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 nm.  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.

Visible (left) vs. Infrared 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 are sensitive to IR light  Must remove filter  How to check camera:  Turn off auto-focus & set camera to bulb using a wide aperture  In a dark room: open shutter, point TV remote control at lens & press button on remote  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 sensitivity o Low pass filter o Removing requires complete teardown of camera, removal of imager  Using an IR lens Example #87 Wratten filter o Can put high-pass filter on imager behind shutter in place of Wratten filter Use a tripod  Example of exposure on sunny days  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 Ultraviolet Region Short wave Long wave Visible Region > Infrared Region Violet BlueGreenYellowOrangeRed IR Near IR – NIR ~ nm Far IR – FIR > 1200nm

Maglite Infrared Light Sources  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

Blocking Light Entering Camera Bandpass Filters  Block wavelengths of light from hitting the digital sensor  Allow unblocked wavelengths into the camera  Different Filters have different characteristics Camera Sensor Bandpass Filter Blocks Visible & UV Light Allows Near IR Light to Pass Through Near IR Light Light: Visible/UV/IR

Predominately a Red Color Photographed

X-Nite Filters

BPBBPGBPR IR Selective Filters

Forensic IR Applications

Gunshot Residue Black t-shirt T-shirt does not absorb Infrared light GSR Particles

Searching for Crime Scene Dark Surfaces That Do Not Absorb in the Infrared

IR Photography of GSR Through Bloodstains GSR Particles Blood Absorbs In 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

Post-Mortem Photography