1. It is critical that both the component and optical flat are perfectly clean before inspection. Any amount of contamination, regardless how small, will affect the measurement. 1 sodium light band is equal to 0.29 microns, so any particles or smears on the surface of either the component or the optical flat will affect the measurement.
This illustration shows how a human hair compares in size to a 1 micron particle. You will not see particles that are 1 micron in size, but if present, these can affect the flatness reading by as much as 3 light bands. This highlights the need for perfectly clean components and optical flats. When cleaning the optical flats, use a lens grade cleaning tissue and either Isopropyl Alcohol or Kemet CO42 cleaning fluid. Harsh tissues will cause scratches on the optical flat.