The word "infrared" is derived from the word "past red", which means beyond red, and indicates where that wavelength is located in the spectrum of electromagnetic radiation. The word "thermography" was generated using the same root word meaning "temperature image". The origin of thermography is attributed to the German astronomer Sir William Herschel, who conducted some experiments with sunlight in the 1800's. Herschel discovered infrared radiation by passing sunlight through a prism and placing thermometers at various colors and measuring the temperature of each color with a sensitive mercury thermometer. Herschel found that the temperature rises when the light passes over the red color and enters a region he called "dark red heat". The "dark red heat," now known as infrared heat, is in the region of the electromagnetic spectrum known as electromagnetic radiation.
Twenty years later, German physicist Thomas Seebeck discovered the thermoelectric effect. Building on that discovery, Italian physicist Leopoldo Nobili invented the heat multiplier (an early version of the thermocouple) in 1829. This simple contact device worked on the principle that the voltage difference between two dissimilar metals varied with temperature. Soon after, Nobili's partner, Macedonio Melloni, modified the heat multiplier into a thermopile (mounting the heat multiplier in series) and concentrating the heat radiation on the thermopile so that he could detect human body heat at a distance of 9.1 meters (33 feet).
In 1880, the American astronomer Samuel Langley used a radiant heat detector to detect the body heat of a cow 304 meters (1,000 feet) away. Instead of measuring voltage differences, the RHD measured changes in electrical resistance associated with temperature changes, and Sir William Herschel's son, Sir John Herschel, made the first infrared images in 1840 using a device called the "evaporation imager". The thermal image was created by the difference in the amount of evaporation of a thin film of oil, which could be viewed with the help of light reflected off the film.
A thermal imaging camera is a device that detects thermal patterns in the infrared wavelength spectrum without direct contact with the device. Early models of thermal imaging cameras were called "photodetectors". From 1916 to 1918, American inventor Theodore Case experimented with photoconductive detectors, generating signals by interacting directly with photons (rather than heat), eventually leading to the invention of faster, more sensitive photoconductive detectors. During the 1940s and 1950s, thermal imaging technology evolved in order to meet the needs of a growing number of military applications. During the 1940s and 1950s, thermal imaging technology evolved to meet the growing demand for military applications, and made great strides. German scientists discovered that cooling photoconductive detectors could improve overall performance.
It wasn't until the 1960s that thermal imaging was used for non-military applications. Although early thermal imaging systems were bulky, slow to acquire data, and had poor resolution, they were used in industrial applications, such as inspecting large power transmission and distribution systems.
In the 1970s, continued development in military applications led to the first portable systems. These systems were used in applications such as construction diagnostics and non-destructive testing of materials.
The thermal imaging systems of the 1970s were rugged and very reliable, but their image quality was poor compared to modern thermal imaging cameras. By the early 1980s, thermal imaging was widely used in the medical and mainstream industries, as well as in building inspections. After calibration, a thermal imaging system can produce a fully radiometric image so that the radiant temperature can be measured at any location within that image. A radiometric image is a thermal image that contains calculated values of temperature measurements at various points within the image.
Safe and reliable camera coolers have been improved to replace the compressed or liquefied gas that has long been used to cool the camera.
In addition, lower-cost, tube-based thermoelectric photoconductive camera tube (PEV) thermal imaging systems have been developed and mass-produced.
Although not capable of radiometric measurements, PEV thermal imaging systems are lightweight, portable, and can be operated without cooling.
In the late 1980s, a new device called the focal plane array (FPA) moved from military applications to the commercial market. An FPA is an image sensing device that consists of an array of infrared sensing detectors (usually rectangular) located at the focal plane of a lens.
This is a significant improvement over the original scanning detectors, resulting in improved image quality and spatial resolution. Typical arrays on modern thermal imaging cameras have pixels ranging from 16 × 16 to 640 × 480; in this sense, a pixel is the smallest independent element of an FPA that can detect infrared energy. For special applications, arrays can have more than 1000 × 1000 pixels.
The first number represents the number of pixels in each vertical column, and the second number represents the number of rows displayed on the screen. For example, a 160 × 120 array has a total of 19,200 pixels (160 pixels × 120 pixels = 19,200 total pixels). Since 2000, the development of FPA technology using multiple detectors has accelerated. Long-wave thermal imaging cameras are used to detect infrared energy in the 8 μm to 15 μm wavelength range. A micrometer (μm) is a unit of length measurement equal to one thousandth of 1 millimeter (0.001 meters). Mid-wave thermal imaging cameras are used to detect infrared energy in the wavelength range of 2.5 μm to 6 μm. Both longwave and midwave thermal imaging systems are available in a full range of radiometric models, with image fusion and thermal sensitivity typically 0.03 SDgrC (0.054 SDgrF) or less. The cost of these systems has been reduced by a factor of more than ten over the past decade, but the quality has been dramatically improved. In addition, the use of computer software for image processing has grown significantly. Almost all commercial types of infrared systems now use software to assist in analysis and report writing. Reports can be quickly generated and sent electronically over the Internet or saved in a common format (e.g., PDF), and can also be burned to a variety of digital storage devices.