Measurement of moderate wedge angles of optical windows using a wedge shear plate

We present what we believe to be a new technique for the measurement of moderate wedge angles of optical wedge plates using a wedge shear plate (WSP). Using the technique, a wedge angle of up to 5 deg can be measured. The WSP, oriented at an angle to the incident collimated beam, produces shear fringes. The shear fringes are parallel-shifted by rotating a test wedge plate (TWP), in its plane, placed at the input beam of the WSP. The fringe shift is due to the variation of the optical path difference between the interfering beams reflected from the front and back surfaces of the WSP, brought about by the change in the angle of incidence on the WSP due to the rotation of the TWP. The maximum number of fringes shifted as the wedge direction of the TWP (initially aligned perpendicular to the wedge direction of the WSP) is rotated by 180 deg is used for the measurement of the wedge angle of the TWP. Results obtained for a 59.0-arcmin wedge plate with a measurement uncertainty of +/- 4.68 arcsec are presented. The technique is very simple, and the results are not affected by external vibrations. (C) 2010 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3454380]