Characterization of infrared detectors
Infrared (IR) radiation itself was unknown until 218 years ago, when Herschel’s experiment with the thermometer was first reported. The first detector consisted of a liquid in a glass thermometer with a specially blackened bulb to absorb radiation. Herschel built a crude monochromator that used a thermometer as a detector so that he could measure the distribution of energy in sunlight. In April 1800, he wrote :
Thermometer No. 1 rose 7 degrees in 10 minutes by an exposure to the full red colored rays. I drew back the stand... thermometer No. 1 rose, in 16 minutes, 8⅜ degrees when its centre was ½ inch out of the visible rays.