The fringes only appear in the reflection of the light source, so the optical flat must be viewed from the exact angle of incidence that the light shines upon it. If viewed from a zero degree angle (from directly above), the light must also be at a zero degree angle. As the viewing angle changes, the lighting angle must also change. The light must be positioned so that its reflection can be seen covering the entire surface. Also, the angular size of the light source needs to be many times greater than the eye. For example, if an incandescent light is used, the fringes may only show up in the reflection of the filament. By moving the lamp much closer to the flat, the angular size becomes larger and the filament may appear to cover the entire flat, giving clearer readings. Sometimes, a diffuser may be used, such as the powder coating inside frosted bulbs, to provide a homogenous reflection off the glass. Typically, the measurements will be more accurate when the light source is as close to the flat as possible, but the eye is as far away as possible.