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A device is described whereby reflections from a semiconductor block placed in a waveguide are used to measure the photon flux incident on the semiconductor. A germanium sample with associated circuitry acts as a tuned cavity whereby in the null condition complete absorption occurs. As light changes the semiconductor conductivity, changes in reflected power provide a sensitive indicator. Computer calculations have been worked out in an exact analysis and the results are checked with experiments. As a photo detector, this device may be competitive with state-of-the art components. If used as a research tool to study semiconductor properties the scheme has a great deal of sensitivity and flexibility. (Author)