Infrared Imaging or Thermal Imaging : History

History

It would be imperative to know about the discovery of infrared and before we delve any further into infrared imaging. Sir William Herschel, an astronomer from England while trying to measure the heat of different colors of the visible spectrum observed that the hottest lay in an area beyond the red band. He reported this phenomenon to the Royal Society by the name ‘Dark Heat’ in 1800. His son, Sir John Herschel had a keen interest in photography. He created an evaporograph image of the heating rays of IR using carbon suspension in alcohol and named it ‘thermogram’, hence laying the foundations of thermal imaging based on IR rays. The technology then either remained dormant or undocumented due to low success rate for over a century.

Silver halide emulsions used in photography had drastically low sensitivity to longer wavelengths and needed dyes as color sensitizers. The first published infrared images in Royal Photography Society Journal in 1910 edition are credited to Robert W. Wood. The primary development work in IR domain was carried out by militaries. During World War 1, US developed sensitizing dyes and photographic plates for spectroscopic analysis. In 1929, Kalman Tihanyi invented an IR-sensitive television camera for anti aircraft guns for Britain. Infrared photography became popular in 1930s when films were made available for commercial use by companies like Agfa, Kodak and Ilford. False color IR photography gained impetus with Kodak Ektachrome Infrared Aero Film in 1940s.

IR photography and its colors became a fancy of music artists like Hendrix, Donovan et al who released their album covers with IR photos which seemed to grace psychedelic aesthetics genre of late 1960s. Another segment of IR technology, thermal imaging technology remained exclusively with military for a very long time, primarily due to the costs involved. Slowly and steadily, it has been rooting itself in the industry starting with Firefighting departments and then extending to material testing, medicine etc. Companies like Texas Instruments, Hughes Aircraft, Honeywell, Lockheed-Martin, Bullard, etc.

What is Infrared Imaging?

Infrared imaging is a technique of capturing the infrared light from objects and converting it into visible images interpretable by a human eye. The infrared region is spread across the 10 micrometer to 100 micrometer wavelength in the electromagnetic region which can be distributed into three bands: the Near-IR region from 0.7 microns to 1.3 microns; the Mid-IR region from 1.3 microns to 3 microns; the thermal-IR occupying the remaining part of the band. While the first two are used in general electronic applications like remote control and Illumination IR photography, the thermal IR is used in the thermal Imaging. The main difference is that the first two are used in reflective type of applications, while thermal IR is emanated from the object and not reflected by it.

Now the question arises, why only IR and why not any other part of the spectrum? The answer lies at the atomic levels. At absolute zero, perfect order is believed to exist in the atomic structure, no collisions and minimal entropy. Any object above the absolute zero temperature has atomic chaos and collisions resulting in thermal energy being radiated off it most of which falls in the IR band. Thus if these radiations can be detected by some means, objects can be visualized without the need of an optical source through their radiation patters. This forms the basis of infrared imaging.