FIRAS – Far Infrared Absolute Spectrometer – was the instrument on board COBE that registered the blackbody spectrum of the microwave background radiation. The radiation lies in the microwave part of the electromagnetic spectrum and has a wavelength of only a few millimeters. The radiation was gathered by FIRAS in a cone-shaped instrument which compared the wavelength spectrum of the collected radiation with a known (reference) blackbody spectrum. The collected microwave background radiation turned out to have a perfect blackbody spectrum.