Infrared radiation has been studied with detectors of one kind or another since about 1800. The difference between the techniques of the first century-and-a-half and the period since World War II is that the earlier devices were thermal detectors, responding to temperature change caused by integrated absorption of energy, while the more recent devices are true photon detectors. In the latter type, interaction of individual photons (above a certain threshold energy) with the solid detector results directly in electron emission or charge-carrier excitation.