ir photography forensic applications. characteristics of ir photography in digital ir photography...
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IR Photography
Forensic Applications
Characteristics of IR Photography
In digital IR photography the sensor is sensitive to IR light – Near-infrared is not far-infrared, which is for thermal imaging. Wavelengths range from 700- 900nm.
Filters used with infrared-sensitive sensors give interesting “in-camera effects“: false-color or black and white images with a dreamlike appearances known as the "Wood Effect," Caused by foliage (such as tree leaves and grass) strongly reflecting in the same way visible light is reflected from snow. There is a
small contribution from chlorophyll fluorescence, but this is marginal and is not the real cause of the brightness seen in infrared photographs. The effect is named after the infrared photography pioneer Robert W. Wood.
Other attributes of infrared photographs: very dark skies and penetration of atmospheric haze, caused by reduced Rayleigh scattering and Mie scattering, respectively, compared to visible light. The dark skies result in less infrared light in shadows and dark reflections of skies from water Clouds stand out. Near IR wavelengths also penetrate a few millimeters into skin and give a milky look to portraits. Eyes often look black.
http://en.wikipedia.org/wiki/Infrared_photography
Visible (left) vs. Infrared (right @ 900nm) Aerial Photography Old Hickory Lake, TN.
Taken from a passenger airplane seconds apart using Sony H-9 Digital camera
Water Absorbs Near IR Light
Water Absorbs Near-IR Light
Digital IR Photography Most digital imaging
chips sensitive to IR light Must remove filter
covering chip How to check camera
o Turn off auto-focus & set camera to bulb at wide aperture
o In dark room: open shutter, point TV remote control @ lens & press button on remote
o If end of remote glows, camera is sensitive to IR radiation Will be able to
convert camera
Converting cameras Remove IR filter
Manufactures add filter to imaging chip that cuts IR sensitivityo Low pass filtero Removing requires complete
teardown of camera, removal of imager.
Shooting in IR Use #87 Wratten filter & tripod
Can put high-pass filter on imager behind shutter in place of Wratten filter
Exposure on sunny days 1/60@f/11, ISO 200.
What do you Need to Know?
Where in the electromagnetic spectrum is the IR region
What part of the IR region is used What are the sources of IR light How to interpret filter data What are the applications Under what circumstances would you use
IR photography
The Electromagnetic Spectrum Using Light to Find Evidence
190-290 290-400
Ultraviolet Region
Shortwave
Longwave
Visible Region
400-455 455-492 >700492-577 577-597 597-622 622-700Infrared Region
Violet Blue Green Yellow Orange Red IR
Near IR – NIR ~ 700-1200nmFar IR – FIR > 1200nm
Digital IR photography typically relies on a NIR light source Sun Incandescent lamps.
Digital camera sensors based on silicon: NOT sensitive to the far (thermal) IR (typically > 3.0µ )
To photograph in the dark Provide proper NIR illumination External NIR-only flash with no filter
Infrared Light Sources
Maglite
Blocking Light Entering CameraBandpass Filters
• Block wavelengths of light from hitting the digital sensor
• Allow unblocked wavelengths into the camera• Different Filters have different characteristics
Camera Sensor
Bandpass FilterBlocks Visible & UV Light
Allows Near IR Light to Pass Through
Near IR LightLight: Visible/UV/IR
Predominately a Red Color Photographed
X-Nite Filters
BPB BPG BPR
IR Selective Filters
Forensic IR Applications
Gunshot ResidueBlack t-shirt
T-shirt does not absorb Infrared light
GSR Particles
Searching for Blood @ Crime Scene
Dark Surfaces That Do Not Absorb in the Infrared
IR Photography of GSRThrough Bloodstains
GSR Particles
Blood AbsorbsIn the IR
Check Fraud
In 830nm (Deep BW) Infrared light you can clearly see the original amount was $100
In visible light the check looks to be written for $400 dollars
http://www.lifepixel.com/galleries/forensic-photography-gallery
Post-Mortem Photography
http://www.evidencemagazine.com/index.php?option=com_content&task=view&id=106&Itemid=49