A collimated beam consists of plane wave-fronts that neither converge or diverge as spherical wave-fronts do.  Collimation is desirable for many types of optical testing and measurement.  Control Optics Corporation offers an instrument to easily test a beam for collimation, the Model JQG-1.  There is also the more advanced Model JQG-2.

The collimation tester has a single glass plate (called a shear plate) to both divide and recombine the beam.  This produces an offset between two wave-fronts.  The shear plate is therefore a very simple interferometer (see figure 1).  Constructive and destructive interference between the two wave-fronts leads to a series of bright and dark fringes. These fringes tell 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.