Since the flatness tolerances required today can be in the range of a few microns, an effective method of measuring flatness is by using an Engis optical flat with a suitable light source.
To use the Engis optical flat, it is carefully placed on the component, directly under the light. The operator then views the component through the optical flat to see the light and dark bands at the interface of the component and the optical flat. These light bands are used to determine the flatness of the component.