Nowadays the devices of night vision are widely applied not only by army and other structures, but also by hunters, tourists, fishers, security divisions and so on. They give conclusive advantages to their owners during twilight and night time.

They allow to observe various objects, to orientate, to conduct aim shooting, to drive transport, to fly, to perform repair and other works during night or twilight time practically as well as in the afternoon.

With the help of NVD you can see at night because they transform and multiply strengthen the radiation from sources of natural night light exposure, i.e. reflected sunlight from planets, star light and fogs, luminescence of oxygen and ozone in the upper atmosphere.

To see with help of NVD in full darkness, for example, in cellars, caves, tunnels, etc., additional infra-red illuminators are used usually on the basis of light-emitting diodes (LEDs).

In the main the structure of the NVD is shown in figure 1. The NVD consists of lens 1, image tube 2, ocular unit 3, battery 4, body 5.

The basic element of NVD is an image tube. An image tube is an electrovacuum device (fig. 2) which has a photosensitive layer and perceives the faint luminescence reflected from objects. It also has electronic system of amplification and an electroluminescent screen on which the visible image of an object is formed.

Instead of image tubes, CCD matrixes can be used. A CCD matrix is a device (fig. 3) with the charge coupling, made as a monolithic chip which represents a set of the smallest photosensitive sensors, collected in a certain way into a unified matrix. The signals from photosensitive elements are processed by electronic image and transferred to the screen, for example, LC indicator.

Claccification of NVD

NVD are divided into monoculars 1, goggles 2, binoculars 3, sights 4.

Monoculars

Goggles

Binoculars

Sights

Монокуляры Elementary devices. Observation is conducted by one eye. They are widely applicable in tourism, security, hunting, rescue operations, repair work, object observation. They can be fixed on a helmet - mask on the head, thereby having freed the hands for carrying out of various works. Together with the mounted laser target designator on the weapon they can be used for night sighting shooting. With the help of a special bracket some monoculars can be mounted on day sights, thus having transformed them into night ones.

Goggles Observation is conducted by two eyes. At magnification of 1Х they provide stereoscopic effect and real estimation of the surrounding space. They are fixed on the head and leave hands free (with the help of a helmet - mask) for carrying out of various works and operations at night. They are applied in repair and rescue operations, driving of surface and air transport, shooting on the targets illuminated by laser emitters.

Binoculars They are intended for observation by two eyes. They are commonly used for object observation, security. In contrast to goggles of night vision binoculars provide the longer range due to the increased optical magnification, it is usually 2Х-6Х.

Sights They are intended for object observation and sighting shooting at dusk and night. They are usually night monoculars supplied with the mechanism of mounting on the weapon and adjustment mechanism, a special mark within eyeshot of the NVD which has a role of "foresight". At finding the range the mechanism of adjustment provides overlapping of the hit point of the bullet with the target. In connection with heavy impact loads arising at a shot (up to 500 g and higher), there are made increased requirements for the shock resistance of sights. Sights can be structurally made by combination of day and night sights. These are day - night sights. As night sights you can also use usual day sights by mounting of NVD (night heads such as monoculars) on the input or output of the day sight. For performance of night sights function you can use sighting complexes consisting of goggles and laser target designator. The laser radiates in IR range. It is fixed on the weapon and adjusted so that the optical axis of the laser and the hit point of the bullet would be coincided. The observer in goggles will see the point of the laser target designator. The aiming will involve aiming of this point at the observable object. Instead of goggles you can use monoculars which are fixed on a helmet - mask.

The NVD on basis of CCD matrixes.

As a photosensitive element in such devices CCD matrixes are used. The image is formed on the CCD matrix, processed by electronic way and transferred to the screen, it is usually a LC indicator. Actually it is an analog of videocamera. In the night conditions NVD on CCD matrixes can work effectively only with a powerful IR-illuminator.

The NVD with documentation and image transmission through TV-channel

Most simply this problem is solved in the NVD on the basis of CCD-matrixes. The electronic system of image processing allows recording on the media by accessible ways, processing and transmitting the image with a TV-transmitter in the distance depending on the capacity of the TV-transmitter and TV-cannel, satellite communication, etc. In the NVD with image tubes the image from the screen is transferred to CCD matrix by the optical systems, built-in the ocular part, then it is processed electronically and transferred to the liquid crystal or light-emitting diode display. It also can be transferred through a TV-channel and can be observed in the distance by other observers.

Thermal imaging devices

The structure of thermal imaging device is similar to the structure of NVD. They can be used during both day and night. The distinctive feature is that a matrix detector is used as a radiation detector sensitive to average IR-range of radiation of 8-12 microns, i.e. to the intrinsic thermal radiation of bodies not to the reflected one. As detectors of radiation in this range uncooled microbolometric matrixes have found wide application. The image received through lens, shaped on the microbolometric matrix, will be transformed with the help of the electronic circuit to the image which is reproduced on liquid crystal or semi-conductor light-emitting diode screens. The image can also be transferred through the TV-channel or recorded on various carriers/media. The observable picture in thermal imaging device corresponds to the thermal relief of an object. They do not allow identifying small parts of objects because the thermal image changes at the temperature change of the object. Thermal imaging devices detect and distinguish objects in conditions when NVD are not effective. They are effective in a fog and mist and in difficult background conditions. A thermal imaging device shows a thermal picture and owing to the fact that the temperature of alive object is usually higher than of the background temperature, it is easily detected. Thermal imaging devices can be made similar to the NVD as monoculars, binoculars, goggles and sights.

Characteristics of NVD The NVD is characterized by its magnification, angle of vision, removal of exit pupil (eyerelief), diopter adjustment, resolution, detection range and range of recognition.

In table 1 the generalized basic characteristics of NVD irrespective of their purpose are given.

Characteristic	Value range
Magnification, Х	1-7
Lens:
focal length , mm	20-200
relative aperture	1-2,0
Angle of view, degree	6-50
Generation of the image tube	1, 1+, 2, 2+, 3, 4
Resolution, line/mm	25-70
Focusing range, m	0,25 - ∞
Diopter adjustment	±4
Type of IR-illuminator	built-in, hinged
Power supply, V	1,5-9
Time of continuous work, hour	12-40 and more
Weight, kg	0,25-3 and more
Detection range at starry sky, meter	50-500

Detection range (DR) and recognition range (RR)

It is the basic consumer characteristic of the NVD crucial if the NVD is suitable for various problems solving. Detection range of an object is the range at which the observer distinguishes something in the background, not classifying it as a concrete object. Recognition range of an object is the range on which the observable object is classified as a concrete object: a horse, a deer, etc. These parameters depend on the characteristics of the image tube, optics (focal length of the lens and its aperture), atmosphere conditions, mists, humidity, background colour, object colour. Knowing characteristics of the tube, optics it is difficult to speak about advantages of this or that device without their testing and comparing simultaneously in equal conditions. For example, if the object is moving, the detection range can be 1,5 times more than of the motionless object.

Detection range is defined by image tube characteristics and optics. The higher generation of the tube, the more amplification of the tube is and the DR and RR are more. The more focal length to which the NVD magnification is connected, and the more its relative aperture are, the more DR and RR will be. Usually in the NVD of high-quality there used the lens with a relative aperture of not less than 1:1,5 . However in connection with that DR and RR also depend on the atmosphere condition and background, they, as a rule, are given roughly. The range of observation for the NVD with the tubes of various generations is given in table 2.

	Full moon 0,1 lux (meters)	Half moon 0,05 lux (meters)	Quarter moon 0,01 lux (meters)	Starry sky 0,001 lux (meters)	Cloudy sky 0,0001 lux (meters)
Without NVD	230	130	45	-	-
I generation	300	200	150	100	50
II generation	630	630	590	390	145
III generation	810	810	770	530	200
IV generation

Magnification

It shows in how many times the object will be bigger than at observation by naked eye.

The magnification of the device in most cases is defined by three components:

• focal length of the lens, mm;
• magnification factor of the tube (for 2 + and 3-rd generations it is, as a rule, equal 1X);
• focal length of the eyepiece, mm.

The lens aperture is characterized by relative aperture. It is directly connected with the diameter of an entrance pupil of the lens (as a rule, diameter of a front lens) and the lens focal length. The relative aperture is defined by fraction, for example, 1:1,5 where 1,5 is a relation of the focal distance to the diameter of entrance pupil. That is at the same diameter of the front lens at the magnification of the focal length there will be a decrease in size of the relative aperture. The lens will pass less light, and accordingly the characteristics of the device will be lower.

Angle of view

It is measured in degrees. It shows the sector in which objects are observed. For example, for goggles the angle of view is approximately 40? at magnification of 1X. So at magnification of lens focal length in 4 times the angular field of vision will decrease in 4 times.

Focusing range

It shows a distance, a distance limit from an observer to an object, in which it is possible to observe an object precisely. It is carried out by focusing, i.e. moving of the lens or separate components of the lens.

Range of diopter adjustment

It is made by moving of eyepieces before reception of precise observation of the image on the tube screen or other indicator. It allows taking into account various features of sight of the observer.

Resolution

It is the ability of NVD to distinguish two closely located objects and it is measured in lines on millimeter for the tube and in cycles on milliradian as a whole for the device. For the NVD it is the size dependent on characteristics of the tube, optics and the level of night light exposure. The more value specified in lines on millimeter, the more fine details the NVD distinguishes.

Weight of the device

It is also an important characteristic, especially at continuous using of the device. The aspiration to lower weight, for example, of sights, less than 1 kg, not deteriorating other characteristics, is problematic and unreasonably expensive.

The high-quality optics without which it is impossible to realize potential characteristics of the image tube is a multilens system forming the most part of the device weight. In view of that many sights are developed for use at impact loads > 500g, structurally the case, mechanical connections, optics, system of adjustment should provide reliability of work of the device as a whole. It is the optimum complex solution of requirements to all characteristics of NVD that would define their weight. For example, high-quality professional sight F7000 of firm ITT, USA with magnification of 4Х weighs 1,7 kg, and the sight of manufacture of Russia ПН-15К with magnification 4Х weighs 1,5 kg.

Type of the image tube

Image tubes are divided in generations: 1, 1 +, 2, 2 +, 3. This division is connected with design features and the materials used for manufacturing of the photocathode.

With each subsequent generation of image tubes there grows its light sensitivity due to the use of more perfect photocathodes and electronic amplification of the device. There grows resolution and the quality of the image improves. The sensitivity of the photocathode for image tubes of the 1-st generation is 120-250 мА/lumen, and of image tubes of 3-rd generation reaches 900-1800 мА/lumen . The resolution of image tubes of the 2-nd generation in the center of the screen is 35-55 lines/mm, and of image tubes, for example, of the 3-rd generation is up to 70 lines/mm; it is evenly distributed on the screen of the tube. The resolution of image tubes of the 1-st generation is 25-35 lines/mm in the center of the tube screen, however from the center to the edges it goes down to less than 20 lines/mm. The amplification of the image tubes of the 1-st generation is 120-900 times. But image tubes of 2 + and 3-rd generations have it as 25000-50000 times.

Sometimes on the tube screen it is possible to see defects as black dots. It is technologically difficult to make image tubes without these defects. In most cases presence of such dots does not influence on the image quality. It is important if you use NVD for photographing and other similar cases.