Illumination: Infrared illumination
Infrared (IR) illumination can be used to diminish colour effects in monochrome images. The example shows three different views of the same set of coloured marker pens.
- The first image has been illuminated with standard white light and a colour camera has been used.
- The second image is the output from a monochrome camera, still using white illumination. The colours have been reduced to their grey values, but there is still a wide variation in the light intensity.
- The last image has been lit with an infrared light source. This has the effect of reducing the colour information.
Now all colours (except brown and black) show more or less the same brightness level, offering the possibility to identify a black print on the markers with a much better contrast.
Infrared illumination can also be used to look through coloured prints to inspect the surface below. They can also be utilised in combination with infrared sensitive cameras to reduce the effects of ambient illumination, as common indoor ambient light sources tend to have a low IR content. It should be remembered however, that this technique will not work in sunlight, as the sun has high levels of IR light in its spectrum.