A collimated beam consists of plane wavefronts that neither converge nor diverge as spherical wavefronts do. Collimation is desirable for many types of optical testing and measurement. Control Optics Corporation offers two instruments for easily testing a beam for collimation: collimation testers Model JQG-1 and Model JQG-2.

The collimation tester uses a single glass plate (called a shear plate) to divide and recombine the beam. This produces an offset between two wavefronts. The shear plate is therefore a very simple interferometer (see Figure 1). When the beams are recombined, constructive and destructive interference between the two wavefronts leads to a series of bright and dark fringes. These fringes indicate whether the beam is parallel, converging, or diverging (see Figure 2). To make the beam parallel, simply adjust the collimation optics being used on the beam until the fringes line up with a preset reference line on a screen. The direction of misalignment determines the beam’s convergence or divergence.