Universal Multispectral Sight & Observation Device as The Means of Improving The FCS of Armament of Armored Force Vehicles (AAFV) Fiber Optic Devices & Technologies
The night channel is either a thermal imaging camera with IR optics and a sensitive element in the form of a grating allowing to eliminate mechanical scanning, or a channel built on electron-optical image intensifiers operating in the near-IR range.
The main tactical requirement for instrumentation of armored vehicles is the provision of the ability to effectively use the instruments in vibration conditions when the vehicle is moving.
This means that the operator, while protected by the armor of the tower, must perform observation from the moving vehicle, positively detect and identify the target approaching from any direction, and receive information about the parameters of horizontal and vertical pointing.
The aiming axis of sighting and observation systems installed on the armored vehicles, due to linear and angular displacements during the movement of the machine, is subjected to certain angular perturbations. To use the sight for its intended purpose, the position of its aiming axis should be controlled in the horizontal and vertical planes, and synchronized with the gyroscope to minimize the effect of angular disturbances. Precise stabilization makes it possible to get a clearer picture of the observed objects, providing a target recognition.
To control the aiming axis and stabilize the image obtained in the field of view of the sights, flat mirrors or prisms are usually used in the systems with a large range of roll of aiming line (up to 70°). The advantages of mirrors before prisms are obvious, since mirrors are lighter elements and, therefore, have a smaller moment of inertia, which greatly simplifies their use as a stabilizing element. This article does not pose the task of considering cases of systems with large angles of roll of aiming lines, when the presence of a prism element that changes the direction of the aiming line is due to an extreme need.
Current conditions, in addition to the requirements due to the technical features of the design solution of the sighting systems, put forward the requirements for ensuring the multidimensionality of visual information, detailing the features of the observed objects, depending on the spectral range of observation. For example, the known thermal signatures of different objects of observation, depending on the nature of the materials they are made of, make it possible to identify heated objects invisible through the visual channel using a thermal imaging channel.
Multichannel and multispectral modern sighting systems of armored vehicles are not just mood of the times. This is a consistent step in the development of observation and sighting systems, which makes it possible to increase the survivability of the object by increasing the information content of the image obtained, enabling the operational analysis of the situation and taking correct decisions during combat operations.
Significant experience of working with optical systems of sighting devices intended for the completing of the facilities of AAFV, based on the analysis of the market of existing domestic and foreign analog products, shows that there is an urgent need to create stabilized combined multichannel optical systems with one common entrance pupil. This approach solves a lot of problems: from minimizing the head parts, which allow to optimize the system of two-plane stabilization of the aiming line position, to the possibility of simultaneous operation in different spectral ranges, i. e. the acquisition of a complexed image that increases the probability of finding and recognizing targets in difficult observation conditions.
In this article, it is proposed to consider the scheme of a sight, which includes visual, television, thermal imaging and range-finding channels (Fig. 1).
In the proposed scheme, all sight-observation channels and the receiving channel of the laser rangefinder operate through a single input window, and the head part is standardly solved in the form of a swinging mirror with the system of two-plane stabilization of the aiming line.
The effective distribution of the input radiation stream reflected from the head mirror is provided by the three spectral-splitting elements (Fig. 2):
• spectral splitter 1 reflects the spectrum range from 0.45 to 1.1 µm and transmits the range from 1.5 to 13.0 µm (with reference working spectra (1.5 ч 1.6) µm and (8 ч 13) µm),
• the spectrum transmitted through spectral splitter 1 falls on spectral splitter 2, which reflects the range (1.5 ч 1.6) µm, forming a radiation beam of the receiving channel of the rangefinder, and transmits a range of (8 ч 13) µm, forming a beam of radiation from the thermal imaging channel;
• the spectrum reflected from spectral splitter 1 falls on spectral splitter 3, which reflects the spectral range (0.45 ч 0.65) µm, forming a light beam of the visual channel, and transmits a range of (0,65 ч 1,1) mkm, forming a light beam of the television channel.
In this case, the television channel works more efficiently at greater distances when the short-wave region (0.4–0.65 µm) is cut off and visibility in haze, fog, at twilight and at night improves.
A feature of the proposed scheme solution is the need to use highly effective spectral-splitting coatings, since the 1st spectral-splitting element should have good reflection at a wavelength of 1.5 µm and good transmission in two spectral regions on different sides of the wavelength of 1.54 µm – (0.4 ч 1.1) µm and (8 ч 13) µm. Typically, the emission of a reflected wavelength of 1.54 µm is effectively achieved by suppressing the shorter and longer portions of the spectrum without the transmission normalization in these ranges, i. e. mainly for non-spectroscopic purposes, but for filtering, i. e. elimination of spectral backgrounds.
Practically in the spectral splitters the most effective is the separation of the incident radiation into two bands, e. g., into the visible and IR parts of the spectrum .
However, there are also their own peculiarities. For example, two solutions of spectral splitters are possible: high reflection in the long-wavelength range and high transmission in the short-wavelength range or vice versa – high reflection in the short-wavelength range and high transmission in the long-wave spectral range. The implementation of the first solution involves a number of difficulties, while the implementation of the second one is more preferable . It is the latter that is used in the described scheme.
The second feature of the scheme is the complete absence of moving parts when switching channels. In practice, all channels work simultaneously in the device. If necessary, each channel can be operated separately and switching on / off of the corresponding photodetectors is only needed. The conventional rotary mirrors or other additional movable elements introduced into the path of the channel beams are absent in this scheme, which undoubtedly increases the reliability of the system as a whole.
Let us consider consistently the principle of the design and characteristics of the working channels of the sight.
In the presented system, the protective glass of the head part is made of ZnS material transmitting all the necessary spectral ranges. The glass is installed with a "reverse" slope of 6°, reducing the probability of finding the object from helicopters and UAVs and reducing the glare of optics, i. e. the visibility of the system against the background of the surrounding situation.
The visual channel is made with an 8-fold magnification and a field of view of 8° with the inner intermediate plane of the actual image to enable the installation of either two grids (movable and fixed) with impact scales and signs, or one luminous LCD screen with an electronic diagram of the formation of field of view of any kind. The channel scheme is shown in Fig. 3.
It includes an objective lens made in the form of a two-lens gluing, Pekhan Pk‑0° prism for image rotation, a light filter for spectrum correction, an additional lens, a grid system and an eyepiece. The dimensions specified in the figure indicate a sufficient compactness of the system having a resolving power of 7.5". All elements of the channel, except for the light filter, are made of colorless optical glass in accordance with GOST 3514–94. As color filter material, colored glass was used in accordance with GOST 9411–91.
The calculation of the focal lengths of the lenses of the television and thermal imaging channels was made based on known criteria of visual perception. These include the Johnson criteria  given in Table 1 and the current criteria for visual perception given in Table 2.
Johnson identified 4 levels of visual perception, but did not formally define these concepts in any way. They have been later specified in the work of Lucien Biberman  and are actively used at present as the current criteria for visual perception. Some of them are shown in Table 2.
The value of "N" in Table 1 determines the number of periods of an equivalent target, resolved by the observer, for a given level of vision with a 50% probability. Thus, when they say that a target is detected according to Johnson’s criteria, the default value is the threshold value of the signal, in which the probability of a correct decision on the target is 50%, more specifically: if the target is known, the ratio of the detection and misses of the target is 1:1.
To determine the necessary resolution, which provides another value of the probability of perception, it is necessary to use the appropriate coefficients . The value of the recalculation factor for the number N of the periods of the line target, resolved by the observer at the critical size of the object, depending on the required probability of perception, is given in Table 3.
From the tables it can be seen that, e. g., to detect an object with a probability of 0.95, it is necessary 1 Ч 2 = 2 periods of the line target fit on its critical size (or 4 pixels of the photodetector).
Based on this, taking into account the characteristics of the selected television matrix, the following parameters of the electron-optical channels of the system were determined.
The television channel is built on a 1280 Ч 1024 TV matrix with a pixel size of 5.5 µm.
The focal length of the television lens was estimated from a given range of vision for the target of the "tank" type of 5 km and the detection criterion with a probability of 0.95. With these data on the height of the tank, equal to 2.4 m, about 4 pixels of the photodetector must be fit. In this case, the object will stand out against the background of the noise as a blurred spot with a probability of 0.95. With such initial data, the focal length of the television lens F’lens should be at least 46 mm.
According to Fig. 4, the television channel uses a visual channel lens and a spectral-splitting coating on the mirror face of the semipentaprism BU‑45° from the Pekhan’s prism system to reflect the spectral range (0.45–0.65) m in the visual channel and for transmitting the range (0.65 ч 1.1) µm into the television channel. For the execution of the spectral-splitting surface, a gluing of the mirror face of the BU‑45° prism with the hypotenuse side of the rectangular prism is used.
Spectral-splitting coatings are used in the construction, as exemplified by the extremely effective coatings for hyperspectral equipment developed at the Krasnogorsk Mechanical Plant .
Fig. 5 shows the spectral characteristic of the experimentally approved coating design of the spectral-splitting module for remote Earth sounding from space, which is a 27-layer system of ZnS (n = 2,3) and MgF2 (n = 1,38) layers deposited on a K8 glass plate: 0.536H, 0.662L, 1.000H, 0.797L, 0.795H, 0.946L, 0.905H, 0.838L, 0.8906H, 0.924L, 0.893H, 0.870L, 0.897H, 0.916L, 0.897H, 0.870L, 0.893 H, 0.924L, 0.890H, 0.838L, 0.904H, 0.946L, 0.795H, 0.798L, 1.000H, 0.663L, 0.536H.
The thermal channel is constructed using a 640 Ч 512 MBM array with a pixel size of 12 µm.
The focal length of the thermal imaging lens is estimated taking into account the thermal detection features: if the height of the tank is ~2 pixels of the photodetector, then the object stands out against a noise background as a blurred spot with a probability of 0.5.
This condition is satisfied by a thermal imaging lens with a focal length F’lens of 50 mm and a relative aperture 1:1.02.
Such a lens for a photodetector device with a pixel size of 12 µm is used in the system under consideration and is built according to the classical scheme of a three-lens piece using Ge + ZnSe + Ge materials (Fig. 6).
The rangefinder in the system is solved in the form of two separate channels: the radiating and receiving channels.
The receiving channel of the rangefinder has an angle of view of α ~ 0.5° and a photodetector with a sensitive pad with a size of dpr= 0.35 mm.
The focal length lens F’lens = 40 mm with a high transmittance at a wavelength of ~ 1.54 µm and a high aperture and made in the form of a single lens made of silicon is mounted in the receiving channel of the rangefinder (Fig. 7). The image quality of the lens is characterized by a scattering circle, which must be less than one time a measure of the sensitive area of the photodetector equal to 0.35 mm.
To increase the efficiency of the spectral distribution of streams and optimize the layout solution, the slope of the spectral-splitting plate No. 2 is made at an angle of 22.5°.The radiating device of the rangefinder is made as a separate module (Fig. 8) with a telescopic three-lens optical system and a radiating module БЛМ‑1T Ю‑41.90.169.
Since the radiation from a solid-state laser has a high peak power, the radiating path is removed from the common optical system with spectrographic elements and passes only through a common head mirror and a protective glass.
To visualize the image in optical-electronic channels (television and thermal imaging), a six-lens four-component eyepiece with a focal length of f ‘= 15.67 mm is used, conjugated with the SXGA060 microdisplay of 1280 Ч 1024 cells and a pixel size of 9.3 Ч 9.3 µm having active area dimensions of 11.941 Ч 9.56 mm (diagonal = 15.296 mm).
With a significant weight of optical elements (~ 214 g), the eyepiece has a number of features. The main of them are a significant increase (~ 16 times) and a large field of view (over 50°) with a slight distortion (less than 5%) and an acceptable distance of the exit pupil (~ 30 mm), due to the specific application.
In a joint application with such an eyepiece, the angular pixel size of the microdisplay will be 2.04’, which practically falls in the middle of the range (1 ч 3)’, which is the range of recommended values of the minimum angular dimensions of objects observed through the eyepiece.
The technical parameters of optical and optoelectronic channels of the system provide for the observation with sufficiently high magnification values and are consistent among themselves in the field of view.
The specified technical parameters are determined by the following formulas:
For field of view:
F’lens is the focal length of the lens;
F’eyep is the focal length of the eyepiece;
Ddisp is the diagonal of the microdisplay screen;
DPRD is the diagonal of the sensitive region of the photodetector device;
hPRD(VPRD) is the size of the horizontal (vertical) of the sensitive region of the photodetector device
The parameters of the parameters are summarized in Table 4.
Currently, in various optical and electronic protection systems, security and tracking equipment, monitoring and sighting devices, the systems of a circular survey, solving problems and detection of objects in the circular sector, are widely used.
They are very relevant for armored vehicles.
A system of circular vision or panorama in a device equipped with optoelectronic channels is a sophisticated interconnected complex consisting of an optical system, a system of photodetectors, and a digital signal processing system. Considering the methods of effective space review in the framework of constructive provision of the system’s requirements to the destination, a detailed evaluation of the advantages and disadvantages of the specific method of realizing the task is needed.
The most famous and most commonly used methods of space review are the following :
• the theodolite method, providing mechanical scanning of space by the whole device. This scheme is the simplest one. Its main advantage is an extremely simple optical path, which does not contain any compensators for image rotation, nor scanning mirrors, etc. However, in the presence of optical and electronic channels, there is the problem of dumping information on a stationary base, an overview due to the use of several similar optoelectronic modules.
• the view due to the use of several uniform optical and electronic modules. The advantage of such a system is the absence of mechanical displacements, and the disadvantage is the need for a large number of photodetectors, which greatly increases the price of the system and is completely unacceptable in multichannel instruments;
• the formation of a panoramic image through the use of photoreceiving devices and panoramic optical "all sky" and "fish eye" systems. This method, like the previous one, differs in the absence of movable parts, but as a rule such systems differ in the small size of the input window, which is unacceptable for thermal imaging systems
• the view using an optical mirror hinge. In this case, the photoreceiving devices of the optical and electronic channels are fixed, but the rotation of the image is necessary, which complicates the optical path, but excludes the presence of an information reset system.
It is the latter method of implementing the circular panorama which is applied within the framework of the described design of the multi-channel aiming complex.
The variant with the rotating head to provide a circular view is solved using a multi-prism unit consisting of 6 Dove prisms with a partially modified visual channel (Figure 9).
The visual channel in the above-considered version contained an objective, a Pekhan Pk‑0° prism consisting of a BP‑45° half-pentaprism and a Schmidt prism with a BKR‑45° roof, an optical filter, a corrective lens, a movable and fixed grid and an eyepiece.
The visual channel in the panorama version with a rotating head part must contain an objective lens, a Pekhan P‑0° prism (consisting of a BP‑45° semipentaprism and a Schmidt BR‑45° prism), an optical filter, a moving lens and a fixed lens and an eyepiece. The design parameters of the P‑0° prism are calculated in accordance with Fig. 10 .
For the size of the input face a = D = dlight = 40 mm, the path length of the beam in the prism will be l = 184.8 mm.
Before spectral-splitting plate No. 1, i. e. in front of the entrance pupil common to all channels, in a parallel bundle, a multi-prism unit is arranged in the form of six pairwise glued and differently oriented Dove prisms made of zinc sulphide ZnS (Fig. 11).
The multi-prism unit is designed to transmit the maximum light diameter in the thermal imaging channel, and must be made with the dimensions shown in Fig. For the effective functioning of the system, the clearing of the faces of the Dove prisms and the surfaces of the protective glass of the head part must be carried out in a similar way. The initial position of the head mirror, multi-prism unit and Pekhan P‑0° prism is shown in Fig. 12.
In this position, in all channels, the "top-bottom" and "right-left" images correspond to the actual orientation of the image, whereas in the original version of the system in electronic channels on one of the coordinates it was necessary to mirror them on the microdisplay.
When the head part is turned in the horizontal plane by an angle "α", the image tilts to the same angle "α" in the eyepiece of the visual channel that is why the multi-prism unit must be rotated in the same direction as the head, but by a half smaller angle "α / 2".
When the head turns horizontally at all angles (n · 360 °) at any number of revolutions "n", the multi-prism unit compensates for the slope of the image for all pan channels at the same time.An example of head rotation by 90° (lift aiming) and 180° (backward aiming) is shown in Fig.13.
The dimensions of the Dove prisms and the overall arrangement of the device are designed so that when the multi-prism unit is rotated, the laser rangefinder is not covered by the radiating channel.
Obviously, with such a panoramic sight design, an increase in the periscopicity can be achieved: the head can be extended to a periscopic value of ~ 450 mm without a significant increase in vignetting in the channels. It should be noted that the head part, where all the channels are routed through one swivel mirror element, is more compact in comparison with the head parts where each channel is equipped with scanning elements. This fact also contributes to reducing the vulnerability of the head units of the aiming complexes positioned to the armor and increasing the survivability of the object.
The considered variant for solving the optical system of a multichannel sight with a visual and two optical and electronic channels, a laser rangefinder and a two-plane aiming line stabilization both with and without a pan variant was worked out in such overall dimensions that are coordinated with the overall dimensions of a number of sighting and observation devices installed on the facilities of AAFV. This provides for the possibility of upgrading the facilities by placing such a multifunctional system at practically existing locations of standard instruments or locally, with minimal modifications.