Wikipedia
A pyrgeometer is a device that measures near-surface infra-red radiation spectrum in the wavelength spectrum approximately from 4.5 µm to 100 µm.
It measures the resistance/voltage changes in a material that is sensitive to the net energy transfer by radiation that occurs between itself and its surroundings (which can be either in or out). By also measuring its own temperature and making some assumptions about the nature of its surroundings it can infer a temperature of the local atmosphere with which it is exchanging radiation.
These instruments generally have no spectral (frequency/wavelength) measurement capabilities – they use a single (non-frequency resolved) resistance/voltage measurement. They are constructed to be sensitive to the infra-red radiation spectrum that extends approximately from 4.5 µm to 100 µm, thus excluding the main shortwave (solar) spectrum.
Since the mean free path of IR radiation in the atmosphere is ~25 meters, this device typically measures IR flux in the nearest 25 meter layer.