The thermographic imaging characteristics of an IR camera are largely determined by its detector assembly. Infrared radiation (IR) is focused onto the camera’s detector assembly, which is a focal plane array (FPA) that converts IR into a visual image depicting temperature variations across the camera’s field of view (FOV). The FPA consists of thousands of pixels, which could be fabricated from any one of the IR sensitive materials commonly used in these cameras. Most detector materials respond to a selected portion of the IR spectrum (Figure 1). Therefore, a camera selected for any given application should have the appropriate FPA material based on the IR characteristics of objects within the FOV and the user’s study objectives.
Since the atmosphere and other gases can act as spectral filters, an overriding concern is matching the detector’s response curve to the spectral “windows”these gases present. For example, the “standard” atmosphere has two of these windows: one in the 2 - 5.6 μm range, and one in the 8 - 14 μm range. Several detector materials have spectral response curves that can take advantage of these atmospheric windows. (See Figure 1).
Microbolometers: One of the commonly used detector types is based on bolometric principles. A microbolometer FPA can be created economically from metal or semiconductor materials. These detectors respond to radiant energy in a way that causes a change of state in the bulk material (i.e., the bolometer effect). Since they are not based on quantum principles, microbolometers typically do not require cooling, although some form of temperature stabilization may be needed. Cameras using microbolometers have the following general characteristics:
- Relatively low sensitivity (detectivity)
- Broad (flat) response curve
- Slow response time (time constant ≈ 12ms)
Quantum Detectors: For more demanding applications, detectors that operate according to quantum principles are used. The intrinsic photoelectric effect of these materials respond to IR by absorbing photons, which elevates electrons in the material lattice to a higher energy state. This causes some of these electrons to move from the valence energy band into the conduction band, thereby causing a change in conductivity, voltage, or current. The photocurrent carried by the material is proportional to the intensity of the incident radiation.
To work properly, most (if not all) quantum detectors must be cooled to cryogenic temperatures. Under these conditions they can be very sensitive and react quickly to changes in IR levels (time constant ≈ 1μs). This is the type of detector needed to capture transient thermal events, or fast moving objects within the camera’s FOV. Still, quantum detectors often have narrow response curves that depend greatly on IR wavelength, as shown in Figure 1.
Quantum principles have important implications in camera designs. According to these principles, there is a minimum energy state for photons impinging on the detector material that will elevate an electron from the valence band into the conduction band. This energy is related to a cut-off wavelength. Because photon energy is inversely proportional to its wavelength, it is higher in the shorter wavelength portions of the IR spectrum. Typically, this means that the operating temperatures for detectors sensitive to shorter IR wavelengths can be higher than those with peak sensitivities at the longer wavelengths. For instance, an InSb MW detector needs an operating temperature below 173K (-100°C), whereas an HgCdTe (MCT) detector must be cooled to 77K (-196°C) or lower. A Quantum Well Infrared Photon detector (QWIP) typically needs to operate at about 70K (-203°C) or lower. (See red curve in right-hand view of Figure 1.)
Quantum Detectors with Integral Cooling
As alluded to above, the type of quantum detector used in an IR camera affects the cooler design. Originally, IR instruments were cooled with liquid nitrogen, but that is rather cumbersome. Compact camera designs are needed for mobility, and to allow their use where space is restricted. This has led to other methods of cooling, such as Stirling cycle coolers and use of the Peltier effect. 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. Multistage Peltier coolers are often used in IR cameras with quantum detectors, because of cooling efficiency relative to their size.
FPA resolutions can range from about 160×120 pixels up to1024×1024 pixels. For that higher resolution, there are more than 1,000,000 individual detector elements (pixels), each one having micrometer size dimensions. 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.
The FPA is bonded to the IR camera readout electronics (Figure 2), and integrated with a cooler. Some examples of completed FPA/cooler assemblies are shown in Figure 3.
To accurately portray temperatures in a thermographic image, the detector assembly must be calibrated. Although IR cameras are calibrated at the factory, there are certain calibration procedures that can be accomplished by a user. Therefore, a basic understanding of detector calibration is useful.
Non-Uniformity Corrections: One calibration process corrects for non-uniformities of the individual FPA elements, all of which have a slightly different gain and zero offset. To create a useful thermographic image, these must be corrected to a normalized value (Figures 4 and 5). Typical accuracy after calibration is ±2°C, or better.
Emissivity Calibration: The emissivity of a target object is vital to achieving accurate temperatures. (Emissivity or emittance is the radiative properties of an object relative to a perfect black-body.) At the factory, an IR camera is calibrated over its intended temperature range using a black body (Emissivity =1). However, for accurate results in the field, the user needs to enter emissivity values into camera firmware for the target objects of interest.
Selecting the appropriate IR camera depends on several application factors. The key factors relating to the detector are:
- Spectral range,
- Thermal sensitivity,
- FPA resolution,
- Impact on overall camera price.
Different types of detectors have different cost structures due to varying degrees of manufacturability, assembly costs, and calibration complexities. Price aside, spectral sensitivity is often an overriding concern in selecting a detector and camera for a specific application.