Dokument: CLN 06_05 How digital one-shot cameras perceive color

How digital one-shot cameras perceive color

One-shot color CCDs (= charge coupled devices) with a built-in filter grid are the most common image receivers in the ma-jority of today's digital cameras, be it still or motion. They have a regular pattern of optical color filters in the three primary colors R, G, B (Red, Green, Blue) arranged over the light sensitive pixels of the CCD. Such a color filter is a device that lets light of its own color travel through and sharply cuts out all other colors. Thus a red filter would let red light pass through and completely block out blue and green. Each pixel of the CCD has exactly one filter color patch in front of it. It can sense the intensity for this color only. But how can the two remaining color intensities be sensed at the very location of this pixel? They cannot.

They have to be generated instead through interpolation (averaging) by monitoring the signals from the surrounding pixels which have filters of these other two colors in front of them. The interpolation algorithm assumes, that colors and intensities are distributed in some random fashion. In general this assumption and the algorithms based upon it deliver pretty accurate results.

In some special situations, however, they generate artifacts. The picture can show colors where only black and white (bright and dark with no color whatso-ever) should be. This side-effect is unavoidable with one-shot colour CCDs coming with a built-in filter grid. This is not a matter of certain makes or of negligence in design or manufacturing. It is just principally unavoidable.

Camera Lens News No. 6, spring 1999