Increasing the range of working illumination of the video camera.
Arsenyev Mikhail Yuryevich
MODE “COMBI”: INCREASING THE RANGE OF WORKING ILLUMINATION OF THE TV CAMERA.
The magazine “Special Equipment” has already touched upon the issue of television surveillance at high illumination levels. Issue No. 1 for 2001 contains an article by Mr. A. N. Kulikov “Television surveillance in bright sunlight”, which describes in detail the difficulties encountered in such conditions and proposes a number of measures to overcome them.
Therefore, we do not consider it necessary to repeat here the arguments regarding the importance and complexity of the task of obtaining high-quality video images in the entire possible range of illumination.
We propose for consideration the “COMBI” modea significant increase in the maximum illumination value at the observation site for standard CCTV cameras and lenses, based on the simultaneous use of an automatic lens diaphragm and an electronic camera shutter.
It is known that the maximum illumination value on the object for a typical TV camera equipped with a lens without a variable aperture, with a fixed relative aperture of F 1.2, with the electronic shutter mode turned on, with a minimum charge accumulation time of 1/100,000 second, is about 13,000 lux.
The maximum charge accumulation time is 1/50 second.
Using a lens with an automatic aperture in normal mode involves disabling the electronic shutter mode, that is, fixing the charge accumulation time in the CCD matrix at a maximum value of 1/50 second.
In this case, for a relatively inexpensive lens with a relative aperture variable from 1.2 to 128, we have a range of illumination adjustment of 11,000 times. The maximum illumination on the object is about 200,000 lux.
The “COMBI” mode involves turning on the electronic shutter mode when the minimum value of the relative aperture is reached (or the simultaneous operation of the auto diaphragm and shutter).
Then the value of the maximum illumination on the object will be 13,000 lux x 11,000 times = 143,000,000 lux.
This ensures observation of the brightest objects, such as the solar disk. The illumination control range is about 1,400,000,000 times for inexpensive cameras with a sensitivity of 0.1 lux and about 48,000,000,000 times for TV cameras with EXWAVEHAD CCD matrices (minimum illumination of about 0.003 lux).
Photo 1.
For illustration purposes, photo 1 shows images of the filament of the ZK220-500-2 lamp (500 W, 7900 candlepower, equivalent illumination over 107 lux), obtained using the AVC-371A TV camera (400 TVL, 0.1 lux) and a 13FA08T auto iris lens (f=8 mm, F1.2-128).
On the left is the image obtained in the normal auto iris mode with the electronic shutter disabled, on the right is the image obtained in the “COMBI” mode.
As can be seen from the figure, in the first case the image is completely lost, while in the second it remains quite acceptable.
It should be noted that the specified camera and lens are in a fairly low price category.
Are there many more expensive cameras and lenses that can produce at least some kind of image in such lighting conditions in standard mode without using additional filters?
The question of whether it is necessary to turn off the electronic shutter mode at medium and low lighting conditions remains open.
The experiments conducted have not yet revealed any deterioration in the operation of the camera-lens system with both the electronic shutter and the auto diaphragm operating simultaneously across the entire illumination range (provided that the illumination changes smoothly), but they cannot be completely ruled out.
To completely insure against this, you need to turn on the electronic shutter mode only when the lens aperture reaches its minimum value (in our example, F128) and with continued increase in illumination.
The auto diaphragm is then turned off (fixed at the minimum value).
As the illumination decreases, the accumulation time increases and reaches a maximum value of 1/50 second. As the illumination decreases further, the electronic shutter mode is switched off and the auto diaphragm is switched on.
The main criterion for such switching is naturally the video signal level.
Despite its simplicity, the “COMBI” mode practically eliminates the problem of obtaining a high-quality image at the highest illumination levels, since it is difficult to imagine an object of observation with illumination exceeding 100 million lux.
Of course, the maximum contrast value that the camera can detect remains the same. That is, simultaneous observation of objects with illumination levels that differ by a thousand times without loss of video information is not possible.
Implementation of the “COMBI” mode does not require significant additional costs for reworking the equipment, since in the vast majority of cases, modern television cameras equipped with an auto iris control output also have an electronic shutter mechanism, and it is turned on by a corresponding switch or jumper (bridge) on the camera's printed circuit board.
In addition, it becomes possible to use inexpensive lenses with a relative aperture that varies from 1.2 to 64 or even up to 32.
This will reduce the maximum illumination on the object by 4-16 times, but even in this case it will be several million lux, which is usually quite sufficient and is orders of magnitude higher than similar data for traditional systems based on the most expensive television cameras and lenses.