A compact disc (CD) can be used as a diffraction grating, even though its track consists of a series of pits, not a continuous groove. Previous authors described how to measure the track spacing on a CD using an incident laser beam normal to the surface or one at an oblique angle. In both cases, the diffraction pattern was projected on a screen and distance measurements allowed the track spacing to be calculated. I propose an alternative method using a laser ray box, which is also applied to a DVD and a Blu-ray disc.