| 摘要 |
A readhead (18) measures movement in the direction of arrow (20) with respect to a scale (10). The readhead (18) comprises an index grating (12), which interacts with light from the scale (10) to produce fringes in the vicinity of an analyser grating (14). The resulting light modulations are detected by photodetectors (16). In order to reduce the sensitivity of the apparatus to changes in the ride height (h) of the readhead above the scale, at least two different optical path lengths are provided for light travelling from the scale (10) to the index grating (12), and/or from the index grating (12) to the analyzer (14). Various ways to achieve this are disclosed, e.g. a plate (22) of a refractive medium placed in one optical path, or tilting one of the gratings.
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