This chapter begins with broadband instruments for measuring infrared (IR) radiation, including how they function and some similar but slightly different equations that have been used to produce measured IR irradiance from the same inputs. It discusses new developments in calibrating pyrgeometers. The chapter outlines some operational considerations for using a pyrgeometer and estimates uncertainties that one may obtain in routine operations. Downwelling and upwelling IR radiation measurements in the broadband are made with instruments called pyrgeometers. In turn, the uniform blackbody calibration on the instruments provided better agreement among the pyrgeometers than using calibrations that came with the instruments either from the manufacturer or their own laboratories. The thermopile voltage is produced by a temperature difference between the exposed and unexposed junctions of the thermopile. Blackbody-calibrated, sky-scanner-calibrated measurements, and models of downwelling infrared irradiance agreed to within 1–2 Wm-2 at night.