This is the branch of optics which deals with image-forming instruments, including of course spectrographs and interferometers. Such instruments employ lenses, mirrors and prisms and it is the art of combining these elements to make useful devices which is the subject of the next two chapters.

Rays and wavefronts

Image-forming instruments are intended to project images of real objects on a screen or focal surface – usually though not always plane, and instrumental optics is the study of ways of doing this.

The subject comprises two chief parts: instrument design and lens design.

The former is the design of instruments so that light is conveyed through their optical components from one to another to arrive eventually at the focal surface. The technique is essentially a graphical one, using a drawing board or a computer drafting program to lay out the components at their proper places.

The latter involves the accurate tracing of rays through various optical surfaces of different types of glass, and reflections from mirrors of various shapes to achieve a correction of the optical aberrations and to ensure that light of all wavelengths from a point on an object is focused to a corresponding point on its image.

Traditionally this was done by ray tracing, the accurate computation of ray paths using seven-figure logarithms and trigonometrical tables, but now is done chiefly by iterative ray tracing in a small computer, using a program specifically designed for that purpose.