A CCD's primary function is to produce images. At first, it appears to rival photography. Furthermore, it allows luminous flux measurements, and therefore rivals photometers.

It is in comparing its performance with existing detectors, photographic film in particular, that we can easily see in what areas the CCD will assert itself and what new areas can be opened up.

Image quantification and linearity

By its nature, the CCD image is digitized with a regular spatial sampling. Moreover, the digital value representing each image point (after the dark and flat field corrections) is proportional to the amount of light received. Hence, the image is directly usable in digital processing, which makes it accessible to powerful information extraction tools, described in chapter 5.

Digitally processing a photographic image is much less natural. We must first sample the image at regular intervals: a microdensitometer is placed in front of the film which measures its density over an area a few micrometers wide; but this measurement is not proportional to the quantity of light the film received and the area measured must be converted into the amount of light received by the film's standard response curve, for which there is no precise source. Furthermore the mechanism which moves the microdensitometer from one measurement zone to another on the film's surface must be accurate to within a micrometer, which is not easy to achieve.