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IR Photography - PowerPoint PPT Presentation

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IR Photography
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  1. IR Photography Forensic Applications

  2. 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.

  3. Water Absorbs Near IR Light Water Absorbs Near-IR Light 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

  4. Digital IR Photography • Most digital imaging chips sensitive to IR light • Must remove filter covering chip • How to check camera • Turn off auto-focus & set camera to bulb at wide aperture • In dark room: open shutter, point TV remote control @ 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 • Low pass filter • 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.

  5. 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

  6. The Electromagnetic Spectrum Using Light to Find Evidence Ultraviolet Region 190-290 290-400 Short wave Long wave Visible Region Infrared Region 400-455 455-492 492-577 577-597 597-622 622-700 >700 Blue Green Yellow Orange Red Violet IR Near IR – NIR ~ 700-1200nm Far IR – FIR > 1200nm

  7. Infrared Light Sources Maglite • 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

  8. 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 Light: Visible/UV/IR Camera Sensor Near IR Light Bandpass Filter Blocks Visible & UV Light Allows Near IR Light to Pass Through

  9. Predominately a Red Color Photographed

  10. X-Nite Filters

  11. IR Selective Filters BPB BPG BPR

  12. Forensic IR Applications

  13. Gunshot ResidueBlack t-shirt GSR Particles T-shirt does not absorb Infrared light

  14. Searching for Blood @ Crime Scene Dark Surfaces That Do Not Absorb in the Infrared

  15. IR Photography of GSRThrough Bloodstains Blood Absorbs In the IR GSR Particles

  16. Check Fraud In visible light the check looks to be written for $400 dollars In 830nm (Deep BW) Infrared light you can clearly see the original amount was $100

  17. Post-Mortem Photography