A Guide to the Practical Use of Aerial Color-infrared Photography in Agriculture
All life on this planet exists in a sea of electromagnetic-radiation. Most of us are unaware that everything from cosmic and gamma rays to radio and television waves are always moving through our atmosphere. We tend not to notice the movement of these various forms of radiation because they are invisible. In fact, our eyes are sensitive to only a very small part of the total number of energy forms that continually bombard our environment.
Panchromatic and ordinary color film both record essentially what we see with the naked eye, but CIR film uses the light spectrum just beyond the sensitivity of the human eye. This portion of the spectrum is known as the "near-infrared" or photographic-infrared" region (fig. 1).
One must be very careful not to be confused by the term "infrared". The thermal (or heat-emitting) properties of the terrain and vegetation are not recorded on CIR film. This is a common misunderstanding on the part of the casual user of CIR imagery. With CIR film, one is measuring only the amount of near-infrared energy being reflected or absorbed by a given surface, not its temperature.