|Oops moment. Shooting IR photo through auto glass.|
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 and different camera conversions|
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.