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Specific intensity (or spectral intensity) is intensity (per astronomy) per unit wavelength or frequency, i.e., a measure of the amount (energy) of electromagnetic radiation (EMR) striking a surface from a given direction, at a given wavelength or frequency. To measure it, you need an extended source. Like intensity, it has the quality that it does not fall off with distance. The term spectral radiance means the same thing: usage of the terms varies in astronomy, to a large extent according to the type of EMR (e.g., radio vs. visible light).