1 April 2002
Angle and Straightness Measurement with an Interferometer
By changing the optical arrangement, you can measure both angular motion and straightness deviation. One setup for angular measurements adds a fold mirror to the displacement interferometer, thus using the two retroreflectors in a differential mode (Figure A). The angle is determined by taking the arctangent of the phase change and dividing it by the retroreflector separation distance.
Straightness of motion is measured by using an optic called a Wollaston prism and a chevron-shaped mirror (Figure B). The Wollaston separates the laser beam into two components at a fixed angle. The mirror surfaces intercept the beams at right angles. During displacement, any straightness deviation will cause one beam to shorten and the other to lengthen. The resultant phase change can be converted to a straightness measurement according to the relationship of the mirror angle.
|Figure A. Angular interferometer|
|Figure B. Straightness interferometer.|
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