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Infrared sensing may seem a relatively new scientific discipline but in reality the existence of infrared was noted as early as 1800 when Sir William Herschel wrote on the subject. He discovered the "invisible rays" as he called them while developing filters for protecting his eyes while observing the sun. Herschel described this effect in 1801 in two papers. The term "infrared" was not coined until the 1880's. The author of the term is not known but in Latin infra means below or beneath, possibly implying beneath the red. Using a prism and a sensitive mercury - in glass thermometer, William Herschel measured the radiation from fires, candles, and kitchen stoves demonstrating a detector able to measure radiation in this infrared region and raising questions about the connection between light and heat. In 1840, Herschel's son John developed a radiation detection process based on the differential evaporation of a thin film of oil to form a "heat picture." This process was improved by Czerny in 1929 and is still in use today as the "evaporagraph." In 1843 Becquerel found that certain materials phosphoresced when exposed to infrared radiation. During the 1880's several highly sensitive new detectors were developed most notable being the Langley bolometer. In 1901 Langley and Abbot reported the use of a bolometer that could detect the heat from a cow at a distance of 1/4 mile. Case in 1917 developed the thallous sulfide detector, the first use of the photoconductive effect in the infrared. Development continued in detector technology especially by the Germans during World War II. Following the war the efforts were on,radiometry. During the 1950's non-military applications for these devices grew rapidly'. Now, point radiometry is an established quantitative technology with thousands of units in place for remote temperature measurement and control. The instruments are calibrated for temperature with appropriate scale, emittance correction, voltage output, and internal reference. Infrared imaging systems have developed in the 60's and are currently approaching tne quantitative level as they are being integrated into computer processing. Technology improvements provide on-line video tape recording, quantification, computer interfacing, and graphics. Instrument improvement is an on-going active process being pushed by military, space, and industrial applications. Associated with the new technology must be the development of an educational process which provides the user with information sufficient to properly assimilate the information derived from the instrument.
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