[Back]
[Index]
[Next]
|
[Back] |
[Index] |
[Next] |
1. Be acquainted with the visible and infrared portion of the electromagnetic frequency spectrum.
2. Understand the following terms: thermal and selective radiators, blackbody, greybody, absorptance, reflectivity, transmittance, and emissivity.
3. Be able to apply the Stefan-Boltzmann Law and Wein Displacement Law in electro-optic calculations.
4. Understand the universal blackbody curve as it relates to detector bandwidth.
5. Understand electro-optic propagation through the atmosphere, ocean, and liquids/solids, and the limitations imposed by windows, spreading, absorption, and aerosol scattering.
6. Understand the major tactically significant sources of electro-optic energy.
7. Be acquainted with the basic types of electro-optic sensors and detectors, including LLLTV and FLIR.
8. Know the significance of detector temperatures and signal-to-noise ratio as related to electro-optic detection.
9. Be able to employ the principles of power density and beam spreading with those presented in this chapter to calculate detection range for a thermal imaging device.
10. Understand the following terms associated with lasers:
stimulated emission, photons, pumping, coherency, energy levels, active medium, divergence, and monochromaticity, stimulated absorption, spontaneous emission.
11. Be acquainted with military applications of lasers and considerations in their use.
12. Be acquainted with high-energy propagation effects to include: absorption, scattering, turbulence, and thermal blooming.
INTRODUCTION
The first optical sensor of importance to the Navy was the human eye. To a large degree the eye still remains the ultimate optical sensor, but now other optical and electro-optical systems augment it.
Optics have played a major Naval role in the past, and with the recent advances in electro-optics, it will continue to be an important tool in the future. Newly developing technology not only includes devices and techniques for passive sensing of a target's visible and IR radiation, but also extends to active, "radar-type" techniques through the use of lasers. Communication devices with extremely high data rates are now available at these frequencies, and directed energy weapons are closer to being a practical reality.
It is clear that the use of optics cuts across a wide variety of functions, platforms, and missions. The advantages derived from the use of optical and IR frequencies (including precise imaging, tracking, location, and ranging) often offset the disadvantages related to limited ranges and weather. We must develop these systems to complement the other technologies in use today so that one system's disadvantages will be "covered" by another system's advantages.
10.2 FUNDAMENTALS
Before any discussion of a new topic can get started, basic fundamentals and terms must be understood. This section touches on those important to electro-optics.
10.2.1 The Spectrum
The expanded scale of Figure 10-1 shows the portion of the electro-magnetic frequency spectrum important for our discussion. Because the frequencies in this portion of the spectrum are in the millions of megahertz, it is customary to refer to wavelength rather than frequency when describing them. The unit most commonly used to describe wavelengths of visible and infrared radiation is the micron (m or ) which is 10-6 meters long. Visible light extends from approximately 0.4 to 0.76 microns and is broken down into bands according to color. Infrared (IR) extends from 0.76 to 1000 microns and is divided into bands called near (NIR), middle (MIR), far (FIR), and extreme (XIR) infrared. There are, in fact, no clear-cut boundaries to the various portions of the electromagnetic spectrum. The limits used are somewhat arbitrary, and may vary from publication to publication.
Another term frequently used to describe light wavelengths is the angstrom (A) defined as 10-10 meter. We will be using the micron throughout our discussions.
10.2.2 Thermal And Selective Radiators
Optical frequencies are generated by two types of sources, thermal radiators and selective radiators. Figure 10-2 shows the spectral characteristics of each. We'll discuss each type of radiator while covering some important fundamental concepts that help describe them.
10.2.2.1 Thermal Radiators. A thermal source radiates a continuous spectrum of frequencies. Typical thermal radiators include the hot metal of a jet engine or rocket tail pipe, aerodynamically heated surfaces, motor vehicles, personnel, terrain, ships and space vehicles. Another thermal radiator is the sun - the most significant source of optical wavelengths.
Kirchoff stated that an object that is a good energy absorber at a particular wavelength is also a good radiator at that same wavelength. The term "blackbody" was coined to describe the "perfect" thermal radiator - an object that absorbs all incident radiation, and reradiates that energy with complete efficiency.
The theoretical model of a blackbody is the standard with which any other source is compared. Experimental study of the spectral distribution of the intensity of blackbody radiation has resulted in two findings important to the understanding of electro-optic devices. They are the Stefan-Boltzman Law and Wien's Displacement Law.
10.2.2.1.1 Stefan-Boltzmann Law. The Stefan-Boltzmann Law describes the fact that the total emissive power of an ideal blackbody, integrated over all wavelengths, is proportional to the fourth power of the absolute temperature of that body. This relationship is expressed as:
E = T4 (10-1)
where
E is the power radiated per unit area of the body (watts/cm2)
is a proportionality constant (5.67 X 10-12 watts/cm2K4)
T is the absolute temperature of the blackbody ( K = C + 273)
Realistically, we have to consider the blackbody radiating in an environment at a temperature other than absolute zero. The law is then be expressed as
E = (T4 - T4) (10-2)
e
where
Te is the absolute temperature of the environment ( K)
This means that an object in thermal equilibrium with its sur-roundings has a net radiated power of zero!
10.2.2.1.2 Wien's Displacement Law. Planck's energy distribution law successfully modeled the blackbody spectra with what was ex-perimentally measured. Figure 10-3 is the result. One of the important conclusions from this curve has been formulated in Wien's Displacement Law which states that the wavelength at which the max- imum value of intensity occurs varies inversely with the absolute temperature. This is mathmatically expressed as:
m = a (10-3) T
where
m is the wavelength of maximum radiation ()
a is an empirical constant (2898 -K)
T is the absolute temperature of the body (K = oC + 273)
Figure 10-3 illustrates several blackbody curves within the temperature range from 500 K to 900 K. Several characteristics of thermal radiation are evident from these curves. One is that the total output power, which is proportional to the area under the curves, increases rapidly with temperature. Also note that the wavelength of maximum radiation shifts toward shorter wavelengths as the temperature increases.
If the same spectral distribution of power is plotted against the product of wavelength and absolute temperature, the result is Figure 10-4, a universal curve (labeled A) that applies at any temperature. The integral of curve A is plotted as curve B. This provides the fraction of energy radiated below a given value of T. Following are examples of how this curve can be used:
1. A value of T = 4000 K indicates 48 percent of the radiated power occurs below this point. This is true for any combination of wavelength and temperature where the product is 4000 uK.
2. For a detector bandwidth from 8 to 14, what fraction of the radiated energy from an object at 500 K can be processed by the receiver due to bandwidth limitations? This is called the bandwidth factor (f) and will be useful when infrared detectors are discussed.
a. First obtain T for the upper and lower limits of the bandwidth.
T1 = 8 X 500 K = 4000 K (10-4)
T2 = 14 X 500 K = 7000 K (10-5)
b. Entering Figure 10-4, we find that 48 percent of the energy is radiated at 8 and below, and that 80 percent of the energy is radiated at 14 and below.
c. Take the difference between the two figures to determine that amount of energy radiated between 8 and 14 microns.
Bandwidth factor = f = .80 - .48 = .32, or 32 percent of the radiated energy can be processed by the receiver!
As a final point, note that the maximum of curve A occurs when T = 2898 uK, or the constant, a, of the Wien Displacement Law!
10.2.2.1.3 Greybodies. As mentioned before, the blackbody is only a theoretical model. To approximate "real life" bodies, some other factors must be considered. One of these is emissivity. Emissiv- ity is defined as the ratio of the total radiant energy emitted by an object at temperature T, to the total radiant energy emitted by an ideal blackbody at the same temperature, and under the same con- ditions. An object with an emissivity less than one is called a "greybody". It naturally follows that a blackbody has an emissiv-ity equal to one.
With this in mind, Equation 10-2 must now be refined to:
E = (T4 - T4) (10-6)
e
where is the emissivity of the grey body (0 < < 1) and E is now termed the radiant emittance.
There are two other terms that need to be discussed when talk-ing about the optical properties of an object. These are absorp-tance () and reflectivity (). Absorptance can be thought of as that fraction of incident radiation that is absorbed. Assuming an opaque surface with zero energy transmission, then what energy is not absorbed must be reflected. The reflectivity represents that fraction of incident energy that is reflected. Since all bodies can only emit the energy they have absorbed, Kirchhoff's Laws apply and = . Given these conditions, it is clear that + = 1, or + = 1.
Table 10-1 gives some characteristic values for , , and .
Radiation from a metal or other opaque material originates within a few microns of the surface, therefore, the emissivity of a coated or painted surface is characteristic of the coating rather than the underlying material. The visual appearance of a material is not always a reliable guide to its emissivity at infrared wavelengths.
For example, snow appears to be an excellent diffuse reflector to the eye, and we might conclude that it has low emissivity. Since the great bulk of the thermal radiation from a body at the temper-ature of snow occurs above 3u, our visual estimate of its appear- ance based on sensing radiation centered at 0.5u is meaningless. Snow, it turns out, has a high emissivity of about 0.85! Another example is the emissivity of human skin at 32oC, which is close to unity, and is independent of its color.
10.2.2.2 Selective Radiators. A selective radiator is a source where emissivity is a function of the wavelength. It has an out- put concentrated in narrow frequency intervals called line spec- tra. These line emission spectra are produced from gases at high temperature and low pressure where each element emits its own characteristic pattern of lines, uninfluenced by adjacent atoms or molecules. Some examples of selective radiators: the hot gases from jet engine exhaust, gas-discharge sources used in flashing light communications, and the best known example - the laser.
Figure 10-5 shows a schematic representation of a hydrogen atom with the nucleus and its associated electron shells or orbits.
If a hydrogen electron is excited from its normal or ground state to a higher energy level, then that energy must have been provided to the atom at a frequency readily absorbed by hydrogen. The electron will return to its ground state in a very short time (on the order of 10-8 second). This is known as the "lifetime" of that higher energy state. In the transition to the lower energy state, the atom sheds its excess energy in the form of light or other electromagnetic energy.
Quantitatively, the amount of energy absorbed or emitted is given by Planck's Law:
E = hv = E1 - E0 (10-7)
where
E is the amount of energy per photon (Joules)
E1 is the energy at the first of the higher energy states
E0 is the energy at the ground state
h is Planck's constant (6.63 X 10-34J-s)
v is the frequency of the emitted radiation in Hz
Since there are distinct energy levels, ie. no "intermediate" levels, there can only be specific frequencies absorbed or emitted. Because of this we get a distinct line spectra for each element, and can actually identify an element through its own line spectra "fingerprint!"
EXAMPLE: If a hydrogen atom returns to the ground state from its highest state of excitation, it loses 13.6 electron-volts (or eV). One eV = 1.6 X 10-19Joule. The wavelength of light emitted is then = c = hc = hc
v hv E (10-8)
substituting
= (6.63 x 10-34J-s)(3 x 108m/s) = .0912u (10-9)
(13.6 ev)(1.6 x 10-19J/ev)
This ultraviolet wavelength is the limit of the Lyman series and corresponds to the highest energy photon emitted by a hydrogen atom. The remaining series of Figure 10-5 are transitions to other than the ground state.
The spectra of heavier atoms get increasingly complicated with atomic weight. Some elements, such as hydrogen and neon, have relatively few widely spaced lines. Iron (gaseous, not solid) has many thousand spectrum lines. Band-emission spectra are molecular spectra seen in fluorescence and phosphorescence. Each band consists of many closely spaced lines that are irregularly distributed with respect to wavelength and intensity. This characteristic is caused by transitions to lower energy states that take much longer than the "normal" 10-8 second. In some cases, long-lived states may exist for hours or days following exposure to intense light. This gives rise to materials that fluoresce or "glow in the dark" while still at low temperatures. CRT screens, aircraft instrument lighting, and the visual detection of submarines by biological luminescence are three applications of fluorescence in naval weaponry.
10.2.3 Optical Transmission Characteristics
10.2.3.1 Radiation Spreading Or Divergence. The general concept of spherical spreading, waves moving outward in spherical shells, holds true for optics as well as for sound and radar. Diffraction is another name for this spreading - a direct consequence of the wave nature of light energy. As with sound and radar, the power density (power per unit area) on the surface of the expanding sphere decreases as the square of the inverse of the range. As we will discuss in the laser section, mirrors and lenses can be used to focus and reflect light energy to form essentially planar wave fronts. Even though the highly collimated laser light also diver- ges, the narrower beam results in more concentrated power -much like increasing the gain of a radar system. In this sense, lenses and mirrors can be thought of as the antenna of optical systems.
10.2.3.2 Atmospheric Propagation. The atmosphere is a significant limiting factor to the tactical employment of optical frequencies. Figure 10-6 shows the transmission structure of the lower atmos- phere. The propagation losses pictured are caused by two mechan- isms: absorption and scattering.
Absorption is a direct loss process attributed to the constit-uents of air: water vapor, carbon dioxide, oxygen, etc. These con- constituents absorb energy at selective frequencies, or bands of frequencies, converting it to thermal motion and increasing the temperature.
The other loss mechanism, aerosol scattering or simply scat-tering, is caused by the particulate matter in the air: salt part- icles, water droplets, dust, etc. Scattering causes a redistrib-ution of light rather than a true loss. It is certainly a loss for light traveling in a particular direction, e.g., from source to detector, but the energy lost from the line of sight is sent off in other directions rather than lost as heat. As far as attenuation goes, scattering loss is in principle indistinguishable from ab- sorption. One reason for considering it as a separate mechanism is that it is not as selective as absorption and leads to a loss of contrast between target and background. In the visible spectrum, a dark target becomes progressively less black in apperarance as it moves away from the observer, until finally it vanishes against the horizon - regardless of its size. Roughly speaking, the visible range for a large dark object against the horizon is the distance at which the brightness difference between the object and sky is 2%.
On examining Figure 10-6 along with target emission charac- teristics, we find several good transmission windows in the tar- get emission bands of interest. In particular, a good transmis- sion window exists from about 2.9 , to slightly above the 4.4 region of HF chemical laser devices. Two other good windows are located between about 4.4 and 5.2, and from slightly below 8 to about 14 . The fact that these windows are in the infrared portion of the spectrum explains why IR is so useful for target detection and tracking!
10.2.3.3 Ocean Propagation. The ocean is the other major medium in which the Navy operates. It is a very highly attenuating medium as shown in Figure 10-7. The only significant window is in the visible spectrum and is frequently referred to as the "blue-green" window. Underwater ranges are severely limited and may be expected to be measured in tens of meters, with a maximum of perhaps 1000 meters under optimum conditions. Therefore, optics have only very
limited potential applications underwater. One possible use is with air-to-subsurface and subsurface-to-air optical communication links. Morse code light signals have been transmitted about 100 meters underwater to a submerged detector from an aircraft and vice versa. A second possible use is the employment of a blue-green laser for active submarine detection. Carried aboard an aircraft, the laser would be pointed downward to sweep back and forth as the aircraft flew a search pattern. Under certain conditions, the laser beam could reflect off the hull of a shallow submarine.
Ranges are small when compared with acoustic methods, and therefore tactical applications are limited. This method could be used in the same manner as Magnetic Anomaly Detection (MAD) equipment is presently employed.
10.2.3.4 Transmission Through Other Liquids and Solids. Many liquids and solids are employed in optical systems, both as filters and lenses. Each nongaseous material has its own unique band absorption spectrum, similar in concept to that depicted above for the ocean. To act as a filter of unwanted noise, a material is selected that exhibits a strong transmission window at the desired wavelengths and minimal transmission elsewhere. Thus, only the desired wavelengths pass through into the detector. When used as lenses, certain crystalline solids, such as sapphire, lithium fluoride, and quartz, function in the usual optical manner to collect and focus the energy on a detector as well as to select a band width of interest.
Although the path lengths that the incoming energy must transit when passing through these various liquids and solids are insignificant when compared to the ranges between the target and detector, the absorption effects are not. This is due to the greater densities of the lens and filter materials compared to the transmitting environment. The intensity of the incoming energy will decrease roughly as an exponential function of the depth of penetration and is highly dependent upon the type of material employed. Because absorption effects are generally the limiting factor in electro-optic transmission, losses through filters and lenses cannot be ignored.
10.3 VISIBLE LIGHT DETECTORS
10.3.1 Visible Light Sources and Background Noise
By far, the sun is the most significant source of illumination energy. As seen from Figure 10-8, it is similar to a blackbody at 5900 K. But the radiation received on the surface of the earth looks more like selective radiation rather than thermal radiation.
This is due to the fact that the radiation must transit the atmos- phere - a selective transmission medium with numerous absorption bands where various wavelengths are attenuated.
The sun's spectrum has a maximum radiated energy at 0.5, which, unsurprisingly corresponds to the color yellow. This wave-length is also where the human eye has peak light response, and where the atmosphere is an excellent transmission medium.
All passive target detection systems must find their targets against some background with its own pattern of intensity and frequency. The background sources can be considered as noise that competes with the target source, causing false alarms. In the visible spectrum, significant background "noise" is generated by the sun, moon (reflected sunlight), stars, and the wide variety of man made light sources. To successfully detect a target against such a background, there must be a contrast between the target and background. Although not uniformly agreed upon, the concept most frequently used to describe contrast is the ratio of L/L, where L is the difference in luminance or brightness between target and background, and L is the brightness of the background. Low-light- level TV (LLLTV) and starlight scopes are employed to improve the detection capability of the eye (i.e., they reduce the value of L/L required for detection) in the visible region.
10.3.2 Optical Sensors
10.3.2.1 The Eye. The human eye is a key optical detector in the 0.4 to 0.76 region. At low light levels, the fovea, or center of vision, is relatively blind, and a faint source can only be seen by looking slightly away from it -- a fact well known to experienced night time lookouts. Dark adaptation which requires up to 30 min-utes, is not affected by deep red lights, and is rapidly destroy- ed by white and blue lights. Observers can see points separated by 1 to 3 minutes of arc, which is frequently cited as the resolution capability or acuity of the human eye. Due to diffraction limiting one can see a telephone wire that subtends a size far below the "resolution" for two points. In fact, a line that subtends 0.5 seconds of arc can be readily seen! The eye is very good at seeing straight lines, and this fact is frequently used to integrate pat- terns out of displays. The use of binoculars and telescopes to aid vision is a standard technique, but use of these devices, especial- ly for distant objects, will in fact decrease the observed bright- ness. They will, however, give magnification and permit smaller detail to be seen than with the unaided eye. If the target is a point light source such as a star, then use of a telescope will markedly increase the detectability of the point source over that of the unaided eye.
10.3.2.2 Photodetectors. Binoculars and telescopes magnify, but do not amplify existing light. To amplify a dim light source, we need a device sensitive to the weak intensity present. This device is called a photodetector. Two general types are in use.
10.3.2.2.1 Photoemissive Devices. These devices use the photoelectric effect to produce electrons. Briefly, photons strike a light sensitive, electron emitting material causing photelectron emission into a vacuum or gas. This ultimately changes a measured electric potential that indicates relative intensity. Some of the detectors that use this principle are called vacuum photodiodes, gas-filled phototubes, and photomultipliers.
10.3.2.2.2 Solid State Devices. These devices are classified as either photoconductive or photovoltaic. In a photoconductive device (shown in Figure 10-9), incident light energy is absorbed by a solid semiconductor. This changes the resistance across the semiconductor and consequently the voltage read across resistor R. The change in the output voltage is proportional to the incident optical intensity.
Photovoltaic devices indicate light intensity by measuring the voltage across a semiconductor p-n junction. Incident photon energy causes a change in this voltage that is proportional to the inci-dent energy. Examples of solid state photodetector devices include p-n junction photocells, p-n-p phototransistors, and avalanche pho- todiodes.
10.4 INFRARED DETECTORS
10.4.1 IR Sources and Background Noise
Any solid object at a temperature above absolute zero is a source of thermal radiation. The sun is a strong source of IR energy in the near-infrared region (0.7 to 1.3), and in most of this region the atmosphere is a good transmission medium. Gas turbine engines such as the turbojet and turbo fan are tactically significant targets that radiate considerable energy due to the combustion process. There are two main sources of radiation from these engines: the hot metal tail pipe is a thermal radiator, and the stream of high temperature exhaust gases is a spectral radiator. See Figure 10-10.
For engineering calculations, a non-afterburning turbojet engine can be considered to be a greybody with an emissivity of 0.9, a temperature equal to the exhaust gas temperature, and an area equal to that of the nozzle. If, however, the afterburner is used the plume becomes the dominant source. The plume radiance in any given circumstance depends on the number and temperature of the gas molecules in the exhaust stream. These values, in turn, depend on fuel consumption, which is a function of aircraft flight altitude and throttle setting.
Aerodynamic heating is another source of IR radiation. As an object moves at high speed through the atmosphere, it becomes heat-ed, and at speeds above Mach 2, the resulting high temperatures produce sufficient radiation to be of interest. Space vehicles re-entering the earth's atmosphere convert an enormous amount of kin-etic energy into heat. Surface temperatures of 2000o C and more can be expected.
IR detectors must be able to pick out targets against an array of background thermal noise. Reflected and direct infrared energy from the sun is a major contributor, but man made heat sources very often clutter (sometimes on purpose!) the picture as well. In addition to the environmentally generated energy, the IR detector itself generates a significant amount of heat which must be dissipated for the detector to work.
Although the amount of thermal power radiated by a source is important, the detector will only see a target radiating energy at a temperature different from the background. Of equal importance, the energy received by the detector must be at a level higher than the thermal noise generated by the detector circuitry. This is why detector cooling systems are so important. As the wavelength of maximum radiation of the source and the wavelength that character- izes the temperature of the detector approach equality, the sen- sitivity of the detector is degraded by self-noise. According to Hudson, wavelengths greater than 8u require progressively lower detector temperatures that approach absolute zero (0 K). Between 3 and 8 some cooling is required (to 77oC) and below 3 (i.e., at very high source temperatures above 700oC) no cooling is re- quired. In other words, to detect a thermal target, there must be a thermal contrast between the target and the background, as well as between the target and the detector.
With this in mind, we can define the term noise equivalent power (NEP). A detector's NEP is the input radiant flux in watts necessary to give an output signal equal to the detector noise, or the minimum signal strength that would give you an indication of a target. This term is similar to the minimum discernible signal (MDS) and Smin terms used in radar and acoustic theory. All three have been used interchangeably. Obviously, the detector with the lower NEP has the higher useful sensitivity.
10.4.2 IR SENSORS
Infrared sensors can "see" their targets in complete darkness. Low light level systems require some type of illumination since that is what they amplify. Infrared is invisible to the eye, however, so no visible illumination is required.
There are, in general two classes of IR detectors -- those de- pending on the heating effect of IR radiation, and those depending on the quantum nature of photons. The first group of sensors are called thermal detectors and are normally categorized as either radiometers, thermocouples, thermometers, or heat cells.
The second group of sensors are called semiconductor detec-tors, and they are by far the most important type of photon de-tector in use today. There are basically two types of semicon-ductor detectors, intrinsic and extrinsic. Intrinsic photode-tectors involve "pure" semiconductor materials, while extrinsic photodetectors involve a given semiconductor material doped with specific "impurity" materials. Extrinsic photodetectors have been made that are sensitive to wavelengths longer than 200, while in-trinsic semiconductor photodetectors are limited to wavelengths below about 30.
10.4.3 IR Range Prediction
We now have enough information to work out a relationship that will give us a good estimate for the detection range for a specific de- tector looking for a specific target. The same information applied below can be used for detector design as well.
We'll start off with Equation 10-6 which gives us the total radiated energy intensity (watts per share centimeter) for a grey-body at temperature T ( K) in an environment at temperature Te ( K).
E = (T4 - Te4) (10-6)
To get the total energy radiated by the target, multiply Equa-tion 10-6 by the surface area, A, of the target. Note that the units of E change to watts for total radiated energy.
E = (T4 - Te4) A (10-1)
The IR detector will only be able to use the energy that falls within its designed bandwidth. To account for this, we must mul-tiply by the bandwidth factor (f). As discussed earlier in the chapter, the bandwidth factor represents that percentage of the total energy emitted by the target between the lower and upper lim-its of the receiver bandwidth.
E = (T4 - Te4) Af (10-11)
Since the atmosphere absorbs some of the radiated energy, the transmittance (t) factor must be applied to the equation. To find the transmittance, first find the wavelength of maximum radiation for the target using the Wien Displacement Law. Since we are es-timating a maximum range, it makes sense to find the wavelength where most of the energy is being transmitted. If the detector has been properly matched to the target, its bandwidth will include this wavelength! Once the wavelength of maximum radiation is found, enter the atmospheric transmission curve (Figure 10-6) to find the transmittance.
E = (T4 - Te4) A f t (10-12)
Now take into consideration that the energy heading towards the detector will undergo spherical spreading. The symbol "E" once again becomes an energy density (power per unit area), and Equation 10-13 gives the energy density that arrives at the detector from a target at range R.
E = (T4 - Te4) A f t (10-13)
4R2
The IR detector will only pick up that portion of energy that hits the surface of the detector aperture. If we multiply the energy density that arrives at the detector by the surface area of the detector aperture (Ae), E will epresent the energy received by the detector.
E = (T4 - Te4) A f t Ae (10-14)
4R2
If we set E equal to the noise equivalent power (NEP), or the minimum energy required to get more signal than noise (target de-tection), then solve for R, we get a relationship for the estimated maximum detection range for a specific detector against a specific target.
Rmax = (T4 - Te4) A f t Ae 1/2 (10-15)
4 NEP
10.4.4 Example Detection Problem
A NASA substation has been directed to track a space vehicle during reentry. Given the following data, at what range will the target be detected?
Surface area of vehicle: 125 square meters
Detector bandwidth: 3 - 5
Heat shield emissivity: 0.7
Vehicle skin temperature: 575oC
Environment temperature at re-entry altitude: -180oC
IR detector aperture: 1.5 square meters
IR detector NEP: 61.1 X 10-7 watt
One way to solve this, is to take each variable in turn from
Equation 10-15, and find its value.
= 0.7 (given)
= 5.67 X 10-12 watts/cm2 K4 (a constant)
T = 575oC + 273o = 848 K
Te = -180oC + 273o = 93 K
A = 125m2 (given)
f: From Figure (10-4)
T1 = (3u)(848 K) = 2544 -K -> f1 = 0.1
or, 10% of the transmitted energy will be at 3 and below.
T = (5u)(93 K) = 4240 -K -> f2 = 0.5
or, 50% of the transmitted energy will be at 5 and below.
therefore,
f = f2 - f1 = 0.4
or, 40% of the transmitted energy can be received by the IR detector.
t: From Wien's Displacement Law
m = a = 2898 K = 3.4 (wavelength of
T 848 K maximum radiation)
Entering Figure 10-6, the transmittance at this wavelength is approximately 0.8.
Ae = 1.5 m2 (given)
NEP = 61.1 X 10-7 W
Now substituting these values and applying appropriate unit conversions, we solve for the predicted range:
R = (.7)(5.67x10-12)(8484-934)(125)(.4)(.8)(1.5)(104cm2/m2) 1/2
4 (61.1x10-7)
Rmax = 126.63 km
10.5 TARGET IMAGING
The question "What targets can I see with IR viewers or Low-Light- Level (LLL) systems?" is like the question "How far can I see?"; the answer to each is "It all depends . . ." Still, it is possible to indicate a few general order-of-magnitude statements. Various levels of target discrimination have been defined by J. Johnson of the Army Warfare Division, Night Vision Laboratory, to be:
1. Detection (an object is present)
2. Orientation (longitudinal axis can be sensed)
3. Recognition (class discerned)
4. Identification (types in class can be determined)
The better the sensor/imaging systems, the better the target dis-crimination. That brings up another pointless, but often argued question: "Which is "better" -- low-light-level systems or Forward Looking Infrared (FLIR) systems?" The answer is neither. Under varying conditions and targets, either system will have an advant-age over the other. We'll now discuss some of the imaging systems where both visible spectrum and IR detector information is display-ed so that humans can "see" detected targets.
Target imaging systems depende primarily on a camera tube that converts optical images to electronic signals representing the var-ying light intensities detected from the image. Such a wide varie-ty of tubes are in use today that it is not practical to discuss all of them here. Two common systems are discussed below to give a general understanding of how these TV tubes work.
10.5.1 The Vidicon
By far the most widely used camera tube is the vidicon. It is small, simple to operate, relatively slow in response time compar- ed to other systems, and moderately sensitive to low light situa- tions. Figure 10-11 is a schematic representation of the vidicon.
The vidicon's photoconductive target, or light sensor, con-sists of a transparent light conductor on the front with a photo-conductive layer on the "back" side. The transparent side is operated at a positive voltage with respect to the photoconductive layer which operates at the cathode (near zero) potential. The entire face of the photoconductive target can now be modeled as a series of small capacitors. The scanning electron beam initially charges the back side to cathode potential. When a light pattern strikes the sensor, the conductivity increases in the areas of illumination effectively discharging the local area "capacitor," and making the area more positive. As the electron beam is scanned across the back of the photoconductor, it deposits electrons on the positively charged areas. This provides an output signal which in-dicates the relative strength of the light signal.
Vidicons have been used in both low light level TV's and with infrared systems.
10.5.2 The Image Orthicon
Where the vidicon is photoconductive, most other imaging systems are photoemissive. One example of these is the image orthicon (Figure 10-12). The image orthicon is more complicated than the vidicon, but also more sensitive and able to handle a wider range of light levels and contrasts. It's primary disadvantage is ex- cessive noise in the dark areas of the image, due mostly to the electron return beam type of operation.
When a light pattern strikes the photocathode light sensor, a photoelectron image is developed that is focused onto an insulated target surface. When the photoelectrons strike the target surface, secondary emissions occur, leaving positive charges in the local area of incidence. As the electron beam scans the pattern on the target surface, it deposits electrons on the positively charged areas. This modulates the electron beam which is returned to an electron multiplier for amplification. The resultant signal in- dicates relative intensity.
Image orthicons are also used in both low light level TV and and infrared systems.
10.5.3 Applications
There are numerous types of camera tubes in operation in many diff-erent systems. Most are variations or improvements on the two ba-sic systems we have briefly discussed here. Two of the most not-able military applications are Low Light Level Television (LLLTV) and Forward Looking Infrared (FLIR) systems. Although most LLLTV systems use photoemissive tubes, the standard vidicon may also be used after several stages of intensification. This is usually accomplished by using a photocathode on which the scene is focus- ed, then focusing the emitting electrons on a phosphor that emits light in the pattern of the scene which in turn is focused on a-nother photocathode, etc. It should be noted that image inten-sification need not be applied to TV-type systems and that many LLL direct-view systems are in use by the military. In these systems the eye views a phosphor, which is excited by electrons from sev-eral stages of amplification. Light gains of 1000 to 10,000 are readily available. The systems provide the ability of viewing, with the light adapted eye, a faintly illuminated scene that ordinarily would require the eye to be dark adapted. Usually, a considerable degree of detail degradation is experienced in these systems.
It should be noted that no LLLTV systems can "see" in complete darkness -- some visible spectrum illumination is required from stars, moon, sky, etc. If there is no such illumination, then we must use a different part of the spectrum -- infrared -- to detect any targets. That is why neither system is "best". Condi- tions dictate the most appropriate sensor.
FLIR imaging sensors use an array of IR photosensitive de-tectors that are scanned across a scene to provide a TV-type ther- mal image of the scene. They operate in the 8 - 14 spectral region, since as noted earlier, there is a very good atmospheric transmission window in this band. The primary tactical signific-ance of IR imaging systems is that no visible illumination of the scene is required. Unfortunately, FLIR's are not "all weather". Clouds, rain, humidity, or other moisture absorbs and scatters infrared energy, thus reducing FLIR range compared to that of radar. But in contrast to the human eye or other visual sensors, FLIR can more readily penetrate light fog and haze. Under such conditions, infrared range can be three to six times that of visual range.
Performance factors of most interest for a FLIR sensor are its thermal resolution and its angular resolution. The thermal resolution is the temperature difference between two adjacent parts of a scene (which are temperature resolvable) at which two points can just be distinguished. This is similar to range resolution in radar theory. The relationship between thermal resolution and angular resolution is an inverse relationship, so that the smaller or better the angular resolution, the larger or poorer the thermal resolution. The angular resolution is simply the field of view on an individual detector, and the smaller the detector, the better the angular resolution. Thus, there is a trade off between thermal and angular resolutions that is influenced by detector size. With the FLIR systems, angular resolutions of less than a milliradian are readily achievable, while thermal resolutions of hundredths of a degree centigrade can be achieved. Unfortunately, these high angular and thermal resolutions cannot be achieved with one system.
Since FLIR systems respond to thermal contrasts, they work better at night when warm bodies stand out more clearly against the cooler ambient temperature background. It is also interesting to note that they can detect a submarine periscope in total darkness from the temperature gradient in the periscope's wake!
10.6 LASERS
There are two basic approaches that can be taken to develop light sources with the power of radio frequency devices; either extend electronic oscillator principles to the shorter optical wavelengths, or find some way to get atomic or molecular oscil-lators to work together.
The first approach requires building resonant structures with dimensions on the order of the wavelength being generated. Electronic resonators have been built to generate wavelengths down to one millimeter, while optical wavelengths are about 1000 times shorter. The second approach is the principle on which first the Maser (for Microwave Amplification by Stimulated Emission of Radiation, by C. H. Townes in 1951) and then the Laser (for Light Amplification by Stimulated Emission of Radiation, by T. H. Maiman in 1960) were conceived and built.
Following is a brief discussion of the basics of laser theory. It is designed to give you a general understanding of how and why lasers work, and also an appreciation for their utility.
10.6.1 Laser Fundamentals
Three basic conditions must be satisfied before a laser will operate.
1. An active medium must be present.
2. A condition known as population inversion must
exist.
3. There must be some form of optical feedback.
These conditions are the topic of this section.
10.6.1.1 The Active Medium. As we discussed earlier with selec-tive radiators, each element has its own distinctive set of line spectra. These spectra result from the photons emitted as an atom moves from a higher energy state to a lower one, and also the fact that each element can exist only in discrete energy states. Equa- tion 10-7 gives the energy a photon will have from such a transi-tion.
An active medium, with respect to lasers, is a collection of selective radiators (atoms, molecules, or ions) that emit radiation in the optical part of the electromagnetic spectrum. If you desire laser radiation at a specific frequency or wavelength, you "simply" use a medium with predicted transitions that give the desired frequency. Of course, the process is much more complicated than that, but essentially, that's what is done.
Now a few terms to describe how the atoms in the medium get to these upper energy, or excited states, and back down to their rest, or ground state. First is the term transition lifetime. This is basically a measure of how long it takes a certain transition from an upper to a lower energy level to occur. Typical transitions happen in periods on the order of a microsecond, while others take a significantly longer time (a thousand to a million times long-er!). The longer transition times are associated with an excited state called a metastable state. The transitions from metastable states are of great importance to lasers as will be seen later.
Quantum physics tells us that an atom will only emit or ab- sorb energy at discrete frequencies. The process where an atom absorbs incident photon energy and is excited to a higher energy level is called stimulated absorption (see Figure 10-13). Since the higher energy level is generally not a stable state, the transition lifetime to the ground state will be extremely short, resulting in the spontaneous emission of a photon at the frequency appropriate to the difference in the energy levels. If while the atom is in an excited state it is struck by a photon at the transi-tion frequency, then the atom will emit a photon at the frequency of, and in phase with, the incident photon. This process (shown in Figure 10-13) is called stimulated emission. The "trick" is to design an active medium in which most of the atoms can be placed in an excited, or more accurately, a metastable state so that a wave of photons of the right frequency passing through stimulates a cascade of photons, and amplifies the original wave. Obviously, the stimulated emission transition is the most important for laser operation.
As an example, the first laser to be constructed by Maiman consisted of a single crystal of synthetic pink ruby, Al2O3, doped with the addition of and impurity, Cr2O3. The inert aluminum and oxygen atoms suspend the chromium atoms that actually form the active medium.
10.6.1.2 Laser Pumping and the Population Inversion. In the last section we mentioned that we wanted a majority of the atoms in the active medium to be in a metastable state. In this condition, the number of atoms in this excited state exceeds the number of atoms in the ground state, and a population inversion results. But how do we achieve a population inversion? The answer is a process called laser pumping.
To explain the pumping process, we'll refer to Figure 10-14 which illustrates a three energy level system. Figure 10-14a shows the populations in each of the three levels when the active medium is in thermal equilibrium. The distribution follows the Boltzmann Principle which specifies what fraction of atoms are found, on the average, at any particular energy state for a given equilibrium temperature. Some of the atoms are in the excited states because of the thermal energy present. The transition from energy level E2 to E1 is assumed to be very fast, while E1 is a metastable state with a relatively long transition time to the ground state, Eo. A laser pumping source is an energy source with an emission spectrum high in the frequencies that the active medium readily absorbs.
Specifically, we want the source to emit the energy required for the Eo to E2 transition. Once these atoms reach the E2 level, they quickly decay to E1. Since the transition from there to the ground state is slow, the number of atoms in the metastable state starts to increase, while the population in the ground state decreases. As the energy being pumped into the active medium reaches a certain threshold level, the population in E1 exceeds that in E0. We have now met the second condition for laser operation, the population inversion.
There are a number of mechanisms used for laser pumping. These include electrical discharge, fuel combustion, various high inten-sity lamps, as well as pumping by other lasers. As long as the emission spectra of the pumping source matches the absorption spectra of the active medium, any method can be sufficient.
10.6.1.3 Optical Feedback. To this point, we have an active medium capable of emitting the desired laser frequency, and a pumping source so that the population inversion condition can be reached. From this we will get photons from stimulated emission transitions, but they will be travelling in any number of directions, and the measured power out is insignificant. The next step is to focus the laser output and amplify it so it will be useful. That is what optical feedback is all about.
Optical feedback is achieved by placing the active medium inside a resonating cavity. The resonator has mirrors on each end placed an integer number of half-wavelengths apart. This reflects photons travelling along the axis of the cavity back into the active medium to stimulate more photon emissions. It also reinfor-ces or amplifies the energy by setting up a standing wave type of pattern. Only the photons travelling along the axis are reflected, so only longitudinally directed energy is amplified. All other photons are normally lost -- one reason lasers are so inefficient.
Now that the laser energy is focused and at a useful level, we have to get it out of the cavity. Only one of the two mirrors is normally 100% reflective. The other is some fraction less. The situation is analagous to a ping pong ball (photons) and two pad- dles (the mirrors), one missing a fraction of the shots sent its way! Once the laser energy reaches a threshold level, the escaping photons transmitted from the less reflective mirror become the laser beam we've been looking for. In operation, the output laser energy and the pumping energy get into an equilibrium which main-tains the population inversion and the other conditions necessary for laser operation.
Figure 10-15 shows some of the numerous types of laser cavit- ies possible. Each has its own advantages/disadvantages that must be taken into account in laser design.
10.6.2 Laser Light Characteristics
10.6.2.1 Monochromaticity. The term implies a single frequency.
Since laser light comes predominantly from a particular energy transition, and the laser cavity is selective about which frequency is amplified, laser light comes very close to being monochromatic. In actuality, the thermal motion of the active medium's atoms, as well as the presence of impurities, cause natural, collision, and doppler "line broadening." Simply, this means that instead of transmitting only the desired laser frequency, a small band of fre-quencies centered at the laser frequency is transmitted. The cen- ter frequency is significantly more amplified than the secondary fequencies.
10.6.2.2 Coherence. Coherency basically means that the light energy is travelling in the same direction with the same polariza-tion and phase. This is the one characteristic of laser light that sets it well apart from other light sources. When laser light e- merges from the laser output mirror it is coherent, and will remain so for a certain distance (called coherence length) from the mir-ror. Each laser has a different coherence length which must be considered. For example, the coherence length of a He-Ne gas con-tinuous wave laser, known for its stability, is typically about 30 kilometers for a shift in frequency of 1/2 the line width. The ruby laser is fairly unstable, having a coherence length of about one meter.
10.6.2.3 Divergence. One of the laser cavity's contributions to laser light characteristics (related to the coherence), is that the laser wavefront approaches a planar shape. If you think about a point light source, as the spherical wave moves away from the source, its surface of course expands, and in a local area, looks more and more planar. Now "fold" that distance into a laser cavit-y, and you have the same effect as the light bounces from mirror to mirror. Because the laser light emerges perpendicular to the out- put mirror, the beam has very little divergence, typically on the order of one milliradian. This is why the laser is so directional over long distances. With careful design, a laser beam can be made to have a divergence nearly equal to the diffraction limit or o = 1.22 (10-16)
d
where
0 is the angle from the first null to the center of the diffraction pattern.
d is the diameter of the beam emerging from the laser or any subsequent lenses.
10.6.2.4 Power and Efficiency. Most lasers operate at rather low efficiencies, usually only a few percent at best. As an example, a typical 1 cm diameter, 4 cm long pulsed ruby laser pumped with a xenon flash lamp discharging at 1000 Joules may have a reasonable overall efficiency of only 0.1%! Such a laser yields an output energy of only 1 Joule, but with a pulse width of 0.5 milliseconds, the power output is 2 kW. Such a laser also needs a PRT of about ten seconds to allow for the dissipation of the other 99.9% of the input energy, mostly in the form of heat.
A distinct advantage of laser light, especially due to the coherency and small divergence, is the ability to focus its energy.
Unlike incoherent sources which can not be focused to achieve images with higher intensity than originally emitted, the laser can be concentrated into a spot as small as one wavelength in diameter. The nearly parallel beam from a 50 kW infrared neodymium laser can be focused down to a radiant power density of 1012 watts/cm2, or about 100 million times the power density at the surface of the sun!
Table 10-2 lists some characteristics and uses of some of the more common laser in use today.
10.6.2.5 High Energy Propagation Effects. When high energy elec-tromagnetic waves are propagated through the atmosphere, a number of propagation effects present problems. These are discussed below from the standpoint of laser weapon applications.
Absorption of beam energy, principally due to water vapor and carbon dioxide, is a problem. Water vapor absorption is of greater consequence at the surface and at low altitude than at high alti-tude, where radiation weapons might be used for bomber defense. Al-so significant, especially at the shorter wavelengths, is scatter-ing due to water vapor and airborne particulate matter.
Turbulence induced beam spreading resulting from variations in the refraction index along the laser beam path is more severe for lasers operating at shorter wavelengths. The problem is again, more severe at sea level than at higher altitudes.
Thermal blooming is a beam-defocusing effect that occurs be-cause the air in the beam path is heated by radiation energy, changing the index of refraction. Under certain conditions this
can waste 90% of the beam energy. The thermal blooming problem can be eased by using a pulsed laser where the pulses are sufficiently brief that the air does not have time to heat up enough to cause serious defocusing. If the time interval between successive pulses is sufficiently long, motion of the laser beam as it tracks a mov-ing target in combination with natural air mass movement will cause the next pulse to transit through a new shaft of cooler air.
10.6.3 Laser Applications.
10.6.3.1 Laser Weapons. It was originally thought that the most powerful lasers to be built would be of the solid state type because in a solid, the active medium is more highly concentrated than it would be in a gas. Instead, the problem limiting the power levels turned out to be the considerable waste heat generated by laser pumping processes. The wasted heat adversely affects the performance of the optical medium and poses a fundamental limitation on the attainment of high average power output on any laser device.
Because heat transfer by diffusion and conduction to the cavity walls limited static lasers to a maximum output of about one kilowatt per meter length (independent of CW/pulse mode or diameter), dynamic lasers with flowing active media were developed.
Three different types of high energy laser devices are con-sidered potential candidates for continuous (CW) and pulsed high power applications suitable for weapons applications. These de-vices, which include the Gas Dynamic Laser (GDL), the Electro- Discharge Laser (EL) and the Chemical Laser (CL) use inter-mol- ecular collision, electrical discharge, and the heat of forma- tion of a chemical reaction (hydrogen fluoride) as respective methods of exciting their active media. The GDL may operate as either a closed or open cycle device, achieving supersonic flow velocities to aid in both pumping the active medium to an excited state, and to rid the device of waste heat. Figure 10-16 depicts a typical GDL optical train.
At first, it was thought that the only significant obstacle to radiation weapons was achieving sufficiently high energy levels.
Today it is recognized that the concept faces a very broad spectrum of technical challenges, possibly more difficult than those that faced the nation's ICBM and nuclear weapon programs. Foremost of these is that high energy laser technology is not easily translated from the relatively benign environment of the laboratory to the rigors of military vehicles. Other problems include the extremely precise beam aiming/tracking system and optical elements that can reflect/pass high energy laser radiation without damage to them-selves, and the development of lightweight power supplies, suita-ble for military use.
Additionally, there are obstacles to high energy laser prop- agation through the atmosphere, imposed by nature, and the complex effects of pulsed radiation on target materials, some of which function to screen the target from the laser. There is cautious optimism that these problems will soon be overcome. Laser weapons have possible application in everything from the Strategic Defense Initiative (SDI) to close in air defense where they will complement more conventional missiles and guns. The destructive mechanisms of these weapons systems will be addressed in Chapter 13.
10.6.3.2 Rangefinders and Laser "Radars". Laser radars are, in a systems analysis sense, identical to microwave radars, i.e., the same equations apply in regard to operting range, jamming vulnera-blity, scanning, etc. The major difference is that the shorter wavelength provides better range and angular resolution and narrow beam divergence. Because of the limited field of view the laser radar can handle, it is not practical for search, but much better as a tracker. Laser radars are also potentially useful for imaging.
In the area of laser rangefinders, the high peak powers and monochromaticity made them an instant success. The exceptionally narrow transmitted bandwidth allows for a correspondingly narrow receiver bandwidth, eliminating a lot of noise and increasing the signal to noise ratio.
Both pulse and CW systems can be used for ranging. The pulse techniques simply involve measuring the time interval between the pulse leaving the laser and a reflected return from some target.
Field ranging units have been built that are small, easily porta- ble, and can rang targets 10 kilometers away within an accuracy of 10 meters. Other rangefinder functions may include range gating and multiple target recognition. Continuous wave laser rangefind-ers use the delay between transmission and reception of a modulated signal to derermine the range. The maximum range capability of CW systems is less than the pulsed systems because of the lower peak powers.
Optical heterodyne systems, employed to obtain range rate in-formation, carry the information by slight variations in the fre- quency of the radiation. The reflected radiation is made to in-terfere with the transmitted radiation or with some reference rad- iation on a photodetector. The resulting fringe pattern movement on the detector generates a beat frequency equal to the frequency difference. From this range rate data can be computed.
10.6.3.3 Laser Target Designators. The development of laser quid- ided weapons has dramatically improved the accuracy of weapon quid- ance and delivery. Target designators are semi-active illuminators used to "tag" a target. Typical designators, like rangefinders, possess high peak power, short pulsewidth, and narrow beam charac- teristics for precise target designations. Lasers with a coded train of narrow pulses are used to allow weapons to distinguish between multiple targets.
Laser search/track receivers are used to detect and locate the laser energy reflected off an illuminated target. Deployed in air- craft, or in the weapons themselves, a laser tracker can be expec-ted to have an angle tracking precision of about 20 microradians. Since the initial target location is unknown, a wide field of view, typically 10 to 20 degrees, must be used.
Typical laser guided bomb receivers use an array of photodi- odes to derive target position signals. These signals are trans- lated into control surface movements to direct the weapon to the target. An airborne detector can provide steering information to the pilot, via his gunsight, for example, and lead him on a direct heading to the target, finally giving him an aim point for a con-ventional weapon. Alternatively, a laser guided "smart" bomb or missile may be launched when a pilot is satisfied that the detector head has achieved lock-on and the launch envelope requirements are satisfied. In either of these cases, the pilot may never see the actual target, only the aim point as indicated by the laser.
10.6.3.4 Laser Communication Devices. In the field of communi- cations, the laser offers two unusual advantages. The first of these pertains to the bandwidth. It is known that the rate at which information can be transmitted, or the number of channels that can be multiplexed on an information carrier, is proportional to the bandwidth, which is, in turn, proportional to the carrier frequency. Thus, the step from microwaves to the optical region expands the available bandwidth by a factor of 10,000 or more. By developing and employing the proper methods of modulation and demodulation, a few lasers could replace all the present informa-tion carrying systems between the east and west coasts of the United States!
The second immediate advantage of lasers in connection with long range information transmission is the ability of aiming and concentrating the carrier energy in the proper direction. A point to point communication link requires a narrow beam, and the nar-rowness of a microwave beam is limited by the size of the antennas employed. Antennas of considerable size are required to produce a beam only a few degrees wide in the centimeter wavelength region. For laser light, the dimensions of the radiator required for the same gain are decreased by a factor of 10,000! In fact, at a wave-length of one micron or less the practical beamwidth is limited by secondary considerations, not by the size of the radiator. It is not at all difficult to obtain a beamwidth of a few milliradians.
One of the important advantages of laser line of sight com- munications is that the laser beam has no sidelobes. This factor, coupled with its narrow beamwidth, will provide covert communica-tion capabilities in tactical situations.
10.6.3.5 Ring Laser Gyro. This application of lasers has already ben discussed in Chapter 5. Weapons systems require one ring laser gyro for every axis of rotation available to the system.
10.7 TACTICAL CONSIDERATIONS
Electro-optic equipment will function in many areas such as target detection, tracking and identification, fire control, surveillance, threat warning, damage assessment, and communications. It should again be noted that even though E-O systems are designed to perform the same functions as RF and microwave systems, their characteris- tics will be different so that they provide a complement to these systems rather than a simple redundancy.
Passive detection systems for detecting targets in a search mode continue to be of great interest and potential importance to the military planner. The infrared search system is a highly de- sirable device, particularly for use in EMCON conditions and a- gainst incoming aircraft and missiles. Consider that it has the potential for better detection than radar against incoming low- level attacks and that the high resolution may offer an evaluation of the number of incoming targets. The system would not be affect-ed by enemy electronic countermeasures -- although IR countermea-sures have been developed. The systems will work at short (25 km) range against low targets, and will work independently of the search radar. They are not all weather and will not detect targets through heavy clouds or fog.
The concept of using an active optical illuminator for large volume search is simply not feasible. The average power required becomes far too large. Accordingly, optical radar would be more valuable providing precise bearing, range, and doppler on a spec- ific target.
Passive tracking is possible and has been carried out. Day- time tracking in the visible spectrum is possible with TV-type contrast trackers with very high precision. Because the precision is usually obtained with magnification, the target is readily iden-tified. Identification should be achievable at 20 kilometers, and tracking to 20 microradians seems quite feasible.
The infrared guided missile is probably the best known mili- tary application of the infrared spectrum. Laser designation and TV guidance came out of the Vietnam War and the technology quickly spread. A review of weapons employing E-O guidance shows that the U.S.S.R. has deployed a sizable array of such weapons. This trend indicates the increasing importance of optical countermeasures.
The application of the laser as a radiation weapon is under intense investigation. There are many technological questions that must be answered before a laser weapon is deployed, but the devel- opment of such a weapon will undoubtedly revolutionize tactics in many warfare areas. Perhaps the primary arena for the laser weapon is in space, because there is no attenuating atmosphere, and line of sight distances are vast.
The requirements for E-O countermeasures are dictated by the severity of the E-O threat. As the tactical deployment of E-O missile seekers, fire control sensors, reconnaissance, surveil-lance, and other E-O seekers proliferates, the need for an effec- tive means of countering them will become more urgent. These countermeasures can take the form of active or passive devices, techniques, or procedures. Active devices include such items as TORCH, and laser jammers. Passive approaches include such items as camouflage, engine IR suppression, and absorbing materials.
This chapter has given the student a very brief taste of the fast moving and important field of electo-optics. A basic know-ledge of how and why these systems work is essential if we are to use them to maximum advantage, and prevent "the other guys" from using their systems to their advantage.
10.8 REFERENCES/BIBLIOGRAPHY
Commander, Naval Ordnance Systems Command, Elements of Weapons Systems. NAVORD OP 3000, vol. 1, 1st Rev. Washington, D.C.;
GPO, 1971.
Editor, High Energy Laser Systems. Concord, Mass.: General Enterprise, 1978.
Gebbie, H. A., et al. "Atmospheric Transmission in the 1 to 14u Region," Proc. Roy. Soc. A206, 87 (1951).
Hudson, Richard, Infrared System Engineering, New York: John Wiley & Sons, 1969.
Kemp, Barron. Modern Infrared Technology. Indianapolis, Ind.: Howard W. Sams & Co., 1962.
Meyer-Arenot, Jurgen. Introduction to Classical and Modern Optics, Englewood Cliffs, N.J.: Prentice-Hall, Inc. 1972.
Morgan, Joseph. Introduction to Geometrical and Physical Optics.
New York: McGraw-Hill, 1953.
Naval Operations Department, U.S. Naval War College. Technological Factors and Constraints in System Performance Study-Electro- Optics. Vol. I-1, 1975.
O'Shea, Donald C.; Callen, W. Russell; and Rhodes, William T. Introduction to Lasers and Their Applications. Reading, MA: Addison-Wesley Publishing Company, 1978.
RCA Corporation. RCA Electro-Optics Handbook. Technical Series EOH-11, 1974.
Wolfe, William L., ed. Handbook of Military Infrared Technology. Washington, D.C.: GPO, 1965.
Wolfe, William L., and George T. Ziss, eds. The Infrared Handbook. Washington, D.C.: GPO, 1978.
|
[Back] |
[Index] |
[Next] |