Night vision devices: history of generations..

Night vision devices: history of generations..

Salikov Vyacheslav Lvovich

NIGHT VISION DEVICES: HISTORY OF GENERATIONS

The imperfection of one's own nature, compensated by the flexibility of the intellect, continually pushed man to search. The desire to fly like a bird, swim like a fish, or, say, see at night like a cat, were embodied in reality as the required knowledge and technologies were achieved. Scientific research was often spurred by the needs of military activity, and the results were determined by the existing technological level.

Expanding the range of vision for visualization of information inaccessible to the eye is one of the most difficult tasks, as it requires serious scientific preparation and a significant technical and economic base. The first successful results in this direction were obtained in the 1930s. The problem of observation in low-light conditions became especially relevant during the Second World War. Its practical implementation provided the opportunity to act in the twilight and at night without using visible light sources.

The first successes of night vision technology, not yet understood by the public, made war by starlight a dream of military specialists. Enormous resources were spent to achieve results, allocated by both governments and leading firms of developed countries. People started talking about “victory over night” already during the Gulf War. Subsequent conflicts in Yugoslavia and Chechnya made night combat an inevitable attribute of modern warfare.

Naturally, the efforts expended in this direction have led to progress in scientific research, medicine, communications technology and other areas. Adapted for individual use, analogues of military equipment are increasingly used for the needs of law enforcement agencies, security services, rescue services, navigation tasks, among night hunting enthusiasts, etc. The change in the market situation, which was a consequence of the global restructuring of the economy due to the fall of a number of political barriers in recent decades, has led to the rapid commercialization of products of modern high-tech production. As a result, the results of scientific and technical developments based on knowledge of the waves of the optical range of not only the visible spectrum, but also infrared (IR) radiation, have today become available for consumer goods.

Unfortunately, the issues of night vision technology, openly discussed in wide circles of developed countries, in the USSR (Russia) were focused only on representatives of the military-industrial complex and direct developers. A brief retrospective of the history of night vision devices (NVD) and an overview of the current state of this segment of the optoelectronic products market are intended, in part, to fill this gap.

The principle of operation of a classic NVD is based on the conversion of IR radiation generated on the observed object by the glow of the night sky, stars and the moon, into visible light. The functional block diagram of the optical path of a modern NVD is shown in Fig. 1.

Fig. 1. Functional block diagram of the optical path of a modern NVD

1. Object of observation
2. NVD body
3. Lens
4. Electrooptical tube with built-in MCP, VOE and VIP
5. Eyepiece
6. Power elements, usually finger batteries (type AA)
7. Built-in IR illumination

The image of the observed object is projected upside down through the lens onto the input glass of an electron-optical converter, which is a “high-vacuum lamp with two flat ends, the input and output windows, respectively. On the inside of the input window there is a thin translucent layer of light-sensitive material (photocathode), which emits electrons when absorbing light quanta. On the inside of the input window there is a layer of phosphor, a material that emits light when an electron hits it (screen). The transfer of electrons emitted by the photocathode is provided by an electrostatic field, for which a voltage of several kV is applied to the photocathode and screen. The image obtained on the screen is viewed through an eyepiece.

In modern IOP designs, a secondary emission amplifier or microchannel plate (MCP) installed between the photocathode and the screen is used to amplify the image. The MCP allows for gains of tens of thousands of times, and in some special-purpose IOPs, up to 107 times, which is sufficient to register single photons.

The input and output windows of the image intensifier tube are made on flat glass or on a fiber-optic plate (FOP). To rotate the image by 180°, a fiber-optic erecting element (FOE), also known as a twister, is used as the output FOP. In more complex designs, a binocular eyepiece or an additional lens erecting element is used to rotate the image.

Despite the simplicity of the design and the minimum number of units, each element of the NVD is subject to fairly high and often contradictory requirements. Obviously, the most complex and critical unit of the NVD, determining both its limit parameters and price, is the image intensifier tube. The history of the birth and improvement of this unit should be considered indicative of the technocratic era.

“Kholst Glass”

The first converter was developed by Holst and his co-authors at the research center of the Philips (Holland) company in 1934. It remained known as the Holst glass. Its diagram illustrating the operating principle is shown in Fig. 2.

Fig. 2. Operating principle of the Holst glass”

This EOP consisted of two glasses nested one inside the other, onto the flat bottoms of which the photocathode and phosphor were applied. High voltage applied to these layers created an electrostatic field, providing direct transfer of the electron image from the photocathode to the screen with the phosphor. A silver-oxygen-cesium photocathode (or S-1) was used as a photosensitive layer in the “Kholst glass”, which had a rather low sensitivity (Fig. 3), although it was operational in the range of up to 1.1 μm. In addition, this photocathode had a high noise level, to eliminate which cooling to minus 40 oC was required.

Fig. 3. Spectral sensitivity curves of image intensifier tube photocathodes

1. S-1;
2. S-20;
3. S-25;
4. S-25R (2+);
5. GaAs;
6. Near IR GaAs

These shortcomings allowed the use of the image intensifier tube only in active mode, i.e. with illumination of the observed image by an IR spotlight. In addition, the image on the screen was blurry. The distance between the photocathode and the screen had to be made as small as possible due to the scattering of electrons leaving the photocathode at different angles. In the “Kholst glass” it was several mm and it was impossible to reduce it for technological reasons.

The appearance of the first image intensifiers in the pre-war environment aroused considerable interest. The “Kholst glass” was developed to the level of serial production by EMI (England), and from 1942 to 1945 several thousand of them were produced (Fig. 4).

Fig. 4. The first serial samples of the “Kholst glass”.

Due to the “bouquet” of shortcomings of early image intensifier tubes, the first night vision devices were distinguished by significant mass-dimensional parameters and energy consumption, as well as low image quality.

Nevertheless, they were actively used during the Second World War by all sides. The use of NVGs with IR searchlights by Germany to support the actions of combat vehicles was quite successful. As a result, the Soviet Army suffered serious losses in the battles in the area of ​​the Hungarian Lake Balaton. The desire to equalize the chances and deprive the enemy of the advantage that had arisen forced the Soviet command to illuminate the battlefield with anti-aircraft searchlights when crossing the Oder River*. It should be noted that for the needs of the German army, more modern EOPs with electron-optical focusing were used, providing a screen resolution of up to 20 microns, and in more complex versions even up to 1 micron.

American and British companies achieved major successes. The Sniperscope night sights for small arms are well known, successfully used during the American landing on Okinawa Island. A rare photograph of American periscope glasses is shown in Fig. 5.

Fig. 5. Rare photograph of American periscope glasses

Zero generation.

The achievements of electron optics in the mid-1930s made it possible to replace direct image transfer with focusing by an electrostatic field. Zvorykin, Farnsworth, Morton, and von Ardenna worked actively in this direction abroad, and G.A. Grinberg and A.A. Artsimovich worked in the USSR. As a result, three- and then two-electrode systems were developed, providing amplification of the order of hundreds of times with simultaneous image wrapping (Fig. 6).

Fig. 6. Design of a three-electrode image intensifier.

1 – photocathode
2 – cuff
3 – housing
4 – focusing electrode
5 – anode
6 – screen

Subsequent work led to the discovery of the “multi-alkali photocathode” (S-20), consisting of sodium and potassium arsenides activated by cesium. This photocathode has been the basis of most image intensifier tubes of almost all types for 40 years. Image intensifier tubes with electron image transfer and a multi-alkali cathode today belong to the zero generation, in the slang of specialists “zero”. The most common representatives of this family in Russia are the B-8, the famous “eight”, and the K-4, which is of interest as a simple converter.

The efficiency of such image intensifiers can be determined through the amplification of the luminous flux hф (conversion coefficient).

hф(l) = Sk x U x g,

where
Sk is the cathode sensitivity, usually expressed in μA/lm;
U is the applied voltage, V;
g is the screen luminous efficiency, lm/W.

For example, for B-8, the integral sensitivity of a multi-alkali photocathode can be 200 μA/lm, U is about 20 kV, g is about 30 lm/W. The luminous flux amplification will be 120 times. In a similar way, the conversion factor of a monochromatic radiant flux hф(l) can be determined in lm/W, i.e. at a specific wavelength. Spectral sensitivity is also indicated in lm/W.

This type of image intensifier tubes have already been discontinued worldwide and replaced by more efficient, but also more expensive converters of subsequent generations. The production of “zeroes” that remained in Russia supported the national optical industry, which lost the market for its own products during the crisis of the early 90s. Mass production of cheap NVDs was quickly established, filling the shelves. Today, converters of the 0th generation can be purchased for $ 20 each, and in an assembly with a high-voltage power supply (VPS) for about $ 50. In combination with low requirements for the optical units of such NVDs, their cost is $ 100-200. Insufficient characteristics allow us to consider such devices only as souvenirs or toys, which is often underestimated by buyers. Nevertheless, they have found their niche in the market, defining the lower price range of NVDs.

The biggest drawback of the EOP with electrostatic image transfer is the sharp drop in resolution from the center of the field of view to the edges due to the misalignment of the curvilinear electron image with the flat photocathode and screen. To solve this problem, they began to make them spherical, which significantly complicated the design of lenses, usually designed for flat surfaces.

First generation

The development of fiber optics in the USA in the 60s allowed to improve the image intensifier tube. Based on fiber optic plates (FOP), which are a package of many light-emitting diodes, plano-concave lenses were developed, which began to be installed instead of the input and output windows. The optical image projected onto the flat surface of the FOP is transmitted to the concave side without distortion, which ensures the conjugation of the flat surfaces of the photocathode and screen with the curvilinear electron field.

As a result of using the VOP, the resolution across the entire field of view became the same as in the center. IOPs with VOP and electrostatic focusing in mass production belong to the 1st generation. In the manufacture of these IOPs, a sensitive photocathode S-20 began to be used. In addition, mirror-lens objectives began to be used in the design of first-generation NVDs, which improve the weight and size parameters.

At present, first-generation IOPs are still used in night sights for hunting rifles and are successfully used where only the conversion of near-IR wavelengths into visible light is required, for example, for visual inspection of the assembly of optical communication systems, in medicine, where YAG lasers with a radiation wavelength of 1.06 μm are used.

Multi-cascade image intensifier tubes

While fiber optics technology was developing abroad, in the USSR, priority was given to cascade image intensifier tubes by M.M. Butslov. The diagram of one of the most successful models, the U-72, is shown in Fig. 7. In this design, the total gain is equal to the product of the gains of all the chambers and can reach 107 times.

Fig. 7. Design of a two-cascade IOP with electrostatic focusing of electrons of the U-72 type.

The production of such IOPs was associated with significant technological difficulties, in particular, it required the use of only highly qualified glassblowers. In addition, the resolution at the edges of the field of view deteriorated to 2-3 lines/mm. Nevertheless, the mass use of cascade converters ensured the tactical superiority of the armed forces of the USSR in the period of the 50-60s.

Using an image intensifier tube to connect cameras simplified assembly and improved image quality, while using metal ceramics instead of glass significantly increased the strength of the structure. Such image intensifier tubes were successfully manufactured by RCA, ITT (USA), Philips (Netherlands) and some others. They were not afraid of bright light, and their only drawback was their considerable length along the optical axis.

The decade-long dominance of cascade EOPs was replaced by a rapid refusal to use them and the displacement of EOPs of the next generations. Today, these converters have no commercial application; military equipment left over from the USSR period is equipped with modern small-sized EOPs. Overcoming the technological impasse that arose in the USSR was only possible in the 80s.

Second generation

In the 1970s, based on the VOP technology, US companies developed a secondary emission amplifier in the form of a microchannel plate (MCP). This element is a sieve with regularly spaced channels with a diameter of about 10 μm and a thickness of no more than 1 mm. The number of channels is equal to the number of image elements and is about 106. Both surfaces of the MCP are polished and metallized, and a voltage of several hundred volts is applied between them.

The operating principle is well illustrated by Fig. 8. When entering the channel, the electron collides with the wall and knocks out secondary electrons. In a pulling electric field, this process is repeated many times, allowing for a gain of Nx104 times. To obtain the MCP channels, optical fiber with a heterogeneous chemical composition is used. After obtaining the washer, the fiber cores are dissolved in chemical reagents.

Fig. 8. Operating principle of a secondary emission amplifier in the form of a microchannel plate.

The production of MCPs, as well as VOPs, is considered high technology, ensuring the production of compact and energy-efficient image intensifiers suitable for use in head-mounted NVDs, i.e. glasses and monoculars. The image rotation in the image intensifiers from MCPs, classified as the second generation, is still carried out by means of electrostatic focusing (Fig. 9). The prologue to the successful use of binocular glasses to support the actions of special forces of NATO armies was the AN/PVS-5B model by Litton (USA) (photo 1).

Fig. 9. Design of an image intensifier tube with an electrostatic lens

1 – photocathode
2 – anode
3 – microchannel plate
4 – screen

Photo 1. AN/PVS glasses -5V from Litton (USA)

In the late 1970s, biplanar-design MCP image intensifiers were developed, i.e. without an electrostatic lens, a kind of technological return to direct, as in the Holst glass, image transfer (Fig. 10). Similar designs, including multimodal ones, are produced by Praxitronic (Germany). The resulting miniature image intensifiers, in a modern design, already classified as the II+ generation, made it possible to develop night vision goggles (NVG) of a pseudo-binocular system, where the image from one image intensifier is divided into two eyepieces using a beam-splitting prism. The image is rotated here in additional mini-lenses (photo 2).

Fig. 10. Flat image intensifier tube design

1 — photocathode
2 — microchannel plate
3 — screen

Photo 2. The device of the pseudo-binocular ONV scheme (using the 1PN74 as an example).

1 — ONV body
2 — eyepiece
3 — erecting objective
4 — mirror
5 — collimator (magnifier) ​​with prism
6 — ONV body
7 — IR illumination
8 — image intensifier
9 — objective body
10 — objective
11 — objective cover

Pseudo-binocular — the design is not only ergonomic, but also very economical, due to the use of one image intensifier, which is the most expensive unit — about 50% of the cost. The weight of such NVGs is within 500-700 g. Today, these are the most widely used NVGs, used in the Armed Forces and special services of various countries around the world. For example, AN/PVS-7 in the USA and NATO, 1PN74 — in Russia. It should be noted that serial production of such systems began in the USA in the mid-80s, and in Russia only now, although the development of the domestic model was completed in the early 90s.

Third generation

The next step in the development of the EOP was determined by increasing the sensitivity of the photocathode. It became possible due to purely scientific research. As a result of fundamental research, which began in the 70s, it was established that the optimal material for creating a photocathode is gallium arsenide, which is capable of effectively emitting electrons with an initial radiation wavelength of 0.9 μm or less.

However, the implementation of AsGa-FC was hindered for a long time by the presence of an energy barrier that did not allow electrons to break away from the surface of the semiconductor layer (the potential barrier of electron affinity). This problem was successfully solved by Scher and Van Laar, employees of the Philips Research Center, as well as Williams and Soyman, who proposed the theory of NEA (negative electron affinity).

Obtaining AsGa-FC is possible only under conditions of ultra-high vacuum of the order of 10-10 10-11 mm Hg, and the entire process must be carried out under the control of complex diagnostic equipment. Due to the rapid oxidation of the photocathode surface in air, the assembly of the third-generation IOP must also be carried out in a vacuum chamber using manipulators. As a result, more than 400 technological operations are required to produce the third-generation IOP. All this determined the extremely high cost of these converters.

Initially, the industrial technology AsGa-FK was developed by the American company «Varian», from which it was purchased for serial production by ITT Night Vision and Litton, leading companies in the USA, manufacturers of NVG for military purposes for NATO needs.

The high characteristics of the EOP III allowed these companies to develop aviation binocular NVG — ANVIS/AVS-6 for piloting helicopters, and AVS-9 for aircraft in night conditions at low altitude, this allows flying in close formation, recognizing targets and obstacles on the ground.

The long scientific development and complex manufacturing technology that determine the high cost of the third-generation EOP are compensated by the extremely high sensitivity of the photocathode. The integral sensitivity of some samples reaches 2000 mA/W, the quantum yield (the ratio of the number of emitted electrons to the number of quanta falling on the photocathode with a wavelength in the region of maximum sensitivity) exceeds 30%! (Fig. 3).

Of course, when developing the EOP III, the achievements of technologies of all previous generations were applied, which made it possible to create a superminiature design. The standard diameter of the photocathode/screen is 18 mm, much less often 25 mm — for sighting systems. High-voltage power supplies (VPS) are already built into the housing of such EOPs. Current consumption does not exceed 20 mA, with a supply voltage of 3 V, which allows modern NVDs to work continuously for almost a day from two ordinary AA batteries. In addition, these EOPs have very high reliability indicators (the mean time between failures is about 10,000 — 19,000 hours).

The high sensitivity of the new photocathode made it possible to see in the worst conditions, called “cloudy starlight”, which means the presence of clouds and the absence of the moon. The illumination in this case is 5×10-4 lux. NVGs with EOP II were designed to work in conditions of “natural night illumination (ENI)” – 5×10-3 lux, that is, in the light of stars without clouds and moonlight. Since 1992, ITT and Litton NV have been supplying PVS-7B and ANVIS glasses with EOP III for the needs of the US and allied armies, under multi-year industrial contracts Omnibus III (Omni IV since 1996 and Omni V – since 1998).

EOP III is today considered a key military technology. Their presence creates a huge advantage for the army and aviation over a potential enemy in combat operations at night. At present, security services, law enforcement, and rescue services of developed countries also make large-scale purchases of these NVDs.

Apart from the USA, only Russia produces converters based on AsGa-FC. The development of EOP III was greatly delayed due to some technological backwardness, which led to a crisis that became obvious with the beginning of the war in Afghanistan. The embargo prevented the acquisition of the necessary equipment abroad. Nevertheless, the existing obstacles were overcome.

Currently, two Russian companies, Katod (Novosibirsk) and Geofizika-NV (Moscow), produce EOP III, offering them at prices ranging from $1,500 to $1,800, depending on the design and specifications. Geofizika-NV is also the most advanced Russian company in the development and production of aviation goggles. The 1PN74 goggles with EOP III used in the Russian army are manufactured by the Alfa State Unitary Enterprise (Moscow), developed by SKTB TNV, and the same company supplies ONV-1 helicopter goggles for aviation needs. It should be noted that the distribution of such high-tech products is controlled by the state.

 EOP II+ and SUPER II+

The lack of domestic markets required to sell such expensive components as the III image intensifier tubes led most NVG manufacturers to doubt the possibility of recouping costs when launching them into production. An alternative was to increase the efficiency of existing converters. The development of this direction led to a return to the multi-alkaline cathode, initially with increased sensitivity in the IR region (S-25), while maintaining the design solutions achieved in the III generation. Subsequently, a photocathode with especially high sensitivity (S-25R) was developed (Fig. 3). Based on such cathodes, II+ and SUPER II+ generation image intensifier tubes are currently manufactured, respectively. A similar classification is also used for the I generation.

Manufacturers of III image intensifier tubes admit that there are no fundamental differences in efficiency between the new Super II+ and III generation systems. The advantages of third generation converters become obvious as these devices age, as S II+ photocathodes lose sensitivity (degrade) with use. The service life of such image intensifier tubes is about 3,000 hours, and the cost ranges from $600 to $900, depending on the design.

Knowledge of the principles of the EOP and the technology of their production allows us to determine the main characteristics of the NVD and its expected cost. For quick orientation within the framework of the considered classification, you should use the table, which summarizes the main characteristics of the EOP. However, for a more complete assessment, it is necessary to get an idea of ​​the specific requirements for the optical units and the design of such devices. The achieved quality of the optical components did not limit the development of the EOP. The resolution limit, which determines the minimum angular dimensions of the object accessible for observation, is determined by the resolution of the applied MCP, that is, the diameter of the channels. Today, the NVD on average provides 30-40 lines/mm, the best examples of EOP III, intended mainly for aviation, reach 64 lines/mm. The pore diameter in such MCP is 5-6 µm with a thickness of hundredths of a mm. Due to their high fragility, these plates are extremely difficult to manufacture and process.

Only high-aperture lenses are used in the design of NVGs. The optimal lens is one with a relative aperture (F-factor) of 1:1.4, the best models have 1:1.1 (for head-mounted systems with an image magnification of 1x, i.e. glasses, monoculars). Knowing the standard photocathode diameter, 18 mm for II+ and higher, it is easy to determine other basic parameters of modern NVGs: field of view angle of 40°, focal length of 25 mm. Today, lenses are produced with a field angle of 50°, and even 60°, with a proportional decrease in focal length, which corresponds to the angle of the high-definition field of view of the eye. Ergonomic requirements for minimizing the weight and size parameters of NVGs and image quality force the development of multi-component (usually up to 10 thin lenses), difficult to manufacture and, therefore, expensive lenses. An exception are «zero» lenses. — usually inexpensive four-component designs. Differences in the anthropometric structure of the head force developers to introduce a mechanism for adjusting the eye base (the distance between the optical axes of the eyes of different people varies from 56 to 72 mm), or to achieve significant diameters of the exit pupils of the eyepieces, more than 14 mm, which also complicates the design of head-mounted NVDs.

There are also problems in developing night vision sights. In particular, it is necessary to introduce a sighting mark and ensure that the eyepiece exit pupil is more than 50 mm, which leads to an increase in the dimensions and weight of the glass; the requirements for mechanical strength are high. Modern night sights and binoculars provide 3-5x image magnification at focal lengths of 75-120 mm and a relative aperture of about 1:1.5. To use pseudo-binocular glasses as binoculars, afocal attachments are used, installed on the main lens (supplied in the kit or by special order). To reduce the weight of NVDs, mirror-lens lenses are often used, although traditional lens schemes remain the most common.

In conclusion, it should be noted that the history of NVDs is not limited to the level achieved. Continuous expansion of production and sales volumes, increased interest in new products from all participants in the high-tech market indicate broad prospects for night vision technology. Despite the fact that NVDs with EOP III are capable of performing tasks on the darkest nights, active work is currently underway to develop both EOP IV generation and to improve the circuitry of the NVDs themselves. Most of the work is related to improving the energy-efficient characteristics, design and expanding the functionality of the devices. Of significant interest is also the development of photocathodes with sensitivity extended into the long-wave region of the IR range. Litton has achieved a good result here, having developed an «advanced in IR» IOP III, which can be used to detect radiation from a YAG laser with a wavelength of 1.06 µm, used in all armies for rangefinding purposes.

Table 1. Main characteristics of the IOP (selectively*)

* This table does not include a number of key characteristics, such as resolution limit (lines/mm), current consumption (mA), etc. These characteristics can be taken into account in a similar way or as part of the product.

** Multi-chamber image intensifier tubes and image intensifier tubes with an increased photocathode diameter (25 mm versus 18 mm) are not taken into account; they require special NVG designs.

*** ENO — standardized «natural night illumination», 5×10-3 lux, starlight without moonlight and clouds; EOP III — the same, but at 5×10-4 lux, «cloudy» starlight, sky in clouds.

Unfortunately, the limited volume of the magazine article does not allow for a more detailed coverage of the numerous, often dramatic events that accompanied the stages of development and improvement of night vision technology. A reader who has not come into contact with the world of night vision devices will undoubtedly be amazed to learn about the scale of influence of these famous works of technoculture on many military and political decisions of the second half of the 20th century. Even specialists may be equally surprised by the scale of expenses for the development and production of both traditional and the latest models of night vision devices using virtually all types of the described EOPs. The widest range of NVGs of the same design, for example, indicates the lack of qualified marketing in the NVG market that has become international, with the exception of representatives of the USA, of course. It is quite possible that the problems that have arisen can be overcome by those several articles devoted to the domestic and foreign fleet of night-vision equipment, as well as the prospects for its development, which the author plans to offer in the following issues of the journal.

LITERATURE

1. Kurbatov L.N. Brief essay on the history of the development of night vision devices based on electronic optical converters and image intensifiers//Issues of Defense Technology. Series 11. — 1994 — Issue 3(142) — 4(143).
2. Koshavtsev N.F., Volkov V.G. Night vision devices//Issues of Defense Technology. Series 11. — 1993 — Issue 3(138).
3. Goodman G. Modern night vision goggles used in the US Army.//Translation of the article in the Armed Force Journal International, July 1998, pp. 42-45.