When using optical flats to assess the flatness of processed components, the field of view must be illuminated with monochromatic light. Monochromatic light sources are very effective for viewing Brewster's fringes and Newton's rings. This monochromatic lamp uses a standard commercial 35W low pressure sodium SOX lamp which gives very economic running costs and a long operating life. The effective monochromatic wavelength is 589.6nm, so λ/2 is approximately 0.3 microns.
In front of the bulb is an opalescent diffusing screen with a straight line engraved across its face. By suitable adjustment of the flat, work and eye position, the reflection of the engraved line provides a straightness reference with which the bands can be compared.