Comment by _ph_

I was talking here about the typical foto cameras. Of course you can measure the wavelength of the light with other devices like spectrometers. I was specifically talking about camera sensors which have separate filters in usually 3 colors in front of the pixels. The sensors, which are made by Sigma, formerly Foveon, use a different principle. They determine the wavelength by measuring how deep electrons are generated by the photons in the silicon. The depth depends on the wavelength of the light. However, it is more difficult to get a precise color response that way as you cannot just use predefined color-filters.