Setting the Wedge Factor

In an interferometer the source beam is divided into two wavefronts. One wavefront is directed to a reference and the other to the test optic. The two beams are then reflected back to the interferometer and combined, creating an interference fringe pattern from which the wavefront difference can be determined.

This measured wavefront difference, however, may or may not equal the desired measurement quantity (i.e., the measurand) because the geometry of the test setup will act to scale the measurand. The Wedge Factor (also called “Scaling Factor,” or “Interferometric Scaling Factor”) converts the measured wavefront error to the desired quantity.

Wedge Factor is calculated as A*B*C, where:

A equals 1 when measuring transmitted or reflected wavefront, or 1/2 when measuring surface height.

B compares the number of passes through, or reflections from, the optic in use versus the number of passes/ reflections in the test setup. When the measurand is surface height or reflected wavefront:

When the measurand is transmitted wavefront:

C compares the angle of incidence θ for the incoming beam, in use and in the test setup:

 The following chart shows the correct wedge factor settings for common measurement setups:

For a more thorough explanation of wedge factor settings and how they are derived please download the Setting the Wedge Factor 4D technical note.