4 ChemMatters, FEBRUARY 2006 http://chemistry.org/education/chemmatters.html

Imagine needing eight hours totake a single photograph! That’s how long it tookFrench scientist and inventor Joseph Niepce to take theworld’s first photograph in 1826. And the end resultdidn’t win any prizes—it was a

grainy image of some buildings viewedfrom a third-floor window. We havecome a long way since then! Today, anyamateur photographer can produce aglossy full-color photo in a matter ofminutes using a digital camera andcomputer. In just the past 10 years, dig-ital photography has taken the world bystorm, threatening to do to film whatthe DVD has done to video.

Most of your family photos were probably taken theold-fashioned way, with film that had to be taken to aphoto shop to be developed. There is a fascinatingbunch of chemistry involved in this process. All photo-

graphic film is coated with a thinlayer of a silver halide compound,such as silver bromide (AgBr).When light strikes this layer, animage is recorded on film, which ismade visible during the developingprocess. If you have ever beeninside a darkroom, you have proba-bly seen all sorts of mysteriouschemicals such as developers, fix-ers, and baths. Even if you don’t

The Chemistry ofDigital Photographyand Printing

ChemSumer

Once upon a time, people putstuff called film in the their

cameras. First, they paid for it.Then they took photos, butcouldn’t preview them on a

screen. No deleting, no computerediting—they paid strangers todevelop every miserable photo,

hoping that a few were OK! So primitive! So last-century!

By Brian Rohrig

The world’s first photograph!

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ChemMatters, FEBRUARY 2006 5

quite know how it all works, you can stillappreciate the fact that a lot of chemistrygoes into developing pictures.

Does digital processing mark the end ofchemistry in photography? There is actuallyplenty of fascinating chemistry going on—it’sjust on a much smaller scale.

Sensing lightAll cameras work by focusing light

through lenses to create an image. A conven-tional camera records this image on film. A dig-

ital camera records thisimage on a permanent partof the camera known as asensor. A typical sensor ina digital camera measuresonly 4.4 mm × 6.6 mm.This is about the size of afingernail.

Sensor technologyhas enabled manufacturersto make digital cameras sosmall they can even beincorporated into cellphones. Similar sensordevices are used in faxmachines, scanners, copymachines, and bar codereaders at the grocerycheckout.

The sensor is a semi-conductor. Silicon is thematerial of choice for mostsemiconductors. This isironic because siliconbarely conducts electricityat all in its pure form. But

if a small amount of impurity is added,through a process known as doping, then sili-con becomes a fair conductor of electricity.

The sensor in a digital camera com-prises many tiny semiconductors known asdiodes. Diodes allow current to flow in onedirection, but not another. Diodes are com-posed of two different types of doped siliconlayers sandwiched together. One type of sili-con is doped with phosphorus or arsenic.Both of these elements contain five valenceelectrons. Because silicon atoms only havefour valence electrons, the doping agentsprovide the extra electrons that movethroughout the material. With its excess ofelectrons, this type of silicon is known as n-type, with the “n” referring to the negativecharge resulting from the free electrons.

Another type of silicon isdoped with either boron orgallium, which only have threevalence electrons. These dop-ing agents create a deficiencyof electrons in the structure,since silicon atoms have fourvalence electrons. This elec-tron deficiency creates elec-tron “holes” in the structure.Silicon doped with these defi-cient atoms is referred to as p-type silicon, with the “p”standing for the positivecharge resulting from the defi-ciency of electrons.

When placed together,these two types of silicon forma diode, the one-directionalconductor described above.Think of a diode as a one-waystreet for electrons. At the p-njunction, a positive chargebuilds on the n side, and a neg-ative charge on the p side untilthe internal electric field counteracts the ten-dency of the electrons to fill the holes. Theinternal electric field then permits current topass in one direction.

PhotositesEach diode in a sensor is a photosite.

Each photosite represents one picture ele-ment—better known as a pixel. The greater

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As more and more holes are filled with electrons, a region, neitherP nor N, forms, called the depletion zone. Holes are shown as .Electrons are shown as .

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SuperCCD SR structure diagram, one microlens, one color filter, two photodiodes per photosite.

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the number of pixels, the greater theresolution and overall quality of thepictures you take. For example, a typi-cal digital camera may have a resolu-tion of 640 × 480 pixels, for a totalresolution of 307,200 pixels. The bestdigital cameras on the market todayhave a resolution of more than 10 mil-lion pixels (10 megapixels). For com-parison, you should take pride in yourpersonal sensor. The human eye con-tains 120 million pixels!

The pixels of any photo can beclearly seen through the low power ofa microscope. The larger the pixel sizein a photo, the poorer the quality, aslarger pixels mean fewer pixels withina certain area. If you compare a normalcolor photo with a newspaper photo,you can see a huge difference in pixel size.Newspaper photos will have larger pixels,representing poorer quality.

When you take a picture with a digitalcamera, each tiny photosite on the sensor isexposed to light. When a photon is absorbedby the semiconductor, it promotes an elec-tron to a higher energy level. What thismeans is that the high-energy electron actslike an electron that was added by doping: Itis free to move about the semiconductor.Normally, the electron would just relax backto its lower-energy state. However, if it isnear the p-n junction, it is attracted to thepositive side, and migrates there, where it iscollected.

As more photons strike a photosite,more electrons are knocked free. The greaterthe intensity of the light that strikes a photo-site, the more electrons accumulate. A usefulanalogy is to think of the photosite as a tree,the photons as balls that you throw into thetree, and the electrons as leaves on the tree.Suppose that every time you throw a ball intoa tree, a leaf is knocked loose. The more ballsthat you throw into the tree, the more leaveswill accumulate on the ground below. A pho-tosite that has been exposed to very brightlight will contain far more electrons than onethat has been exposed to dimmer light. Gen-

erally, each image sensor can record 256 dif-ferent shades of gray, ranging from purewhite to pure black.

Seeing in colorSo then, how do digital cameras take

color photos, if the sensors can only recordshades of gray? The trick is to use filters, thatcombine to produce any color imaginable.Most cameras use the 3-color system to pro-duce color. The three primary light colors arered, green, and blue. Together, these threecolors make white. Any other color can beproduced by mixing together various shadesof these three colors. To accomplish this feat,

either a red, blue, or green filter is placed overeach photosite on the sensor of a camera. Themost common pattern is known as the Bayerpattern, which alternates a row of red andgreen filters with a row of blue and green fil-

ters. This configuration gives you twice asmany green filters as blue or red. Because thehuman eye is not sensitive to all three colorsequally, extra green filters must be used toproduce the best color for our eyes.

Next, the information at each of thesephotosites is converted to digital form. Bythemselves, electrons that accumulate at eachphotosite do not represent digital informationthat can be read by your computer. So everydigital camera carries its own built-in com-puter that converts information to digital formand stores it on your memory card.

PrintingOnce an image is recorded digitally by a

camera and downloaded onto a computer, itcan be printed. Or, it can be manipulatedusing software on a computer and thenprinted. The ability to choose, alter, and cropphotos on screen before printing gives even acasual photographer unprecedented power toprint only the images they want.

There are two basic types of printersthat can print photos: laser and inkjet. Thelaser printer works by using static electricity.The underlying principle involves positivelycharged toner sticking to negatively chargedpaper, since opposite charges attract. A laser

Information from photosites is converted to digital form and stored on memory cards for later retrieval.

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beam projects a negatively chargedimage of whatever is to be printedonto the light-sensitive drum. Thedrum is then coated with positivelycharged toner, which is attracted tothe negatively charged image on thedrum. An analogy would be writing amessage on the outside of a coffee canwith glue, and then rolling it in flour. Theflour will stick to the glue but not to the“unglued” parts of the can.

A piece of paper then passes over acharged roller, giving it an even strongernegative charge than the drum. The drumthen rolls over the sheet of paper. Thestrongly negatively charged piece of paperpulls off the positive toner from the drum.Finally, the paper passes through a pair ofheated rollers known as the fuser, whichfuses the toner to the paper. After the paperattracts the toner from the drum, a dischargelamp bathes the drum in bright light, erasingthe original electrical image.

Color printers work the same way,except the above process is repeated fourtimes. Four types of toner are used: cyan(bluish), magenta (reddish), yellow, and black.By combining tiny dots of these four colors,nearly every other color can be created.

A photocopier works according to thesame basic principle, except the electrostaticimage that forms on the drum is formed bybright light that reflects off the paper to becopied. The drum is manufactured with a pho-toconductive material on its surface thatmakes it sensitive to light. White areas of thepaper are reflected onto the drum. The blackink on the paper to be copied absorbs light, soparts of the drum do not receive an electricalcharge. These uncharged parts of the drumwill form the photocopy. Just like in a laserprinter, the negatively charged toner isattracted to the positively charged imageimprinted by light on the drum. A stronglypositively charged piece of paper then attractsthe toner from the drum. Your copy is com-

plete once it passes through the heated rollersof the fuser.

Inkjet printers, as the name implies, workby spraying tiny droplets of ink onto the sur-face of the paper. Each drop is very tiny, beingonly about 50–60 micrometers in diameter. A

micrometer (!m) is a thousandth of a mil-limeter. A human hair has a diameter of

about 70 !m. Thereare two maintypes of inkjet

printers onthe markettoday. Bub-ble jet print-

ers use heatto vaporize inkto form a bub-ble. This

expanding bubbleforces some of the

ink onto the paper.

When the bubble pops, a vacuum is created,causing more ink to flow from the cartridgeinto the print head. A piezoelectric printerworks using piezo crystals (such as quartz).Piezoelectric crystals generate an electric fieldwhen distorted, but conversely, they can bedistorted by an electric field. Thus, to get thenozzle to deform and eject the ink, an electricfield is applied. This electric charge causes thenozzle to vibrate, forcing ink out on the paper.

Digital photos can be printed usingeither laser or inkjet printers, but inkjet print-ers tend to produce better quality photos. Aninkjet photo printer will generally use six col-ors as opposed to the four that are normally

employed in a color printer. Other types ofphoto printers use a dye sublimation tech-nique. Sublimation is the process of chang-ing phase from a solid to a gas, skipping theliquid phase altogether. Heat is used tovaporize solid dyes, which permeate thepaper before they return to the solid form.Thermal autochrome photo printers requirethe use of special paper that already containsthe ink. A print head delivers variousamounts of heat to the paper, causing vari-ous pigments to appear.

Amazingly, experts agree that digitalphotography is still in its infancy. We will no

doubt see huge advances in digital quality andconvenience in the near future. Will digitalcameras completely replace conventionalcameras? There are photographers whoremain devoted to the artistic and visualeffects of developed film and darkroom pro-cessing. For most of us, it’s nice to know wehave plenty of options available for recordinglasting images of our big moments. And it’sall due to—you guessed it—chemistry!

ChemMatters, FEBRUARY 2006 7

Inkjet printers work by spraying tiny droplets in ink on the surface of the paper and tend to producebetter quality photos.

Brian Rohrig teaches chemistry at Jonathan AlderHigh School in Plain City, OH. His most recentChemMatters article “There’s Chemistry in GolfBalls” appeared in the October 2005 issue.

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