IR Photography

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 Region400-455 455-492 >700492-577 577-597 597-622 622-700

Infrared 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