Thermal imaging devices, also called infrared devices, detect the heat given off by an object or person.
Everything we encounter gives off thermal energy. The hotter the object, the more thermal energy it emits. This emitted thermal energy is called a “heat signature”. When objects have different heat signatures, they are displayed in different colors, which make it easy and quick to detect warm-blooded animals (including people) against cooler surroundings.
Thermal imaging devices have lenses, just like visible light devices. But in the case of thermal detection, the lenses focus waves from infrared energy onto an infrared sensor array. Thousands of sensors on the array convert the infrared energy into electrical signals, which create a video image. The infrared device measures and displays a “thermal profile” of objects in relation to the temperature of any surrounding objects.
Thermal imaging technology also allows users to detect targets by cutting through snow, dust, smoke, fog, haze, and other atmospheric obscurants.
Understanding Light
The amount of energy in a light wave is related to its Wavelength: Shorter wavelengths have higher energy and longer wavelengths have lower energy. Within the visible light spectrum, violet has has the most energy and red has the least. Just next to the visible light spectrum is the Infrared Spectrum.
Infrared light is just a small part of the light spectrum and can be split into three categories:
Near Infrared
Near IR, closest to visible light, has wavelengths that range from 0.7 to 1.3 microns (700 billionths to 1,300 billionths of a meter.
Mid Infrared
Mid IR has wavelengths ranging from 1.3 to 3 microns. Both Near IR and Mid IR are used by a variety of electronic devices, including remote controls.
Thermal Infrared
Thermal IR occupies the largest part of the infrared spectrum with wavelengths ranging from 3 microns to over 30 microns. The main difference in the three infrared categories is that Thermal IR is actually emitted by an object instead of reflected off it. This stems from what is happening at an atomic level.
After the electrons in an atom achieve a high energy orbit (once they are heated), they eventually return to their ground state. This is achieved by releasing energy in the form of Photons – a particle of light. The photon emitted has a very specific wavelength (color) that depends on the state of the electron’s energy when the photon is released. Anything that is alive uses energy and therefore, generates heat. Many inanimate objects also generate heat, firing off photons in the Thermal Infrared Spectrum. Objects that become very hot will begin to emit photons in the visible light spectrum, such as flame.
Thermal imaging takes advantage of this infrared emission.
Thermal Imaging
All objects emit infrared energy. The amount of infrared energy emitted is proportional to the amount of heat an object or organism produces. Thermal imagers sense this infrared energy and provide a thermal signature of a scene.
How it works:
A special optical lens focuses the infrared light emitted by all objects in view.
The focused light is scanned by a phased array of infrared detector elements. The detector elements create a very detailed temperature pattern called a Thermogram. It only takes about one-thirtieth of a second for the detector array to obtain the temperature information to make the thermogram. This information is obtained from several thousand points in the field of view of the detector array.
The thermogram created by the detector elements is translated into electric impulses.
The electric impulses are sent to a signal-processing unit, a circuit board with a dedicated chip that translates the information from the elements into data for the display.
The signal processing unit sends the information to the display, where it appears as various colors depending on the intensity of the infrared emission. The combination of all the impulses from all the elements creates the image.
Most thermal imaging devices scan at a rate of 30 times per second. They can sense temperatures ranging from -4 degrees Fahrenheit (-20 degrees Celsius) to 3,000 F (2,000 C), and can normally detect changes in temperatures of about 0.4 F (0.2 C). There are two common types of thermal imaging devices:
Un-Cooled
Un-Cooled is the most common type of thermal imaging device. The infrared detector elements are contained in a unit that operates at room temperature. This types of system is completely quiet, activates immediately and has the battery built right in.
Cryogenically Cooled
More expensive and more susceptible to damage from rugged use, Cryogenically Cooled Thermal Imaging Devices have the elements sealed inside a container that cools them to below 32 F (zero C). The advantage of such a system is the incredible resolution and sensitivity that result from cooling the elements. Cryogenically-cooled systems can “see” a difference as small as 0.2 F (0.1 C) from more than 1,000 ft. (300m) away, which is enough to tell if a person is holding a gun at that distance.
Unlike traditional image-enhancement night vision technology, thermal imaging is great for detecting people and vehicles in near-absolute darkness with little or no ambient lighting. Even the best Gen 3 PINNACLE image intensifier tubes require some ambient light to function. Thermal Imaging focuses the infrared light emitted by all objects at all times and in all lighting conditions – even when there is no light to speak of, bodies and vehicles are still warm. Another advantage of thermal imaging is that it can be used effectively in broad daylight whereas image-intensifier tubes performance is severely degraded in these conditions.
Development
1960’s
- Development of cooled Forward-Looking Infrared (FLIR)
1972
- Joint development with the US Army Night Vision Laboratory develops common module (cooled) FLIR system.
1978
- Development of uncooled thermal imaging technology and demonstration to the US Army Night Vision Laboratory.
1981
- Custom Readout Design business founded. Customer-specific requirements are 100% of orders.
1985
- Development of battery-operated uncooled sight for the US Army through the Short Range Thermal Sight (SRTS) program.
1986
- Indium Antimonide (InSb) detector processing begins.
1988
- Industry’s first commercial IR camera introduced.
1990
- 256 x 256 focal plane array camera introduced.
1992
- Raytheon acquires Amber as a wholly-owned subsidiary.
- Radiance 1 camera developed.
- 512 x 512 focal plane camera demonstrated.
- Successful demonstration of Low Cost Uncooled Sensor Prototype (LOCUSP) system as a surveillance sensor and a battery-operated weapon sight.
1993
- Developers prototype commercial uncooled IR products and demonstrate them in the marketplace.
1994
- Radiance 1 sales eclipse all other products.
- Extensive customer testing occurs and the NightSight product family is developed for commercial users. Hand-held and weapon sight products are developed for the military.
1995
- NightSight is introduced and production deliveries to first customers begin.
- Emergent detector technologies brought to commercial market.
1996
- First uncooled microbolometer camera to market.
1997
- First radiometric uncooled camera to the market.
- Introduction of Palm IR 250, the world’s most affordable hand-held IR imager.
- Introduction of Series 200, the most affordable remote-controlled thermal imager for Law Enforcement and surveillance.
2000
- Introduction of the world’s first automotive thermal imaging driving aid, in the year 2000 Cadillac DeVille.
- Amorphous Silicon Bolometer technology product introduced.
2001
- Updated IR products portfolio to digital electronics.
- Produced the first uncooled IR camera with zoom.
Introduced the first uncooled radiometric camera with pocket PC interface.
An in-depth article on How Thermal Imaging Works can also be found here
