While testing different samples of colored glass that gave similar reductions in brightness, he was intrigued to find that some of the samples passed very little of the Sun’s heat, while others passed so much heat that he risked eye damage after only a few seconds’ observation.
Herschel was soon convinced of the necessity of setting up a systematic experiment with the objective of finding a single material that would give the desired reduction in brightness, as well as the maximum reduction in heat. He began the experiment by actually repeating Newton’s prism experiment, but looking for the heating effect rather than the visual distribution of intensity in the spectrum. He first blackened the bulb of a sensitive mercury-in-glass thermometer with ink, and with this as his radiation detector, he proceeded to test the heating effect of the various colors of the spectrum formed on the top of a table by passing sunlight through a glass prism. Other thermometers were placed outside the Sun’s rays and served as controls.
As the blackened thermometer was moved slowly along the colors of the spectrum, the temperature readings showed a steady increase from the violet end to the red end. This was not entirely unexpected, since the Italian researcher Landriani, in a similar experiment in 1777, had observed much the same effect. It was Herschel, however, who was the first to recognize that there must be a point where the heating effect reaches a maximum, and that measurements confined to the visible portion of the spectrum failed to locate this point.
Moving the thermometer into the dark region beyond the red end of the spectrum, Herschel confirmed that the heat continued to increase. When he had finally found the maximum point, he discovered that it lay well beyond the red end in what is known today as the “infrared wavelengths.”
When Herschel revealed his discovery, he referred to this new portion of the electromagnetic spectrum as the “thermometrical spectrum.” The radiation itself he sometimes referred to as “dark heat” or simply “the invisible rays.”
The thermal imaging cameras used today are based on technology that was originally developed for the military. Infrared technology provides the ability to see and target opposing forces through the dark of night or across a smoke-covered battlefield. The properties that have made infrared detection valuable to military forces around the world have also made it valuable to firefighters and law enforcement.
In the late 1950s and 1960s, Texas Instruments, Hughes Aircraft, and Honeywell developed single-element detectors that scanned scenes and produced line images. The military had a lock on the technology because it was expensive and had sensitive military applications. These basic detectors led to the development of modern thermal imaging. The pyroelectric vidicon tube was developed in the 1970s by Philips and EEV and became the core of a product first used by the Royal Navy for shipboard firefighting.
In 1978, Raytheon’s R&D group, then part of Texas Instruments, patented ferro-electric infrared detectors that used barium strontium titanate, or BST, which is the material that coats the thermal imager’s sensor. Raytheon first demonstrated the technology to the military in 1979. In the late 1980s, the federal government awarded hIgh-density array development or HIDAD contracts to both Raytheon and Honeywell for the development of thermal imaging technology for practical military applications. Raytheon went on to commercialize BST technology, while Honeywell developed vanadium oxide (VOx) microbolometer technology. Later, federal programs such as LOCUSP (Low-Cost Uncooled Sensor Program), provided funding for both companies to develop their thermal imaging technologies into equipment systems, including rifle sights and drivers’ viewers. After the 1991 Gulf War, production volumes increased and costs decreased, so the use of thermal imaging was introduced to municipal firefighting services.
In late 2004, Raytheon’s Commercial Infrared Division was sold to L-3 Communications. Meanwhile, the Honeywell microbolometer was awarded a patent in 1994. Boeing, Lockheed-Martin (who sold its infrared business to British Aerospace, or BAE), and others licensed VOx technology from Honeywell developed infrared detectors for military applications. Thermal imagers based on both BST and microbolometer technologies are now available for non-military applications. In fact, thermal imaging has expanded for used in law enforcement, commercial and industrial applications, security, transportation, and many other industries. Bullard introduced its first thermal imager specifically designed for firefighting in 1998.
The American Society of Non-Destructive Testing developed and approved standards for teaching thermal imaging courses in 1992. These classes are called Level I, II and III. By the early 2000s, infrared camera prices continued to fall and the cameras were getting smaller, so new uses for the building industry began to emerge in earnest. By 2006, thermal imaging using infrared cameras by home inspectors and contractors became more common.
In 2022, Nick Gromicko won a 7-year legal battle that resulted in allowing home inspectors to use infrared cameras.
Special thanks to: Flir Systems, Erich Black of the Infrared Tech Institute, and Jim Sefferin of the Infraspection Institute.