Infrared camera
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An infrared camera is a device that forms an image using infrared radiation, similar to a common camera that forms an image using visible light.
All objects emit a certain amount of black-body radiation as a function of their temperatures. Generally speaking, objects that are hot by human standards emit more infrared radiation as black-body radiation than those that are cold. This radiation can be detected by a special camera in a way similar to that in which a common camera detects visible light. An infrared camera normally shows warmer areas as brighter and cooler areas as darker. This is independent of the level of ambient light, so such a camera can work in total darkness. This makes it useful for rescue operations in smoke-filled buildings and underground.
Images from infrared cameras tend to be monochromatic, because the cameras are generally designed with only a single type of sensor responding to a particular wavelength of infrared radiation. This is simpler than a colour camera, which has multiple types of sensor (usually three, tuned to red, green and blue light) allowing it to show colour pictures, but is adequate for night-vision applications.
For use in temperature measurement, the image from an infrared camera is sometimes processed to convert it to colour (as the eye can discern colors with finer resolution than grayscale levels). This is called false colour, and has no relationship to the true colours in the scene, or the true wavelengths of the radiation (visible colours are seen due to different wavelengths). Each colour is simply assigned arbitrarily to a certain range of intensities of the monochrome signal, in the same way that arbitrary colours on a relief map represent different height ranges. For example, the brightest (warmest) parts of the image might be coloured white, intermediate temperatures might be shown as reds and yellows, and the dimmest (coolest) parts as blue. A scale should be shown next to a false colour image to relate colours to temperatures.
Infrared cameras can be broadly divided into two types: those with cooled infrared image detectors and those with uncooled detectors.
Cooled infrared detectors
Cooled detectors are typically contained in a vacuum-sealed case and cryogenically cooled. This greatly increases their sensitivity since their own temperatures are much lower than that of the objects from which they are meant to detect radiation. Without cooling, these sensors (which detect and convert light in much the same way as common digital cameras, but are made of different materials) would be "blinded" or flooded by their own radiation. The drawbacks of cooled infrared cameras are that the vacuum container is often expensive and difficult to produce, and the cooler requires a lot of power to function, and time to bring the sensor down to the operating temperature, as the camera may need several minutes to cool down before it can begin taking pictures. These devices make cooled infrared cameras generally bulky and expensive, but they also provide superior image quality compared to uncooled cameras.
Materials used for infrared detection include a wide range of narrow gap semiconductors. This list includes:
Uncooled infrared detectors
Uncooled thermal cameras use sensors that operate at room temperature. Modern uncooled detectors all use sensors that work by changing electrical properties of the material when heated by infrared radiation. These changes (in resistance, voltage, or current) are then measured and compared to the values at the operating temperature of the sensor. Uncooled infrared sensors can be stabilized to an operating temperature to reduce image noise, but they are not cooled to low temperatures and do not require bulky, expensive cryogenic coolers. This makes small, relatively inexpensive infrared cameras possible. However, they tend to have low resolution and poor image quality relative to cooled detectors. This is due to different processes used in their fabrication, which are still limited by available technology.
Uncooled detectors are mostly based on pyroelectric materials or microbolometer technology.
Modern digital cameras are based on CCD chips. By the nature of the materials used in them, these chips are sensitive not only to visible light but also to "near" infrared radiation (that is, infrared radiation that is just below visible light.) For example, the camera can 'see' the light of your TV remote control when pressed.
Many modern digital cameras have a filter to remove infrared light to avoid the colours being wrong.
Some other cameras have a special night mode or night vision mode, where an infrared LED illuminates the scene and the camera is in infrared (only) mode. The resulting picture is black-and-white.
A so-called X-ray effect has been described using an infrared filter (800-1000nm wavelength) with night mode in bright sunlight. Clothes then appear semi-transparent.
Companies producing infrared cameras
US
- Raytheon
- DRS
- BAE SYSTEMS NA
UK
- BAE SYSTEMS
- Thales
France
- Sofradir
- CEDIP
Germany
- AIM
Israel
- SCD
Russia
- Orion
This list is not exhaustive.