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Precision optical systems for the infrared (IR) require optical grade IR materials. Materials mostly used are Ge, ZnSe, ZnS, Chalcogenide glasses. An instrument is des-cribed where dn/dx, the inhomogeneity of the refractive index can be detected and quantitatively interpreted by means of measuring the shift of a fine-line image. The influence of spatial variation of absorption to these results can be eliminated by a double scan at first with the measurement slit in the left flank and secondly in the right flank of the fine-line image. On the other hand, if the slits are removed this instrument enables measurement of local transmission. Results obtained with IR materials are discussed. The same instrument can be used to measure the absolute value of the refractive index by means of the shift of the fine-line image caused by a small wedge. Beside inhomogeneity optical quality of IR materials may be degraded by birefringence. A setup for measurement of birefringence in the IR is shown. Image quality in the IR can be determined by measuring the MTF. An instrument is described with a small slit as test. First the IR fine-line image is measured and the MTF by Fourier Transform. Hereby noise is eliminated with a lock-in amplifier. With a small test slit it is easier to fulfil the theoretical requirements, especially to realize an isoplanatic region and an incoherent illumination. By this means no ground glass is needed. With a precision scanning unit no additional magnification of fine-line image is required. Both remarks are important for IR measurement technology. Furthermore, the influence of the colour temperature of the radiation source onto the accuracy of results is shown. A comparison is made between obtained measured results and the MTF calculations. The possibility to measure the fine-line image is important to describe the image quality of IR systems used in connection with detector arrays.
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