Lighting in video surveillance systems.
You have all had to be present at advertising battles, when one of the companies starts to declare that its video cameras are 10 or 1000 times more sensitive than others. Of course, it always quickly becomes clear that the laws of physics have not been repealed and if the camera works without liquid helium cooling, then its sensitivity is objectively limited by thermal noise and for a format, for example, 1/2 inch, it cannot be better than 0.1 lux (for details, I recommend looking at the classic article by N. Uvarov, it has been repeated on the Internet about 1500 times). Nevertheless, some cameras are really a little better, a little closer to the theoretical limit, some are a little worse. Does it really matter? Sometimes they sacrifice the frame rate, for example, averaging over 10 frames, the resulting frame rate is reduced to 5 frames per second, this allows you to increase the sensitivity by another factor of 10. Sometimes they sacrifice the resolution, averaging the signal over neighboring pixels, this allows you to increase the signal several times more. In total, the most sensitive cameras and the least sensitive ones differ by about 20 times.
Is it a lot or a little? Let's look at the daylight statistics. During the day, sensitivity does not affect image quality. At sunset, illumination drops by an order of magnitude in a few minutes, so a more sensitive camera can work for a few minutes longer. And it is for the sake of a few minutes that advertisers break spears, and techies invent complex methods that worsen temporal or spatial resolution?
Not really. Everything is done to simplify the lighting system. Even if the video surveillance system clearly does not include a lighting subsystem, there are some bulbs nearby. For example, in urban conditions, street lighting provides several lux, which is more than enough for any video camera, and for a good enough one, the fraction of lux that remains from street lighting in a dark yard will be enough. So, the question is whether you will have to install a special lighting system or what the city services installed is enough.
In some cases, you can't count on them. For example, in a military unit in the remote taiga, you will inevitably have to install your own lighting system. That's when you will fully experience what sensitivity is. To provide 3 lux, you will have to install 50-100 W bulbs every 50 m, like on city streets. And if 1 lux is enough for you, the power can be reduced proportionally, to 20-30 W. And if you firmly believe (it is better if you have experimentally verified) that your video camera can work at 0.1 lux, it is enough to install 10 W bulbs every 100 m. Do not take my data as dogma — these are approximate figures for gas-discharge lamps in a high-quality lamp with a reflector that forms a beam. Incandescent lamps, of course, will require almost an order of magnitude more power. An unsuccessful choice of lamps or a height and angle that do not correspond to the lamps can also lead to serious losses.Let's discuss why lighting power is so important. Of course, in your particular system it may not be essential: big deal, an extra 200 watts turned on at night is less than a thousand rubles per year. However, in most systems the problem is not the cost of electricity. First of all, you must calculate an uninterruptible power supply system sufficient for the entire system, including the lighting system. It is of little use if the video cameras work, but work in complete darkness — the result is as sad as if they were not there at all. And here it turns out that the main consumption is often formed by the lighting system. A video camera consumes units of watts (well, 10-20 watts, if it is a heated outdoor one) and provides control of 100 m of the perimeter. And lighting these 100 m may require 1-2 kilowatts, if the camera is not very sensitive, and the lamps are not very economical. It seems that a standard 2 kilowatt UPS is not expensive at all. However, the standard battery in this UPS will last only 10-15 minutes. Is that enough for you? Or do you really need several hours? Then you will have to install additional batteries, which are much more expensive than the main UPS unit, and not every cheap UPS will allow you to increase the batteries. And with a good video camera and a carefully calculated lighting system, 20 W could be enough, which the same UPS-2000 in the basic configuration could produce for almost a day.
For the mentioned military unit in the taiga the situation is even worse. With a perimeter length of more than 10 km, if you spend several kilowatts per kilometer, you will not be able to provide low-voltage (I mean 220 volts) power supply to distant sections of the perimeter. Do you think that 2 kilowatts is 10 amperes, and to transmit 10 amperes, only 1 sq. mm is enough? Alas, the losses are such that even a cable with a cross-section of 50 sq. mm over a length of 10 km has a resistance of 20 ohms, i.e. all 220 volts will drop on this cable and your bulbs in the distant section of the perimeter will have nothing left. You will have to design a number of transformer substations with a voltage of 6 kilovolts between them, no matter how unpleasant it is. By the way, I do not know of any commercially available uninterruptible power supplies with an output of 6 kV, so providing uninterruptible power supply for such a system will not be easy at all.
What does a properly designed lighting system mean? First of all, it means that it is well matched in spectral and energy characteristics with the video camera. Invisible infrared lighting uses only the edge of the sensitivity range of the video camera. For any video camera, a similar effect from white lighting will require 10 times less power. Sometimes hidden lighting makes sense, for example, if you do not want to attract the attention of intruders. This is if we are talking about protecting a private apartment, especially if the video camera itself is installed secretly, so as not to advertise the financial capabilities of the apartment owner. The second case when IR illumination makes sense is if you do not want bright white light to bother people. For example, if your neighbors complain about floodlights turning on due to false alarms. Or if the backlight of a video camera for recognizing license plates when entering a garage shines directly into the eyes of drivers. In this case, switching to IR illumination will solve the problem. In all other cases, it is more important to fully utilize the spectral characteristics of the video camera. By the way, since most cameras have a red-shifted sensitivity range, it is better to use warm sodium lamps rather than cold mercury lamps. However, if you are designing a color system, excessively yellow-shifted lighting can greatly distort color rendition.
Now let's look at the issue of illumination distribution. The simplest solution is to place a spotlight next to each video camera. Unfortunately, this is not a very good solution, because the illumination will be inversely proportional to the square of the distance from the spotlight, and therefore, if the camera needs to monitor an area from 10 to 100 m away, the illumination difference will be 100 times. However, conventional CCD video cameras can handle a maximum of 10-fold illumination difference across the frame. It is necessary to either use spotlights with a narrow beam and direct them in such a way as to specifically reduce the illumination nearby, or use street lighting technology and use lamps located relatively close together and mounted on high masts. Then the illumination will be more uniform and the image quality will improve.
In addition, in fog, frontal illumination is like driving with high beams. The brighter the headlights, the worse it is to see.
Another disadvantage of placing a spotlight next to a camera is the “flat faces” effect known from photography from a flash located on a camera. It is not without reason that professionals use flashes placed to the side or at least directed to the side – this produces a much more natural image. Naturalness is not so important for security purposes, but the essence of the problem – less information content of the image with frontal lighting – manifests itself in lower contrast and, accordingly, worse image legibility. Sometimes, especially if we are not talking about monitoring a narrow strip along the perimeter, but about monitoring a relatively wide adjacent area, it can be convenient to place the lamps on the side, for example, on the wall of a building, so that they emit light across the axis of vision of the video camera.
I will note that careful calculation of the illumination field using special programs sometimes allows you to reduce the calculated power by 2 times. Such programs are quite simple and exist, including in free versions. Many leading lamp manufacturers provide them on their websites. As an example, I will cite the programs Calculux, DiaLux or the domestic “Formula of Light”.
When calculating, you should be aware that even the most honest manufacturers indicate the «maximum illumination» at which something can be seen as the sensitivity of a video camera. Often this means that it is difficult to distinguish white objects on black with 100% contrast. Real objects have a contrast of 20% at best (this is if the enemy is not trying very hard to camouflage himself, but just put on blue jeans and walks on gray asphalt). At best, the sensitivity is given for a signal-to-noise ratio of 20 dB. For those who don't know: such a signal-to-noise corresponds to a very old, worn-out videotape with Voldarsky's translation. Something can be seen, but in reality much more is needed.
Therefore, if you have not checked the actual sensitivity of the specific video cameras used together with the specific lamps used, you should allow a reserve.
In addition, it is necessary to remember that all calculations were carried out in ideal conditions. Over time, the light output of any lamps decreases, the transparency of glass (including video camera lenses) and the mirror properties of reflectors deteriorate. So after choosing lamps, they should be replaced with others of double power.
In conclusion, take into account that the video surveillance system itself will not protect anyone. And if a security group arrives at the site, they will not be robots, but people. And, as a rule, not equipped with night vision devices, which means that they need illumination of at least 5 lux. In addition to helping the alarm group, good illumination itself is a deterrent for many criminals. I remember the story of one friend who, instead of a security alarm on his car, hung a spotlight on the balcony aimed at his car, and had no problems with theft of wheels or batteries even in the wildest 90s. In order to save energy, it may make sense to design separately the lighting system “for people” and “on duty”, since it is enough to keep only the on duty system for video cameras on uninterruptible power supply, and people in extreme cases will be able to use lanterns and headlights.