Adding Infrared to Red, Green and Blue
As you probably know, a colour image can be broken down into three separate images showing the red, green and blue parts (or channels) of the scene since red green and blue are the three primary colours for light.
This does not perfectly reproduce the scene since not all colours we are able to see can be built this way, but it suffices for most purposes and is used in television and computing. Because you add the red, green and blue images together to make full colour, this is known as additive colour. For printing and most colour film, subtractive colour is used where the secondary colours - yellow, cyan and magenta - are used. For printing, black ink is often used as well which is where CMYK comes from since K is the black channel.
You can think of near infrared light as being another light colour channel which gives you infrared, red, green and blue to play with. First, let's consider the three visible colour components of a scene.
The colour image above is made from the red, green and blue channels which, when you split them out, are subtly different monochrome images in their own right. The blue one is darkest, because this scene is dominated by trees and grass and the sky was cloudy at this time.
The two types of infrared film were black and white and colour. The black and white simply captured the infrared light of a scene based on what kind of filter was used. Any filter from a deep red (Wratten 25) down to true IR-pass filters, which are visually opaque, could be used. The deeper the infrared-pass filter was, the lower would be the exposure.
Colour infrared film was more complicated. The basic idea here was to shift the colours around in such as way that the resulting image represented something useful. The final configuration of Kodak's Ektachrome Infrared film (EIR) worked by coupling infrared to the red-sensitive layer of the film. Red from the scene was mapped to the green layer and green to the blue. Blue was filtered out: so a yellow (Wratten #14 minus-blue) filter was usually used.
Healthy foliage, which reflects infrared strongly, would show strongly in the resulting red channel and, being green, would also show strongly in the new blue channel. How these two were balanced could tell you how healthy, or stressed, the foliage was. Stressed foliage tended towards a magenta colour whereas healthy foliage, would tend towards red. The early use of such film to detect camouflage made use of this effect.
The first image here is the scene in near-infrared light. In this case, it's as seen through a filter with a 50% rolloff at 720nm (a Hoya R72).
The second image here is the scene as it would show using EIR film, with IR mapped to the red channel, red to green and green to blue.