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.
- 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.
Additionally, a spectral band may be selected because of the dynamic range concerns. Such an extended dynamic range would not be possible with an infrared camera imaging in the MWIR spectral range. The wide dynamic range performance of the LWIR system is easily explained by comparing the flux in the LWIR band with that in the MWIR band. As calculated from Planck’s curve, the distribution of flux due to objects at widely varying temperatures is smaller in the LWIR band than the MWIR band when observing a scene having the same object temperature range. In other words, the LWIR infrared camera can image and measure ambient temperature objects with high sensitivity and resolution and at the same time extremely hot objects (i.e. >2000K). Imaging wide temperature ranges with an MWIR system would have significant challenges because the signal from high temperature objects would need to be drastically attenuated resulting in poor sensitivity for imaging at background temperatures.
- Image Resolution and Field-of-View
2.1 Detector Arrays and Pixel Sizes
High speed infrared cameras are available having various resolution capabilities due to their use of infrared detectors that have different array and pixel sizes. Applications that do not require high resolution, high speed infrared cameras based on QVGA detectors offer excellent performance. A 320×256 array of 30 micron pixels are known for their extremely wide dynamic range due to the use of relatively large pixels with deep wells, low noise and extraordinarily high sensitivity.
Infrared detector arrays are available in different sizes, the most common are QVGA, VGA and SXGA as shown. The VGA and SXGA arrays have a denser array of pixels and consequently deliver higher resolution. The QVGA is economical and exhibits excellent dynamic range because of large sensitive pixels.
More recently, the technology of smaller pixel pitch has resulted in infrared cameras having detector arrays of 15 micron pitch, delivering some of the most impressive thermal images available today. For higher resolution applications, cameras having larger arrays with smaller pixel pitch deliver images having high contrast and sensitivity. In addition, with smaller pixel pitch, optics can also become smaller further reducing cost.
2.2 Infrared Lens Characteristics
Lenses designed for high speed infrared cameras have their own special properties. Primarily, the most relevant specifications are focal length (field-of-view), F-number (aperture) and resolution.
Focal Length: Lenses are normally identified by their focal length (e.g. 50mm). The field-of-view of a camera and lens combination depends on the focal length of the lens as well as the overall diameter of the detector image area. As the focal length increases (or the detector size decreases), the field of view for that lens will decrease (narrow).
A convenient online field-of-view calculator for a range of high-speed infrared cameras is available online.
In addition to the common focal lengths, infrared close-up lenses are also available that produce high magnification (1X, 2X, 4X) imaging of small objects.
Infrared close-up lenses provide a magnified view of the thermal emission of tiny objects such as electronic components.
F-number: Unlike high speed visible light cameras, objective lenses for infrared cameras that utilize cooled infrared detectors must be designed to be compatible with the internal optical design of the dewar (the cold housing in which the infrared detector FPA is located) because the dewar is designed with a cold stop (or aperture) inside that prevents parasitic radiation from impinging on the detector. Because of the cold stop, the radiation from the camera and lens housing are blocked, infrared radiation that could far exceed that received from the objects under observation. As a result, the infrared energy captured by the detector is primarily due to the object’s radiation. The location and size of the exit pupil of the infrared lenses (and the f-number) must be designed to match the location and diameter of the dewar cold stop. (Actually, the lens f-number can always be lower than the effective cold stop f-number, as long as it is designed for the cold stop in the proper position).