Why do most webcams have an Infra Red blocking filter?
Most webcams and video cameras have an IR blocking filter, either built into the lens or mounted behind it, in front of the detector chip The reason is that the detector chips (CCD or CMOS) are sensitive to Infra Red light well beyond the range of the human eye. If this is not filtered out, it can cause two problems 1. Colour rendition is affected. This is because the red green and blue colour filters built into colour detector chips are transparent to IR and so IR goes straight through to the chip, dilutes the visible colours and causes colour casts. 2. Focus is affected. Lenses do not focus all wavelengths (colours) of light to the same point. Lens design can minimise this for visible wavelengths (eg achromatic, apochromatic designs) but not for Infra Red. Without an IR filter, unfocussed IR causes soft focus and halos round bright objects.
