In the 1920s Vasco Ronchi developed the well-known method of testing optical systems now named after him. The essential features of the Ronchi test may be described by reference to Figure 44.1. A lens (or more generally, an optical system consisting of a number of lenses and mirrors) is placed in the position of the “object under test”. The lens is then illuminated with a beam of light, which, for the purposes of the present chapter, will be assumed to be coherent and quasi-monochromatic. These restrictions on the beam may be substantially relaxed in practice.

The lens brings the incident beam to a focus in the vicinity of a diffraction grating, which is placed perpendicular to the optical axis, i.e., the Z-axis. The grating, also referred to as a Ronchi ruling, may be as simple as a low-frequency wire grid or as sophisticated as a modern short-pitched, phase/amplitude grating. The position of the grating should be adjustable in the vicinity of focus, so that it may be shifted back and forth along the optical axis. The grating breaks up the incident beam into multiple diffracted orders, which will subsequently propagate along Z and reach the lens labeled “pupil relay” in Figure 44.1.

The pupil relay may simply be the lens of the eye, which projects the exit pupil of the object under test onto the retina of the observer.