Recording of thermal radiation by a detector with double frequency conversion

A discussion is given of the recording of thermal radiation by means of two-stage frequency conversion in AgGaS₂ and α-HIO₃ crystals. It is shown that the sensitivity of a detector of this kind is limited by the thermal noise of the nonlinear crystal in the first stage and by the background radiation. At room temperature the contribution of the thermal noise in this crystal is 80% and the background noise is 20%.