In diffraction-limited optical systems the number of optical components is rather high. Local changes in the refractive index of the order of 10−6 must be detected to guarantee an adequate performance of the system as a whole. Therefore, the Twyman-Perry method for the evaluation of refractive-index deviations from uniformity—further developed by Roberts and Langenbeck—has been programmed for a digital Twy-man-Green interferometer. If the glass block to be tested is slightly wedge shaped (wedge angle ~10−3 rad), the measurement can be carried out without coating the glass block. With the help of four interferograms the variations of the refractive index apart from a linear slope, as well as the thickness variations of the glass block, can be measured and displayed.
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