Sunday, September 22, 2013

Auto Glass and Infrared Photography

I was on the Huron Parkway bridge waiting for the traffic light to change when I noticed some interesting clouds in the distance. The infrared camera (standard 720 nm conversion) with a 20 mm wide angle lens was on the seat beside me so I popped off the lens cap and shot a few frames at f/8 with matrix metering and auto focus. I did not use the viewfinder or LiveView because the camera was in front of my chin and pointed toward the left window of the car.   

Oops moment. Shooting IR photo through auto glass.
When I looked at the frames, I noticed that the side window was only partially down and the view through the window portion was exceedingly dark. It was one of those duh! – slap yourself on the forehead - moments. This experiment reminded me to lower the window completely because auto glass is treated to block infrared in order to reduce the heat load in the vehicle.
 
According to the U.S. Department of Energy, some 230 million vehicles in the U.S. consume 7 billion gallons of gas annually to power air conditioning (AC). By reflecting the sun’s heat and maintaining a cooler cabin temperature, IR reflective glass reduces AC power consumption up to 20 percent, increase miles-per-gallon by up to 5 percent, and lower emissions. By the 2014 model year, all new cars and trucks sold in California must have infrared-reflective glass under the California Greenhouse Gas Emission Reduction directive.

Information published by the Guardian Industries Corp. (Auburn Hills, Michigan, USA) indicates that IR reflective films block about 85% of the near IR light (about 3 f/stops) while reducing the visible light (what your meter sees) by 25% (half a stop). Grey tinted windows will reduce the visible light a little more. This means that the IR photographer who uses an IR-converted SLR camera will need to add at least 3 EVs of exposure compensation when shooting through auto glass. It also means that there will be about 3 stops less IR light in the vehicle when photographing interior objects with the windows closed. IR-converted mirrorless cameras will not have this difficulty because these cameras meter what the sensor sees.

The selective nature of the auto glass filtering – passing visible light and blocking IR – made me curious about the performance of the 590nm (SuperColor) conversion. The SuperColor IR filter passes some yellow, orange-yellow, and red visible light in addition to the IR wavelengths. To find out more, I took photographs through the partially lowered side window of a car (this time on purpose) towards a large photographic reflector. The reflector minimized any changes in luminosity due to the scene beyond the window (i.e., bright sky and dark foreground). All shots used a 20mm lens at f/8, with matrix metering, and focusing on the edge of the window. EV compensation was adjusted for each camera to balance the luminance values in the histogram. Cameras were placed on a tripod to assure the same field of view. My friend Vicki was kind enough to hold the reflector for me.

Effect of auto glass with different IR camreras
Effect of auto glass and different camera conversions
The composite photo shows the images captured with an unconverted D90 (VIS), a SuperColor (590) converted D90, standard infrared (720) converted D90, and a D90 with the deep infrared conversion (830). The visible light photograph shows very little darkening of the scene when viewing through the auto glass. The other images show significant darkening of the scene in the auto glass portions. While it is difficult to quantify the amount of near IR blockage, my visual assessment suggests that the 590 nm SuperColor conversion had the least IR attenuation of the three IR cameras and the 830nm deep infrared camera had the most.

Shooting IR photographs through auto glass will result require significant EV adjustments before one can obtain an acceptable photograph. This could be an advantage in extremely bright situations and when additional contrast is needed.

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