| 摘要 |
A precision reference point detector includes a pair of relatively displaceable, wedge-shaped, birefringent prisms juxtaposed with their respective optic axes in orthometric relationship and having their thick sides and thin sides correspondingly oriented. Polarized light rays are passed through the prisms, a pair of coplanar quarter-wave plates and an analyzer onto a photoreceptor where they are converted into electrical signals. A rotatable light chopper, interposed in the light path between the quarter-wave plates and the analyzer, selectively blocks the light emerging from alternate quarter-wave plates such that the light impinging on the photoreceptor is alternately advanced and retarded 90 DEG in phase. When a relative displacement at right angles to the light path is imparted to the prisms, detector circuitry coupled to the photoreceptor receives and demodulates a symmetrical amplitude modulated signal having a carrier at the chopping frequency and an envelope having diminishing intensity peaks with sharply defined nulls therebetween. A reference position for the quartz prisms may be defined by the null between the two highest peaks as observed on a measuring instrument at the detector output. By affixing one of the prisms to a movable instrumentality or workstage while the remainder of the apparatus, including the other prism, remains fixed in space, the null may be utilized to define a reference position for the instrumentality.
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