Abstract
In order to measure geometrical shapes by a scanning method, any datum line is necessary. In general, the accuracy of the datum must be ten times as high as that of measured workpieces. The datum line is disturbed by the scanning motion and vibration or thermal deflection, even if the original datum line is accurate enough. Influence of these disturbances, can be successfully decreased by the laser autocollimation method which has been introduced by A. E. Ennos and M. S. Virdee. The autocollimation method is essentially free from the scanning error in the Z-direction. In order to suppress pitching error also, they used a pentagonal prism projecting a laser spot on a measured mirror. In this report we try to correct pitching error by measuring it through photo-electrical autocollimator. This is a kind of differential laser autocollimation methods in which the pitching error is canceled by two outputs of probes. This kind of differential laser autocollimation method can measure straight line of mirror with a repeatability of 3 nanometer. And this result promises high accuracy of a differential laser autocollimation method with two probes in measurement of straight or aspherical mirrors with large area.
