Diffraction grating film (holographic type). A diffraction grating is a plane surface that contains literally tens of thousands of microscopic lines or grooves. A phenomenon called “diffraction” causes white light to be split or diffracted into its component colors. These are replica gratings (copies) placed in 2" × 2" cardboard mounts. Each mount provides approximately one square inch of exposed film. Sold in packages of 10 mounts. 1,000 lines/mm.