John Della Porta described the presence of reflected heat in 1698, but the original pioneering work in thermal radiation can be attributed to the German Sir William Herschel, Royal Astronomer to King George III of England, and already famous for his discovery of the planet Uranus, in 1800 (Herschel 1800). Knowing that sunlight was made up of all the colours of the spectrum, and that it was also a source of heat, Herschel wanted to find out which colour(s) were responsible for heating objects.

Hersel devised an experiment using a prism, paperboard, and thermometers with blackened bulbs, which he used to measure the temperatures of the different colours. Other thermometers, placed outside the sun’s rays, served as controls. As the blackened thermometer was moved slowly along the colours of the spectrum, the temperature readings showed a steady increase from the violet end to the red end, and in the dark region beyond the red end of the spectrum, the heating continued to increase. Herschel showed that not only was there a ‘dark heat’ present, but that heat itself behaved like light; it could be reflected and refracted under the right conditions.

His son, John Herschel, created the first thermal image in 1840, using a suspension of carbon in alcohol and a lens placed in a beam of natural sunlight (Herschel 1840). He called the image a thermogram, a term that is still in use today.