With the increasing demand for night vision surveillance in the security video surveillance system project, infrared cameras have entered the mainstream market of cameras, and the sales volume is increasing day by day. Problems in the application of infrared night vision technology are gradually exposed.
In fact, night vision technology is an optoelectronic technology that enables nighttime observation by means of optoelectronic imaging devices. It includes low-light night vision and infrared night vision: low-light night vision technology is a kind of night vision technology with image enhancement tube, which actually enhances the weak target image illuminated by night light for observation. Photoelectric imaging technology. Here mainly talk about active infrared night vision technology, which is a kind of night vision technology that realizes observation by actively illuminating and utilizing the infrared light of the target reflection infrared source. The corresponding equipment is active infrared night vision apparatus, and the actual core is the infrared camera.
The combination of the infrared light source, the camera composed of the non-direct-view photoelectric imaging device CCD or CMOS image sensor and the lens required for the imaging picture are the three main factors that dominate the infrared camera night monitoring effect.
First, the choice of infrared light source
Infrared light is an invisible light having a wavelength greater than 780 nm. Generally, there are three methods for producing such invisible light.
1. Directly use infrared light from incandescent or xenon lamps. That is, the visible light filter is installed on the two lamps to filter out the visible light, and only the invisible infrared rays are emitted;
2. Use infrared light emitting diode LEDs or LED arrays to generate infrared light. Such devices generate infrared light by recombining electrons and holes in a gallium arsenide (GaAs) semiconductor;
3. Use infrared laser diode LD, also can be used as infrared light source. However, it is necessary to excite or pump electrons in a lower energy state to a higher energy state, and maintain the stimulated radiation infrared light by reversing a large amount of particle distribution and resonating.
The first type of light source is a thermal infrared light source, and its greatest advantage is that it can be made into a relatively large power and a large irradiation angle, so that the irradiation distance is long. The biggest shortcoming is that it contains visible light components, that is, there is red storm, and the service life is short. If you work 10 hours a day, 5000 hours can only be used for more than one year. If you consider insufficient heat dissipation, the life expectancy is short. In order to improve the life of the heat radiation infrared lamp, a light control switch circuit must also be used in order to reduce the working time. In addition, a delay switch circuit has been added to prevent ambient light interference.
The second is an infrared lamp composed of a semiconductor gallium arsenide LED array, especially an array-type integrated light-emitting chip LEDArray that is now developed using a new technology. Its LED-Array has an optical output of 800mw-1000mw, which has become a replacement for ordinary LEDs. The LED-Array has a half-power angle of 10-120° (variable angle).
Since the LED-Array is a highly integrated LED and the size is only one penny coin, it can evenly illuminate the entire space indoors, with a life of 50,000 hours. It was originally used in aviation aircraft. In recent years, due to the development of the civilian night monitoring market, LED-Array has gradually moved to the civilian market, becoming an ideal choice for high-quality night monitoring.
Although infrared LEDs and LED-Arrays have eliminated thermal infrared light sources, infrared LD light sources have to be selected for monitoring ultra-long-range scenes of more than 1 km. Because semiconductor lasers have higher brightness and better directivity than LEDs.
In general, pay attention to the following points when selecting an infrared source.
A, infrared light distance selection should leave a margin
When selecting an infrared lamp, pay special attention to the difference between the nominal distance and the actual irradiation distance. Because some manufacturers have a nominally large visible distance, actual testing is required when selecting the IR lamp irradiation distance, and sufficient margin is required to make the illumination of the observed scene reliable.
B. The source of the steady current source should be selected.
Because the LED is a current-driven device, its luminance and power are proportional to the drive current, not the voltage. Therefore, in order to keep the luminous intensity constant, it is necessary to use a light source driven by a steady current source.
C, to choose a good heat source
Because the light-emitting tube has the problem of heat generation and heat dissipation, especially the light source with a large power distance is large, so it is necessary to select a light source with good heat dissipation to ensure stable and reliable operation.
D. To use an infrared light source with a power supply circuit of the light control switch automatic control circuit and separated from the light board
Because the power supply circuit of the infrared lamp is not separated from the lamp plate, the heat generated by the operation of the light-emitting tube affects the performance of the component of the power supply circuit, thereby causing the illumination of the light-emitting tube to be unstable. With the circuit of the light-controlled switch, the infrared light can be made. The lights are off during the day and open at night.
E, to choose infrared light according to the monitoring distance
Because the infrared LED can be mounted on the lens, it can be used with the infrared camera or directly on the casing around the camera and lens interface. Obviously, these two assemblies have fewer infrared LEDs, and the distance they are illuminated is certainly not as far as that of a separate infrared LED lamp. Generally, more than 50m, it is better to use a separate infrared light, and the other LD is used for night vision illumination of a monitoring scene with a distance of more than 1km, but the beam of the LD is thin and strong, so that the infrared beam illuminates a certain range of scenes, use It is also necessary to expand the beam through a beam expander lens.
Second, the choice of camera
At present, the camera's photosensitive device has two kinds of CMOS and CCD. Like the CCD, the CMOS image sensor chip also responds to infrared light waves, but it is much more sensitive than the CCD image sensor chip in the 890-980 nm range, and the gradient of attenuation is slower as the wavelength increases. With the rapid development of CMOS image chips, the noise signal has been further reduced, and star-level CMOS cameras have also been available. Therefore, it is not necessarily limited to the selection of CCD cameras, and can be fully utilized to select their respective advantages. In general, the selection points of the infrared camera are as follows.
A, should choose low illumination camera, its illumination requirements are generally ≤ 0.02Lux
Some camera manufacturers or vendors falsely report the minimum illumination, so that the night vision effective distance is greatly reduced, so it is best to test specifically.
Luminous and star-level sensitization cameras can work in very dark conditions, but some areas with small reflection coefficients are still not available, such as deserts, green areas, and forest areas. In this case, a low-light-level night vision camera directly coupled by a fiber panel and a light cone by a high-performance image intensifier and a CCIR black-and-white CCD is required. In short, the lower the illumination, the better. Because it is not standardized at present, it is impossible to believe that the minimum illumination of the factory is nominal. It is better to test under practical conditions. Generally, the illumination requirement is generally ≤ 0.02 Lux.
B. The size of the selected infrared camera image sensor is as large as possible.
Because 1/4 CCD can not be used for the effective distance of infrared night vision above 15m, because the luminous flux of 1/4 CCD is only 50% of 1/3 CCD. The size of the CCD is large, and the received luminous flux is large; the CCD size is small, and the received luminous flux is small. Therefore, the night vision camera selects 1/2 of the CCD, and 1/3 of the CCD can be tested, but it is absolutely impossible to select 1/4.
C, infrared camera should have automatic electronic shutter and automatic gain control (AGC) function
Because of these functions, the signal can be adjusted to a better state to meet the observation effect.
D, need to cover the occasion to choose CMOS ultra-miniature camera
Due to its low power consumption, high integration, and small size, only CMOS image sensors can be used, so it can be made into a shirt button and a suit button-sized CMOS camera. Coupled with the miniaturization of the corresponding infrared source and the introduction of high-performance batteries, the third eye will be everywhere. In this way, with a pair of night vision goggles and a hat with an infrared source and a CMOS ultra-miniature camera, the night will be as white. Obviously, this will change the face of our entire social life.
E, matching the camera with a small regulated power supply
The selection of the power supply of the camera should pay attention to the following two points.
1. Select the regulated power supply. Because the AC220V of the mains is unstable, if only a transformer with an alternating current of 12V is selected and not stabilized, the circuit in the camera will be unstable, which will make the camera unstable and the required performance will not meet the requirements.
2. It is better to choose a small power supply that is more than double the current of the camera. If the current of the camera is 200mA or 250mA, it is usually enough to select 500mA. If the selection is too large, such as 1A for a camera, the volume is too large and the price is too expensive; if it is too small, the current of the camera may be hot (because the continuous working time is too long) and affect the reliability and working life.
Third, the choice of lens, etc.
The camera lens is the key equipment of the infrared camera. Its quality (indicator) directly affects the imaging effect of the system. Therefore, whether the lens selection is appropriate is related to the system quality and the engineering cost. Therefore, pay attention to the following points when choosing a lens:
A, the best choice of infrared lens
Due to the ordinary optical lens, the infrared light reflected back to the lens by the object cannot be effectively focused on the CCD target surface, and the infrared night vision effect is greatly reduced, so it is preferable to use the infrared lens. Especially for color-turning black cameras, it is impossible to make day-night surveillance focal planes consistent without using an infrared lens, so that day and night images cannot be kept clear.
B. The imaging size of the selected lens is preferably the same as the size of the image sensor in the camera.
The size of the selected lens should be the same as the size of the image sensor in the camera. If the size of the image sensor is 1/2", the imaging size of the lens must be 1/2".
When the imaging size of the lens is larger than the size of the camera's photosensitive surface, the imaging will not be affected, but the actual imaging angle of view is smaller than the nominal field of view of the lens, and when the imaging size of the lens is smaller than that of the camera When the size is small, it will affect the imaging, and the image is surrounded by the lens barrel, and black corners appear at the four corners of the picture.
It can be seen that for 1/3" cameras, 1/3", 1/2" and 2/3" lenses can be selected; for 1/2" cameras, 1/2", 2/3" can be selected. The lens, but not the 1/3 lens. Because the CCD is like a human eye, the lens is like people's glasses, the glasses are too small, the eyes will not see the surrounding things.
C, the best choice lens and camera interface installation method
The interface between the lens and the camera is installed in two types: C-type and CS-type. The two are best. If you choose a C-type lens, you need to add a 5mm thick ring. If you connect directly to the CS interface camera without adding a ring, the rear mirror surface of the lens may touch the protective glass of the sensitive surface of the CCD, which may cause damage to the CCD camera. This is particularly noticeable in practical use.
D, it is best to choose an automatic iris lens to adapt to the great changes in day and night illumination
Due to the day and night monitoring, the illumination varies widely, so it is best to use an auto iris lens that automatically changes according to the illumination to keep the image clear.
E, the lens resolution and transmittance must meet the requirements
The resolution of the imaging lens used must be greater than the resolution of the camera, otherwise the required sharpness will not be achieved. And the transmittance of the lens in the lens is better, that is, the attenuation of light is very small.
F, monitoring distance of more than 1 km, also need to use laser beam expansion collimating lens
According to the size and distance of the scene to be observed, select the appropriate laser beam expanding collimating lens so that the laser beam can illuminate the scene to be monitored, so that the reflected light of the monitoring scene can be received by the camera.
G. Pay attention to the protective cover of the window glass with good performance.
In addition to selecting infrared lamps, cameras, and lenses, you should also pay attention to the comprehensive considerations of protective covers and power supplies. Because the protective cover has an effect on the effect of the infrared lamp, such as infrared light passing through different media, the transmittance and reflectivity are different. For example, different window glass, especially automatic defrost coated glass, has different attenuation of infrared light, so care should be taken to select a protective cover with good window glass.
Fourth, the conclusion
It can be seen from the above that the three infrared light sources have their advantages and disadvantages, but the thermal infrared light source is basically eliminated by the infrared LED and the LED-Array light source. Because the LED-Array can also be used to fully illuminate the distance of several hundred meters of monitoring scenes. As for the ultra-long-distance monitoring scene of 1km or more, it is better to use an infrared LD light source. Because of its high brightness, a LD tops many LEDs, so the infrared LD light source is small and light, which is the best choice for ultra long distance.
The most important issue in the selection of infrared cameras is the complete set. In addition to selecting the infrared light source, the matching of the camera, lens, protective cover, power supply, etc. should be considered comprehensively. Only by considering the selection points mentioned in the article, can we get the effect with half the effort.