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 see can be built this way, but it suffices for many 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 colour channel which gives you infrared, red, green and blue to play with. The black and white photographs on this site are made by combining infrared and red light, although it is possible to use only infrared light. Let's use this scene on the left as an example. I used a digital video camera to record it because that allowed me to capture all four of the channels I wanted since the CCD in the camcorder was able to 'see' infrared light if I filtered out all visible light.

Red	Green	Blue

The colour image above is made from the red, green and blue channels which, if 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 near infrared light in the same scene is, however, somewhat different. The leaves on some of the trees and the grass reflect a lot of light and so appear 'white'. The sky, being cloudy, is 'white' as well but the tree trunks and the garden shed are dark. This is the result you would get by using infrared film and shooting through a visually opaque filter such as the Hoya R72.

Next: Options for combining the 'colours'

	More background on infrared photography
Technical background Thermal Imaging NASA IR Video Camera IR 'Colour' Mapping 2 Imaging Abstracts Noctovision