A wide range of optical filters is available for use with cameras. Filters are used to modify the transmitted spectral characteristics of the image-forming light by selective absorption. Other attachments used in front of the lens can be spectrally non-selective, but usually alter the nature of the image.
A colour filter consists of a transparent flat sheet of coloured material, usually placed in front of, within or behind a camera lens. It acts by selective absorption of the light incident upon it, as shown in Figure 11.1. A coloured opaque object such as a yellow sheet of paper reflects green and red light and absorbs blue, so a yellow transparent object transmits green and red light and ...