POSSIBILITIES OF BUILDING A SECURITY TELEVISION CAMERA FOR SURVEYING IN COMPLEX LIGHTING CONDITIONS..
Smelkov Vyacheslav Mikhailovich, candidate of technical sciences, associate professor
POSSIBILITIES OF CONSTRUCTING A SECURITY TELEVISION CAMERA FOR OBSERVATION IN COMPLEX LIGHTING CONDITIONS
Source: magazine «Special Equipment» No. 1 2006.
Works [1 – 3] proposed methods for minimizing distortions of a CCD-matrix TV camera when used in conditions of light overload, work [4] provides an estimate of its recovery time after the end of such exposure.
However, the situation of high and uniform illumination in the camera's field of view is a special case of complex illumination of the observed scenes and objects. Examples of complex illumination conditions can be:
- observation through a window or against the background of open doors, when it is necessary to simultaneously distinguish objects on the street and in the room;
- observation against diffuse sunlight;
- observation against the background of glare, streetlights, etc.
In foreign literature, complex lighting conditions are called Back light, which predetermines the presence of a “back light” from the object or work “against the light”. In this case, with the automatic sensitivity control (ASC) operating in the camera for any of the three parameters, namely: the relative aperture of the lens, the accumulation time, the gain coefficient of the video path — there is a limitation of the dynamic range of brightness gradations for dark and/or low-light details of the image transmitted by the camera, since the video signal from them can either be significantly reduced or completely lost.
It should be noted that the idea of combating video signal distortions under conditions of this a priori uncertainty of the plot is known [5] and consists of the following. The dynamic range of the TV camera is divided into several parallel channels. Each of them uses a “light-signal” converter with a light range shift device. In this case, the minimum level of the light range of each channel must correspond to the limitation level of the light range of the previous channel so that the sum of the ranges of all conversion channels is equal to the range of brightness and/or illumination.
The abbreviation BLC stands for the mode of compensation of video signal distortions accompanying the given observation conditions. The organization of automatic BLC by the digital method was proposed by the company Matsushita (trade mark – Panasonic) [6] and is called super dynamic technology. The advertising name accurately conveys the task of the proposed technical solution, which is aimed at expanding the dynamic range of brightness gradations transmitted by the camera of the television image. This technology implements, at the rate of the television standard, two serial-parallel channels of photoelectric conversion with “long” (1/50 s) and “short” (1/2000 s) exposure time on a single CCD matrix.
The basic result of this technology is the production of a new CCD matrix with the organization of «line-frame transfer» (LFT), and in addition to it — a specialized processor for digital processing of video signals. However, the undoubted costs of this technical solution are: the need for a twofold increase in the frequency of line-by-line and element-by-element transfer in the CCD matrix and the formation of a multiplexed video signal at its output.
An analog method of automatic BLC was proposed as an alternative [7, 8]. Double exposure of the photodetector during a frame is provided here by using a three-section CCD matrix with the organization of “frame transfer” (FT) or with the organization of FFT + FT. The advantages of this solution should be considered the absence of operations of multiplexing and demultiplexing of the video signal of the photodetector, as well as the rejection of a digital processor for this reason. Unfortunately, the current state of the Russian economy does not allow us to hope for the rapid implementation of this method.
If the operating conditions of the television camera at the facility allow for its preliminary orientation so that highly illuminated and/or bright objects are perceived in the central part of its field of view, then the BLC mode can be implemented on the basis of two synchronously and in-phase operating camera modules by forming a combined image signal. This image is the result of the synthesis of images generated by each of the modules. In the central “window” of the combined image, the central fragment is transmitted from one module, and around the “window”, i.e. outside it, from the other module. With the same scale of the component images, the scale of the combined image remains unchanged over the entire area of the raster. An example of television surveillance using a combined image is shown in Fig. 1.
Fig. 1. Example of an image formed by a television camera
The necessary modules are currently available on the market from Russian manufacturers. Essentially, they are frameless cameras, developed on the basis of previously developed CCD matrices with the organization of the SCP and have an AFC with photometry for a selected area of the photodetector target.
Below is the technical solution to the problem. The structural diagram of the TV camera is shown in Fig. 2.
Fig. 2. Structural diagram of a television camera
The television camera contains a sequentially located and optically connected lens 1 and a beam splitter 2, the first television signal sensor 3, the second television signal sensor 4 and the switch-mixer 5. The beam splitter 2 with a mutually perpendicular arrangement of the phototargets of the sensors 3 and 4 contains a semitransparent mirror 2-1, (Fig. 3a) the input of which is the input of the beam splitter, and the first and second outputs of the semitransparent mirror are the first and second outputs of the beam splitter, respectively. Beam splitter 2with a mutually parallel arrangement of the photo targets of the sensors 3 and 4 contains a sequentially located and optically connected semi-transparent mirror 2-1 and a reflective mirror 2-2 (Fig. 3b), where the input of the semi-transparent mirror is the input of the beam splitter, and the output of the reflective mirror and the second output of the semi-transparent mirror are the first and second outputs of the beam splitter, respectively. Input optical image along the optical path: lens 1, input of the beam splitter 2, first output of the beam splitter 2 is projected onto the photo target of the first sensor 3. At the same time, this image is projected along another optical path: lens 1, beam splitter input 2, second beam splitter output 2 is projected onto the photo target of the second sensor 4.
a – semitransparent mirror 2-1;
b – translucent mirror 2-1
Fig. 3. Beam splitter device
Photoelectric conversion of the optical image of each of the sensors into the corresponding video signals is carried out using the AFC. The main adjustable parameter of the AFC is the accumulation time of the photodetector on the CCD, and the additional parameter is the gain of the video amplifier. Note that both sensors operate in the frequency and phase synchronization mode of the frame and line scans from the receiver synchronization signal (RSS) of sensor 3. For the AFC of each of the sensors, a preset of different and mutually exclusive photometric areas is provided. For sensor 3, the photometric area is the central area of its phototarget (Fig. 4a), and for sensor 4 — the entire area of its phototarget minus the central one (Fig. 4b).
Fig. 4. Photometric areas of camera sensors
Note that the size and location of the photometric area of sensor 3 determine the placement of the “window” signal in the frame, and therefore the area of the “window” and its position in the combined image.
For the plot observed in our example (Fig. 1) within the selected photometric areas, the TV camera will automatically set different, but optimal, values for the accumulation time of each of the photodetectors and the gain for their video signals. Here, these parameters satisfy the relationships:
Tн1 < Tн2; Ku1 < Ku2, where Tн1, Ku1 and Tн2, Ku2 are the accumulation duration and the gain of the video amplifier, respectively, for sensors 3 and 4.
As a result, television signals from both sensors will be prepared without distortion in the required fragments. The formation of the video signal of the combined image is provided in block 5, where the signals from both sensors are sequentially switched by the control signal “window”. Taking into account that the signal-to-noise ratio changes proportionally to the accumulation time of the photodetector and the gain of the video amplifier, the proposed solution guarantees an increase in the signal-to-noise ratio for dark and/or low-light objects.
Literature
1. Kulikov A.N. Television surveillance in bright sunlight/Special equipment, 2001, No. 1, pp. 11 – 20.
2. Smelkov V.M. Method for minimizing television camera distortions when operating under light overload conditions/Special equipment, No. 5, 2001, pp. 20 – 22.
3. Patent No. 2231233 RF. MKI7 HO4N 5/335, 3/14, 5/202. Image signal formation device/Smelkov V.M. /B.I. – 2004, No. 17.
4. Smelkov V.M. Evaluation of recovery time of a CCD-matrix television camera after exposure to light overload/Special equipment”, No. 1, 2004, pp. 38 – 40.
5. Patent No. 2199827 RF. MKI7 H04N 5/202. Method for expanding the dynamic range of transmitted gradations of brightness and/or illumination in a television system
6. Vilenchik L.S., Goncharenko B.G., Kurkov I.N., Razin A.I., Rozval Ya.B./B.I. – 2003, No. 6.
7. Superdynamic Panasonic/“Security Systems”, No. 19, 1998, pp. 24–25.
8. Smelkov V.M. Analog method for automatic operation of a security television camera for surveillance in difficult lighting conditions. Systems and means of communication, television and radio broadcasting, 2002, No. 1, 2, pp. 35–39.
9. Patent No. 2235443 of the Russian Federation. MKI7 H04N5/335, 3/14, 5/202. Television camera on a matrix of devices with charge coupling/Smelkov V.M./B.I. – 2004, No. 24.