A METHOD OF ANALYZING ZONE-PLATE INTERFEROGRAMS USING A NEMATIC LIQUID-CRYSTAL CELL

A nematic liquid-crystal cell is used to analyze zone-plate interferog rams. The liquid-crystal cell is located in front of the vertex of the mirror under test. The cell is vibrated in the direction perpendicula r to the optical axis by a vibrator in order to eliminate noise in the interferograms. One problem is variation in the properties of the cel l. Therefore the following method is used: A function generator suppli es a saw-tooth signal with a carrier frequency to the liquid-crystal c ell. The phase of the reference light transmitted through the cell var ies according to the change in the voltage of the signal. The intensit y of a zone-plate interferogram varies according to the phase differen ce between the reference light and the test light. Four interferograms with different phases are recorded by a charge coupled device camera, and are fed into a computer one at a time. Distribution of the phase differences caused by the error in the shape of the mirror under test is obtained by analyzing the four interferograms. Errors in the shape of two spherical concave mirrors were measured by phase shifting inter ferometry using a nematic liquid-crystal cell.