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Choosing an appropriate detector involves many technical considerations.

A simplified block diagram shows how an IR camera works. Click to enlarge.

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. Most detectors have a response curve that is narrower than the full IR range (~ 900–14,000 nm or 0.9–14µm). 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 micron-sized 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 passes through the atmosphere with little attenuation. Essentially, there are two of these windows, one in the 2-5.6 µm range, the short/medium wavelength (SW/MW) IR band, and one in the 8-14 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

Detectivity (D*) curves show how different detector materials respond. Click to enlarge.

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 curves, and slow response time (time constant ~ 12 ms).

For more demanding applications, quantum detectors are used, which 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 response time constant 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

An integrated Stirling cooler works with helium gas, cooling down to –196ºC or sometimes even lower temperatures.

In materials used for quantum detectors, there are electrons at different energy levels at room temperature. Some electrons have sufficient thermal energy and 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 which 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 173K (–100ºC), although it may be operated at a much lower temperature. An HgCdTe (MCT) LW detector must be cooled to 77K (–196ºC) or lower. A QWIP detector typically needs to operate at about 70K (-203ºC) or lower. Quantum detector wavelength dependence is such that the incident photon wavelength and energy must be sufficient to overcome the band gap energy, ?E.

Cooling methods

These samples of cooled focal plane array assemblies are used in IR cameras.

The first detectors used in infrared radiometric instruments were cooled with liquid nitrogen. The detector was attached to the Dewar flask that held the liquid nitrogen, thus keeping the detector at a very stable and low temperature (-196ºC).

Later, other cooling methods were developed. The first solid state solution to the cooling problem was presented by AGEMA in 1986, when it introduced a Peltier effect cooler for a commercial IR camera. In a Peltier cooler, DC current is forced through a thermoelectric material, removing heat from one junction and creating a cold side and hot side. The hot side is connected to a heat sink, whereas the cold side cools the component attached to it.

For very demanding applications, where the highest possible sensitivity was needed, an electrical solution to cryogenic cooling was developed. This resulted in the Stirling cooler. Only in the last 15-20 years were manufacturers able to extend the life of Stirling coolers to 8,000 hours or more, which is sufficient for use in thermal cameras.

A QWIP FPA is mounted on a ceramics substrate and bonded to the external electronics.

The Stirling process removes heat from the cold finger and dissipates it at the warm side. The efficiency of this type of cooler is relatively low, but good enough for cooling an IR camera detector. Integrated Stirling cooler, working with helium gas, cooling down to –196ºC or sometimes even lower temperatures.

Regardless of cooling method, the detector focal plane is attached to the cold side of the cooler in a way that allows efficient conductive heat exchange. Because focal plane arrays are small, the attachment area and the cooler itself can be relatively small.

FPA assemblies
Depending on the size/resolution of an FPA assembly, it has from (approximately) 60,000 to more than 1,000,000 individual detectors. For the sake of simplicity, this can be described as a 2-D pixel matrix, each pixel (detector) having micron-sized dimensions. FPA resolutions can range from about 160 x 120 pixels up to1024 x 1024 pixels.

In reality, assemblies are a bit more complex. Depending on the detector material and its operating principle, an optical grating may be part of the FPA assembly. This is the case for QWIP detectors, in which the optical grating disperses incident radiation to take advantage of directional sensitivity in the detector material’s crystal lattice. This has the effect of increasing overall sensitivity of a QWIP detector. Furthermore, the FPA must be bonded to the IR camera readout electronics. A finished QWIP detector with IC electronics assembly is incorporated with a Dewar or Stirling cooler in an assembly.

An IR image from a 1024x1024 InSb detector camera displays relative temperatures across the target object.

Another complexity is the fact that each individual detector in the FPA has a slightly different gain and zero offset. To create a useful thermographic image, the different gains and offsets must be corrected to a normalized value. This multi-step calibration process is performed by the camera software.

To normalize different FPA detector gains and offsets, the first correction step is offset compensation. This brings each detector response within the dynamic range of the camera’s A/D converter electronics.

After offset compensation, slope correction is applied.

After gain factors are brought to the same value, non-uniformity correction (NUC) is applied so that all detectors have essentially the same electronic characteristics.

The ultimate result is a thermographic image that accurately portrays relative temperatures across the target object or scene. Moreover, actual temperatures can be calculated within ±1°C accuracy.

Application criteria

Relative response curves for various IR cameras with different detectors are shown. Click to enlarge.

As indicated earlier, different types of detectors have different thermal and spectral sensitivities. In addition, they have different cost structures due to various degrees of manufacturability. Where they otherwise fit the application, photon detectors such as InSb and QWIP types offer a number of advantages:

• High thermal sensitivity
• High uniformity of the detectors, i.e. very low fixed pattern noise
• A degree of selectability in spectral sensitivity
• High yield in the production process
• Relatively low cost
• Resistance to high temperatures and high radiation
• Very good image quality

Camera electronics can handle wide variations in absolute detector sensitivities. For example, high sensitivity that might saturate a detector at high thermal intensities can be handled by aperture control and neutral density filters. Both of these solutions can reduce the radiant energy impinging on the FPA.

Price aside, spectral sensitivity is often an overriding concern in selecting a detector and camera for a specific application. Once a detector is selected, lens material and filters can be selected to alter somewhat the overall response characteristics of an IR camera system. 

Resources:
FLIR Systems, Inc., North Billerica, Mass., 800-464-6372, www.flir.com


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