some implementations also use an MPEGt Group) compression scheme which looks for redun- to the next. Any lossy compression method can alter

2001 CRC Press LLC

fine details of the imagcompression of digital s

However, even withvideo is still much bettcan, under ideal conditiowidth for a color image.elements. Note that thithe chip, which for a typchip detector is used, coland blue light in differeend DV cameras use a sampling as much, andR, G, B images. They achangeable lenses, have

Figure 14

Two represen

(a)

(b)e, so all of the same cautions as noted above for thetill images apply. the lossy compression, the image resolution of DVer than that of VHS analog video. VHS recordingsns, achieve 240 resolution elements across the screen

DV more than doubles that, to about 500 resolutions is still much less than the number of detectors on

ical DV camcorder is more than 750 (but if a singleor filters are employed to separately detect red, green,nt locations, which must then be interpolated). Highcompression scheme that does not reduce the color have three chips with a light prism to separate thelso generally have better optics and/or accept inter-

tative DV camcorders: (a) pocket type; (b) professional. comprehensive exposure adjustments, etc.

2001 CRC Press LLC

A DV camcorder wrecording crime and fira useful accessory for rshow the layout and locfor fire or accident scenvideo recording is an recording apply, of couzooms, which are disor(unless separate micropfrom the scene) to avophotographer which arwould be duplicated alon the tape).

It would not, howeclose-up details in lieu camera, because of the

Scanners

The flat bed scanner hacquiring digitized imagfrequently encountered itizing X-ray films (botsional gels as used in

Figure 15

One frame froould be a better choice than a VHS camcorder fore scenes in which motion is important, and may beecording the overall appearance of a crime scene toations of detail images by panning. This is also truees, and for surveys of property damage (Figure 15)excellent tool. The usual strictures for any videorse. The photographer should avoid rapid pans andienting. It is wise to turn off the sound recordinghones are being used to pick up important sounds

id recording comments and other noises from thee apt to be distracting and perhaps misleading (butong with the video evidence if they were captured

ver, be acceptable to use a DV camcorder to recordof acquiring still photographs with a digital or filmpoorer resolution and concerns about compression.

as become a widely used computer accessory fores from photographs and other flat objects. The mostforensic laboratory applications for scanners are dig-h medical and industrial) and one- or two-dimen-

m a video recording of hurricane damage. electrophoresis and other protein separation

2001 CRC Press LLC

technologies. They maytional photographic filmsion, enhancement, or m

Flat bed scanners coon the same side of the only for reflected lightprints. Others, which geopposite side of the objto scan negatives. It is athan prints, because theresolution) as well as suare the more original socifically for scanning ph

The light detector inrather than the two-dimmay cover up to the ful(dedicated film scanneralong the document imalso about 600 per inchinch another phrasedifferent situations). Somore than 1000 dpi, bumuch greater than the stion is not real resolutiperformance of the devito the computer (SCSI ofiles containing all of th

For color imaging, red, green, and blue lighfilters or turn on differedifferent colors. Some swhile others make threeadvantages and drawbademands on the mechaarate images. But the sinless stable illumination three sets of detectors msample, which producewith rough surfaces or

Some inexpensive c

256 levels) dynamic racompared to a digital c also be used in some situations to convert conven- to digital form, for example to facilitate transmis-easurement.

me in a variety of types. Some have the light sourceobject being scanned as the detector and can be used imaging of opaque objects such as photographicnerally cost more, have a second light source on theect and can record transmitted light, thus being ablelways preferable to scan the original negatives rather negatives have a much greater dynamic range (tonalperior spatial resolution, and, of course, because theyurce of evidence. Special scanners are available spe-otographic film such as 35 mm slides and APS rolls. a scanner is a single linear array of CCD detectors,

ensional array used in camera chips. This linear arrayl width of a page with 600 or more sensors per inchs use much finer sensor arrays). It is scanned slowlyaging one line at a time, with the spacing of the lines to yield an overall resolution of 600 dpi (dots per

which, like pixels, has more than one meaning inme scanners have higher spatial resolution, up tot many advertise an interpolated resolution that ispacing of the actual sensors. This empty magnifica-on and should not be used to judge the quality orce. Scanners usually have high speed digital interfacesr USB are the most common) and produce very largee individual pixel values.some scanners use three sets of detectors to capturet, while others use a single detector array and insert

nt light sources as the array is scanned to capture thecanners do this during a single pass over the object, passes, once for each color. Each method has bothcks. The three pass method is slower and places

nical precision of the scanner to align the three sep-gle pass method with different filters or lights givesand makes the color fidelity poorer, and the use ofeans that they have different view angles onto the

s color offsets and shadows, particularly for objectswith a finite thickness, such as color film.onsumer-oriented scanners have only an 8 bit (28 =nge, but the slower readout from these devices as

amera (a scan can take a minute or more) usually

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allows images to be capeach channel of red, gdynamic range. Twelve

required to deal with tspecification may also bmeasured, and 1 part inX-ray films contain a lardifferences, as for exambetween metal and cracical X-ray scanners prov

density of 4.2).The detector outpu

are to photographic maof the data linearly intoadditional complication

There is another wtaken with film. Kodak that uses their scannersalong with (or in lieu oto be an attractive, inexinto many computer imthe professional scanneHowever, the Kodak Phto pack more images oabove this would not beimportant. It has been conventional film photoimages, but for the samsion, it is wise to avoid

Taking Good Pictu

There is no intent to rbecome a skilled photogbasic skills but is new ttexts such as the followrecording images of cri

D. R. Redsicker, 199

CRC Press, Bocatured with greater dynamic range. For color images,reen, and blue intensity is recorded with the same bits (212 = 4096) per channel is common, and ishe dynamic range of X-ray films in particular. Thee given as the maximum optical density that can be 4000 corresponds to an optical density of 3.6. Somege amount of silver and so can capture larger densityple between bone and tissue in medical X-rays or

ks or gaps in industrial X-rays. Some dedicated med-ide 14 bits (214 = 16384 shades of grey, or an optical

t is linear, but many of the applications of scannersterials whose response is logarithmic, so the transfer computer memory is appropriate and introduces nos for interpretation.ay to obtain digital representations of photographsoffers a service through its film development centers to digitize photographic negatives and deliver themf) traditional prints. Kodak Photo CD would seempensive way to obtain these files, which can be readage processing programs. It would be logical to expectrs to be of the highest quality and well maintained.oto CD format uses a lossy compression algorithmnto the CD, and so for the same reasons discussed a preferred method for images in which details are

suggested in some texts that Photo CD recording ofgraphs provides a low-cost method to acquire digitale reasons as presented above for avoiding compres-this.

res

eproduce here a comprehensive course on how torapher. It is assumed that the reader already has the

o digital photography, or that he or she will refer toing to learn about the particular requirements of

me scenes, fires, evidence, etc.:

4, The Practical Methodology of Forensic Photography, Raton, FL, ISBN 0-8493-9519-4

2001 CRC Press LLC

S. Staggs, 1997,

Cri

Publishing, TemeL. S. Miller, 1998,

P

Cincinnati, OH,

This has much morabout each picture, andscene, evidence, surfaceof photography, which

It is a common misjuries when images are pone knows how to seepreferably no close-upstrue for a variety of reasand only a very small normally see is retainedin nature: the brightnepersons can be judged same lighting and withoptical illusions demontricked, and that compafraught with peril.

Photographs can seBut they can also brinnoticed and perhaps noto accomplish both of tjury, and to highlight imobserved in that scene a

There are a few poparticular needs of digprocessing. First, rememspatial resolution and means that you will havthat capture different paso that regions are not toften possible to depenthe dense or thin regiolatitude for under- or o

Also, more separateimages will show the plshow their exact relationsurface details and mame Scene and Evidence Photographers Guide, Staggscula, CA, ISBN 0-9661970-0-3olice Photography, 4th edition, Anderson Publishing,ISBN 0-87084-816-X

e to do with accurate recording of the information acquiring images that reproduce the details of the markings, etc., than it does with the usual concernis the taking of attractive pictures.conception, which attorneys often try to urge uponresented that are damaging to their case, that every- things in pictures so that no explanations, and

, processing, or enhancement are needed. This is notons. First, people are not particularly good observersamount of the information in the raw images they or interpreted. Second, human vision is comparativess, color, size, distance, etc., of several objects or

rather well when they are seen side by side with theout distracting surroundings. But many commonstrate that this comparative capability can be easilyring one object or person to a remembered image is

rve simply as a reminder of events and things seen.g to our attention details of a scene that were nott visible originally. It is the role of the photographerhese purposes: to make the crime scene visible to the

portant evidence which may not have been readilyt the time. ints that are worth including here, because of theital cameras and the possibilities of digital imageber that the digital camera has less resolution (both

tonal resolution) than a typical film camera. Thise to take more pictures. Separate pictures are neededrts of the scene with an appropriate exposure settingoo bright or too dark to show detail. With film, it is

d on darkroom techniques to bring out the detail inns of the negative, but digital imaging provides lessver-exposure. pictures are needed at different scales. Overall sceneacement of objects, but close-ups will be required toship to their immediate surroundings, and to revealrkings. With film, it is often possible to enlarge a

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wide-field negative to results may be obtainedflash to eliminate brightmarks. Digital imaging above) places more rest

With any photograpscene, placed close to owould be, at a minimusize. If color is of potenalso wise to place a refethat it has the same illu

When the size, shaptire marks, as shown incapture images that havportion of the scene. Fofootprints, with an apprmakes the marks evidensometimes difficult to aor a digital one. Large osometimes be photograpening or perspective dis

As will be shown inand remove perspectiveit requires that an apprthan a simple ruler is nthe same foreshorteninphotograph. An ideal sitvisible square, p