Cyclops see at night.
V.P. Beguchev, Ph.D.,
A.L. Chapkevich, Ph.D.,
State Research Center «Orion» Scientific and Production Association
«To see» means to distinguish between enemy and friend and the surroundings in all details.» — this is how one of the chapters of S.I. Vavilov's remarkable book «The Eye and the Sun» begins.
The most informative sense of human perception of the external world — vision, being a very perfect optical-biological instrument, has, unfortunately, limited spectral sensitivity. From a wide range of the optical radiation spectrum (from 0.001 to 1000 µm), the eye perceives a very narrow section from 0.38 to 0.78 mim, and even then starting from a certain level of illumination: at illumination less than 0.01 lux, the eye does not perceive color and distinguishes only large nearby objects.
Modern photoelectronics has come to the aid of the eye, making it possible to create devices capable of detecting, amplifying and visualizing radiation invisible to the human eye.
By allowing «seeing» in the fundamentally invisible ultraviolet (UV) and infrared (IR) ranges, as well as increasing the brightness of the night image many times over, these devices give their owners an invaluable advantage in military, search, rescue and other special operations.
The successful night operations of the anti-Iraq coalition troops during the Gulf War are one of the few «open» confirmations of the effectiveness of such photoelectronic devices.
The operation of these devices is based on the phenomena of external and internal photoeffects. The external photoeffect (otherwise known as photoelectron emission) consists of the emission of electrons from a solid (in practice, from thin translucent special semiconductor layers) into a vacuum under the influence of quanta in the optical range of the spectrum. The operation of an electron-optical converter (EOC) is based on this effect — a vacuum photoelectronic device that amplifies weak light in the visible range by thousands of times and also converts X-ray, UV and IR radiation into visible (with simultaneous amplification). The EOC is the main element of devices for observation in these ranges, including night vision devices (NVD).
The internal photoelectric effect (photoconductivity) consists of a change in the electrical conductivity of semiconductors under the influence of optical radiation quanta. The operation of photodetectors (PD) and more complex photodetector devices (PDD) is based on this effect; the signals they generate activate various actuators.
Signals from multi-element photoelectric detectors, generated by the objects' own radiation, are used after electronic processing to create a visible thermal image (the so-called thermal imaging). The internal photoelectric effect is also the basis for the operation of charge-coupled devices (CCDs) — multi-element self-scanning matrices that convert an image into a sequence of electrical signals and then into a standard video signal.
The first part of the article will consider the main types of night vision devices (NVDs) based on an EOP, and the second — devices and equipment that use photoconductivity phenomena, including thermal imagers of the internal photoelectric effect.
1. PRINCIPLE OF OPERATION OF NVD
The design and principle of operation of NVD based on EOP are explained schematically in Fig. 1, and is a telescopic optical system with an EOP built into it.
The lens 2 creates an image of a poorly illuminated (night sky, stars, moon) object 4 on the photocathode of the EOP 3. The photocathode 3 is a thin semiconductor layer applied to the inner surface of the input window of the vacuum housing 5 of the EOP.
The photocathode emits electrons into the vacuum, and the number of electrons emitted from each point is proportional to the brightness at that point of the image projected by the objective 2.
The electron flow carrying the «electron image» is accelerated and focused by the electron-optical system 6 onto the cathodoluminescent screen 7. The acceleration of photoelectrons occurs under the action of a voltage of about 10,000 volts generated by the power source 8. It is due to the conversion of photons into electrons and the acceleration of the latter in the EOP that the brightness is enhanced, since it is fundamentally impossible to enhance the energy of neutral photons.
Accelerated and focused photoelectrons, hitting the luminescent screen 7, cause it to glow in the visible spectrum (in almost all EOPs — in green).
Since the brightness of the screen glow at each point will be proportional to the number of photoelectrons that hit it, a visible amplified and transformed image of the observed object 4 is created on the screen. This image is observed using an eyepiece (or magnifying glass) 9.
Since the brightness amplification in modern image intensifier tubes reaches several tens of thousands of times, then, even despite certain losses in brightness in the input lens, modern night vision devices allow for clear observation of images in night lighting conditions, including moonless nights.
Simultaneously with amplification, the NVD also transforms the image. The transformation occurs due to the difference in the spectral characteristics of the sensitivity of the EOP photocathode and the human eye. Modern photocathodes have an extended (compared to the eye) IR sensitivity (up to 0.9 μm).
In this area, there is a significant difference in the coefficients of light reflection from natural and artificial objects. Therefore, a person in protective clothing, indistinguishable to the eye in the dark against the background of grass or foliage, will be clearly visible in the NVD as a dark object against a light background.
2. MAIN TYPES OF NVD
Currently, various types of NVDs are produced and freely sold, the designs of which are optimized in accordance with their purpose. The main types of NVDs are:
— observation devices (monoculars, pseudo-binoculars, binoculars),
— sights for small arms,
— glasses,
— devices that allow you to document the observed image (night photo and video cameras).
Since the optical elements of NVDs have been well-developed for a long time, the main parameters of NVDs and their cost are largely determined by the image intensifier tubes used in them. Most domestic NVDs use:
— single-chamber image intensifiers with a glass vacuum housing and flat input and output windows. Such image intensifiers provide brightness amplification up to 1000 times with high definition (resolution) only in the center of the field of view. As you move away from the center, the definition drops sharply, which reduces the information content of the observation. However, the low price of such image intensifiers makes them preferable for manufacturers of inexpensive, mass-produced night vision devices. Such image intensifiers will be further (conditionally) called «zero generation image intensifiers»;
— single-chamber image intensifier tubes with fiber-optic plates (FOP) at the input and output, using a microchannel electronic image amplifier, and also having a built-in power supply. Such image intensifier tubes, called «second-generation image intensifier tubes», amplify image brightness by 30-50 thousand times, have good image clarity across the entire field of view and have high noise immunity to bright local illumination. Expensive components of these image intensifier tubes determine their much higher price.
Miniature IOPs with a «flat» electron-optical system and photocathodes with increased efficiency (the so-called «third generation») have even higher characteristics (and, accordingly, price). These IOPs are used mainly in special equipment devices. 6 commercial NVDs, their variety with a simpler photocathode is more often used: «2-plus generation».
The so-called first generation IOPs with flat-concave IOPs at the input and output are easily assembled into two- and three-module assemblies. A three-module IOP has parameters and a price comparable to a second-generation IOP, but is used less often due to its greater length: a three-module first-generation IOP with a working diameter of 25 mm has a length of 195 mm, while a similar second-generation IOP is 76 mm.
2.1 .Monoculars
Night vision monoculars (visors) have one input lens, one image intensifier and one eyepiece (magnifying glass). Observation through such a night vision device is carried out with one eye, and the device is held with one hand.
The most common in domestic sales are monoculars based on a zero-generation image intensifier, which determines their acceptable price for the average buyer at a level corresponding to 150-200 US dollars. Such night vision devices have input lenses with a light intensity of about 1:1.5 — 1:2 and a focal length of 80-100 mm with a field of view angle from 10 to 20 degrees.
The power source is usually powered by two 1.5 volt batteries. The weight is about one kilogram.
The main parameter of both observation and all night vision devices is the range of vision. Its proper definition should include the following factors: illumination of the observed scene, the size of the observed target and its contrast with the background, the detail of vision: detection, recognition, identification. Such a multifactorial definition of the range of vision often leads to a discrepancy between the advertised and real values of this parameter.
In some cases, the buyer is offered to check the operation of the night vision device with a protective cap with a «pinhole» simulating night illumination put on the lens. Such a check gives an idea of the image quality (clarity and purity of the field of view), but cannot be used to assess the range of vision, since it dramatically changes the parameters of the input lens.
From work experience and elementary calculations it follows that observation NVDs based on a zero-generation image intensifier and input optics with the above parameters provide, at an illumination of 0.01 lux (the illumination created at night by a quarter of the moon), the detection of a full-length human figure against a green background from 150-200 m and the recognition of its details from approximately 70-100 m.
Some NVDs of this type are equipped with small-sized infrared illuminators (maximum radiation of about 0.8 μm), with a power of about several tens of milliwatts/steradians.
The effective range of such illuminators is about 50 m, the NVD with such an illuminator allows work in complete darkness (caves, basements), but exposes the observer to similar or more advanced NVD.
Night vision devices using the second generation image intensifiers have greater capabilities. With high-aperture (1:1, 5 — 1:2) lenses with a magnification of 3-5 times, these night vision devices provide the ability to observe even on a moonless night, which corresponds to the illumination on the ground of (1-5) x 10-3 lux. In this case, a human figure can be detected from a distance of 400-600 m, and its details — from 250-300 m.
NVDs with second-generation image intensifiers have good noise immunity: bright backlighting, falling into the field of view of such NVDs, is local in nature on the output image: it does not create halos and does not interfere with observation across the entire field of view. The weight of such NVDs, mainly due to the large size and weight of the second-generation image intensifiers, is at least 1.5-2 kg. The price is from one to several thousand dollars, depending on the quality of the image intensifier and optical components, as well as the power supply circuit. including automatic adjustment of output brightness (AOR).
2.2 Binoculars
During the normal visual process, a person uses two eyes. The images created by each eye, complementing and reinforcing each other, create in our minds one visual image, which also carries information about the spatial nature of the image (stereoscopic effect).
Pseudo-binocular NVDs use one objective lens, one image intensifier tube and a binocular panoramic eyepiece at the exit. Such an eyepiece faces the observer with the convex side of the flat lens, with a field of view angle of 90 degrees or more, through which the image on the image intensifier tube screen is viewed with both eyes. This ensures greater convenience of observation and, accordingly, less eye fatigue.
Night binoculars of the classical design contain two objectives, two image intensifiers and two eyepieces. Unlike pseudo-binoculars, such a design provides stereoscopic images of distant objects, for which the distance between the input objectives is made significantly greater than the distance between the eyes (base). When observing, night binoculars, like regular ones, are held with two hands using special design elements: handles, holders, lugs.
Depending on the quality of the optical components and the type of image intensifiers used (zero, first or second generation), the price of pseudo- and binocular NVDs ranges from 800 to 2500 dollars.
2.3. Sights
In certain circumstances, it is necessary not only to observe an object in the dark, but also to shoot at it in such conditions (for example, night hunting). This task is solved by night sights. In principle, a night sight is similar to a night monocular, but has the following significant differences.
The sight has a weapon mounting mechanism and a «verification» mechanism. The latter ensures that the aiming (firing) line of the weapon is aligned with a special mark («mark») in the NVG field of view, which plays the role of a «front sight» during night shooting. The mark can be dark or luminous, which is preferable, since a luminous mark is easier to distinguish on a dark object (target). Due to the high impact loads that occur at the moment of firing (up to 100 and higher «g»), the EOL and the entire design of the device are subject to increased requirements for impact resistance and shock resistance compared to other NVGs.
The range of night sights should be reasonably correlated with the range of aimed fire of the weapon, which for light small arms is 200 — 400 meters. Night sights can also be used in the daytime, for which a special diaphragm (cover) with a «pinhole» is put on the lens, protecting the image intensifier from bright light and at the same time providing good visibility of the target.
A more complex design of sights also determines their higher price by 25-30 percent (compared to monoculars).
2.4. Night vision goggles
The fundamental difference between night vision goggles (NVG) and the above-described NVGs is that NVGs are attached to the observer's head or special headgear, leaving his hands free to perform various jobs and operations at night. The scope of the latter is wide and varied: repair and rescue work, driving ground and air transport, shooting at targets illuminated by laser emitters.
In the latter case, the object (target) is «marked» with an invisible laser beam to the naked eye, aligned with the weapon, and the shooter, seeing the «mark» with the help of the NVG and aiming it at the target, hits the latter without the usual aiming. For the usual and quick orientation, almost all types of NVG have a single magnification and a wide field of view: 40 degrees or more. The classic NVG design contains two lenses, two image intensifiers and two eyepieces; in some NVGs, in order to reduce the price, one lens and one image intensifier are used (the so-called «cyclops» type).
The most accessible and inexpensive NVGs are based on the zero and first generation image intensifiers, and the most effective in terms of range and the lightest (less than 500 g) are based on the 2+ and 3rd generation image intensifiers. Due to the high price of such glasses, comparable to the price of an average car, they are used mainly for night piloting of helicopters or for tasks carried out by special services.
The latest achievement in the field of NVG is holographic NVG. Due to the use of holographic mirrors and filters in such glasses, almost completely (08 percent) transmitted, enhanced image of dimly lit objects, while simultaneously reflecting the same amount of light interference that interferes with observation, which are visible as if through very dark glasses. The lower part of the mask of such glasses has increased transparency, which makes it convenient to observe the dashboard when using glasses for driving at night.
2.5 NVG with image documentation
In some cases, it is necessary to document (photograph, video) scenes, objects and their actions observed with the help of NVDs.
The simplest solution is to attach to the NVD instead of the eyepiece of a photo or movie camera. Some NVDs are equipped with adapters for attaching cameras, which is easily done by the user of the device.
A more advanced and multifunctional system is one in which the image from the NVG image intensifier screen is optically transmitted to the CCD matrix. The transmission is carried out using focons (fiber-optic «image reducers») or lens optics «transfer». The electronic circuit («frame») of the CCD matrix converts the received image into a video signal in analog and, if necessary, digital form.
The video signal can be observed on a TV screen (monitor), which is more convenient and less tiring than observation (especially long-term) through the eyepiece of a night vision device. Simultaneous recording on a video recorder and transmission to several monitors for several operators is possible.
The video signal can be transmitted via a cable (up to 200 m without intermediate amplifiers), or using a miniature transmitter built into the surveillance device, the signal of which is received on one of the channels of a regular TV.
The quality of such systems is determined by the number of television lines transmitted at a certain illumination of the observed scene.
When using a night vision device with a zero-generation image intensifier, 300-350 lines are transmitted at an illumination of 0.01 lux, for a night vision device with a second-generation image intensifier, the same number of lines are transmitted at an illumination of 1…5×10-3 lux, and with a third-generation image intensifier, at 1×10-4 — 5×10-5 lux.
Such devices can be equipped with adapters for connecting modern lenses for CCD cameras with remotely adjustable iris (auto-iris), variable zoom and subfocus to the NVD input. A device with such a lens and an image intensifier with a good AGC circuit provides almost round-the-clock (from a moonless night to a bright day) observation with the necessary documentation.
Conversion of the video signal into a digital code gives the systems «ПНВ+П3С» additional capabilities. The resulting image can be recorded by a digital photo or video camera, processed to enhance contrast, eliminate light and dark defects, and painted in conditional colors.
A more complex system of two NVDs with a CCD with special light filters after digital electronic signal processing creates an image of the observed night scene in natural colors on the monitor. This significantly increases the information content, speed and value of visual perception. Such a device, which refutes the well-known proverb «all cats are gray at night», was demonstrated at the exhibition «IDEX-97» (Abu Dhabi) by the Belgian company «Delft Sensor Systems». According to the developers, the image in natural colors increases the efficiency of detection and recognition of objects at night by 30-60 percent.
In Russia, the development and production of systems based on NVDs and CCDs is carried out by the State Research Center «NPO Orion» together with a number of co-executors.
3. Devices with UV and IR photocathodes.
Above, we considered NVDs with image intensifiers sensitive in the range of 0.4-0.9 µm and intended for observation of distant objects, which corresponds to the main task of these devices: to see at night as far and well as during the day.
At the same time, there are many other applications of image intensifier tubes based on their unique ability to transform an image that is fundamentally invisible to the eye into a visible one.
For example, an image intensifier tube with a photocathode sensitive to the «solar-blind» UV region (from 0.1 to 0.3 µm) allows for important environmental observations of ozone holes, glow of objects and areas with increased radioactivity, and toxic engine exhaust in daylight.
In forensics, UV-EOPs are used to detect counterfeits when genuine documents are provided with special marks that reflect UV radiation.
A method of reading documents covered in ink or ink using an EOP with oxygen-silver-cesium photocathodes sensitive to 1.2 — 1.3 µm has long been known. Many liquids and materials that are opaque in the visible region are transparent in this IR range, and a covered or counterfeit inscription is clearly visible using such an EOP.
X-ray image intensifier tubes (REOTs), which have a complex «sandwich» instead of a photocathode: «X-ray phosphor-layer-photocathode», are capable of visualizing X-ray radiation.
The main area of application of large-diameter (up to 320 — 400 mm) NVGs is medical X-ray diagnostics. Small-sized NVGs with a field diameter of up to 50-100 mm in combination with miniature radioisotope radiation sources can be used for operational field X-ray diagnostics, luggage scanning, defect control of electrical and microassemblies, and other tasks of operational introscopy with portable equipment.
Thus, modern photoelectronics significantly expands the capabilities of human vision in terms of vision in low-light conditions and in ranges fundamentally inaccessible to the eye. The complexity of this equipment, the large number of types of NVGs available for sale, the difference in their classes and prices, as well as the inevitable advertising «overexposure» in the descriptions of the devices make it highly desirable to have qualified and objective advice when choosing and purchasing such devices.
GNI «NPO» Orion», which has extensive experience in developing and producing night vision devices and image intensifiers for them and is the leading organization in Russia on night vision, offers, on agreed terms, both consultations and developments in this area, as well as the sale of ready-made night vision devices of almost all types described in this article.