Thermographic Imaging Selecting an IR Camera with the Right Detector
- Tuesday, 01 April 2008
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 dif- ferent 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.