Lecture 21 - Lecture 21 Lecture 21 Twyman-Green interferometer M1 reference mirror laser imaging lens test surface camera If the test surface is

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Lecture 21 1 © Jeffrey Bokor, 2000, all rights reserved Lecture 21 Twyman-Green interferometer If the test surface is perfect, then the path length is identical across the beam, and the intensity is uni- form on the camera. can be translated, and then like the Michelson interferometer, the intensity varies uniformly over the camera. If the mirror has aberration, then a fringe pattern appears on the camera that gives a signature of the aberration. A very accurate measurement of the aberration is made by varying . At each point in the image there is an oscillation in intensity as moves. With aberration, there are variations in the phase from point to point. This can be determined very accurately. This technique is called Phase-Shifting Inter- ferometry (PSI). Thin film interference The phase difference between the reflected rays can be shown to be For , we get a bright fringe; for , we get a dark fringe. Variations in , or give rise to fringes. camera imaging lens laser reference mirror test surface M 1 M 1 max min M 1 M 1 1 P d n thin film, thickness d, index n  4 n --------- d P cos + = 2 m = 2 m 1 +  = d n 
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Lecture 21 2 © Jeffrey Bokor, 2000, all rights reserved
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Lecture 21 - Lecture 21 Lecture 21 Twyman-Green interferometer M1 reference mirror laser imaging lens test surface camera If the test surface is

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