It was the year 1800 when astronomer Sir William Hershel (at age 61) was playing around with temperature measurements of the different colours in the spectrum when sunlight is split by a prism into the colours of the rainbow. William noted that the colours increased in temperature from violet to red and got even warmer when measuring just outside the red. William was no fool, so he determined that there must be more thermal energy in this area which we cannot see with our own eyes. It was the discovery of what we now call the Infrared wave band. It was rumoured that at the time, Williams son John who was very interested in photography, was playing around with what could be the first Thermograph (image of thermal radiation) using paper with an oil emulsion where the differences in evaporation would give an image of the incoming radiation.

It took almost a century before the discovery was put to good use. It apparently happened first within military uses where a bolometer, a device for measuring radiation using a material which changes resistance depending on temperature, could detect a temperature change in the field of view, the size of a human, at a distance of 300 metres (or a cow at 400 metres!). This device was invented by another astronomer Samuel Pierpoint Langley in 1880 and the basics of this is being used in many of today's infrared cameras.

First ca. 1960 was a scanning camera invented for medical uses, where an image would take 15 minutes to record, before around 1965 where Swedish AGA (Later AGEMA, one of the companies now part of FLIR) came up with a camera which could "take" 16 images a second. Finally, the race was on.

But measuring temperatures remotely would not have been possible without the theory behind thermography. We find that back in 1864 the Irish Physicist John Tyndall made a discovery when comparing temperature and colour of a platinum filament (at glowing temperatures). This was followed up by the Slovene physicist Josef Stefan, who in 1879 found the physical relationship between the radiated energy and temperature: The energy radiated from a blackbody is proportional to the fourth power of the absolute temperature (Kelvin).

Austrian physicist Ludvig Boltzmann, being a student of Josef Stefan, was also interested in this area of thermal physics and in 1884 derived the same relationship theoretically, and the equation:

j* = (sigma)T^4

is now known as the Stefan-Boltzmann law

j* is the blackbody radiant emittance

(sigma) is the Stefan-Boltzmann constant (5.670373 x 10-8 W/m^2 K^4 )

T is the absolute temperature of the object (Blackbody) in Kelvin

By using this equation, Stefan determined the temperature of the surface of the sun to be 5,430°C or 5,703 Kelvin - and should you Google this number you will come up with the number 5,778 Kelvin today!

It should be mentioned that John Tyndall in 1859 was the first to prove the connection between CO2 in the air, and what we know as the "greenhouse effect" as we call it today. Later in life he often took chloral hydrate to make him sleep, but at 73 his wife Louisa eventually (accidentally?) gave him an overdose and John died.

Well to be totally honest – apparently American Eunice Foote discovered the absorption of IR by CO2 and water vapour already in 1856 and wrote about it in Scientific American – but being a Lady, only a few at the time would listen! has an interesting history lesson about IR and Camera History


Sir William Hershell and Joseph Stefan