Night vision devices for detecting glare elements..
VOLKOV Viktor Genrikhovich, Candidate of Technical Sciences, Associate Professor
NIGHT VISION DEVICES FOR DETECTING GLARE ELEMENTS
It is known that when optical retroreflective elements are illuminated by radiation, part of the illumination energy is reflected from them and returns towards the illumination source, creating a light response – a glare.
Nowadays, we often encounter situations where optical or optoelectronic surveillance and aiming devices are hidden. However, they can be detected by glare from the probing radiation reflected toward the observer from the optical surface of the aiming mark or scale of a daytime surveillance or aiming device, or from the photocathode of the electron-optical converter of a night vision device or the CCD matrix of a television (TV) system [1].
This effect is most clearly observed when illuminating people or animals at night with a flashlight. The backlight radiation is reflected from the retina of the eye and creates bright luminous spots that are clearly visible at night. In principle, any night vision device (NVD) in combination with an illuminator can be used to detect glare under these conditions. However, the greatest effect should be expected from active-pulse night vision devices (APNVD) [2, 5]. They allow you to detect an object by glare both in normal and low-transparency atmospheres, in a wide range of illumination changes (up to daytime conditions) and when exposed to light interference. If necessary, APNVDs allow you to measure the distance to the objects of observation with high accuracy.
When visually observing a glare in an APNV, the threshold range of its detection depends on the threshold contrast sensitivity of the eye and the angular size of the glare.
Currently, the most complete and reliable data on the values of the threshold contrast and angular dimensions as a function of background brightness were obtained in laboratory conditions by Blackwell [3]. According to the data of [4], the threshold contrasts increase by 30-50 times compared to Blackwell's data when observing in a device based on an image intensifier tube. For the operation of the laser illuminator AI NVD in a single-pulse mode, the threshold contrast value depends on the duration of a single illumination pulse. Taking into account the real curve of the increase and decrease of the image intensifier tube screen [5], we adopt the effective duration of the radiation pulse affecting the eye equal to 1 ms. Then, in accordance with the data of [5], it is necessary to increase the threshold contrast according to the Blackwell curves by 20 times. Taking this into account, it is possible to carry out calculations for monopulse illumination using the method described below.
The energy flux Ф (W) of reflected radiation entering the lens of the receiving part of the AI NVD is determined by the formula:
Ф = Iбле p dвх2 tа tоб tф (4 D2)-1, (1)
where Iбле is the power of reflected radiation, W/Ср;
dвх – diameter of the entrance pupil of the lens, m;
ta, tоб, tф – transmission of the atmosphere, lens, filter respectively;
D – distance to the glare object, m.
Let's use the concept of the retroreflection index R (m2/Ср):
R = Iбле Евх. зр.-1, where Евх. зр. – irradiance on the glare element of the object, W/m2.
Accordingly:
Iбле = RE = RPtа (wD2)-1, (2)
where Р – average radiant power of the backlight, W;
w – solid angle of the backlight, Ср.
Taking into account equation (2), expression (1) takes the form:
Ф = pRP dвх2 tа tоб tф (4wD4)-1
Illumination Eфк (W/m2) in the plane of the EOP photocathode:
Eфк = ФSбле-1 = pR dвх2 tа tоб tф (4wD4 Sбле)-1, (3)
where Sбле is the area of the glare image.
The brightness Lбле W/m2 Ср) of the glare image (spot) on the EOP screen is determined by the formula:
Lбле = Eфк Sl h (SS p Гэ2)-1, (4)
where Sl is the spectral sensitivity of the EOP photocathode, A/W;
h is the EOP conversion coefficient;
SS – integrated sensitivity of the EOP photocathode, A/lm;
Ge – electron-optical magnification of the EOP, times.
After substituting equation (3) into formula (4) and replacing t = tatobtph, we obtain:
Lbl = RPdvkh2 t Sl h (4wD4 Sbl SS Ge2)-1 (5)
The brightness of the background Lф of the EOP screen, on which a luminous glare is observed, is made up of the brightness Lфе of the screen glow created by natural illumination and the dark background of the EOP (Lт):
Lф = Lфе + Lт
Lфе = (Q-1 + Kз-1)rеl DlО2 t hSl (4 p SS Ge2)-1, (6)
where
Q is the duty cycle of the image intensifier tube shutter;
Kz is the locking coefficient of the image intensifier tube shutter;
r is the background reflectance;
el is the spectral radiation of the night or day sky, W/m2 μm;
Dl is the filter passband, μm;
O is the relative aperture of the lens;
t is the transmission of the entire optical-electronic path.
Given the threshold contrast Kth at a certain adaptation brightness Lph, we determine the required brightness of the glare:
Lbl = Kth Lph
From formula (5), taking into account equation (6), we find:
dвх2 = 4 Кпор Lф w D4 Sбле SS Гэ2 (R P t h Sl). (7)
The required luminous intensity of the illuminator:
I = P w-1. (8)
From formula (7), taking into account expression (8), we have:
I = 4 Кпор Lф w D4 Sбле SS Гэ2 (R dвх2 t h Sl)-1
From formula (8), the threshold range of glare detection is determined by the equation:
D = (P R dвх2 t Sl (4 Кпор Lф w Sбле SS Гэ2)-1)0.5. (9)
The angular dimensions of the retroreflective elements of the observed objects may be within the range g = 2» + 40». The diameter of the image of the reflective element dbl is determined by the formula:
dbl = (dфк2 + dоб2 + dэ2)0.5, (10)
where dфк is the diameter of the image of the reflective element on the photocathode of the image intensifier tube, mm, dоб, dэ are the diameters of the scattering circles of the lens and the image intensifier tube, mm.
Usually dоб = (1-5)х10-2 mm, then:
dфк = fоб’ + tg(g), (11)
where fоб’ – focal length of the lens, mm.
dэ = Nэ-1, (12)
where Nэ is the resolution of the image intensifier, lines/mm.
Taking into account formulas (10) – (12), moving from millimeters to meters, we obtain Sбле = 0.25 p dбле2.
Knowing that tg(g)= dбле Гок/250, where Гок is the magnification of the ocular part, in times, we find, according to Blackwell’s data [3] (Fig. 1), the value of the threshold contrast corresponding to the value of g for a given Lф. Increasing the obtained value of the threshold contrast by 20 times, we obtain Кпор, used in formula (9).
Fig. 1. Dependence of the threshold contrast on the background brightness and the angular size of the object
If a TV channel is used instead of an ocular image output, we have:
t = tatobtftop
where ttop is the transmission of the transfer optics.
dbl = (dfk2 + dob2 + dэ2 + dоп2 + dтв2)0.5,
where dоп is the diameter of the image of the glare element created by the optics for transferring the image from the image intensifier tube screen to the light-sensitive element of the TV camera, mm;
dtv is the diameter of the image of the glare element created by the TV channel, mm.
dоп = (Nоп)-1, dтв = (Nтв)-1,
where Nоп, Nтв – resolving power of transfer optics and TV channel respectively, lines/mm.
tg(g) = dбле (lопт)-1,
where lопт is the optimal distance from the pupil of the eye to the TV monitor screen, mm (lопт = 5 – 6 diagonals of the TV monitor screen).
The value of Lф is set based on the technical characteristics of the TV monitor.
The experiments showed that a glare detection range of up to 5000 m can be achieved with normal atmospheric transparency (ta і 0.8) and in the rain (ta = 0.69). In light fog, this range decreases to 3000 m [1]. For a more objective assessment of the possibility of detecting an object by glare when it is located at different angles relative to the optical axis of the AI NVD, it is necessary to know the retroreflection functions of the glare elements, i.e. the distribution of R as a function of the angle between the normal to the object's surface and the optical axis of the AI NVD.
Let us now consider a number of NVDs for detecting objects of observation by glare.
One of them is a small-sized laser location equipment for remote reconnaissance of optical and optoelectronic means Antisniper-1” [6]. The results of its experimental studies are shown in Photo 1. The range of the device for R = 5 m2/Cp is 500 m. The laser illuminator generates a radiation power of 0.7 W at a wavelength of 0.8 µm. The radiation receiver is a TV camera based on a CCD matrix with a sensitivity of up to 10-3 lux. The field of view of the device is 5×70 [6]. Further development of these devices were the devices “Antisniper-M” [7] and Antisniper-M2” [8]. In the “Antisniper-M” device, the laser illuminator generates a radiation power of 1 W at the same wavelength. The range of the device for R = 5 m2/Cp is 800 m. The “Antisniper-M2” device has even higher characteristics. To increase the sensitivity of the receiving channel, the latest CCD matrix of the SONY company is used, manufactured using the ExviewHAD technology with a digital image processing unit. In this case, the increase in sensitivity is achieved due to the spatial and temporal integration of the signals of the storage elements of the matrix. This provides a gain in the signal/noise ratio at night by 100 times. The laser illuminator uses a semiconductor emitter with a power of 2 W, operating in a pulse mode and controlled by a built-in microcomputer [8]. The block diagram of this device is shown in Fig. 2. The maximum detection range of reflectors with R = 1 m2/Cp (in the range of working illuminations of 10-3 – 7×104 lux) is not less than 2000 m. The laser illuminator operates at a frequency of 50 Hz and has an illumination angle of 2×30. A zoom lens with variable magnification and auto diaphragm is used in the receiving channel. The field of view angle of this channel along the horizon changes from 30 to 180. The minimum working illumination of the area at night is 10-4 lux. The weight of the device is 1.4 kg, dimensions are 230x120x90 mm.
Photo 1. Results of experimental studies of the “Antisniper-1” device [6]
Fig. 2. Block diagram of the “Antisniper-M2” device [8]
The optical-electronic device “ANTISVID” is designed for remote detection by glare of working or disabled hidden video surveillance systems, disguised in interior details, premises, clothing, personal belongings, etc. [9]. The range of the device is 0 – 15 m, the minimum diameter of the lens of the hidden video surveillance system is 1 mm, the working range of natural background illumination reaches 1000 lux. Viewing angles are 3600 horizontally and ±600 vertically, power consumption is 5 W. The device operates in the near IR region of the spectrum.
The ANTISVID device has a similar purpose [10]. Its appearance is shown in photo 2, and the results of experimental studies are shown in photo 3. The range of the device is up to 10 m with a probability of reliable detection of 0.99, detection accuracy of 1 cm, continuous operation time of at least 8 hours, power is supplied from a DC voltage of 12 V with an energy consumption of no more than 10 W, dimensions are 230x140x80 mm, weight is no more than 2 kg. A further development of this device is a device with the same name (photo 4) [11]. The device provides a detection distance of hidden video surveillance systems with a lens pupil diameter of 1 mm from 1 to 15 m with a reliable detection probability of 0.99 and a detection accuracy of 1 cm. Continuous operation time of at least 6 hours, power is provided by a battery = 6 V or an external power source = 12 V, weight 1.6 kg, dimensions 275x120x75 mm.
Photo 2. Device “ANTISVID” [10]
Photo 3. Visualization of the location of portable
video surveillance systems camouflaged in interior objects [10]
Photo 4. The “ANTISVID” device [11]
The ANTISNIPER” [12] remote detection device for optical and optoelectronic devices, sights, and long-focus lenses (photo 5) has a detection range of sights up to 1000 m at R = 0.5 – 20 m2/Avg. The estimated detection accuracy at a distance of 50 m is 10 cm, and at a distance of 1000 m – 2 cm. The weight of the device does not exceed 1.5 kg, and the dimensions are 273x110x110 mm.
Photo 5. The ANTISNIPER” [12] device
The Almaz [13] detector (photo 6) is designed to detect hidden micro-video cameras of all types in the on and off state – hidden inside the packaging, in the walls and ceilings. Inside the electromagnetic screen. The weight of the device is 0.2 kg, the dimensions are 50x50x100 mm, the continuous operation time is up to 30 hours, the output power of the illumination laser is less than 10 mW.
Photo 6. Almaz [13] device
The LAR-1 laser reconnaissance device [14] (photo 7) is designed to detect optical and optoelectronic devices and systems during manual scanning of the terrain. The range of the device is 50 – 1000 m (Fig. 3), the scanning speed in azimuth is up to 1 deg/s, the instantaneous field of view is 6×30, the probability of detecting a typical device at a distance of 1 km is over 90%, the weight is 1.8 kg.
Photo 7. LAR-1 device [14]
The “LUCH” optical system indicator [15] (photo 8) is designed to detect optical devices and night vision devices, as well as to measure the range to the detected object. The receiver of laser radiation reflected from the object in the glare detection unit is a photodetector. The detection unit is mounted on the observation device. The fact of detection is indicated by sound. The message about the range to the detected object is given by a synthesized voice from the built-in sound source or through the headphone. The effective detection range of the PSO-1 optical sniper sight is 800 m with a maximum detection range of 2000 m. The maximum speed of space scanning is 90?/s. The accuracy of measuring the range to the detected optical object is ±10 m. The continuous operation time from an autonomous power source is up to 18 hours. The mass of the glare detection unit is 0.9 kg. [15].
Photo 8. The “LUCH” device [15]
The active-pulse device for detecting optical and optoelectronic systems “Mif-300 [16] (photo 9) has a glare detection range of 1 to 300 m, a field of view of 60, power consumption from the built-in power source of 4 W, and a weight of 1.5 kg.
Photo 9. The “Mif-300” device [16]
A device of the same type “Myth-350” [17] has a range of 0.5 – 400 m, a field of view angle in passive mode of 60, in active-pulse mode of 50, the depth of the viewed space in this mode of 15 m, supply voltage = 9 – 36 V or ~100 – 240 V with a frequency of 50 – 60 Hz (via a network adapter – up to 1200 m) with a current consumption of 0.6 A and 0.07 A, respectively, a continuous operating time of 3 hours, a weight of 1.1 kg and dimensions of 175x100x75 mm.
The active-pulse television device “MIRAGE-1200” [18] (photo 10) has a range of 0 – 1785 m with a viewing depth of 15, 30, 60, 600 m. The minimum viewing distance is 3 m. The field of view in passive mode is 4.5×3.30, in active-pulse mode – 4×30. The supply voltage is =9 – 36 V or ~10 – 240 V with a frequency of 50-60 Hz (via a network adapter) with a power consumption of 13 and 16 W, respectively, and a continuous operation time of 1.5 and 3 hours. The weight of the device is no more than 2.3 kg, dimensions are 325x140x80 mm.
Photo 10. Device “Mirage-1200” [18]
The SET-1 company (RF) [19] has developed an optical search device (photo 11). It is designed to detect and identify hidden pinhole lenses for video cameras and other optical embedded devices. The device, based on a household video camera, contains a pulsed white light source and a laser illuminator with a wavelength of 0.61 µm. The detection range is 0.5 – 7 m, the continuous operating time from the built-in battery is over 1.5 hours, weight is 3 kg, dimensions (in a carrying bag) are 250x150x140 mm.
Photo 11. Optical search device from SET-1 [19]
The PAPV glare detection device [20, 21] (photo 12) has a range of 300–1500 m with a laser illumination frequency of 6 Hz. The device operates from an autonomous power source and weighs 56 kg. With the help of powerful laser radiation, it damages the aiming scales and photosensitive elements of the detected devices. The device can be used for anti-terrorist operations to combat snipers. According to the developer, the device is not intended to have a non-selective effect on human vision that is not equipped with optical devices. The PAPV device is similar to the Stingray device developed for the US Army [20]. In France, the CILAS company has developed the same device based on a low-power laser for detecting snipers. This device was used in Sarajevo in the mid-nineties [21].
Photo 12. PAPV device [20, 21].
In the USA, a glare detection device AN/PLQ-5 (AN/PLQ-4) was developed, mounted on the M16 assault rifle [22]. The device provides detection of ground and air targets with measurement of the range to them.
If the retroreflective elements form a certain configuration, characteristic only of a given object of observation, then it is possible not only to detect it by glare, but also to recognize it. Fig. 4 shows a diagram of the AI NVD, which provides a solution to such a problem. In the AI NVD TV system, implemented on the basis of a CCD matrix, an image is created in which all the brightness points corresponding to the glare elements are present. The coordinates of these points in the image frame are recorded in the operational memory devices of the microcomputer built into the device. After this, the belonging of these points to individual sets is determined. It can be considered that the sets of points with coordinates x1i, x1i . . ., xni, y1i, y2i, . . ., yni, belong to the i-th set if xi+1, i – xj,i Ј k, yj+1, I — yj, i Ј l, 0 Ј j Ј n, i.e. the coordinates xj,I of adjacent points differ by no more than k elements, and the coordinates yj,I of these points differ by no more than l lines. In the ideal case, k = 1, l = 1. However, to eliminate the influence of impulse noise, a reserve of several line elements can be given. Then, the coordinates of the points within the sets are averaged:
Fig. 4. Block diagram of the AI NVD with automated recognition of the observation object by glare: 1 – observation unit, 2 – lens, 3 – pulse image intensifier, 4 – image transfer optics, 5 – transmitting TV camera, 6 – video amplifier, 7 – TV monitor, 8 – synchronization generator, 9 – pulse laser illuminator, 10 – radiation shaping lens, 11 – pulse laser semiconductor emitter, 12 – pumping unit, 13 – control and synchronization unit, 14 – pulse master generator, 15 – adjustable delay unit, 16 strobe pulse generator, 17 – microcomputer, 18 display; T, C, K – respectively, outputs of clock, line and frame synchronization of the synchronization generator 8
The average values of the coordinates of the sets are taken as the coordinates of the vertices of geometric figures. Then, the obtained data are systematized in order of increasing xi and the corresponding values yj : x1, x2 . . ., xi, . . . , xn. After this, the lengths of the sides of the geometric figure are calculated:
li = ((xi+1 – xi)2 + (yj+1 yj)2)0,5
To ensure greater reliability of recognition, it is necessary to bring the observed image to a single scale. For this purpose, a signal input into microcomputer 17 from the output of adjustable delay unit 15 is used. The signal value is proportional to the distance to the observed object. Having data on the distance (by the delay value), we number the lengths of the lines of the geometric figure:
li =D-1 ((xi+1 – xi)2 + (yj+1 – yj)2)0.5,
where D is the signal carrying information about the distance;
j is the serial number of the vertices of the geometric figure.
The normalized values of the lengths of the sides of a geometric figure are determined in a similar manner. Next, based on the number of its vertices, after a preliminary assessment of the coordinate values, the microcomputer selects from its memory a reference image that is closest in configuration to the observed one. After this, the ratios of the lengths of the sides of the geometric figures of the observed and reference images are determined:
gi = (dj D)-1 ((xi+1 – xi)2 + (yj+1 – yj)2)0.5,
where dj is the length of the corresponding side of the reference geometric figure.
The case of similarity of figures gi = 1 corresponds to recognition with a probability of 100%.
However, the inaccuracy of measurement and calculation of the coordinate almost always has an effect. Therefore, it is advisable to use the mean square criterion in the form:
where m is the number of sides of the geometric figure (equal to the number of vertices);
e is the specified threshold.
According to the given criterion, if the standard deviation ? of the lengths of the sides of the observed image does not exceed a certain specified threshold ?, then it is considered that the system has recognized the image of the object of observation. In this case, in accordance with the configuration of the geometric figure in the microcomputer, a transition is made to the corresponding program, which displays text and (or) graphic information about the recognized object on the display.
If s > e, the reference object does not correspond to the observed one, the microcomputer switches to another subroutine, which extracts the parameters of another image with the closest configuration from the microcomputer memory and compares it with the observed image according to the above criterion. The specified algorithm is repeated as many times as the microcomputer memory contains reference images with a given number of coordinates. If, after going through all the reference images, the object is not recognized, the microcomputer will display information about this. After the recognition process is complete, a new process of recording the coordinates of the glare points on the image in the microcomputer's RAM begins, and after the frame is finished – information is read and recognition is performed.
Thus, this AI NVG allows to recognize, when operating in the active-pulse mode, the object of observation at the distance at which its glare is detected. Calculations show that this range is at least twice as great as the range of object recognition by its contour when the AI NVG operates in the same mode. This solves the problem of automating the recognition process. This is of undoubted interest for the development of advanced devices.
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