Despite the great progress which is being continuously made towards direct photoelectric recording, it is almost certain that for many years to come photographic plates will remain the most common means for detecting images of stars and stellar spectra, whether directly or through the intermediary of an image intensifier.

The photographic image of the spectrum of a star contains an enormous wealth of information whose utilization is one of the most important tasks of observational astrophysics. To achieve this task many difficult problems must be solved, the first being to express the information in a form which lends itself to further processing.

This is usually done by means of two different kinds of instruments.