| 摘要 |
PURPOSE:To correct an error in measurement due to variation in the refracting index of the 1st liquid and to take an accurate measurement by providing the 2nd medium in an optical path of reflected light and setting a specific relation between its refractive index and the rate of variation in refracting index or thickness. CONSTITUTION:The 2nd medium 8 having thickness D is arranged in the optical path of the reflected light from the horizontal surface 3 of the 1st liquid 2. Then, an equation holds, where K is the ratio of variation in the refractive index between the medium 8 and liquid 2, (t) is the thickness of the air part between an image forming lens 6 and a photodetector 7, and N is the refractive index of the medium 8. Consequently, even if the liquid 2 varies in refracting index with temperature, etc., the error is corrected by the medium 8, so a slanting surface is measured accurately.
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