High-sensitivity TV cameras.
High-sensitivity TV cameras
One of the problems of the security television market, according to a group of respondents to the survey, namely video system installers, is the lack of awareness of consumers about the capabilities of CCTV. This leads either to unjustified costs for the client or to a decrease in the efficiency of security systems. The article examines the features of highly sensitive television cameras that must be taken into account when selecting the optimal composition of equipment for the operation of a surveillance system in low-light conditions.
Quantum efficiency, sensitivity and illumination
The task of a TV camera is to convert the light reflected from an object and hitting the CCD matrix into an electrical signal, which can then be transmitted over a distance, displayed on a monitor, recorded on magnetic tape, i.e. one that can then be worked with. Therefore, one of the most important characteristics of a TV camera is the parameter describing the camera's ability to convert photons falling on the CCD matrix into electrical current.
In physics, there is a concept called quantum efficiency of a CCD matrix. Quantum efficiency is the ratio of the number of registered charges to the number of photons hitting the crystal. In terms of quantum efficiency, charge-coupled devices (CCDs) have no equal. For comparison: out of every 100 photons hitting the pupil of the eye, only one is perceived by the retina (quantum yield is 1%), in CCDs this parameter can take values from 4% (low-quality CCDs, usually used in inexpensive video cameras) to 50% (a typical video camera of «white» assembly) and higher — up to 95%.
In the description of cameras for security television systems, the parameter sensitivity is used instead of the parameter quantum efficiency. These parameters are linearly dependent. If quantum efficiency relates the number of incident and registered photons, then sensitivity determines the response of the light receiver in electrical units of measurement (for example, in milliamperes) to a certain value of the incident light flux (measured, for example, in watts or in lx/s). In principle, this is the same value, only expressed in units of measurement more convenient for the engineer.
As a rule, consumers of security television systems understand sensitivity as the minimum illumination on the object (scene illumination), at which it is possible to distinguish the transition from black to white, and much less often — the minimum illumination on the matrix (image illumination).
From a theoretical point of view, it would be more correct to indicate the minimum illumination on the matrix, since in this case there is no need to specify the characteristics of the lens used, the distance to the object, its reflectivity, etc. It is very difficult to operate objectively with a definition of sensitivity based on the illumination level of the object.
However, when selecting a camera, it is more convenient for the user to work with the illumination of the object, which he knows in advance and which can be measured, so this parameter is often used. The formula linking the illumination on the object and on the matrix is given below:
Iimage=IsceneR/(3.14 F)
where Iimage, Iscene are the illumination of the CCD matrix of the object; R is the reflection coefficient of the object; F is the aperture ratio of the lens.
The values of Iimage and Iscene differ by approximately an order of magnitude.
However, the differences in the definition of sensitivity do not end there. Sensitivity can also be bolometric (summary over the entire spectrum) and monochromatic (measured in a certain spectral range).
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Table 1. Outdoor illumination levels, lx. |
The unit of measurement of illumination «lux» is defined for monochromatic radiation with a wavelength of 550 nm. Therefore, special attention should be paid to the spectral dependence of sensitivity. In most cases, the sensitivity of black and white cameras compared to the human eye is significantly extended into the infrared range up to 1100 nm. Some modifications have sensitivity in the near infrared region even higher than in the visible. These cameras are designed to work with infrared spotlights and are close in some parameters to night vision devices. |
What is an «acceptable signal»
In order to avoid misinterpretations and get a real picture of the sensitivity ratio of different camera models, it is necessary to obtain information about the conditions under which the value recorded in the passport was obtained. Some standard measurement conditions adhered to by equipment manufacturers help with this.
In some cases, the sensitivity measured for the so-called «acceptable signal» is indicated — the signal-to-noise ratio equal to 24 dB (the signal-to-noise ratio is calculated as S/N = 20log (/).
Since the main load on security television systems falls during the dark hours of the day, when the signal is extremely weak, the signal-to-noise ratio (S/N = signal/noise) becomes a particularly important parameter.
With a signal-to-noise ratio of 50 dB or more, the monitor will have an almost perfectly clear picture without visible interference; at 40 dB, flickering dots are sometimes noticeable; at 30 dB — «light snow» across the entire screen; 20 dB — the image is almost unacceptable, although large contrasting objects can still be seen through a solid «snow» veil.
A signal-to-noise ratio of 24 dB is an empirically found limiting value of noise, which still allows us to hope to see something when playing a video recording.
In addition, it is generally assumed that when measuring, the reflectivity of the object is 0.75; the lens aperture is 1.4; automatic gain control and gamma correction are disabled. As the illumination decreases, the signal becomes smaller, and the noise, especially due to the action of AGC and gamma correction, increases accordingly.
IRE scale
Another attempt to clarify the definition of illumination, which is used, in particular, by Sony and Philips, is to use the so-called «IRE scale» (the abbreviation comes from the name of the Institute of Radio Engineers). The full video signal (0.7 V) is taken as 100 IRE units. A signal of about 30 IRE is considered acceptable. Some manufacturers specify sensitivity for 25 IRE, others — for 50 IRE (signal level 6 dB).
Ways to increase sensitivity
We can conditionally distinguish several main approaches used to improve the sensitivity of television cameras.
1. Increasing the charge accumulation time.
There are two problems here. The first is caused by the growth of the so-called «dark current of the matrix», which can be successfully combated by some technological methods, for example, by using a cooling system. The second concerns non-stationary objects of observation: an increase in the exposure time leads to the appearance of discreteness of movement.
2. Expansion of the range of the recorded light signal.
For example, it is possible to specially cover the matrix with thin films of substances that absorb ultraviolet photons and re-emit in the visible range.
3. Use of electron-optical converters (EOC).
Cameras that use such technologies have unique properties. On the one hand, their sensitivity is 100-10,000 times higher than that of conventional cameras, on the other hand, they are extremely capricious. The mean time between failures is about one year. Moreover, these cameras should not be turned on during the day, it is even recommended to cover their lens during the day to protect the cathode of the image intensifier from burning out. During operation, the camera must be regularly slightly rotated to avoid «burning» the image on the cathode of the image intensifier.
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It is interesting to note that, unlike a conventional CCD matrix, the cathodes of the image intensifier tubes are very sensitive to maximum illumination. If the light-sensitive area of the CCD matrix returns to its original state relatively easily after bright illumination (it is practically not afraid of overexposure), then the cathode of the image intensifier tube recovers very slowly after bright illumination (sometimes 3-6 hours). During this recovery, even with the input window closed, a residual, «burned» image is read from the cathode of the image intensifier tube. |
Sony
Sony is a recognized leader in the production of CCD matrices for security systems, including matrices used in high-sensitivity models. Many manufacturers use Sony matrices in their cameras.
Sony has developed the HuperHAD technology, which allows to increase the parameters, including the sensitivity of mass-produced CCD matrices. Hyper-HAD matrices have a microlens on each light-sensitive cell, which collects and focuses additional light flux.
Today, Sony is promoting its new development — ExWaveHAD CCD matrices. Compared to Huper-HAD, cameras based on the new matrix have better sensitivity.
Panasonic
To increase sensitivity, Panasonic uses two methods. Firstly, microlenses are installed on the matrix, similar to Sony's Hyper-HAD. Secondly, a well-developed digital video signal processing system allows for an electronic increase in sensitivity (up to 32 times in some models).
Cohu
Just a few years ago, the Cohu trademark was firmly associated with highly sensitive, but very expensive television cameras. There are practically none of them in Russia, but foreign magazines write a lot about them.
Cohu produces cameras not only for security systems, but also for scientific, medical and other purposes.
Cohu's successes are impressive (for example, there is a model that operates at 0.00015 lux, albeit with a 30% video signal). But this is achieved by using an image intensifier, accumulation systems, cooling, etc., which leads to the emergence of new disadvantages, as well as a significant increase in price.
Philips
For comparison (see Table 2), one of the latest models of Philips LTC 0500 TV cameras was taken. It has very high parameters, including sensitivity. For a signal of 50 IRE and a signal-to-noise ratio of 40 dB, the sensitivity on the matrix is 0.005 lux.
Table 2. Characteristics of a number of models of highly sensitive cameras
The camera is based on a 1/2-inch CCD matrix with interline transfer. A fairly powerful digital signal processing system is used. In addition, the camera implements very useful image correction functions, such as: backlight compensation, automatic contouring, automatic black level, brightness control, etc.
Philips also produces cameras with image intensifier tubes designed exclusively for night surveillance. As already noted, cameras with image intensifier tubes are extremely sensitive and must be protected from bright light.
Watec
In 1999, the Japanese company Watec released a new modification of the 902 series television camera and called it WAT902H (the addition of «H» in the name is most likely an abbreviation for «high»).
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The main advantages of this product are uniquely high sensitivity and resolution at a relatively low price. Just a few years ago, only scientific journals wrote about mass-produced cameras with such parameters, when they described individual samples with unique parameters and at fantastic prices or cameras with electron-optical converters. |
The WAT-902H resolution is 570 TV lines, the sensitivity on the object is 0.0003 lux F1.4 (AGC Hi). The company keeps secret how it managed to achieve such characteristics. It is only said that Watec, together with Sony, developed and implemented a fundamentally new technology: the camera does not use a photoelectric amplifier or other similar devices. Externally, the camera differs little from other models of the 902 series, it is as miniature as all Watec products.
EVS
EVS is a Russian company successfully operating in the television surveillance systems market.
EVS cameras use foreign components and original Russian technical solutions. For example, a special «night mode» microcircuit is used in highly sensitive camera models. In low light conditions, the microcircuit switches the CCD matrix and video amplifier to a mode that sums the charges from neighboring elements. As a result, sensitivity improves by an order of magnitude (up to 0.0008 lux on an object when using an aspherical lens). However, this reduces image clarity (resolution deteriorates by about 3 times).
EVS also announced the release of a camera with a different version of the «night mode», in which sensitivity is increased by increasing the exposure time.
The following Internet sources were used in preparing this article: soling, ru; http://philipscss. corn; http://bpgprod.sel. sony corn; panasonic. corn/cctv; evs.metrocom.ru; cohu/cctv