Infrared Radiation Detectors for Thermographic Imaging
- Monday, 01 September 2008
Thermographic imaging is accomplished with a camera that converts infrared radiation (IR) into a visual image that depicts temperature variations across an object or scene. The main IR camera components are a lens, a detector in the form of a focal plane array (FPA), possibly a cooler for the detector, and the electronics and software for processing and displaying images (see Figure 1). Most detectors have a response curve that is narrower than the full IR range (900 to 14,000 nanometers or 0.9 to 14 μm). Therefore, a detector (or camera) must be selected that has the appropriate response for the IR range of a user’s application. In addition to wavelength response, other important detector characteristics include sensitivity, the ease of creating it as a focal plane array with micrometer-size pixels, and the degree of cooling required, if any.
In most applications, the IR camera must view a radiating object through the atmosphere. Therefore, an overriding concern is matching the detector’s response curve to what is called an atmospheric window. This is the range of IR wavelengths that readily pass through the atmosphere with little attenuation. Essentially, there are two of these windows, one in the 2 to 5.6-μm range (the short/medium wavelength (SW/MW) IR band), and one in the 8 to 14-μm range (the long-wavelength (LW) IR band). There are many detector materials and cameras with response curves that meet these criteria.
Quantum vs. Non-Quantum Detectors
The majority of IR cameras have a microbolometer type detector, mainly because of cost considerations. Microbolometer FPAs can be created from metal or semiconductor materials, and operate according to non-quantum principles. This means that they respond to radiant energy in a way that causes a change of state in the bulk material (i.e., the bolometer effect). Generally, microbolometers do not require cooling, which allows compact camera designs that are relatively low in cost. Other characteristics of microbolometers are:
- Relatively low sensitivity (detectivity)
- Broad (flat) response curve
- Slow response time (time constant ~12ms)
For more demanding applications, quantum detectors are used that operate on the basis of an intrinsic photoelectric effect. These materials respond to IR by absorbing photons that elevate the material’s electrons to a higher energy state, causing a change in conductivity, voltage, or current. By cooling these detectors to cryogenic temperatures, they can be very sensitive to the IR that is focused on them. They also react very quickly to changes in IR levels (i.e., temperatures), having a constant response time on the order of 1 μs. Therefore, a camera with this type of detector is very useful in recording transient thermal events. Still, quantum detectors have response curves with detectivity that varies strongly with wavelength.
Operating Principles for Quantum Detectors
In materials used for quantum detectors, at room temperature there are electrons at different energy levels. Some electrons have sufficient thermal energy that they are in the conduction band, meaning the electrons there are free to move and the material can conduct an electrical current. Most of the electrons, however, are found in the valence band, where they do not carry any current because they cannot move freely. When the material is cooled to a low enough temperature, which varies with the chosen material, the thermal energy of the electrons may be so low that there are none in the conduction band. Hence, the material cannot carry any current. When these materials are exposed to incident photons, and the photons have sufficient energy, this energy can stimulate an electron in the valence band, causing it to move up into the conduction band. Thus, the material (the detector) can carry a photocurrent, which is proportional to the intensity of the incident radiation.
There is a very exact lowest energy of the incident photons that will allow an electron to jump from the valence band into the conduction band. This energy is related to a certain wavelength, the cut-off wavelength. Since photon energy is inversely proportional to its wavelength, the energies are higher in the SW/MW band than in the LW band. Therefore, as a rule, the operating temperatures for LW detectors are lower than for SW/MW detectors. For an InSb MW detector, the necessary temperature must be less than 173 K (-100°C), although it may be operated at a much lower temperature. An HgCdTe (MCT) LW detector must be cooled to 77 K (-196°C) or lower. A QWIP detector typically needs to operate at about 70 K (-203°C) or lower. The incident photon wavelength and energy must be sufficient to overcome the band gap energy, ΔE.