Cooled thermal camera mounted on pedestal

Cooled Thermal Camera Systems

Sierra Pacific Long range thermal flir imaging, Long Range PTZ Thermal FLIR zoom imaging camera, Cooled Thermal Camera 2 Comments

Cooled thermal camera systems are the clear choice for long range imaging. This is due to the highly sensitive FPA technology they employ. Typically, a cooled thermal camera will utilize either InSb or MCT sensor materials. These are very good materials for a thermal array in the 3-5 micron range. However, Focal Plane Arrays made from these materials do require cryogenic cooling in order to function at their peak performance level. That is why they are called “cooled thermal camera” systems. But, when these exotic sensor materials are combined with an appropriate cooling system, the result is a thermal imaging system that boasts MRTD’s on the order of 20mk or less. In practical terms this equates to 3-4 times the sensitivity of uncooled thermal camera systems. The sensitivity of cooled sensors, allows us to use much smaller lenses (f/4-f/5.5). It follows that, given a certain envelope of space, we can fit a much longer focal length lens into a cooled thermal camera design than any other technology would allow. When ultimate distance matters, long range cooled thermal camera systems are the only option.

Cooled thermal camera mounted on pedestal

Cooled Thermal Camera Sensor Types

Exotic materials are used to produce the focal plane array sensors that are used in cooled thermal cameras.  The most common are Indium Antimonide (InSb) and Mercury Cadmium Telluride (HgCdTe).

InSb

The InSb (commonly referred to as Ins-Bee or reticulated insb) sensor material is a narrow gap semiconductor in crystaline form.  It is sensitive to wavelengths of 1µm to 5µm but is typically filtered, in cooled thermal camera systems, to 3µm to 5µm. InSb detectors have long been the leading choice for imaging systems that feature long range thermal optics.  It is a reliable and mature sensor technology that is manufactured in various countries around the world. The InSb detector does require cryogenic cooling to operate at peak efficiency and produce a usable thermal image. In the past ,this was accomplished by a dewar that needed to be filled with Liquid Nitrogen. The dewars are long gone. Now,highly efficient cryo coolers manage the task of cooling down to ~80°K.

The M9 Cooled MWIR Camera System uses InSb Detectors and is the pinnacle of cooled Long Range thermal camera systems, The Unit has extremely long range cooled MWIR lenses which
provides Very long range detection Ranges up to 60 Kilometers

HgCdTe

The HgCdTe (commonly referred to as MCT or Mer-Cad) sensor material is a narrow direct bandgap zincblende.  MCT sensors are tuned to operate in wavelengths from 1µm all the way to 14µm. They are a versatile sensor material.  Unlike InSb sensors, MCT sensors produce cooled thermal camera systems that access both atmospheric transmission windows of 3µm to 5µm and 7µm to 14µm. Most people think of cooled cameras as always being MWIR (3µm-5µm).  MCT materials produce a cooled thermal camera that is MWIR or LWIR (7µm to 14µm).  MCT focal plane arrays also require cryogenic cooling to ~80°K.

HOT HgCdTe (The New Kid On The Block)

Hot MCT is a relatively new technology utilizing Mer Cad based sensors.  The new “HOT” sensors operate at warmer temperatures than the InSb and traditional HgCdTe chips. Instead of cooling to 80°K the HOT MCT only needs to be cooled to 160°K. In practical terms this greatly reduces the load on the cryo cooler. This creates a cooled thermal camera that starts up faster, has a longer cooler lifespan and uses less power.  This is fantastic for SWaP efforts.  The Hot MCT is a new development that is very promising but only time will tell if it proves to be the future of cooled thermal.

MCT cooled thermal camera module

 

In conclusion, cooled thermal camera systems incorporate different sensor materials but offer distinct advantages when used in long range and ultra long range thermal imaging platforms. The key is the increased sensitivity and smaller lens sizes.  Next time, we will discuss lens options for your cooled thermal camera.

 

long range cooled MWIR thermal camera with laser range finder-SPI

SPI long range cooled MWIR thermal camera with laser range finder-SPI

 

UP NEXT COOLED THERMAL CAMERA LENSES >>>

Examples of systems using the above technology. This is only a partial list of whats possible with cooled thermal imaging. Review the site for more options or contact us to discuss thermal imaging. Thats what we do here. Make sure you tune in next time for a complete review of lens and optical systems for long range thermal camera systems.

Spi Uav drone aerial gimbal Flir CMOS llltv nit vision sensors

cooled thermal image of a plane

 

Comments 2

  1. SPI Corp announces the Longest Range Cooled Thermal Camera with a whopping 2550mm efl lens. The biggest hurdle in todays long range imaging is the ability to get a big enough lens to really break through the 25km human identification barrier. Even, highly sensitive cooled sensors require relatively fast lenses at f/4 or f/5 and uncooled thermal cameras need ridiculously fast optics on the order of f/1. SPI Corp is proud to announce a new breakthrough in cooled thermal camera system integration that will shatter the 25km barrier and should produce human detection at unheard of 30-40km ranges or more. By leveraging 20 years of experience and industry breakthroughs, SPI Corp has leap frogged the competition to be the first manufacturer to offer thermal optics with an efl of 2550mm. But, that is just the beginning of the story.

    Our breakthrough technology uses the latest in space derived sensor technology and new lens designs to bring customers not only longer range but lower price and increased reliability. We have pivoted from older technologies like reticulated InSb, Lattice Mesh Indium Antimonide and traditional HgCDTe sensors in favor of the future. The new sensor (in fab today) gives us all the sensitivity of the reticulated InSb but at a fraction of the power consumption and with more than double the expected lifespan. By reducing thermal load on the sensor support electro mechanical systems we can do more with far less. By coupling the new sensor technology with our exclusive lens package we can achieve an effective 2550mm focal length in a ground based cooled thermal camera system. This is almost double any competing system in the world. We are very excited about this new technology and the promise it has to make the world a safer place. We imagine M10 Ultra Long Range Thermal 2550 systems watching over every sea port, border and critical facility in the world. Low cost long range thermal imaging is now a reality. Advanced orders are being taken now for priority delivery. Contact SPI CORP for more information on this groundbreaking technology package.

  2. All objects with an absolute temperature over 0 K emit infrared (IR) radiation. Infrared radiant energy is determined by the temperature and emissivity of an object and is characterized by wavelengths ranging from 0.76 (the red edge of the visible range) to 1000 μm (beginning of microwaves range). The higher the temperature of an object, the higher the spectral radiant energy, or emittance, at all wavelengths and the shorter the peak wavelength of the emissions. Due to limitations on detector range, IR radiation is often divided into three smaller regions based on the response of various detectors.SWIR: 0.9-1.7 μm
    SWIR is also called the «reflected infrared» region since radiation coming from a light source is reflected by the object in a similar manner as in the visible range. SWIR imaging requires some sort of illumination in order to image an object and can be performed only if some light, such as ambient moon light or stars light is present. In fact the SWIR region is suitable for outdoor, night-time imaging.SWIR imaging lenses are specifically designed, optimized, and anti-reflection coated for SWIR wavelenghts. Indium Gallium Arsenide (InGaAs) sensors are the primary sensors used in SWIR, covering typical SWIR band, but can extend as low as 0.550 µm to as high as 2.5 µm.A large number of applications that are difficult or impossible to perform using visible light are possible using SWIR InGaAs based cameras: nondestructive identification of materials, their composition, coatings and other characteristics, Electronic Board Inspection, Solar cell inspection, Identifying and Sorting, Surveillance, Anti-Counterfeiting, Process Quality Control, etc… When imaging in SWIR, water vapor, fog, and certain materials such as silicon are transparent. Additionally, colors that appear almost identical in the visible may be easily differentiated using SWIR.MWIR: 3-5 μm / LWIR: 8-14 μm
    MWIR and LWIR regions are also referred to as “thermal infrared” because radiation is emitted from the object itself and no external light source is needed to image the object. Two major factors determine how bright an object appears to a thermal imager: the object’s temperature and its emissivity (a physical property of materials that describes how efficiently it radiates). As an object gets hotter, it radiates more energy and appears brighter to a thermal imaging system. Atmospheric obscurants cause much less scattering in the MWIR and LWIR bands than in the SWIR band, so cameras sensitive to these longer wavelengths are highly tolerant of smoke, dust and fog.MWIR collects the light in the 3 μm to 5 μm spectral band. SPI’s MWIR cameras are employed when the primary goal is to obtain high-quality images rather than focusing on temperature measurements and mobility. The MWIR band of the spectrum is the region where the thermal contrast is higher due to blackbody physics; while in the LWIR band there is quite more radiation emitted from terrestrial objects compared to the MWIR band, the amount of radiation varies less with temperature (see Planck’s curves): this is why MWIR images generally provide better contrast than LWIR. For example, the emissive peak of hot engines and exhaust gasses occurs in the MWIR band, so these cameras are especially sensitive to vehicles and aircraft. The main detector materials in the MWIR are InSb (Indium antimonide) and HgCdTe (mercury cadmium telluride) also referred to as MCT and partially lead selenide (PbSe)
    LWIR collects the light in the 8 μm to 14 μm spectral band and is the wavelength range with the most available thermal imaging cameras. In fact, according to Planck’s law, terrestrial targets emit mainly in the LWIR. LWIR systems applications include thermography/temperature control, predictive maintenance, gas leak detection, imaging of scenes which span a very wide temperature range (and require a broad dynamic range), imaging through thick smoke, etc… The two most commonly used materials for uncooled detectors in the LWIR are amorphous silicon (a-Si) and vanadium oxide (VOx), while cooled detectors in this region are mainly HgCdTe.
    Thermal radiation principle
    An object reacts to incident radiation from its surroundings by either absorbing, reflecting or transmitting the radiation incident upon it. Therefore:α + ρ + τ = 1α = absorption coefficient 0 < α < 1 ρ = reflection coefficient 0 < ρ < 1 τ = transmission coefficient 0 < τ < 1Kirchoff’s law At thermal equilibrium, the power radiated by an object must be equal to the power absorbedBlackbody A blackbody is defined as a perfect radiator which absorbs and re-radiates (as stated by Kirchoff’s law) all radiation incident upon it. For a Blackbody α=1, ρ=0, τ=0Blackbody spectral radiant emittance (Planck’s Law) The higher the temperature of an object, the higher the spectral radiant emittance (at all wavelengths) and the shorter the peak wavelength of the emissions. Emissivity describes the efficiency with which a material radiates infrared energy compared to a blackbody. Real-world objects have emissivity values between 0 and 1.00 and are selective radiators, i.e. their emissivity varies both with wavelength and temperature. Moreover emissivity is also dependent on emission angle, surface treatment and material thickness.In general, the duller and blacker a material is, the higher its emissivity. On the other hand, the more reflective a material is, the lower its emissivity. Therefore, the same material c an show extremely different emissivity values depending on the surface treatment. For example polished aluminium, which is highly reflective, has a much lower emissivity than anodized aluminium.Thermal imaging cameras calculate an object temperature by detecting and quantifying the emitted energy over the operational wavelength range of the detector. Temperature is then calculated by relating the measured energy to the temperature of a blackbody radiating an equivalent amount of energy according to Planck’s Blackbody Law. Because the emissivity of an object affects how much energy an object emits, emissivity also influences a thermal imager’s temperature calculation.Material Emissivity Human Skin 0,98 Water 0,95 Aluminium (polished) 0,10 Aluminium (anodized) 0,65 Plastic 0,93 Ceramic 0,94 Glass 0,87 Rubber 0,90 Cloth 0,95 Tab. 1: Emissivity values of common materialsAtmospheric windows Water vapor and gases that make up the Earth’s atmosphere tend to absorb infrared radiation coming form an object, which becomes therefore severly attenuated if radiation must be detected at great distances from the object.Thus, in order to detect the IR signal, one must use the so-called atmospheric windows (Fig. 3). Essentially two infrared atmospheric windows (bands) are available: the short/medium-wave windows spanning form 2 to 5,6 μm and the long-wave window, spanning from approximately 7,5 to 14 μm. Composition of detectors material is selected for sensitivity to one band.Types of infrared detectors An infrared detector is simply a transducer of radiant energy, converting radiant energy in the infrared band into a measurable form. There are many detector materials with response curves that fit within the above mentioned infared windows. Infrared detectors are classified into thermal types, that have no wavelength dependece, and quantum types that are wavelenght dependent.THERMAL / NON-QUANTUM TYPESThermal IR detectors include thermocouple, thermopile, bolometer, and pyroelectric detectors. Thermal detectors, as the name suggests, change their temperature depending upon the impacting radiation. The temperature change creates a voltage change in the thermopile and a change in resistance in the bolometer, which can then be measured and related to the amount of incident radiation. Thermal detectors are much slower (response time order of ms) than quantum detectors due to the self-heating required. One of the most attractive characteristics of thermal detectors is the equal response to all wavelengths. This contributes to the stability of a system that must operate over a wide temperature range. Another significant factor is that thermal detectors do not require cooling.A microbolometer is a specific type of bolometer, i.e. a detector that measures the power of electromagnetic radiation incident upon a material which possesses the specific property of changing its electrical resistance when heated. Basically, infrared radiation strikes the detector material, heating it, and thus changing its electrical resistance, which is then measured. Microbolometers detectors are used in thermal cameras operating in the LWIR (7.5 – 14 μm) range and do not require cooling. The two most commonly used materials are amorphous silicon (a-Si) and vanadium oxide (VOx). Advantages include: – Broad and flat response curve (wavelenght independent),- Do not require cooling,- Small and lightweight, allows compact camera designs, – Less expensive,- Low power consumption relative to cooled detector thermal imagers, – Very long MTBF (Mean Time Between Failures), while disadvantages are – Relatively low sensitivity (detectivity), – Slow response time (time constant 12 ms)QUANTUM TYPESQuantum detectors operate on the basis of an intrisic photoelectric effect and interact directly with impacting photons. These materials respond to IR radiation by absorbing photons that elevate the material’s electrons to a higher energy state, causing a change in conductivity, voltage or current.In materials used for quantum detectors, electrons are either in the conduction band, where they are free to move (and therefore conduct electrical current), or in the valence band, where they cannot move freely. When the material is cooled below a certain temperature, no electrons can be found in the conduction band and no electrical current is carried. In these conditions, when incident photons hit the material they stimulate electrons to move up into the conduction band, thus carrying a current which is proportional to the intensity of incident radiation. Since IR radiation has small energy when compared to Visibile or UV rays (energy is inversely proportional to wavelength), these detectors are cooled down to cryogenic temperatures in order to increase infrared detection efficiency/sensitivity. Cooling methods include Stirling cycle engines, liquid nitrogen and thermoelectric cooling (). Cooled thermal imaging cameras are the most sensitive type of cameras to small differences in scene temperature. Quantum detectors react very quickly to changes in IR levels (response time order of μs), however they have response curves with detectivity that varies strongly with wavelength. Cooled quantum detector materials include – InSb, – InGaAs, – PbS, – PbSe, – HgCdTe (MCT). Short-wave infrared (0.9 to 1.7 µm): mainly InGaAs detectors cover this region Mid-wave infrared (3 to 5 µm): covered by Indium antimonide (InSb), HgCdTe and partially by lead selenide (PbSe) Long-wave infrared (8 to 14 µm): this region is covered by HgCdTe and microbolometers IR detectors performance parameters Signal-to-noise ratio (S/N) Responsivity R Responsivity is the ability of the detector to convert the incoming radiation into an electrical signal. Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, responsivity measures the electrical output per optical input. Noise equivalent power (NEP) A photodetector produces some noise output with a certain average power even when it does not get any input radiation. This noise output is proportional to the square of the r.m.s. voltage or current amplitude. The noise-equivalent power (NEP) of a detector is the optical input power (P) which produces an additional output power identical to the noise power for a given bandwidth (Δf). In other words, the NEP is the light power required to obtain a signal to noise ratio S/N of 1, that is, the light level required to produce a signal current equivalent to the noise current. The units of NEP are watts per square root hertz. NEP indicates the lower limit of light detection: a smaller NEP corresponds to a more sensitive detector. Specific detectivity D* (D-star) D* is the photo sensitivity per unit active area of a detector. D* is conveniently used to compare the performances of various detector types since it is area-independent. D* is the signal-to-noise ratio at a particular electrical frequency, and in a 1 Hz bandwidth when 1 Watt of radiant power is incident on a 1 cm² active area detector. In other words it is equal to the reciprocal of the noise-equivalent power (NEP), normalized per unit area. P = Incident radiant power received by the detector [W] A = Detector active area [cm2] Δf = Noise bandwidth [Hz] S/N = Signal to Noise ratio In general the measurement conditions of D* are expressed in the format of D* (X, Y, Z), where X is the temperature [K] or wavelength [μm] of a radiant source, Y is the chopping frequency [Hz], and Z is the noise bandwidth [Hz]. The units of D* are centimeter-square root-hertz per watt, sometimes referred to as “Jones” units. The higher D*, the better the detector. D* values are very high Noise equivalente temperature difference NETD NETD is a widely used performance parameter that characterizes the sensitivity of thermal imaging sensors. NETD is the amount of incident signal temperature that would be needed to match the internal noise of the detector (such that the signal-to-noise ratio is equal to one). Essentially, it specifies the minimum detectable temperature difference. Typically NETD is expressed in units of Kelvin (K). Cooled infrared camera systems typically have low noise levels, in the range of 10 – 30mK. Uncooled infrared cameras systems are typically noisier, in the range of 30 – 120mK. One important parameter that needs to be taken into account when specifying the NETD value of a thermal imaging camera is the lens aperture (or f-number). In fact, the lens f-number will directly affect the sensitivity of the camera. NETD values of different detectors can be compared only by using a lens with the same f-number. Common infrared (IR) materials Zinc Selenide (ZnSe) Zinc Sulfide (ZnS) Zinc Sulfide MultiSpectral (ZnS MS) Germanium (Ge) Gallium Arsenide (GaAs) Silicon (Si) Optical coatings ANTI-REFLECTIVE AR COATING Anti-reflective (AR) coatings are thin films applied to surfaces to reduce their reflectivity through optical interference. An AR coating typically consists of a carefully constructed stack of thin layers with different refractive indices. The internal reflections of these layers interfere with each other so that a wave peak and a wave trough come together and extinction occurs, leading to an overall reflectance lower than that of the bare substrate surface. Anti-reflection coatings are included on most refractive optics and are used to maximize throughput and reduce ghosting. Perhaps the simplest, most common anti-reflective coating consists of a single layer of Magnesium Fluoride (MgF2), which has a very low refractive index (approx. 1.38 at 550 nm) HARD CARBON ANTI-REFLECTIVE HCAR COATING HCAR is an optical coating commonly applied to Silicon and Germanium designed to meet the needs of those applications with optical elements exposed to harsh environments, such as military vehicles and outdoor thermal cameras. This coating offers highly protective properties coupled with good anti-reflective performance, protecting the outer optical surfaces from high velocity airborne particles, seawater, engine fuel and oils, high humidity, improper handling, etc.. It offers great resistance to abrasion, salts, acids, alkalis, and oil. ATHERMALIZATION Any material is characterized by a certain temperature expansion coefficient and responds to temperature variations by either increasing or decreasing its physical dimensions. Thus, thermal expansion of optical elements might alter a system’s optical performance causing defocusing due to a change of temperature. An optical system is athermalized if its critical performance parameters (such as Modulation Transfer Function, Back Focal Length, Effective Focal Length, …) do not change appreciably over the operating temperature range. Athermalization techniques can be either active or passive. Active athermalization involves motors or other active systems to mechanically adjust the lens elements’ position, while passive athermalization makes use of design techniques aimed at compensating for thermal defocus by combining suitably chosen lens materials and optical powers (optical compensation) or by using expansion rods with very different thermal expansion coefficients that mechanically displace a lens element so that the system stays in focus (mechanical compensation). Basic optics definitions FOCAL length d = Focal Plane Array diagonal (mm), f = focal length (mm), FOV = field of view (degrees). FOV is the angular subtense (expressed in angular degrees or radians per side if rectangular, and angular degrees or radians if circular) over which the optical system will integrate all incoming radiant energy. According to the above formula, as the focal length increases, the field of view for that lens will be narrower and viceversa. For instance, long range thermal infrared surveillance applications require long focal length lenses. F/# The f/number determines the light gathering power of the lens and therefore affects the sensitivity of the optics-camera system. The f/number of an optical system is the ratio of the focal length of the lens to the diameter of the front lens element. f = focal length A = diameter of the front lens element As the focal length of a lens is increased, the diameter of the front lens element must be increased to keep the system f/number constant. Sensitivity of IR cameras can be increased by choosing the appropriate lens. Uncooled cameras equipped with uncooled microbolometer detectors are typically less sensitive than cooled cameras equipped with quantum detectors. Therefore a camera equipped with a low-sensitivity detector must be run with a lens that has a low f/number (i.e. wide aperture) to have comparable sensitivity to a cooled camera. However, using such wide-aperture lenses limits the depth of field that can be obtained by the imaging system. In contrast, a cooled camera system can be operated at higher f/numbers without significantly compromising system sensitivity. Long range thermal infrared surveillance applications require long focal length lenses, and the cost of lenses increases rapidly with focal length for uncooled camera systems and rather slowly for cooled systems Spatial resolution Diffraction limits the resolution possible with an objective lens. Each point of the object to be viewed is imaged as a spot pattern called the an Airy disk. Its diameter is given by the following formula. Clearly, as the FN / wavelength increases, the resolution limit increases proportionally. Therefore, in order to achive a similar resolution limit, LWIR lenses working at λ = 10 μm will require a much lower FN (larger apertures) than MWIR lenses working at λ = 4 μm.

Leave a Reply