September 5, 2018 By anglian
Recent developments in cooled mercury cadmium telluride (MCT or HgCdTe) infrared detector technology have made possible the development of high performance infrared cameras for use in a wide variety of demanding thermal imaging applications. These infrared cameras are now available with spectral sensitivity in the shortwave, mid-wave and long-wave spectral bands or alternatively in two bands. In addition, a variety of camera resolutions are available as a result of mid-size and large-size detector arrays and various pixel sizes. Also, camera features now include high frame rate imaging, adjustable exposure time and event triggering enabling the capture of temporal thermal events. Sophisticated processing algorithms are available that result in an expanded dynamic range to avoid saturation and optimize sensitivity. These infrared cameras can be calibrated so that the output digital values correspond to object temperatures. Non-uniformity correction algorithms are included that are independent of exposure time. These performance capabilities and camera features enable a wide range of thermal imaging applications that were previously not possible.
At the heart of the high speed infrared camera is a cooled MCT detector that delivers extraordinary sensitivity and versatility for viewing high speed thermal events.
1. Infrared Spectral Sensitivity Bands
Due to the availability of a variety of MCT detectors, high speed infrared cameras have been designed to operate in several distinct spectral bands. The spectral band can be manipulated by varying the alloy composition of the HgCdTe and the detector set-point temperature. The result is a single band infrared detector with extraordinary quantum efficiency (typically above 70%) and high signal-to-noise ratio able to detect extremely small levels of infrared signal. Single-band MCT detectors typically fall in one of the five nominal spectral bands shown:
• Short-wave infrared (SWIR) cameras – visible to 2.5 micron
• Broad-band infrared (BBIR) cameras – 1.5-5 micron
• Mid-wave infrared (MWIR) cameras – 3-5 micron
• Long-wave infrared (LWIR) cameras – 7-10 micron response
• Very Long Wave (VLWIR) cameras – 7-12 micron response
In addition to cameras that utilize “monospectral” infrared detectors that have a spectral response in one band, new systems are being developed that utilize infrared detectors that have a response in two bands (known as “two color” or dual band). Examples include cameras having a MWIR/LWIR response covering both 3-5 micron and 7-11 micron, or alternatively certain SWIR and MWIR bands, or even two MW sub-bands.
There are a variety of reasons motivating the selection of the spectral band for an infrared camera. For certain applications, the spectral radiance or reflectance of the objects under observation is what determines the best spectral band. These applications include spectroscopy, laser beam viewing, detection and alignment, target signature analysis, phenomenology, cold-object imaging and surveillance in a marine environment.