A frustrated multiple total internal reflection absorption spectrophotometer employs a circular rod (20) having conical end faces (38, 40) in a sample chamber (24) through which a fluid sample flows. Unit magnification mirror relay optical systems comprising a pair of facing spherical mirrors (44, 46, 80, 82) having aligned circular apertures in the centre thereof are employed between the source of radiation (22) and one conical end face of the rod (40) and/or the other conical end face of the rod (38) and the detector of radiation (88) for imaging a conical end face of said rod at said source or detector. A rotating disc (66) is located between the source and the detector containing apertures employing one or more radiation filters. The relay systems employ baffles (60, 90, 96) preferably located near their exits extending within the narrowest cone of rays reflected by the mirrors. The system is particularly useful in the infra-red but may also be used at optical and ultraviolet wavelengths. IMAGE
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