TV cameras for security purposes.

TV cameras for use in security purposes.

TV cameras for use in security purposes

The Scientific and Research Center «Security» of the VNIIPO of the Ministry of Internal Affairs of Russia is currently developing GOST R «Video monitoring systems. General technical requirements and testing methods.» This document will create a regulatory framework for conducting certification tests of this type of equipment. According to the editors, the recommendations for choosing closed-circuit television systems proposed in the article have undoubted practical value.

The «eyes» of any television video surveillance system (TSV) are its television camera (TC). The choice of TC determines what the operator will see on the video monitor screen (and whether he will see anything at all). If you want to observe not only the general situation in the room, but also recognize a familiar or unfamiliar person, read a car number, etc., then cheapness should not be the main criterion when choosing a camera.

Although television systems have recently ceased to be a rarity (rather, they have become a necessity when forming a security system for a facility), in most cases the Customer is not prepared enough to understand the variety of TCs presented on the Russian market and choose the cameras that will be most optimal for performing the task.

This article attempts to describe in the most complete manner the technical characteristics of the cameras necessary for choosing a TC, as well as the principles of their selection and application.

Television camera (TC) — a device that converts the light flux reflected from an object into electrical signals using the physical and chemical properties of photosensitive materials. The purpose of the camera is to ensure fast, reliable video information.

The camera contains the following main components (Fig. 1):
— an image sensor (currently, the image sensor in the vast majority of TCs is a CCD matrix, so below we will only talk about such sensors):
— synchronization device:
— video amplifier;
— control device (AGC circuit),
and in some cameras — a built-in power supply.

An equipped television camera is a device containing the television camera itself, the corresponding lens and the necessary auxiliary equipment (brackets, special housings, rotary devices, power supplies, etc.).

Complete video signal (LAN)— consists of an image signal (video), a blanking signal and a sync component (synchronization signal). The amplitude of the full video signal at the camera output (not cheap cameras of unknown origin) should be 1 Vpp at a matched load of 75 0 m.

Image signal — part of the full video signal containing information about the image.

Synchronization signal— a signal sent to the cameras that causes them to generate images synchronously. All equipment in a television system must switch from frame to frame simultaneously to avoid image interruptions. This is especially important when there are a large number of television cameras in the system. Synchronization can be either internal (from a quartz generator built into the camera) or external. In turn, a camera with external synchronization can be synchronized in several ways:
— from the power supply:
— from a special device — a synchronization generator;
— from another, selected leading, television camera.

Pixel— a resolution element (a light-sensitive element) that is the smallest detail of an image. A CCD matrix consists of a certain number of pixels vertically and horizontally, which must be specified by the camera manufacturer in the passport. The number of pixels (usually from 270,000 to 440,000) determines such an important characteristic of the CCD as resolution. The more pixels are placed on the CCD matrix, the higher the clarity and quality of the image obtained from the camera.

The resolution of the CCD determines the maximum number of transitions from black to white and back that can be obtained from the camera in the central area of ​​3/4 of the screen, and characterizes how close two point objects of observation can come to each other so that their images do not merge. Measured in television lines (TVL).

As mentioned above, the resolution of the TC depends on the number of pixels in the CCD matrix vertically and horizontally and is in no way more than 3/4 of their number. In fact, the vertical resolution of the camera is not specified, but is determined by the output video signal standard. For example, for a CCIR camera, the vertical resolution is 625 TVL, and for an EIA camera — 525 TVL.

When determining the resolution, the signal-to-noise ratio, modulation coefficient and illumination at which this value was obtained must be specified.

TC sensitivity— the minimum illumination on the image sensor required for the camera to produce a certain amplitude (usually 1 V) of a full (color) video signal with a certain signal-to-noise ratio. Television camera manufacturers may measure and indicate its sensitivity in different ways:
— as the minimum illumination on the TC lens (in this case, the lens aperture must be indicated);
— determine the sensitivity for -6 dB video signal (i.e. with this sensitivity at the TC output you will receive only half the amplitude of the video signal);
— under other conditions, usually known only to manufacturers.

Image sensor size — the size of the light-sensitive surface of the sensor. Its value is expressed in inches. Currently, most cameras presented on the Russian market use image sensors with the following dimensions:
1 inch 12.8 x 9.6 mm (16 mm diagonal);
2/3 inch 8.6 x 6.6 mm (11 mm diagonal);
1/2 inch 6.4 x 4.8 mm (8 mm diagonal);
1/3 inch 4.8 x 3.6 mm (6 mm diagonal).

1/4″ image sensors are typically used for ultra-miniature cameras, such as those used for covert surveillance.

Black Level— the level of the electrical signal in the overall video signal that represents optical black. A television camera may use a special automatic black-linking device that identifies the darkest part of the image as optical black, increasing the contrast of the image in some cases.

When using a camera in conditions of greatly varying illumination, special devices and circuits are provided in cameras to maintain the output video signal within certain limits:

Electronic shutter— a device built into a camera's CCD that changes the camera's sensitivity by controlling the accumulation time of the electronic charge. Typically designated as 1/50, 1/100000, etc.

Electronic Aperture — an automatic electronic shutter that changes the camera's sensitivity to changing lighting conditions so as to maintain the video output within certain limits.

Gamma — correction— non-linear signal processing that corrects the gray scale of an image. Typically, this coefficient has a value from 0.45 to 0.7. Using this correction improves the visual perception of the image, as if emphasizing its contour.

White Limiter — a circuit inside the camera that limits the maximum white voltage in the output video signal to a certain level.

Back Light Compensation (BLC) — the ability of a camera to automatically set shutter speed, aperture, and gain parameters based on a certain fragment of the image (usually the center).

Imagine a situation: you need to monitor the entrance door, for example, to a bank. It's a bright sunny day outside, a visitor opens the entrance door, and instead of the expected image of the visitor, the operator sees only his dark silhouette. In order to see not a dark silhouette, but a clear image of the visitor, it is necessary to have a backlight compensation circuit in the camera.

Sound channel— provides acoustic monitoring of the monitored room using a microphone built into the camera. To organize a bidirectional audio channel, in addition to the microphone, a speaker is built into the camera.

Lens mount unit — if the camera does not have a built-in lens, its design includes a connection unit for installing interchangeable lenses. Two types of standard connection unit designs are used:

type «C» — thread 2.54×0.8 and distance to the reference plane of the CCD matrix 17.5 mm;

type «CS» — thread 2.54×0.8 and distance to the reference plane of the matrix 12.5 mm.

Cameras with a C-mount can only work with C-type lenses (if you try to attach a CS-type lens to a C-camera, the image will be blurred, since the lens will focus the image not on the CCD matrix, but before it), while cameras with a CS-mount allow connection of any type of lens (in this case, a C-type lens must be used with a mount adapter).

Camera mount unit— is designed to install the camera in a casing, on a bracket, etc. Most of the produced TCs have a 1/4″ -20 UNC or 3/8″ -16 UNC mounting unit.

Power supply voltage — Most TV cameras are powered either from an AC 220 V/50 Hz or 24 V/50 Hz network, or from 12 V DC sources. Less often, a DC voltage of 9 V is used.

Dead zone — the unviewable part of the video surveillance zone under the TV camera (Fig. 2).

The size of the dead zone can be determined by the formula: L=L1+h-tg(b-av/2), where L1 is the distance from the wall to the CCTV lens, m:

h is the height of the CCTV installation, m;
V is the vertical viewing angle of the CCTV lens:
I is the angle between the vertical axis and the CCTV axis (CCTV tilt angle).

Television cameras can be classified according to the following criteria:

1. By the type of video image produced:

— with a black-and-white image of the output video signal, the following standards can be used:

a) CCIR;

б) E1A;

— with a color image of the output video signal, the following standards are used:

a) PAL;

б) SECAM;

c) NTSC.

2. By sensitivity:

Class I — television cameras operating in daylight, i.e. before sunset (approximately 50 lux);

Class II — television cameras operating in low light, i.e. before twilight (approximately 4 lux);

Class III — television cameras operating in moonlight, i.e. at an illumination level corresponding to the light of a quarter of the moon on a cloudless night (0.1-0.4 lux);

Class IV — television cameras operating in starlight, i.e. at an illumination level corresponding to the light of the stars on a cloudless night (0.0007-0.002 lux);

Class V — infrared TCs, i.e. television cameras that use infrared sources to operate in the complete absence of visible light.

Examples of typical illumination levels:
— well-lit motorway at night: 10 lux
— stairs or corridor: 60 lux
— office or shop: 250-500 lux.

Not all television cameras can operate with infrared light. The characteristics of color cameras correspond to the characteristics of human vision, and most black and white cameras use part of the red and infrared region. Therefore, when choosing a television camera for a specific object, it is necessary to take into account the type of light source and the spectral characteristics of the television camera image sensor.

3. By resolution:

a) normal resolution (up to 380 TVL);

6) high resolution (from 380 to 500 TVL);

c) high resolution (over 500 TVL).

4. By type of protection from environmental conditions:

Class I — indoor — living space or office (heated);

Class II — indoor general (warehouse, etc.);

Class III — outdoor, protected from direct rain and sunlight, or indoor with extreme conditions;

Class IV — outdoor general.

To ensure that the television system does not become just an expensive toy, before purchasing it, you need to decide on a number of issues in advance: the composition of the system, its tasks, location, and the number of TCs. You can try to do this yourself (if your qualifications allow) or invite specialists to select the necessary equipment.

The first step is to determine the zones that the consumer wants to protect with a television video surveillance system, i.e. video surveillance zones. As a rule, these zones are the most likely places for a criminal to enter (doors, windows, etc.) and places where material assets are directly concentrated (warehouse, cash register, sales area, management offices, etc.). The number of video surveillance zones depends only on the consumer's «appetite» or the funds he would like to spend on a video surveillance system. There can be any number of video surveillance zones: from one or two (for example, the entrance door and sales area) to several dozen or hundreds, including most of the premises of the facility. In this way, the approximate number of cameras in the system is determined and, consequently, other necessary equipment for equipping the TC, signal processing, displaying video information, etc.

After determining the number of cameras, the consumer must decide what kind of television cameras (black and white or color) he needs. Color television cameras provide more informative images than black and white ones, but they have worse sensitivity, resolution (although there are high-resolution color television cameras, but they are significantly more expensive than regular ones) and are more expensive. In addition, the choice of a color camera will entail the corresponding choice of all other equipment for processing, recording and displaying video information, which will significantly increase the cost of the entire system as a whole. Color cameras are installed at sites where it is necessary to know the color of the object of control (for example, the color of the clothes or hair of a criminal, the color of a car, etc.)

When operating a television camera indoors, the consumer only needs to think about how the installed cameras will fit into the interior of his premises. It is necessary to determine where the camera should be installed hidden, and where in special decorative casings. In any case, it is necessary to strive to ensure that the installed television cameras do not distract employees from their work or museum visitors from exhibits.

It's a different matter if the camera is to be installed outdoors. And the outdoors in Russian conditions means large temperature fluctuations from -40 to +50 °C, (high humidity, large fluctuations in illumination (Fig. 3). The possibility of physical destruction of the camera should not be discounted. Large fluctuations in illumination lead to unacceptable changes in the level of the camera's output video signal (for example, with insufficient or excessive illumination, we will not see anything on the monitor screen). To work in a wide range of illumination, it is necessary to choose a camera with video signal stabilization devices, such as an electronic shutter. If the range of the electronic shutter is insufficient to compensate for excessive illumination, lenses with an automatic diaphragm are used. However, one should not forget about the compatibility of the camera and lens in terms of the method of controlling the automatic diaphragm. There are two methods of controlling the automatic diaphragm:
— video signal control: all automatic control is located in the lens, to which only the video signal is fed from the camera:
— direct control: a control method in which the video signal is processed in the camera, and a constant voltage is supplied to the lens, which controls the auto iris motors.

To compensate for harsh outdoor climatic conditions beyond the TC's operational limits, special climatic casings are used. Suitable conditions for the camera inside the casing are created by sealing it, using special heaters and fans.

To protect the video camera from vandalism, special casings made of high-strength alloys with armored glass are used. Such casings can also contain the necessary thermal devices.

When solving general surveillance tasks, it is sufficient to use a standard resolution TC, and when it is necessary to identify a control object or to observe it at a great distance, it is recommended to choose a high-resolution TC.

The required sensitivity of the television camera is established during the inspection of the object, and the consumer must determine the illumination at the object not only during the daytime (working) time, but also at night and, if necessary, decide on the presence of emergency lighting or infrared illumination.

The algorithm for determining sensitivity is as follows:

a) using a lux meter (or in some other way), measure the illumination in the control zone of the protected object;

b) determine the value of the reflection coefficient of the real object of control OK according to Table 1:

c) the transmission coefficient is determined by the specified in the description aperture ratio of the lens selected for this TC (according to Table 2);

d) the minimum illumination on the image sensor (Esensor) is calculated, which can be obtained in the camera control zone according to the formula:

Esensor = Escene*R*K

where Esensor is the illumination on the image sensor:
Escene — illumination in the TC control zone;
R — reflection coefficient of the controlled object;
K — transmission coefficient.

The obtained result (illumination of the image sensor) should be higher than the sensitivity of the TC. If this is not the case, the consumer either decides to have emergency lighting in the video control zone, or chooses a more sensitive (i.e. more expensive) camera.

If there are no special requirements (Customer, State Fire Supervision Authority, etc.) for the use of low-voltage equipment at the facility or other restrictions, it is recommended to use TCs with a supply voltage of 220 V. Such TCs are conveniently synchronized via the power supply network (there is no need for additional equipment such as synchronization generators or an additional cable). When powered from a DC source, it is necessary to ensure that the supply voltage specified in the passport (no more and no less) is supplied to the camera from the source. Since with a different voltage the camera can and will only work for a certain time. It is also necessary to ensure that the power source provides the required TC power (especially when powering several cameras from one source). It is also necessary to resolve the issue of the possibility of the system (or part of the system) operating when the supply voltage is disconnected, that is, to organize a backup and uninterruptible power supply.

Rotary devices are widely used in TSV for use both outdoors and indoors. But when placing the TC on a rotary device, it is necessary to think about the compliance of the scanning speed of the rotary device with the speed of movement of the controlled object (it may turn out that the rotary device will not have time to track the movement of the controlled object). In most cases, it is more advantageous (and cheaper) to install two or three television cameras with a fixed position and viewing angle than one located on a rotary device.

These include:

The standard of the output video signal of the television camera. In this case, the main condition is that all equipment selected for the TSV is of the same standard.

The optical format of the television camera.

The presence of gamma correction.

The consumer can leave these and many other questions to the discretion of the design company or supplier of TSV components.

Let's consider what types of television cameras are presented on the Russian market. Most of the TCs produced by the industry have a standard design and are supplied without a lens, power source and other auxiliary equipment, which the consumer must select separately in accordance with his needs. However, recently more and more equipped TCs have been produced, such as:
— miniature frameless and framed television cameras with a built-in lens (possibly of the «eye of a needle» type), intended for covert surveillance, as well as for intercom systems and video peepholes;
— office television cameras with a built-in lens, bracket and original decorative casing, and, if necessary, an audio channel: microphone (one-way audio channel) or microphone and speaker (two-way audio channel). Such cameras match the style of the room, are easy to install, carry and are intended mainly for monitoring the general situation in the room;
— sealed television cameras. Consist of the television camera itself, lens and durable sealed casing, inflated with dry nitrogen, with a built-in heater;
— high-speed PTZ cameras of the AutoDome type with presets. The TC includes the television camera itself with a lens equipped with a zoom, a high-speed PTZ device and a decorative casing with a bracket. The ability to remember a large number (up to 99) of positions, including all TC and lens settings, and then reproduce any of them with high accuracy (up to 0.5 degrees) in less than 1 second makes such a TC equivalent to a system of 99 cameras with different lenses directed in different directions;
— cameras with built-in radio frequency video signal transmitters or a transmitter via a telephone line.