Patent ID: 6956655

Claim:
A method comprising: providing a test interferometer which during operation directs two beams along different paths and then combines them to produce an output beam, wherein the output beam comprises information about an optical path length difference between the two beams, wherein imperfections in the test interferometer cause a measurable interferometric test phase derived from the output beam to deviate from the expression Φ=pknL, where p is an integer, k is the wavenumber of the output beam, and nL corresponds to the optical path length difference, wherein the deviation comprises a non-cyclic error term that varies in a nonperiodic way on the optical path length difference; using the test interferometer to measure the test phase as a function of a test parameter that causes the optical path length difference to vary; comparing the measured test phase to another expression indicative of the optical path length difference; and determining a representation indicative of the non-cyclic error term for the test interferometer based on the comparison.