There are many electro-optical warfare systems, which are analogous to radio frequency (RF) EW systems. These EO EW systems operate in the optical portion of the electromagnetic spectrum. Electro-optics (EO), as the name implies, is a combination of electronics and optics. By one definition EO is the science and technology of the generation, modulation, detection, and measurement, or display of optical radiation by electrical means. Most infrared (IR) sensors, for example, are EO systems. In the popularly used term “EO/IR,” the EO is typically used to mean visible or laser systems. The use of EO in this context is a misnomer. Actually, almost all “EO/IR” systems are EO systems as defined above. Another often-used misnomer is referring to an EO spectrum. EO systems operate in the optical spectrum, which is from 0.01 to 1000 micrometers. EO systems include but are not limited to: lasers, photometry, infrared, and other types of visible, and UV imaging systems.
As can be seen from the diagram, Electro-optical sensors cover the frequency range from 10^4 GHz to 10^6 GHz. Due to the very big number involved with frequency, the specification of EO systems is often referred to by wavelength. The common unit of wavelength for EO system is micron:
The visual light spectrum is between 0.3 and 07 microns
The IR spectrum is spread out from 1 to 1000 µm, which is often sub-divided into three regions:
- Shortwave IR: 0.76–2 µm
- Medium-wave IR: 2–6 µm
- Long-wave IR: 6–1000 µm
In general, with IR sensor (and to a lesser extent, CCD sensor in daylight) no illumination is required because radiation come from targets itself. This direct emission enables passive detection. Passive detection and tracking of a target’s radiation by an airborne or ground optical sensor give no warning to the target unlike tracking by radar.
The most fundamental property of infrared (IR) is also the one most important to the military: warm materials emit a significant amount of IR radiation. As explained earlier, IR is electromagnetic radiation with wavelengths longer than those of visible light and shorter than those of microwaves. IR cannot be seen with the human eye but can be felt by the skin as warmth. It is easy to see that: the higher the temperature of a material, the stronger the radiation.
As the area of the spectrum in which Infrared sensors operate is close to visible light, they experience many of the same shortcomings to a greater or lesser extent: obscuration due to water vapor or other gasses, scattering due to haze and smoke, etc. Therefore, a major problem confronting the use of sensors in the IR region, in particular, is the severe attenuation that occurs in certain parts of the spectrum, allowing only certain windows to be used. An example of IR transmission characteristics in the atmosphere are presented in the table below:
This table shows the percentage transmission over the IR band from 1 to 15 µm at sea level. As shown in the table, at sea level there are a number of areas where the attenuation is significant, particularly in a region between 5.5 µm and about 7.6 µm or 14 µm to 15 µm where there is no transmission at all – mainly owing to water vapor and CO2. Thus, to reduce atmosphere attenuation, IR systems tend to use the IR windows such as SWIR (1–2.5 µm), MWIR (3–5 µm) and LWIR (8–14 µm) regions only instead of the whole spectrum. It is important to remember that since infrared attenuation is affected by gas and aerosol in the atmosphere, the transmittance will get better at higher altitude. Table below shows infrared transmission at different altitude in clear day.
It is important to note that adverse weather conditions or clouds can reduce Infrared radiation transmission significantly.
Not only have the ability to absorb infrared radiation, in LWIR region, clouds are also the source of infrared emission themselves.
Infrared Basics Terms and Laws
All IR detectors respond to irradiance, that is, to the density of the radiant power that is incident on their surface. The SI unit for radiant power is the watt. The SI unit for area is the square meter (or centimeter).The conventional symbol used for irradiance is the capital letter E. Irradiance for aviation application usually has units of watts per square centimeter.
Intensity (also known as radiant intensity) is the most widely used measure of the IR signature or the susceptibility of an aircraft to detection by threat IR sensors. In that sense, intensity is analogous to radar cross section (RCS) in the radar world. However, it is important to remember that the target aircraft in the IR world is an active emitter rather than the passive reflector of a distant RF illuminator. For this reason, intensity is actually more closely related to RF effective radiated power (ERP) of radar, which combines transmitter power with antenna beam width.As we learned earlier irradiance is area power density at the receiver. Intensity is defined as the angular power density from the source (or in other words: a measure of power per unit of solid angle). The units of radiant intensity are watts per steradian. The conventional symbol for intensity is the capital letter I.
What is solid angle ?. In geometry, the ratio of the area on the surface of a sphere to the square of the radius is the unit of solid angle or steradian in the SI system of units. Steradian is usually abbreviated as sr, and the symbol most often used for solid angle is the Greek letter omega (Ω).
Irradiance and intensity are related by the square of the distance.
Radiance is comparable to the quantity brightness in the visible wavelength, while intensity specifies radiation from the total visible area of a source, radiance specifies that from only a small area. It can be thought of as intensity per unit area.
Reflectance can be understood as the efficiency of a surface in reflected off radiation illuminate it. Reflectance is generally categorized as either specular (mirror-like) or diffuse (scattered by reflection from a rough surface). Most surfaces exhibit both types of reflection, but one typically dominates. If an object reflects energy from another radiating source with a higher temperature, the apparent temperature that is measured for the object by a thermal sensor will be higher than its true temperature. On the other hand, if an object reflects energy from another radiating source with a lower temperature, the apparent temperature that is calculated for the object will be lower than its true temperature.
It is important to remember that only perfect radiators (in technical terms, “black bodies”) actually radiate all of their internal thermal energy. With other types of objects, the amount of energy radiated also depends on factors other than the temperature of the object, such as the properties of the material and surface reflection. The efficiency an object in emitting infrared radiation compared to a perfect emitter is called emissivity. The value of emissivity ranged from 0 to 1. Given two objects with the same true temperature but different emissivity, the one with the low emissivity will radiate less energy. Thus, a higher apparent temperature will be calculated for the object with higher emissivity
In general, the emissivity of an object depending a lot on the color and materials properties of the object. Objects with light colors, made from metal often have low emissivity while objects with a dark color, made from organic often has high emissivity. Objects with higher emissivity are not only better at emitting infrared radiation, they are also better at absorbing infrared radiation.
Low emissivity paint can be applied on fighter’s engine nozzle to reduce detection range of IR sensor
Energy can neither be created nor destroyed but only transformed through interactions with matter. The most common transformation is the transformation from heat to infrared radiation. Any object with the temperature above absolute zero will radiate in the infrared. The distribution of radiant power as a function of temperature was derived
mathematically by Max Planck in 1909,
Planck’s law allows us to calculate the amount of energy given off at a certain wavelength by a blackbody at a certain temperature.
Also known as Wein’s displacement law, was discovered by Wilhelm Wien, it states that the black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature. In layman terms, a hotter object can emit IR radiation with a shorter wavelength and higher intensity.
Below are 2 graphs formed using Wien’s equation:
It is important to remember that even though, SWIR (short wave infrared) radiation is often only emitted by objects with temperature 700 ºC or above, SWIR sensors can be used to observe objects much colder such as building or vehicles. The reason is that SWIR radiation from the sun, the moon, or stars reaching object can reflect toward SWIR sensor. Unlike LWIR and MWIR sensors which observe targets by their own heat radiation, SWIR sensors operate mainly thanks to reflected IR radiation. In layman term, SWIR sensor is very similar to a visible camera with better clarity.
Main advantages of SWIR over MWIR and LWIR are high resolution, small size, and light weight sensor, SWIR sensor can also be used to detect laser illumination making it useful in detecting LRF (laser range finder). On the other hand, SWIR sensors require either very hot target or natural light reflection.
MWIR and LWIR regions are sometimes referred to as “thermal infrared” because radiation is emitted from the object itself and no external light source is needed to image the object. While LWIR sensors can be used to observe extremely cold targets (such as ICBM missiles in mid phase outside the stratosphere), MWIR sensors are more commonly used for aviation and Navy applications because MWIR sensors operate in the region of the spectrum where the thermal contrast is much higher due to black body physics. Whereas, in the LWIR band there is quite more radiation emitted from terrestrial objects and the amount of radiation varies less with temperature. In perfect condition, MWIR can see 2.5 times as far as LWIR sensor of similar aperture size, MWIR will perform much better than LWIR especially in high humidity condition.
On the other hand, LWIR can outperform MWIR in dirty battlefield conditions consist of hot targets, burning objects, smoke, obscurants. A good example of the dirty-battlefield power of LWIR is burning barrels as shown in the photo below. Hot targets shift left on Planck’s curve with huge energy in the MWIR that blooms the detector and provides veiling glare, whereas the LWIR image is still functional with burning barrels in the field of view.
For a photon infrared sensor (detail will be explained later) quantum efficiency is the percentage of photons hitting the device’s photoreactive surface that will produce charge carriers. It is measured in electrons per photon or amps per watt. Since the energy of a photon is inversely proportional to its wavelength, QE is often measured over a range of different wavelengths to characterize a device’s efficiency at each photon energy level.
Specific Detectivity ( D* )
The principal issue usually facing the system designer is whether the system will have sufficient sensitivity to detect the optical signal which is of interest. Detector manufacturers assist in making this determination by publishing the figure of merit D*. D* is defined as follows:
Noise Equivalent Power (NEP)
Noise equivalent power or NEP expresses the minimum detectable power per square root bandwidth of a given detector, in layman terms, it is a measure of the weakest optical signal that can be detected. Therefore, it is desirable to have a NEP as low as possible, since a low NEP value corresponds to a lower noise floor and therefore a more sensitive detector
Infrared sensors can be simply understood as electronic systems designed to capture infrared radiation and through that form a picture of outside world. In general, most infrared sensors consist of following basics components:
- Optics: with the role of collecting radiation. Irradiance (power density) is increased by collecting radiation over a large area and focusing down to a small area. The other role of optics is to form an image in which information can be analyzed.
- Filters: can be either spectral or spatial filter
- Spectral filter: A spectral filter restricts response to a limited band of wavelengths to help distinguish known target features from natural background.
- Spatial filter: A spatial filter distinguish/separate targets by features such as size or position
- Detectors: is an electronic device that helps converting received radiant power into an electrical signal. On the early infrared sensors, the detector is often only a single element whereas modern Infrared sensors often have an array of detectors that can determine spatial information in addition to generating a signal.
- Electronics: with the role of amplifying and conditioning the detector signal to perform required action.
IR sensors on aircraft and missiles are protected from weather conditions, aerodynamic pressure and aerodynamic heating by domes (window). They are the outter most layer of any infrared sensor.
There are several factors that need to be considered when choosing a material for IR sensor’s domes.
.Since conventional glass will block infrared radiation with a wavelength longer than 3 µm, the dome is often made from an alternative material that is transparent to infrared radiation. The infrared window of various material are as follow:
No solid materials can be 100% transparent, there is always a certain amount of infrared radiation that will get absorbed when they traveled through the material. The percentage of left over energy is measured by a variable called the transmittance. The transmittance of infrared radiation at different wavelength through various materials are as follow:
- Aerodynamic heating
When aircraft/ missiles traveling at high speed, air friction will heat up on the window. Thus, it is important to note that the absorption edge of infrared windows (domes) will shift to shorter wavelength region as the temperature of the window increase. The reason is that intensity of weak absorption will increase.
Moreover, as windows get heated by aerodynamic heating, they will also emit infrared radiation. This emission from the window can be so great that radiation from targets is obscured. Alternatively, the radiation from the domes can be so great that it over saturate the detector with photons, making them unresponsive against signals. It is generally agreed that it is emission rather than absorption that significantly limit performer of Infrared systems at high temperature.
Emittance of some sample Infrared domes at different temperatures are as follows:
Emittance comparison between domes making from different materials at 700K (426°C):
* Yttria ( Y2O3 ) has very low emittance and low absorption in the Mid-infrared wave. However, due to lower thermal shock than Sapphire, it has not replaced Sapphire in infrared applications.
As previously shown, hotter objects will emit infrared radiation with higher intensity, this raised an important question for designers, how hot do the domes of IR sensor on aircraft and missiles get in flight?. Here are some sample values taken from experiments:
*Depending on altitude, stagnation temperatures will take different time to reach.
The degradation of an infrared sensor when their window reach high-temperature mainly come from the reduction of signal to noise ratio. The higher the noise, the higher the signal need to be so that they can be detected.
It is important to remember that, the degradation due to rising domes temperature also change significantly depending on operating wavelength of the infrared sensor. As a general rule long-wave infrared sensor can tolerate much greater emittance from the dome than a mid-wave sensor.
*-3 dB is equal to 50% reduction in signal-noise ratio.
So how come long wave infrared sensor can tolerant higher domes temperature than Mid-wave infrared sensor ?. The answer lies in the spectral radiance of objects in Mid-wave and Long-wave region. The Mid-wave emittance from the domes are very insignificant at low temperature and rise dramatically as the dome is heated up ( around 2000 times between 300K and 900K ). On the other hand, the Long-wave emittance from the domes (compared to background radiation) are already significant at low temperature and only rise slightly as temperature increased ( around 30 times between 300K and 900K ).
Most aircraft cruising at speed below Mach 1 and has top speed less than Mach 2 so their infrared sensors rarely need protection from aerodynamic heating. By contrast, missiles speed can reach between Mach 4- Mach 5, thus measurements against aerodynamic heating are important for high speed Infrared guided missiles.
The majority of measures based on the fact that the temperature of the domes will decrease very significantly with distance from the stagnation point.
Some common methods are as follow:
This is a device attached to the front of missiles nose so as to create a detached shock ahead of the body. The shock cone is wider than the body of the missile, as result not only the dome of missiles get colder but drag also reduced. The main disadvantage of Aerospike is that they become inefficient at angle of attack higher than 5º as the shock will be reattached to the missiles domes. The spike will also partly block the field of regard of the sensor.
Side flat window:
As the temperature of the domes will fall rapidly off the center of the nose. One way to reduce the effect of aerodynamic heating on missiles seeker is to put the dome to one side of the missiles and a certain distance from the nose. The main disadvantage of this method is that the field of regard of the seeker will very limited compared to nose mounted seeker.
A pyramidal domes is a dome made up of a heat-resistant metal nose tip and several side panels. The face of the pyramid is much cooler than the metal nose and this design also offers a better field of regard than a side mounted flat window design, and better aerodynamic than infrared guided missiles with a blunt nose. However, the main disadvantage is of this design is the multiple internal reflections of sunlight whenever the sun is in the forward hemisphere.
- Refractive index
A very important aspect when choosing domes material for an infrared sensor is the refractive index of the material. The refractive index n of a material is a dimensionless number that describes how radiation propagates through that medium. In simple terms refractive index is the ratio of the speed of light in a vacuum to the speed of light within a given material. It is a means of quantifying the effect of light “slowing down” as it enters a high index medium from a low index medium. For example refractive index of diamond is 2.42 that mean light travel 2.42 faster in vacuum compared to in Diamond
The refractive index values are important because it indicates how much light bend when traveling from air to a medium. In general, the higher the refractive index, the more light would bend toward the normal line.
Refractive index for some common domes materials are as follow:
* Author would like to remind readers that chemical formula of Sapphire is Al2O3.
Dispersion is a measure of how much the refractive index of a material changes with respect to the wavelength of light. In other words, dispersion value indicates the variation in bending angle when lights with different wavelength traveling through the material. One phenomenon that shows the effect of dispersion very well is the separation of white light into several colors when shined through a prism.
Dispersion is a very important parameter when choosing lens and domes material. Because dispersion affects chromatic aberration.
Chromatic aberration is the phenomenon that occurs when a lens is either unable to bring all wavelengths of light to the same focal plane, or when radiations with different wavelengths are focused at different positions in the focal plane.Thus, failure to focus.
Quantitatively, dispersion is inversely given by the Abbe number (vd), which is a function of the refractive index of a material at the f (486.1µm), d (589.2µm), and c (656.3µm) wavelengths
Bigger values for Abbe number is considered better. Materials with an Abbe number greater than 55 (less dispersive) are considered crown materials and those with an Abbe number less than 50 (more dispersive) are considered flint materials. The Abbe number for visible transparent materials ranges between 20 and 80, while the Abbe number for IR transparent materials varied between 20 and 1000.
- Index Gradient
The index of refraction of a medium varies as the temperature changes. This index gradient (dn/dT) can be problematic when operating in unstable environments, especially if the system is designed to operate for one value of n (refractive index ). Thus, lower index gradient is more desirable.
- Coefficient of thermal expansion ( CTE )
The coefficient of thermal expansion (CTE) indicates how much a material expand or contract as respond to change in temperature. Material with a higher value of CTE will expand/contract more when temperature change.
CTE is an important parameter as it shows the degradation of optical performer depending on temperature. For IR and visual application, lower CTE is considered better.
- Comparison between various materials for domes and lens
In IR system, lenses are often referred to as pieces of IR transparent or reflective material that allows the system to collect and distribute IR radiation in a specifically desired way.
In IR guided missiles the domes sometimes act as the first objective lens, whereas, in FLIR (Forward Looking Infrared) system, the cover window is separate from the objective lens. Most material requirements for domes such as refractive index, Abbe number and transmissivity will also apply to lenses.
The two most common kind of lenses are the concave and convex lens
A convex lens cause radiation to converge at a focus point while a concave lens will cause radiation to diverge. The focal point of a lens is normally illustrated by letter “F”. It is the point in space where the light rays will converge to after passing through a converging lens. A diverging lens will have a negative focal point where the rays originate from before diverging through the lens. The distance from the lens to the focal point is called the focal length. Some common kind of converge and diverge lenses are as follow:
A lenses system can either be in reflective or refractive fashion.
Refractive lenses system can often be made with a lower f-number and afford greater light-gathering power than equivalent reflective lenses system but have chromatic aberration that must be corrected with multiple elements of different materials (but cannot be totally suppressed). Reflective lenses system made from curved mirrors, do not have chromatic aberration but are often physically larger for the same focal length and f-number.
With a single lens, the focus point of the system is fixed (the field of view and magnification is constant). Thus, to allow the system to magnified and minimize the scenery, several lenses are often put together with some of them are movable.
Above is an example of an optical zooming system. When the concave lens moves closer to the objective lens (outermost optical elements) and further from detector then more of radiation from the scenery will be diverged toward the tube wall and only the radiation from the center of the lens will come to the sensor. This is also known as zoom in action, where the focused object is magnified and the field of view is reduced. Zoom in help the system see object clearer as it increases signal-noise ratio by eliminating unwanted signal from the background. On the other hand, when the concave lens moves toward the detector and further from the objective lens, then more of the radiation from the scenery will come to the sensor and less will be directed toward the tube casting. This is also known as zoom out action, where the field of view is increased but the magnification of target object is reduced. For infrared sensors, this means they can either have long detection range or wide field of view but not both at the same time.
In general, longer lenses system allow higher zoom, but the length required for desirable zoom could be too long for the system to fit in vehicles. Thus, one way to get around this problem is to install more diverging lenses, the drawback is that the strength of radiation will be decreased when they pass through each lens.
Optical zoom should not be confused with digital zoom, however. Optical zoom is provided by the magnification of the focused object by the lenses, so the final image uses all of the detectors arrays to maximum effect. Digital zoom simply takes a part of the image from the detector and digitally enlarges it, so the end result is a much lower resolution image.
For IR systems, bigger apertures (lens) are generally considered better since they can capture more radiation, systems operate at longer wavelength will often require bigger optics for a similar resolution.
- Spectral filter
To enhancement of target to background contrast and avoid unwanted signal such as radiation from the sun (to improve signal-noise ratio), spectral filters are commonly used on infrared systems. Most spectral filters are thin-film interference type. Layers of dielectric material are vacuum deposited on a substrate window material. The thickness of the filter is designed to have constructive interference to pass desired radiation at desired wavelengths and to have destructive interference to block undesired wavelengths.
- Spatial filter
The main purpose of a spatial filter is to separate information in a scene image by features such as size or position, a spatial filter will also help determine target direction. The most common and important kind of spatial filters is mechanical modulators, also known as Reticle, often used on non-imaging infrared missiles. In layman terms, a reticle is a circular lens with sequentially-arranged transparent and opaque or parts on it (this should not be confused with the reticle with cross hair often seen on sniper rifles).
The most simple form of reticle has 2 parts on it, one-half transparent, the other half opaque. As the reticle spins, the target radiation falling on the opaque portion is blocked and produces no detector signal. A target image falling on the transparent portion is passed on to the detector. As a result, when the reticle spinning, IR radiation from a target off center is alternately passed and blocked, resulting in amplitude modulation (AM). The phase of this modulation relative to a spin reference is used to tell target direction from center.
Finding target direction is not the only purpose of reticles, as introduced earlier, some forms of reticle also help seeker distinguish targets from the background signal. For that purpose, these reticles divided into very small opaque and transparent slides, this is often called full spokes reticles. The design is based on the assumption that the signal from real targets such as aircraft is often a point or very small, while false signal such as cloud reflection is often distributed over a large area. When the reticle rotates, it will chop the radiation from optics before they can go to the detector. If the targets are very large, such as clouds, the energy will transmit through most transparent slides, resulting in detection but with very little change in output signal. But if the target is small, IR radiation will pass through a single transparent slide only, resulting in an output signal in style of separate square pulses. This helps the missile distinguish between massive clutter such as clouds and real targets such as aircraft.
To give the seeker ability to determine the direction and at the same time distinguish between the signal from real target versus cloud, one method is to combine the 2 patterns introduced earlier into a single reticle, this form of reticles are called rising sun reticles. A rising sun reticle has one-half semi-transparent while the other half consists of fan blades shape sectors, these sectors are divided into transparent and opaque parts.
The method of determining target direction and rejecting clutter of rising sun reticles is the combination of two methods introduced before. It is important to note that, the way missiles scan will be the deciding factor for the pattern on their spatial filters. Most early IR missiles used spin scan tracker combined with rising sun reticles. For this scanning method, the goal is to get the target to the center of the reticle. When the target is at the center, the signal is not modulated by the reticle’s spoke, as a result, generate zero voltage for the detector and the missile knows it flying toward the correct direction.
The main problem with spin scan design and rising sun reticle design is that because the seeker always looking at the target, it is more vulnerable to decoys.
One way to get around this problem is by using conical scanning (con-scan) and full spokes reticles. Conical scanning in Infrared guided missiles is very similar to conical scanning of radar.
In a conical scan tracker, the missile’s instantaneous field of view rotates around the target so that target is always at the center of the rotating “beam” pattern and the radiation from target is at the certain point on the edge of the reticles.
Using the same principles as introduced earlier, when the reticle rotates, thanks to the opaque spokes, it will chop the radiation from optics before they can go to the detector. If the target is at the center of the nutating beam, the detector output will have a fixed pulse width that equal to the chopping frequency of the reticles. On the other hands, if the target is not at the center of the nutating beam, the output of the detectors will have a varied pulse with.
Because conical scan seekers do not view the target area continuously, they are more resistance to decoys. It is important to remember that, there are many others kind of reticles pattern and scan pattern exists apart from these ones that have been introduced but due to length constraints, they will not be mentioned here.
In IR field, a detector is a device that transforms Infrared radiation into the electrical current so that they can be processed and analyzed easier. There are 2 main kinds of infrared detectors:
Thermal detectors absorbed the infrared energy produce the change in temperature, this temperature change is then indirectly detected by measuring the temperature dependent property of the detector materials. Photosensitivity is of thermal detectors independent of wavelength. Thermal detectors do not require cooling but have a very slow response time and low detection capability. Some common kind of thermal detectors are as follow:
A bolometer operates based on the fact that some materials change their electric resistance according to the change of the temperature. Hence, for high sensitivity, a high-temperature coefficient of resistance (TCR) is needed.
Thermopiles are built from a series of thermocouples of different materials attached to each other at two junctions. When the junctions are held at different temperatures, an electrical voltage is produced in the circuit. The physical principle of this is called the thermoelectric effect, or the Seebeck effect. The magnitude of the voltage generated across the thermopile junction depending on the type of materials and the temperature difference between the junction.
Pyroelectric detectors are designed based on the fact that in some crystal, the change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the electrons within the crystal move to one side. Thus, create a voltage different between two sides of the crystal (the total charge still cancel out to be zero). The voltage different due to temperature change can then be measured by a voltmeter attached to the crystal. It is important to remember that if the temperature stays constant at its new value, the pyroelectric voltage different will gradually disappear due to leakage current.
Quantum detectors (also known as Photon detectors)
Quantum detectors absorb the Infrared radiation directly, produce free electron and holes. As a result, generate a photon-induced current. Quantum detectors have much faster response time and detection capabilities, however, they normally need to be cooled to very low temperature unless they work in the Short-Infrared region. Some common kind of quantum detectors are as follow:
Photoemissive detectors are based on the photoelectric effect, in which the incident photons give electrons enough energy so that they are released from the surface of the detector material. The free electrons are then collected in an external circuit.
- Intrinsic Photovoltaic
To understand how a photovoltaic detector work. Let first, have a look at the molecule world of the atoms. Every atom consists of a nucleus with the positive charge protons and zero charge neutrons.
The electrons are found a certain distance from the nucleus in a series of levels called energy levels (despite the photo above electrons do not orbit around the nucleus like planets orbit the sun, the circle is used to represent energy level only). Each energy level can only hold a certain number of electrons (energy level are also called the shell or band).
When the outermost shell (valence shell) of atoms hold 8 electrons, they will be stable. As a result, atoms will tend to share/take electrons to have 8 electrons in their valence shell, this is also known as the octet rule. In solids, atoms are brought close together. In such a case, outer shell electrons are shared by more than one atom.
Semiconductor materials such as silicon (Si) and germanium (Ge) have 4 electrons in their valence shell, thus they are stable in a group because their 4 electrons can be shared with the neighbor atoms result in 8 electrons total in valance shell.
In a process called doping, the impurity (atoms of different material) is introduced to the semiconductor material, something interesting happens. If the new impurity atoms have 5 electrons in their valence shell, then 4 electrons will be shared with neighbor silicon atoms to form a bond while 1 electron would freely moving around the composite. The doped semiconductor material with free moving electrons here is called N-type (N stand for negative).
On the other hand, if the impurity atoms have 3 electrons in their valence shell then the bond with neighbour atoms will have a “hole”. Since atoms will tend to take electrons to fill that “hole” so that they can have a full valence band, the hole will also freely moving around the composite. The doped semiconductor material with moving “holes” here is called the P-type (P stand for positive).
When an N-type and a P-type semiconductor are joined together, we have a P-N junction. At the connection point/surface of the N-type and the P-type semiconductor, the free electrons from the N-type will jump toward the P-type area to fill up the hole. The closer holes will be filled up first. As result, the volume near the contact surface of the N-type will be robbed of their electrons, hence, have a positive charge. By contrast, the volume near contact surface of the P-type will have excess electrons, thus, have a negative charge. This creates a barrier prevent further free electrons from the N-type to move toward the P-type. The middle area where all holes are filled up is called the depletion area.
A photovoltaic detector contains a P-N junction. When infrared radiation (photon) strike an electron in the depletion zone, it gives the electron enough energy to jump out of depletion zone. If a wire connects the N-type and the P-type silicon together, the electrons flow will create a current, thus result in detection.
- Intrinsic Photoconductive
As we learned earlier, the nuclear of atoms are orbited by electrons. The highest level of energy that electrons could have while still orbit a single atom is called valence band. Once the electrons are given energy higher than that, they become free moving electrons, taken a new energy level called conduction band. Different materials have a different energy gap between their conduction and valence band.
In general, insulators such as plastic have a very wide band gap between valence and conduction band. Conductors such as metals have overlap conduction and valence band. Whereas, semiconductors such as silicon has a very small gap between conduction and valence band.
Photoconductive detectors are often fabricated from semiconductor materials such as silicon or Germanium. The incoming Infrared radiation can give electrons enough energy to move within the material. Since the conductivity of any material depending on a number of free moving electrons within it, thermal radiation can change the semiconductor material’s resistance depending on the radiation intensity. Photoconductive detectors operate as resistors in a circuit unlike photovoltaic detectors, so an external voltage. In general photoconductive detectors are less susceptible to electrostatic discharge and to physical damage due to handling compared to photovoltaic detectors but they also have less signal-noise ratio and responsivity.
Extrinsic detectors are very similar to Intrinsic detectors (both photoconductive and photovoltaic type exist), however, they use doped (impure) semiconductor material instead of undoped semiconductor materials. When doping a semiconductor, an extra allowed state is introduced in the bandgap. This extra state can be located close to the conduction band. The electrons can be excited by infrared radiation from this extra state (also called impurity level) into the conduction band, where they contribute to an increased conductivity. As for the previously described photoconductive detector, an external voltage is needed. Since the energy needed for exciting the electrons to the conduction band is small, the detector can operate at a much longer wavelength. On the other hand, the electrons can readily be thermally excited at elevated temperatures. If all the electrons are excited into the conduction band due to thermal excitation, none can be optically excited and no change occurs in the current due to illumination. Therefore, the device has to be cooled down to extremely low temperatures. This is the main drawback of extrinsic type detectors compared to intrinsic.
- Quantum Well Infrared Photon (QWIP)
The fourth main type of IR detector is the quantum-well IR photoconductor (QWIP). The
operating principle in these devices is similar to that for extrinsic detectors. QWIP detectors consist of semiconductor materials with narrow band gap sandwiched between semiconductor with a wider band gap, thus creates a potential well to trap electrons (this is called a quantum well). With an adequate layer composition and thickness in the nanometer range, quantized energy levels occur within the quantum well (due to the wave function of chargers carrier within a box-shaped potential). These energy levels are called the subbands. The energy gaps between subbands are much smaller than the energy gap between the energy gaps between the two semiconductors. However, Quantum well has very low quantum efficiency, thus to increase the efficiency the structure of multiple quantum well is often used.
A QWIP detectors generally consist of around 50 potential well, and a voltage is applied to tilt the conduction band so that electrons can be collected when they are given enough energy.
The main difference between QWIP and others kind of detectors are their various way to detect photons.
Bound-to-Bound QWIPs have intersubband absorption occurring between two bound states with a lower potential than the top of the well. In this case, the electrons often escape the well by either quantum tunneling (the barrier need to be very very thin) or thermionic emission.
Bound-to-Continuum QWIPs operate similarly to extrinsic photoconductive detectors.
Bound-to-Quasibound QWIPs the wave function of electrons in upper excited states aligned with the top of the well, thus the barrier height (conduction band gap) will limit the cut off wavelength for this kind of detectors
Bound to Miniband QWIPs the transition from the localized bound ground state in individual well to a resonant coupled miniband of superlattice barrier.
Some advantages, disadvantages of QWIPs compared to others LWIR detectors are as follow:
LWIR QWIP cannot compete with HgCdTe photodiode as the single device, especially at higher temperature operation (> 70 K) due to fundamental limitations associated with
intersubband transitions. QWIP detectors have relatively low quantum efficiencies, typically less than 10%. The spectral response band is also narrow for this detector, with
a full−width, half−maximum of about 15%. All the QWIP detectivity data with a cutoff wavelength about 9 μm is clustered between 10¹° and 10¹¹ cm Hz½/W at about 77 K operating temperature. However, the advantage of HgCdTe is less distinct in temperature range below 50 K due to the problems involved in HgCdTe material (interface instabilities ). Their main advantages of QWIP are fast response time, performance uniformity and availability of large size arrays. The large industrial infrastructure in III–V materials/device growth, processing, and packaging brought about by the utility of GaAs−based devices in the telecommunications industry gives QWIPs a potential advantage in producibility and cost
While spectral response bandwidth of thermal detectors only depending on the transmittance of domes and lens, spectral response bandwidth and sensitivity of photon detectors highly depending on the materials made up the detectors themselves. Different materials have different effective sensitivity range and value.
- Lead Sulphide (PbS)
Lead Sulphide (also known as Lead Sulfide) is an inorganic compound with the formula PbS, it was the first practical infrared detector material deployed in a variety of applications. It is commonly used in MANPADs such as Sa-7 or early infrared guided missiles such as AA-2, AIM-9B. Uncooled PbS is sensitive to infrared wavelengths between 1-2.5 μm. Cooled PbS is sensitive to wavelength up to 3 μm.
- Silicon (Si) and Germanium (Ge)
Silicon and Germanium detectors have inherent advantages of manufacturing due to
compatibility with semiconductor production techniques. The use in IR detector stems from variations possible due to doping. In case of silicon doping with gold gives it an energy band gap of 0.02 eV and cut off wavelength of 5μm. These figures get altered to 0.05 eV and ~20μm with phosphorus doping. Similarly the quantum efficiency, electric field and thickness can be varied from 20% to40%, 100 to 500V/cm and 1 to 3 respectively. Doping silicon with boron, arsenic, or gallium, for example, introduces different energy levels into the host material bandgap. Electrons can then be knocked off the dopants at energy levels well below the cutoff wavelengths for silicon or germanium, and IR detection at longer wavelengths becomes possible
- Mercury Cadmium Telluride ( HgCdTe or MCT )
MCT have been the most important semiconductor for mid and long-wavelength (3-30 μm) infrared photodetectors for decades. No known material surpasses MCT in fundamental flexibility. MCT (Hg1-xCdxTe) is a combination of mercury telluride and cadmium telluride. Relative concentrations of two molecules i.e. x and 1-x are deliberately adjusted in growth process to obtain the desired mixture. This helps to adjust cut off wavelength (maximum wavelength of response). Hence, MCT exhibits extreme flexibility. It can be tailored for optimized detection at any region of the IR spectrum. Both photoconductive and photovoltaic types are available. Limitations of MCT are compositional nonuniformity, difficulty to grow on silicon and fragility. For high-performance applications such as thermal imaging and radiometry, photoconductive, MCT provides better sensitivity, faster response, and lower bias voltage. Three-stage or even four stage cooling is used for maximum performance.
- Indium antimonide (InSb)
Indium antimonide (InSb) is a crystalline compound made from the elements indium (In) and antimony (Sb). This compound is the most commonly used III-V material, which provides high-performance detectors in the wavelength region from 2 to 5 μm. The material typically offers very sensitivity as a result of its very high quantum efficiency (80%-90%). With InSb, the detectors swamp in a few microseconds, but then the rest of the photons must be dumped. As a result, for most applications, there is little benefit to the added quantum efficiency. Another drawback is that InSb infrared FPAs have been found to drift in their nonuniformity characteristics over time and from cool down to cool down, thus requiring periodic corrections in the field. As a result, the system becomes more complex by requiring thermoelectric coolers, and additional electronics in the camera. Thus, few manufacturers use InSb FPA detectors for measurement applications. The added complexity of an InSb system is generally warranted in applications where extreme thermal sensitivity is required, for example, long-range military imaging. InSb is commonly found in FLIR sensors such as LANTIRN Pods or all aspect IR guided missiles such as ASRAAM, AIM-9L.
- Indium arsenide antimonide (InAsSb)
InAsSb offers several advantages over InSb detectors. First, the addition of arsenic to the compound material, (For example: In As 0.80 Sb 0.20), increases the bandgap slightly and consequently reduces the maximum detectable wavelength to 5 μm compared to almost 6 μm for InSb. In this case, thermoelectric cooling is sufficient, with advantages such as compactness and reduced cost. When compared to ternary compounds such as MCT, manufacturing is more predictable, whereas, for MCT, the variation of the bandgap is much more sensitive to the relative composition of the elements in the material.
- Platinum silicide (PtSi)
Platinum silicide, also known as platinum monosilicide, is the inorganic compound with the formula PtSi and forms an orthorhombic crystalline structure when synthesized. Platinum silicide detectors operate in the short-wavelength region (1-5 μm), have good sensitivity (as low as 0.05°C), and excellent stability. It is manufacturable with semiconductor production techniques, with fairly high detector yields resulting in reasonable costs. Platinum silicide has been desirable for scientific measurement cameras and FPAs because it is a highly stable material that resists drift over time in its responsivity to temperature. One drawback is very low quantum efficiency (<1%).
Early infrared sensors such as those used on MANPADS and old generation short range IR missiles only have a single detector element, while they can detect infrared radiation and estimate target temperature, they incapable of forming a visual image of the scenery. Advance in infrared countermeasure systems and rules of engagements, however, calling for better ways to discriminate targets, hence, imaging infrared systems are developed. Imaging infrared system can generally be divided into scanning array and staring array.
In scanning systems, the image of the scene is generated row by row or column by column similar to a TV screen.
The heart of a scanning system is a two-dimensional mechanical scanner coupled with a mirror. The first generation of scanning array only has a single detector element. The second generation of scanning array has a line of detector elements. The third generation of scanning array has elements that form both vertical and horizontal axis and can generate a rectangular image.
The common characteristics shared by all generations of scanning array is that images are built up gradually as the instantaneous field of view of the detectors will be moved across the field of view of the camera.
The main problem that all scanning array share is the inverse relationship between large pixels number and frame rate. Higher pixels number generate bigger pictures that is better for targets identification, however, the scanner will take a longer time to fully finished scanning the image. Thus, the frame rate is reduced. While in theory, it is possible to keep the same frame rate by increasing the scanning rate. This would mean shorter integration time for each pixel, hence, the signal from targets would decrease. With signal-to-noise ratio decrease, it will be harder to detect targets. Due to this weakness, nowadays most scanning systems are replaced by staring systems. With that being said, scanning array offers some advantages over staring array such as radiometric accuracy (since radiation from all pixels generated by a single or very small number of detector elements)
Staring array (Focal Plane Array -FDAs)
In staring systems, the image is projected simultaneously into all pixels of the detector array.
In staring array, the detectors are arranged in a matrix consist of column and rows. As the detector array of staring array system cover the whole field of view simultaneously for the entire frame time, the signal-noise ratio is increased significantly. In theory, the signal-noise ratio of focal plane array is increased by the square root of the image pixel format ( in reality, due to the non-uniform characteristics of detector elements, the gain is much lower.)
A common FDA consists of infrared radiation sensitive material and a read out integrated circuit (ROIC)
The ROIC has two functions, first they realize the simple signal readout and second, they contribute to signal processing with signal amplification and integration (In layman terms, the ROIC converts the charge to a voltage with an amplifier in each pixel, and transfers the signal to the edge of the array). Unlike FDAs for visible wavelength, FDAs for Infrared sensor need two different material classes to be electrically integrated. This can be done by either monolithic or hybrid integration technology.
For monolithic systems, first the ROIC is often made from silicon. After that, the infrared sensors are built up on top of the ROIC by thin film deposition, lithography and etching. This is a very complex process since all materials and methods used to create the IR sensor matrix must be compatible to the CMOS (Complementary metal–oxide–semiconductor) process used for the fabrication of the ROIC and must not change the ROIC properties
For hybrid systems, both parts are made separately then they will be joined together later by special mounting processes such as flip-chip bonding or loophole interconnection. In this case, we can optimize the detector material and multiplexer independently. Hybrid FPAs also have other advantages such as near 100% fill factor and increased signal processing area on the multiplexer chip. In the flip-chip bonding, the detector array is typically connected by pressure contacts via indium bumps to the silicon multiplex pads. The detector array can be illuminated from either the front side or backside (with photons passing through the transparent detector array substrate).
As mentioned earlier, all object with temperatures higher than absolute zero will emit infrared radiation. Hence, more important than the maximum infrared emission of the object is the contrast between it and the background. It is the contrast that makes detection possible, without contrast then targets would be undetectable. The diagram below shows the contrast difference between human and average earth surface temperature.
There are 3 kinds of contrasts: positive contrast, zero contrast and negative contrast. Positive contrast is the situation when targets have higher temperatures than the background. Negative contrast is the situation when targets have lower temperatures than the background. Zero contrast is the situation when targets temperature is so close to the background that it is indistinguishable.
Below is a sample photo of the same aircraft in 3 different backgrounds with different contrast stages:
Natural source of infrared radiation are the sun, earth surface and clouds (since they got heated by solar radiation). Generally, the sun as a point targets and with emission located around very short infrared wavelength does not limit detection range of imaging infrared sensor. By contrast, Earth surface and clouds with radiation concentrated around MWIR and LWIR region can reduce detection range of infrared sensor significantly, most notably in look down scenario (targets at lower altitude, background are earth surface or low level clouds).
The photo below shows negligible contrast difference between aircraft airframe and earth, cloud surface but significant contrast difference between aircraft airframe and clear sky.
Aircraft IR signature
The total infrared signature of an aircraft is a combination from various source:
Since different parts of aircraft have different temperature, they also emit IR radiation at different wavelength and amplitude
Thus, depending on the aspect of target aircraft to IR sensor, the wavelength and amplitude of IR radiation that they received can be vastly different. It can be seen from the diagram above that the majority of infrared radiation of aircraft came from their engine. Since, most aircraft nowadays using jet engines, the tail aspect of them will normally emit significantly more heat radiation than head on aspect. As a result, the detection range of IR system is much longer if a target running away.
As, the engine is the main source of infrared radiation, main efforts to reduce aircraft infrared signature are concentrated in reducing exhaust plumes radiance. One common method is using high bypass engine. It can be seen that plume radiance reduced significantly when bypass ratio increased from 0.2 to 1.4.
Another common method is to use a serrated or square nozzle instead of conventional round nozzles to turbulence, thus help mix exhaust plume with cool air faster. As a result, reduce total IR signature of the aircraft.
Others methods to reduce aircraft IR signature will be discussed here
Another source of infrared radiation is aerodynamic heating, faster-moving aircraft will generate significantly more IR signature (higher fuselage temperature).
As can be seen from the table, aircraft flying at supersonic speed will be detected from much longer distance compared to aircraft flying at subsonic speed.
Nevertheless, in most case Infrared detector detection range is inferior to radar detection range.
The passive working principles of infrared sensors give user ability to detect and track targets silently without being detected themselves. However, as infrared detectors are totally passive, they have a hard time determine variables such as distance to target, target’s velocity, target’s altitude. Since these variable are very important for successful target engagements, especially beyond visual range engagement, various methods are used to aid infrared sensors in the targeting process.
Laser Ranger Finder (LRF)
A laser range finder is a sensor that can determine the distance to an object by a concentrated beam of light. The basic principles are very simple, the device sends a short laser pulse towards the target and measuring the time taken by the pulse to reach the target, reflected off it and returned. As speed of light is constant, the distance can be measured
Laser range finders are very common and can be found on most Forward looking infrared (FLIR) and Infrared search and track (IRST) systems since it is the fastest and most accurate way to help Infrared sensor determine distance to targets and their velocity. It is however not without drawbacks. As an active system, LRF is not as stealthy as infrared sensor and can be detected by laser warning receiver or SWIR sensor.
Moreover, LRF can also be jammed by adversary laser systems and often have limited effective range, often between 20-30 km depending on atmospheric condition.
Triangulation is a method of determining distance by forming a triangle with the target from two known points. The principles are simple, two or more aircraft using infrared sensors to track the same target, each can let the others know the direction of the target relative to themselves through datalink. Thus, a triangle can be formed and distance to target is solved by trigonometric functions
Triangulation is a simple method with acceptable accuracy, when coupled with a low-probability-of-intercept directional datalink network, it allows a higher level of stealthiness compared to LRF ranging method. However, triangulation method requires several platforms working together and often take longer time to determine target’s velocity.
Kinematic ranging is a method that made popular by submarine warfare. The main idea in kinematic ranging is straightforward: assuming the target have a constant flight path, if sensor aircraft and target do not fly directly at each other or in the same direction, there will be bearing change over time. The sensor platform can perform certain maneuvers to gain various bearing information. By correlating every bearing measurement over time, with known speed of sensor platform, then the distance to target can be estimated. The accuracy of such estimation often depending on bearing change value (which varied depending on aircraft speed or measurement time)
Kinematic ranging is arguably the most stealthy ranging method of all as it does not require transmission toward target direction or communication with others allied platforms. The disadvantages are much lower accuracy and significantly longer measurement time compared to others method. Moreover, as kinematic ranging relies on bearing change measurement, the accuracy heavily depends on target keeping straight flight path and relatively constant speed (range estimation is not possible otherwise). Thus, it is often only suited for VLO assets since they are unlikely to alert targets.
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