The temperature of a thermally emitting target (26) which radiates over a band of wavelengths which includes first and second spaced wavelengths and which has an unknown emissivity characteristic at which the emissivity at one wavelength may but does not necessarily differ from the emissivity of another wavelength is read remotely by [a] deriving signals from those portions of the thermal radiation at these two wavelengths; [b] deriving signals from the reflected portions of two beams of radiation at these wavelengths which strike the target (26); and [c] by using these derived signals to compute the temperature.
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