PROBLEM TO BE SOLVED: To more exactly compensate effects on length measurement accuracy caused by refractive index variations in a length measurement optical path, even when it is not easy to set a compensation-use sensor or a temperature adjusting mechanism near the length measurement optical path or in the whole area of the optical path.;SOLUTION: In a laser interferometer length measurement apparatus including a laser interferometer 2 for measuring a distance from an object 1 to be measured and a control means 9 for controlling at least one of refractive index variation parameters varying a refractive index of the length measurement optical path 3 ranging from the laser interferometer to the object to be measured, a means is disposed which calculates at least one of the refractive index variation parameters, a refractive index or those variations from an operation parameter of the control means, and a means is disposed which compensates a measurement value of the laser interferometer based on the above calculated results.;COPYRIGHT: (C)2008,JPO&INPIT
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