One of the most common methods used in the Flat Lapping sector to measure flatness is by using a Monochromatic Sodium light unit and an Optical Flat. An Optical Flat is a high-precision glass or quartz flat surface that acts as a reference plane. When a monochromatic light source, such as a Sodium light unit, is directed onto the Optical Flat, the light is reflected and forms a series of bright and dark fringes called light bands.