Illumination of a switch or of certain features of a switch often has a functional purpose and must meet specification to satisfy the functional requirements of the switch.

Illumination of the switch can be affected by variations in the quality and design of the overlay and its application.

This test method addresses only the optical and visual appearance of the switch and not its electrical function.

This test method is non-destructive.

If this test method is applied to the entire switch assembly, the results can be applied to the whole device. However, it may be sufficient and practical to apply the test either to a subassembly only, or to the illumination layer only, in which case the results apply to that layer only and the net effect on the fully assembled device must be calculated, extrapolated, or otherwise inferred.