Recent developments in cooled mercury cadmium telluride (Mct or HgCdTe) infrared detector technology have made potential the improvement of high operation infrared cameras for use in a wide collection 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 collection of camera resolutions are available as a consequent of mid-size and large-size detector arrays and discrete 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 consequent in an expanded dynamic range to avoid saturation and optimize sensitivity. These infrared cameras can be calibrated so that the yield digital values correspond to object temperatures. Non-uniformity improvement algorithms are included that are independent of exposure time. These operation 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 astonishing sensitivity and versatility for viewing high speed thermal events.
Optical Encoder Resolution
1. Infrared Spectral Sensitivity Bands
Due to the availability of a collection of Mct detectors, high speed infrared cameras have been designed to control in several determined spectral bands. The spectral band can be manipulated by varying the alloy blend of the HgCdTe and the detector set-point temperature. The consequent is a particular band infrared detector with astonishing portion efficiency (typically above 70%) and high signal-to-noise ratio able to detect very 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 use "monospectral" infrared detectors that have a spectral response in one band, new systems are being industrialized that use infrared detectors that have a response in two bands (known as "two color" or dual band). Examples include cameras having a Mwir/Lwir response exterior both 3-5 micron and 7-11 micron, or alternatively determined Swir and Mwir bands, or even two Mw sub-bands.
There are a collection of reasons motivating the selection of the spectral band for an infrared camera. For determined applications, the spectral glow 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 lookout in a maritime environment.
Additionally, a spectral band may be selected because of the dynamic range concerns. Such an extended dynamic range would not be potential with an infrared camera imaging in the Mwir spectral range. The wide dynamic range operation of the Lwir theory is unquestionably 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 portion ambient temperature objects with high sensitivity and resolution and at the same time very hot objects (i.e. >2000K). Imaging wide temperature ranges with an Mwir theory would have vital challenges because the signal from high temperature objects would need to be drastically attenuated resulting in poor sensitivity for imaging at background temperatures.
2. Image Resolution and Field-of-View
2.1 Detector Arrays and Pixel Sizes
High speed infrared cameras are available having discrete 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 320x256 array of 30 micron pixels are known for their very 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 coarse 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 disagreement 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 extra properties. Primarily, the most relevant specifications are focal distance (field-of-view), F-number (aperture) and resolution.
Focal Length: Lenses are ordinarily identified by their focal distance (e.g. 50mm). The field-of-view of a camera and lens blend depends on the focal distance of the lens as well as the allembracing diameter of the detector image area. As the focal distance 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 coarse focal lengths, infrared close-up lenses are also available that produce high magnification (1X, 2X, 4X) imaging of small objects.
Infrared close-up lenses supply 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 use cooled infrared detectors must be designed to be compatible with the internal optical establish 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 vigor 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 productive cold stop f-number, as long as it is designed for the cold stop in the permissible position).
Lenses for cameras having cooled infrared detectors need to be specially designed not only for the exact resolution and location of the Fpa but also to accommodate for the location and diameter of a cold stop that prevents parasitic radiation from hitting the detector.
Resolution: The modulation exchange function (Mtf) of a lens is the characteristic that helps decide the capability of the lens to decide object details. The image produced by an optical theory will be somewhat degraded due to lens aberrations and diffraction. The Mtf describes how the disagreement of the image varies with the spatial frequency of the image content. As expected, larger objects have relatively high disagreement when compared to smaller objects. Normally, low spatial frequencies have an Mtf close to 1 (or 100%); as the spatial frequency increases, the Mtf at last drops to zero, the ultimate limit of resolution for a given optical system.
3. High Speed Infrared Camera Features: changeable exposure time, frame rate, triggering, radiometry
High speed infrared cameras are ideal for imaging fast-moving thermal objects as well as thermal events that occur in a very short time period, too short for standard 30 Hz infrared cameras to capture literal, data. Beloved applications include the imaging of airbag deployment, turbine blades analysis, dynamic brake analysis, thermal analysis of projectiles and the study of heating effects of explosives. In each of these situations, high speed infrared cameras are productive tools in performing the vital analysis of events that are otherwise undetectable. It is because of the high sensitivity of the infrared camera's cooled Mct detector that there is the possibility of capturing high-speed thermal events.
The Mct infrared detector is implemented in a "snapshot" mode where all the pixels simultaneously integrate the thermal radiation from the objects under observation. A frame of pixels can be exposed for a very short interval as short as <1 microsecond to as long as 10 milliseconds. Unlike high speed visible cameras, high speed infrared cameras do not require the use of strobes to view events, so there is no need to synchronize illumination with the pixel integration. The thermal emission from objects under observation is ordinarily enough to capture fully-featured images of the object in motion.
Because of the benefits of the high operation Mct detector, as well as the sophistication of the digital image processing, it is potential for today's infrared cameras to perform many of the functions vital to enable detailed observation and testing of high speed events. As such, it is useful to recapitulate the usage of the camera including the effects of changeable exposure times, full and sub-window frame rates, dynamic range expansion and event triggering.
3.1 Short exposure times
Selecting the best integration time is ordinarily a compromise in the middle of eliminating any request for retrial blur and capturing enough vigor to produce the desired thermal image. Typically, most objects radiate enough vigor during short intervals to still produce a very high capability thermal image. The exposure time can be increased to integrate more of the radiated vigor until a saturation level is reached, ordinarily several milliseconds. On the other hand, for provocative objects or dynamic events, the exposure time must be kept as short as potential to take off request for retrial blur.
Tires running on a dynamometer can be imaged by a high speed infrared camera to decide the thermal heating effects due to simulated braking and cornering.
One relevant application is the study of the thermal characteristics of tires in motion. In this application, by observing tires running at speeds in excess of 150 mph with a high speed infrared camera, researchers can capture detailed temperature data during dynamic tire testing to simulate the loads associated with turning and braking the vehicle. temperature distributions on the tire can indicate potential question areas and protection concerns that require redesign. In this application, the exposure time for the infrared camera needs to be sufficiently short in order to take off request for retrial blur that would reduce the resulting spatial resolution of the image sequence. For a desired tire resolution of 5mm, the desired maximum exposure time can be calculated from the geometry of the tire, its size and location with respect to the camera, and with the field-of-view of the infrared lens. The exposure time vital is thought about to be shorter than 28 microseconds. Using a Planck's calculator, one can surmise the signal that would be obtained by the infrared camera adjusted withspecific F-number optics. The consequent indicates that for an object temperature estimated to be 80°C, an Lwir infrared camera will deliver a signal having 34% of the well-fill, while a Mwir camera will deliver a signal having only 6% well fill. The Lwir camera would be ideal for this tire testing application. The Mwir camera would not perform as well since the signal yield in the Mw band is much lower requiring whether a longer exposure time or other changes in the geometry and resolution of the set-up.
The infrared camera response from imaging a thermal object can be predicted based on the black body characteristics of the object under observation, Planck's law for blackbodies, as well as the detector's responsivity, exposure time, atmospheric and lens transmissivity.
3.2 changeable frame rates for full frame images and sub-windowing
While standard speed infrared cameras ordinarily deliver images at 30 frames/second (with an integration time of 10 ms or longer), high speed infrared cameras are able to deliver many more frames per second. The maximum frame rate for imaging the entire camera array is dinky by the exposure time used and the camera's pixel clock frequency. Typically, a 320x256 camera will deliver up to 275 frames/second (for exposure times shorter than 500 microseconds); a 640x512 camera will deliver up to 120 frames/second (for exposure times shorter than 3ms).
The high frame rate capability is very desirable in many applications when the event occurs in a short estimate of time. One example is in airbag deployment testing where the effectiveness and protection are evaluated in order to make establish changes that may heighten performance. A high speed infrared camera reveals the thermal distribution during the 20-30 ms period of airbag deployment. As a consequent of the testing, airbag manufacturers have made changes to their designs including the inflation time, fold patterns, tear patterns and inflation volume. Had a standard Ir camera been used, it may have only delivered 1 or 2 frames during the first deployment, and the images would be blurry because the bag would be in request for retrial during the long exposure time.
Airbag effectiveness testing has resulted in the need to make establish changes to heighten performance. A high speed infrared camera reveals the thermal distribution during the 20-30ms period of airbag deployment. As a consequent of the testing, airbag manufacturers have made changes to their designs including the inflation time, fold patterns, tear patterns and inflation volume.
Even higher frame rates can be achieved by outputting only portions of the camera's detector array. This is ideal when there are smaller areas of interest in the field-of-view. By observing just "sub-windows" having fewer pixels than the full frame, the frame rates can be increased. Some infrared cameras have minimum sub-window sizes. Commonly, a 320x256 camera has a minimum sub-window size of 64x2 and will yield these sub-frames at roughly 35Khz, a 640x512 camera has a minimum sub-window size of 128x1 and will yield these sub-frame at faster than 3Khz.
Because of the complexity of digital camera synchronization, a frame rate calculator is a convenient tool for determining the maximum frame rate that can be obtained for the discrete frame sizes.
3.3 Dynamic range expansion
One of the complications of having a very high sensitivity infrared detector is that the allembracing scene dynamic range will be limited. For example, if a raw count corresponds to 5 mK/digital count, a 14-bit signal range will deliver less than 80 degrees C in dynamic range. This range is further reduced because of pixel non-uniformity. As a consequence, the range of object temperatures that can be viewed in one frame may be too narrow for the application.
To increase the apparent dynamic range, a unique clarification can be implemented which allows the user to artificially progress the dynamic range without sacrificing the high sensitivity operation of the camera. (This mode is sometimes called Dynamic Range ExtendIr, Dr-X, superframing, multi-It). When the dynamic range expansion mode is engaged, the camera sequentially captures many frames, each frame having a different exposure time. The short sequence includes frames that are very sensitive (because of long exposure times) and also less sensitive frames for imaging objects at higher temperatures (because of shorter exposure times). For the formula to be effective, the allembracing time for the frame sequence must be short enough to avoid request for retrial blur. If this is the case, then camera software combines the frames into one image frame having the entire dynamic range for the sequence.
As an example, think the following sequence of images showing the process of mixing a cold fluid to a flask of boiling liquid. If an exposure time was selected based on the full temperature range, the thermal resolution of the cooler objects will be poor. Conversely, if the exposure time is selected to heighten the thermal resolution of the cold fluid, the hotter objects may cause saturation. As a result, with dynamic range expansion, many integration times can be selected that span the entire scene dynamic range.
Exposure time 110 microseconds / Frames 1,4,7 / Object temperature Range 65-150 degrees C
Exposure time 600 microseconds / Frames 2,5,8 / Object temperature Range 35-70 degrees C
Exposure time 1375 microseconds / Frames 3,6,9 / Object temperature Range 5-40 degrees C
In this example, three exposure times have been selected (1375 microseconds, 600 microseconds, and 110 microseconds) to cover a wide scene temperature. The camera then cycle straight through each exposure time at the full frame rate. If the camera is operating at 240 frames/second, the first frame will be at the first exposure time, the second frame will be at the second exposure time, the third at the third exposure time. The fourth frame will begin the sequence again at the first exposure time. The theory will effectively create three sequences, three frames apart, each at a rate of 80 frames/second with the three exposures times. straight through image processing, the sequential frames can be recombined into one unblemished sequence making a pixel by pixel estimation as to the apparent signal, further addition the dynamic range. The resulting image is shown below (with a 5-150 degrees C object temperature scale):
The exposure times correspond to different camera sensitivities. In operation, the camera is programmed to agree the standard exposure time frame by frame. The resulting data will whether be many sequences created from many integration times, or a combined sequence that takes the most standard data based upon the scene. In addition, the user can pick to vary the estimate of frames per integration time, as well as have the selection to use an internal filter mechanism for attenuation or spectral data.
Certain applications require very wide thermal dynamic ranges, which may not be potential with a particular integration time. The high speed infrared camera's dynamic range expansion mode will allow the user to cycle straight through exposure times at the fastest rate potential for the camera.
3.4 Event Triggering
In order to capture high speed events, infrared cameras must be properly synchronized. In the tire-testing example in Section 3.1 above, it is potential to have an optical encoder on the rotating tire that allows literal, position location. The Ttl signal generated by the optical encoder can be fed into the infrared camera to trigger the start of the recording sequence for the camera. The consequent is that every time the encoder sends the pulse, the camera exposes the infrared detector for a determined exposure time creating an image. This allows a real-time stop image sequence to be created via software.
In addition to the capability to accept an external Ttl trigger, infrared cameras have other capabilities that heighten their capability to capture high speed events. For example, determined trigger features permit the infrared camera to synchronize the trigger with the desired image capture. Because digital image frames are captured in real time, a pre-trigger permits the software to identify the beginning of a desired sequence that unquestionably occurs before the trigger signal! Post-trigger delays are also available for aligning the frame capture with an event that follows the trigger after a programmable delay.
In addition, most high speed thermal cameras today have the capability to supply a trigger yield to allow external devices to be synchronized with the thermal camera. Therefore the camera can slave or be slaved. Having both a trigger input and yield is useful in an application that involves using many cameras to view the same target from different angles. In this case, the data can be assembled - via software - into a 3-dimensional rendering of the thermal profile.
3.5 Calibration: non-uniformity improvement and radiometry
One of the challenges in obtaining the best data from a high operation infrared camera theory was in maintaining a permissible calibration. Calibration often refers to two different operations. One, non-uniformity correction, is vital to calibrate the sensor for optimal image quality. The other calibration has to do with determining the temperature of objects based on their image brightness.
Non-uniformity improvement is required to assure that the infrared detector array delivers the best potential image quality. Each pixel in the detector array inevitably has a slightly different gain and offset value. In addition, some pixels may have other anomalous properties that deviate from the norm. The gain and offset for all the pixels in the array need to be adjusted so that each pixel performs identically to the others. Variations can occur for a collection of reasons, including detector non-uniformity and optical affects such as the lens illumination non-uniformity that attenuates the apparent glow near the edge of the image. Anomalous pixel signals must be substituted with nearest neighbor averages as is standard for the application.
To literal, for the gain and offset, a calibration called Non Uniformity improvement (Nuc) must be created. The process typically requires that the user expose the detector to a "cold" and "hot" blackbody source. An algorithm then corrects the detector signal non-uniformity. A similar process called Bad Pixel exchange (Bpr) is required for any pixels that are thought about "bad" which means they deviate from determined thresholds set for evaluating uniformity or due to noisy behavior.
Non-uniformity improvement is complex because there are variations in pixel operation for each integration time. Therefore, this process would need to be performed for every integration time that the user selects. As high operation cameras can control from 1us to >10ms, this means that in theory 10,000 calibrations need to be made. However, because of the linear response of the detector, new advances have been potential to make this process transparent to the user. A process called TrueThermal allows the user to agree any integration time and the camera will automatically reference a look up table of both Nuc and Bpr properties that were established whether at the facility or at the user's site. In this situation, once a user selects the standard integration time, the camera theory applies a predefined Nuc and Bpr table to allow instant and seamless operation.
Once the sensor is calibrated for uniform image quality, the camera can be calibrated for radiometry, or temperature measurement. If an infrared camera is properly calibrated, the object temperature can be thought about based on the glow signal in the thermal images, the background ambient temperature, potential atmospheric effects and the objects emissive properties. It is often particularly useful to be able to use the infrared camera to portion the temperature of objects (such as projectiles) traveling at high speeds. This finds applicability in several leading situations, including: tracking of missiles, spacecraft and other objects, in determining the trajectory of bullets and projectiles and automatically identifying their origin based on trajectory information, and in creating thermal signatures for military targets.
Some users require that the thermal data be calibrated for radiometry. Again, this radiometric data will be dependent upon a exact integration time and must include the Nuc and Bpr corrections. In the past, for each integration time, a unique radiometric calibration would be required. Today, the TrueThermal calibration function facilitates the process, not only correcting for Nuc and Bpr, but also applying the standard radiometric calibration table to the data. This now allows the user to, in real time, convert integration times and have fully corrected data for Nuc, Bpr and radiometric calibration.
4. Infrared Camera Applications
Ir Inspection in Design, Test and Manufacturing:
Thermal imaging has become an very vital technology in many industries as a tool to discover and test different designs and processes. The thermal signatures can be a consequent of electrical, electro-mechanical, chemical or other causes. Thermal images recapitulate heat dissipation, thermal conductance, non-uniformities as well as other leading diagnostic factors.
Hyperspectral and Gas Imaging, Remote Sensing:
Broadband infrared cameras are very useful for hyperspectral imaging (which involves the accumulation of a spectral set of times), gas imaging (which occurs at a sometimes very narrow portion of the infrared spectrum) and remote sensing (imaging the backscatter, reflection and emission differences of discrete materials). Noteworthy image processing software is available to facilitate the analysis of the resulting infrared images.
Target Signature estimation and Tracking:
The spectral characteristics of vehicles, weapons and countermeasures have been found to be leading for many applications. Broad spectral range, high resolution and high sensitivity are key features of infrared cameras for these applications. We offer multi-spectral imaging systems with a wide range of optics. In addition, we offer Noteworthy data acquisition systems featuring real-time image capture and radiometric analysis.
Research and Development:
Thermal imaging is used extensively in engineering and scientific explore centers colse to the world. Thermal imaging provides comprehension into vital information about an object's thermal and spectral characteristics. In determined circumstances, information can be obtained on high-speed events (available with high frame-rate cameras) as well as circumstances requiring large dynamic range (available with changeable integration cameras). Key to the use of these imagers is often application-specific software that permits the detailed analysis of both two-dimensional images as well as arrays of image sequences.
Medical Imaging, Body temperature Detection:
Many physiological conditions produce variations in body temperature and temperature distribution across the human body. As an example, the facility of thermographic cameras at airports has become a key Swine Flu and Sars screening tool for many areas colse to the world. Thermography has also been used as a screening tool for applications such as breast cancer and pain management.
Non-Destructive Test (Ndt):
Thermal imaging is a non-invasive technique which when applied with exact stimulus provides a view into subsurface defects in difficult test samples. Inspection of composite aircraft parts is gaining wide acceptance in airframe establish and service. industrialized materials are seeing their way into automotive and consumer products and thermographic Ndt is a fast and wide area screening technique that is very cost effective.
Summary
Because of the impressive operation of Mct detector technology, high operation infrared cameras have become available that enable a wide collection of demanding thermal imaging applications. A selection of infrared cameras are available having mid-format to large-format detectors and with spectral sensitivity ranging in the short, mid and long-wave spectral bands. The cameras owe their versatility to determined features that include: high frame rate imaging, adjustable exposure time, event triggering enabling the capture of temporal thermal events, dynamic range expansion, non-uniformity improvement and radiometric calibration. These operation capabilities and camera features enable a wide range of thermal imaging applications that were previously not possible, including: Ir Inspection in design, test and manufacturing, hyperspectral imaging, gas detection, remote sensing, target signature estimation and tracking, R&D, curative imaging and Ndt.
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