A three-beam lateral shearing interferometer has been developed. The three-beam shearing interferograms consist of two sets of fringes, one of which acts as a carrier whose intensity is modulated by the second set. The depth of modulation is directly proportional to the phase difference between the middle beam and the outer beams. Phase errors on the order of π/2 cause every other fringe to go from complete dark to complete bright. Therefore, phase errors much smaller than π/2 can be detected. The three-beam interferometer is implemented in three ways: (1) thin film thickness measurement, (2) surface roughness measurement, and (3) surface figure measurement. The three-beam interferometer implemented to measure thin film thickness and surface roughness is accurate to 25 Å. Surfaces with different microstructure are characterized. We have found that each of those surfaces may have one or more of the following: (1) random roughness, (2) slow waviness, and/or (3) periodic structure. The three-beam interferometer is also implemented for testing optical surfaces. The three-beam interferometer is more capable in detecting small zonal errors than the two-beam interferometer. Three-beam interferograms of different surfaces are produced and analyzed. The three-beam interferometer has many advantages: (1) it is a stable, common path interferometer, (2) white light can be instead of coherent light to get rid of the effects of speckle and dust particles, and (3) it is a very low cost interferometer.
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