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The Differential Microwave Radiometer measured differences in microwave power between pairs of sky directions to map Cosmic microwave background anisotropy. Comparing two directions suppresses a common absolute brightness offset; repeated comparisons during scanning are then combined into a sky map.

The COBE instrument used paired horn antennas at three frequencies, with independent channels that tested detector noise and frequency-dependent galactic foreground contamination. A differencing instrument does not by itself determine the absolute monopole temperature. Its measurement therefore complements the spectrum measured by the Far Infrared Absolute Spectrophotometer.^[ref]

The Far Infrared Absolute Spectrophotometer measured the spectrum of the cosmic microwave background by comparing the sky with a calibrated reference. Its differential Fourier transform spectroscopy and blackbody calibration tested whether the radiation followed one Planck function, rather than only measuring differences between sky directions.

The key observable is a spectral residual after fitting a temperature and accounting for instrumental and foreground contributions. A small residual constrains departures from a blackbody spectrum and thus energy exchange in the early universe. This absolute spectral test is distinct from the directional map supplied by the Differential Microwave Radiometer.^[ref]