Recent developments in cooled mercury cadmium telluride (MCT or HgCdTe) infrared detector technologies have created attainable the improvement of high functionality infrared cameras for use in a wide range of demanding thermal imaging applications. These infrared cameras are now out there with spectral sensitivity in the shortwave, mid-wave and long-wave spectral bands or alternatively in two bands. In addition, a range of camera resolutions are accessible as a outcome of mid-size and substantial-size detector arrays and different pixel sizes. Also, camera attributes now include things like higher 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 variety to prevent 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 functions allow a wide range of thermal imaging applications that have been previously not probable.
At the heart of the higher 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 wide variety of MCT detectors, higher speed infrared cameras have been developed 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 outcome is a single band infrared detector with extraordinary quantum efficiency (usually above 70%) and higher signal-to-noise ratio able to detect incredibly small levels of infrared signal. Single-band MCT detectors typically fall in 1 of the 5 nominal spectral bands shown:
• Quick-wave infrared (SWIR) cameras – visible to 2.5 micron
• Broad-band infrared (BBIR) cameras – 1.five-five micron
• Mid-wave infrared (MWIR) cameras – 3-5 micron
• Long-wave infrared (LWIR) cameras – 7-10 micron response
• Incredibly Long Wave (VLWIR) cameras – 7-12 micron response
In addition to cameras that make use of “monospectral” infrared detectors that have a spectral response in one band, new systems are being created that utilize infrared detectors that have a response in two bands (identified as “two colour” or dual band). Examples consist of cameras getting a MWIR/LWIR response covering each three-5 micron and 7-11 micron, or alternatively certain SWIR and MWIR bands, or even two MW sub-bands.
There are a wide variety of causes motivating the choice of the spectral band for an infrared camera. For particular applications, the spectral radiance or reflectance of the objects under observation is what determines the best spectral band. These applications incorporate spectroscopy, laser beam viewing, detection and alignment, target signature analysis, phenomenology, cold-object imaging and surveillance in a marine environment.
On top of that, a spectral band could be selected for the reason that of the dynamic variety issues. Such an extended dynamic variety would not be attainable with an infrared camera imaging in the MWIR spectral range. The wide dynamic variety overall performance of the LWIR technique is conveniently 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 broadly varying temperatures is smaller in the LWIR band than the MWIR band when observing a scene having the similar object temperature range. In other words, the LWIR infrared camera can image and measure ambient temperature objects with higher sensitivity and resolution and at the identical time really hot objects (i.e. >2000K). Imaging wide temperature ranges with an MWIR method would have significant challenges for the reason that the signal from high temperature objects would want to be drastically attenuated resulting in poor sensitivity for imaging at background temperatures.
2. silicon lens and window and Field-of-View
two.1 Detector Arrays and Pixel Sizes
High speed infrared cameras are obtainable obtaining various resolution capabilities due to their use of infrared detectors that have various array and pixel sizes. Applications that do not require higher resolution, higher speed infrared cameras primarily based on QVGA detectors offer you outstanding performance. A 320×256 array of 30 micron pixels are identified for their incredibly wide dynamic variety due to the use of comparatively huge pixels with deep wells, low noise and extraordinarily high sensitivity.
Infrared detector arrays are readily available in diverse sizes, the most frequent are QVGA, VGA and SXGA as shown. The VGA and SXGA arrays have a denser array of pixels and consequently deliver greater resolution. The QVGA is economical and exhibits exceptional dynamic variety for the reason that of huge sensitive pixels.
Much more recently, the technology of smaller sized pixel pitch has resulted in infrared cameras getting detector arrays of 15 micron pitch, delivering some of the most impressive thermal photos readily available nowadays. For higher resolution applications, cameras getting bigger arrays with smaller pixel pitch deliver pictures having high contrast and sensitivity. In addition, with smaller sized pixel pitch, optics can also turn into smaller sized additional lowering cost.
two.2 Infrared Lens Traits
Lenses made for higher speed infrared cameras have their personal unique properties. Mainly, the most relevant specifications are focal length (field-of-view), F-number (aperture) and resolution.
Focal Length: Lenses are ordinarily identified by their focal length (e.g. 50mm). The field-of-view of a camera and lens mixture depends on the focal length of the lens as well as the all round diameter of the detector image region. As the focal length increases (or the detector size decreases), the field of view for that lens will lower (narrow).
A convenient on-line field-of-view calculator for a variety of high-speed infrared cameras is readily available online.
In addition to the widespread focal lengths, infrared close-up lenses are also accessible that generate high magnification (1X, 2X, 4X) imaging of smaller objects.
Infrared close-up lenses provide a magnified view of the thermal emission of tiny objects such as electronic elements.
F-quantity: Unlike high speed visible light cameras, objective lenses for infrared cameras that utilize cooled infrared detectors have to be created to be compatible with the internal optical design of the dewar (the cold housing in which the infrared detector FPA is positioned) simply because the dewar is created with a cold cease (or aperture) inside that prevents parasitic radiation from impinging on the detector. Because of the cold cease, the radiation from the camera and lens housing are blocked, infrared radiation that could far exceed that received from the objects beneath 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) will have to be designed to match the location and diameter of the dewar cold stop. (Basically, the lens f-quantity can constantly be reduced than the efficient cold stop f-number, as extended as it is developed for the cold quit in the right position).
Lenses for cameras obtaining cooled infrared detectors need to have to be specially designed not only for the distinct resolution and location of the FPA but also to accommodate for the place and diameter of a cold stop that prevents parasitic radiation from hitting the detector.
