Infrared Radiation Detectors for Thermographic Imaging

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.