| camera parameters details [turn] Camera Parameters Detailed Introduction
A lens that cannot be underestimated
Lens is the eye of the camera, in order to adapt to different monitoring environment and requirements, the need to configure different specifications of the lens. For example, in the indoor focus of surveillance, to carry out a clear and large view of the image capture, you have to configure a wide-angle lens; In the outdoor parking lot, not only to see the full picture of the car park, but also to see the details of cars, this time need wide-angle and zoom lens, in the border, coastal defense line monitoring, need to shoot far away
1, the main parameters of the lens
Focal length (f): The focal length is the distance between the lens and the photosensitive element, by changing the focal length of the lens, you can change the magnification of the lens and change the size of the shot image. When the object is far away from the lens, we can use the following formula to express: lens magnification ≈ focal Length/object distance. Increase the focal length of the lens, enlarge the magnification, you can narrow the vision, the scope of the picture is small, the details of the vision to see more clearly; if you reduce the focal length of the lens, the magnification is reduced, the scope of the picture is enlarged and the scene is larger.
The main parameters of the lens
Field angle: In engineering practice, we often use the horizontal angle of view to reflect the scope of the picture shot. The larger the focal length F, the smaller the field angle, the smaller the range of the picture formed on the photosensitive element; Conversely, the smaller the focal length, the larger the view field angle is, the larger the picture range formed on the photosensitive element.
Aperture: Aperture installed in the rear of the lens, the larger the aperture, the greater the amount of light through the lens, the higher the sharpness of the image, the smaller the aperture, the smaller the amount of light through the lens, the lower the sharpness of the image. Usually expressed in F (luminous flux). f= focal Length (f)/pass light aperture. In the camera's technical specifications, we can often see the parameters such as 6mm/f1.4, which means that the lens focal length of 6mm, luminous flux of 1.4, then we can easily calculate the aperture of the optical pass is 4.29mm. In the case of the same focal length F, the smaller the F value, the larger the aperture, the greater the luminous flux reaching the CCD chip, the better the lens.
2, the lens classification
Classification by size of view
Sort by aperture
Second, improve the clarity of the image is to improve the photographic ability of the camera
1, the role of photosensitive elements
At present, the mainstream surveillance camera's photosensitive element uses the CCD element, actually is the photoelectric conversion element. Compared with the previous CMOS photosensitive elements, CCD is 3 to 10 times times the sensitivity of the CMOS, so the CCD chip can receive more light signals, converted to electrical signals, the video processing circuit filtering, amplification to form video signal output. The stronger the light signal received, the greater the amplitude of the video signal. The video signal is connected to the video input of the monitor or TV to see the video image. The key to improving the image clarity is to improve the photographic ability of the camera.
2, lens and CCD sensor components of the configuration
In Figure one, we can see that the image formed on the CCD sensor is smaller than the original image, and the size of the CCD image surface is different.
CCD imaging size is commonly used in 1/2-inch, 1/3-inch, CCD size determines the specifications of the camera.
Configuration of lens and CCD photosensitive element
CCD imaging dimensions, that is, the camera screen width and height ratio and television screen width and height ratio, usually 4:3. This ensures that the video image of the camera is not distorted on the display.
Lens specifications are also divided into 1/2 inches, 1/3 inches, 1/2-inch lenses can be used for 1/2-inch, 1/3-inch cameras, while 1/3-inch lenses can only be used for 1/3-inch cameras, not for 1/2-inch cameras, because 1/3-inch lens luminous flux is only 1/ 2-inch lens luminous flux 44%, can not meet the 1/2-inch camera luminous flux requirements.
The lens focal length configuration we still use the diagram to explain. Determining the appropriate focal length is an important factor in determining the quality of the image. f=vd/vf=hd/h. Where the F represents the focal length, V represents the height of the CCD imaging dimensions, v represents the height of the object being observed, h represents the width of the CCD imaging dimension, h represents the width of the object being observed, and d represents the distance from the object to the lens. Assuming the 1/3 "CCD camera observation, the object is 500 mm wide, 400 mm high, the lens focus from the object 5000 mm." From the formula can be calculated: focal length f=4.8x5000/500≈48 mm or focal length f=3.6x5000/400≈45 mm.
Iii. How to capture a clear image in a poorly lit environment
The surveillance camera requires a clear image to be photographed in an environment where the lighting conditions are poor or even without light at night. In the camera indicators, we can often see the low illumination of this item.
1, the concept of illumination
Illuminance is the unit of measurement of the camera's speed, which is indicated by luck (Lux), that is, the camera can be photographed under the condition of dark illumination. The lower the value of the Luck (Lux), the better the camera is able to take a clear picture when the lighting conditions are lower. We know that the video camera produced a nominal value of 1v, the standard value of 700mv, such as the use of aperture for F1.2 lens, when the illumination of the scene is 0.02Lux, the camera output video signal amplitude of the standard amplitude of 700mv 33%-50%, At this point the camera's minimum illuminance is 0.02lux/f1.2. Test minimum illuminance value must pay attention to the lens aperture size, the lower f value, the larger the aperture, the need for the lower illuminance. Different aperture, the lowest illuminance value is different.
2, the realization of low illumination image of the scheme
We know that CCD camera can be divided into color and black-and-white camera, the minimum illuminance of ordinary cameras see the table below.
Minimum illumination of a normal camera
The wavelength range of visible light is 380nm~780nm, the wavelength of visible light is divided into red, orange, yellow, green, blue, blue and violet, and the wavelength is shorter than violet, which is called infrared light, which is longer than infrared light. CCD sensor surface has a layer of absorption ultraviolet transparent electrode, so the CCD can not accept ultraviolet light. Ordinary color camera CCD chip has red, green, blue three-color filter, so color camera can not feel the infrared light. The general CCD Black-and-White camera's spectral range is wide, not only can feel the visible light, but also can feel the infrared light.
According to the above principles, in poor lighting conditions, engineers often use the following scenarios to capture a clear image.
(1), ordinary low illumination CCD black-and-white camera + infrared lamp
In the monitoring of the installation of infrared light radiation "lighting", resulting in invisible and ordinary cameras can capture the infrared light, through CCD black-and-white camera can be achieved at night shooting.
(2), color turn black and white camera + infrared lamp
The so-called color of black and white camera refers to the daytime is a color camera, to the evening light conditions are very poor, the use of black and white images of infrared sensitivity of the higher characteristics, automatic switching to black and white, in conjunction with the infrared to shoot. In conjunction with the infrared lamp, the low illumination camera must meet the minimum illumination that the IR lamp supports.
(3), infrared low illumination color camera
IR low illumination color camera's infrared sensitivity is 4 times times higher than the average camera, and can work under 0 illumination (0Lux).
Infrared low illumination Color camera
(4) Low Speed shutter camera
Low-speed shutter camera also called picture Cumulative camera, through the computer continuous storage of multiple frames (up to 128 frames) due to the lack of light and more blurred picture, and accumulated, become a clear picture, with the help of Slowshutter technology, to achieve in the 0.008lux/f1.2 under the camera. This low illumination camera is suitable for museums that prohibit red, ultraviolet damage, nocturnal biological activity observation, and night military shoreline surveillance.
(5), ultra low Illumination camera
Ultra-low illumination camera using Exviewhad technology greatly improve the sensitivity, its color illumination of up to 0.05LUX, black and white can reach 0.003-0.001lux. When equipped with a dedicated infrared device, you can get a high-resolution black-and-white image to achieve 0Lux shooting.
Four, the camera control
In order to enlarge the scope of monitoring, the monitoring camera can realize rotation, zoom, magnification, auto focus and so on. The realization of these functions requires the digital hard disk video recorder to control the camera through the controller.
1. Rotation control
The engineers use the PTZ to install and fix the camera, and the PTZ is divided into fixed ptz and electric PTZ. Fixed PTZ is suitable for monitoring the scope of the situation, the fixed on the platform installed on the camera, adjust the camera level and pitch angle, to achieve the best working state after the locking adjustment mechanism can be. Electric PTZ installed stepper motor, the motor to accept signals from the controller, driving the camera rotation to achieve accurate positioning, suitable for large-scale monitoring.
PTZ according to its characteristics can be divided into only the rotation of the horizontal rotating cloud platform and can rotate around and up and down the omni-directional ptz. Generally speaking, the horizontal rotation angle is 0°~350°, the vertical rotation angle is +90°. The horizontal rotational speed of the constant-speed PTZ is generally 3°~10°/s, and the vertical speed is 4°/s. The horizontal rotational speed of the variable-speed PTZ is generally 0°~32°/s, and the vertical rotation speed is around 0°~16°/s. In some high-speed camera systems, the horizontal rotation speed of the PTZ is as high as 480°/s, and the vertical rotational speed is above 120°/s.
2, the realization of electric zoom, variable times, automatic focus
(1) The so-called integrated camera is to make the lens, CCD chip, video processing circuit, power supply, chassis integration as a whole, you can achieve electric zoom, variable times, automatic focus function. The key to evaluate the quality of integrated cameras is to realize automatic focus quickly and accurately. Good products can be a one-time accurate focus, and poor quality products, in the focus will be back and forth, need several times to set the focus. The current integrated camera with 16, 18, 20, 22, 27, 32 times times as the mainstream, the development trend is that the illumination is getting lower and higher optical multiples. Note that the zoom ratio here refers to the optical variable times.
The key technology of integrated camera is lens, CCD and DSP processing module. Top-grade lenses are mainly held by Japanese manufacturers, such as Canon, Camputar, Avenir and so on. The CCD chip takes the Japanese Sony as the main, the SONYCCD divides into Superhad and exview two kinds, among them Exview is the most new technology, the general use 1/4 inch size, the sexual noise ratio is higher than the Superhad; on the DSP processor chip, Sony's DSP chip can handle the image color very well, make the image look very bright. And the Canon, Nikon DSP in the capture mode and focus on the better.
(2) using electric zoom lens + Normal Camera
Combining the electric zoom lens and the ordinary camera, the principle of video drive of the common camera is used to realize the automatic control of lens focal length, aperture and focus. At present, some manufacturers have developed a super high rate of 60 times times the Electric zoom lens "d60x12.5". Its 750mm (up to 1500mm when using a zoom extension mirror) can clearly identify people 3 of kilometres away.
V. Network Transmission of VIDEO images
1, Analog camera + digital hard disk Recorder + computer network system
This is currently the most widely used network video surveillance system, by setting ports, gateways and routes, the digital hard disk recorder is used as the server to install special monitoring software or plug-ins on the remote client's computer, and the user can see the scene thousands of miles away through the internet to realize the remote monitoring and video recording of single channel and multi-channel video.
2, Analog camera + network video server + computer network system
Analog camera output signal is analog signal, computer processing signal is digital signal, in the network is also a digital signal transmission, Network video server (VIDEOSERVER) analog camera analog signals into digital signals, and then after the efficient compression chip compression, coding, The output can transmit in the computer network The digital signal, realizes in the computer network in the form of the digital signal transmission. Therefore, the network video server can also be called Video encoder (Videocoder). When one end of the video server is connected with the analog camera output signal, the other end plug in the computer network line, and then the Internet in any of the computers set up a good gateway, route, open IE Browser, input IP address or domain name can be seen in the computer monitoring screen. If the analog camera is configured with PTZ, we can also control the camera by zooming, doubling, rotating and so on. In the network video server also has to embed real-time operating system, can be Linux version, also can be a version of Windows, from the stability of the Linux version is more than a chip. The use of Network video server can be selected and equipped with different cameras, with more flexibility.
3. Network Camera + computer network system
The network camera is the integration of the analog camera and the network video server. Inside the camera built-in module/number conversion, video server functions, and network video server, in accordance with network protocol to achieve network communication and data transmission, but also to receive alarm signals and send out the alarm signal. This is more convenient, as long as the network camera installed, plug in the cable can be browsed.
4, CDMA wireless network video Surveillance System
The transmission described above is wired, but in mobile vehicles, remote mines and mountainous areas, it is clearly difficult to use wired transmission, and we can take advantage of sophisticated wireless communications technology. The representative products here are China Unicom's shift-through. The wireless network video surveillance system is a convenient product which integrates the CDMA data communication function and the digital video coding function. The camera image is compressed by video compression coding module, and transmitted to CDMA network through intelligent wireless Communication terminal, which realizes the function of video data interaction, sending/receiving, encryption and decoding, link control and maintenance. The system can transmit real-time dynamic images to the nearest Unicom communication network. Real-time image information can be obtained from the control end of the system via the Internet. The system integrates the advantages of CDMA network and Internet network, and makes remote monitoring and management anytime and anywhere.
Vi. interpretation of common technical indicators
1. Resolution
Image resolution simply refers to the screen level and direction of the vertical direction of the points displayed. For example 1024x728, where "1024" indicates the number of points displayed horizontally on the screen, and "768" indicates the number of points displayed vertically. The higher the resolution, the clearer the image will be. The higher the resolution, the clearer the display of the image.
2, Clarity
The clarity of the camera is expressed by line, which is divided into horizontal and vertical lines, in practical engineering applications we often use horizontal line as the evaluation index of the camera sharpness, the more lines, the higher the definition. Commonly used Black-and-white camera sharpness is generally 450-600, and color camera sharpness is generally 330-480, the larger the number of imaging more clearly. General monitoring occasions, with a 450-line camera can meet the requirements for medical, image processing and other special occasions, with 600-line camera can get clearer images.
3, Automatic gain control (AGC)
In order to enable the camera to output standard video signal under different illumination conditions, the automatic gain control (Autogaincontrol) is introduced into the video processing circuit, and gain feedback control is realized by detecting the average level value of the video signal. A camera with AGC function can increase the sensitivity of low illumination, but also amplify the jamming signal, which makes the image appear clutter.
4. Backlight compensation (BLC)
When the camera is in the backlight environment shooting, the screen will appear black image, however, in the security of the backlight environment is difficult to avoid, this time need for backlight compensation. When the backlight compensation function is introduced, if the camera detects that the video level in an area is low, the AGC circuit described above improves and enhances the video level of the region, and increases the amplitude of the output video signal, which makes the image whole clear and bright. If the theme you want to see is dimmed by a bright background, you can set the BLC to on state to compensate for the strong backlight.
5, electronic shutter (ee/ai) switch
At the rear of the camera we can often see the ee/ai switch. EE means electronic shutter mode, AI means automatic aperture lens. The camera's electronic shutter is generally set to automatic electronic shutter mode, through the electronic shutter mode, according to the intensity of incident light to adjust the exposure time of CCD image sensor, so that a clear image, electronic shutter time between 1/50-1/100000 seconds.
6, signal to noise ratio
The ratio between the signal level and the clutter level, the clutter includes power clutter, random clutter, single frequency clutter, etc. Often expressed in decibels (db). The higher signal-to-noise ratio indicates that the less clutter it produces, the higher the image signal quality. The signal-to-noise ratio shall not be less than 48dB.
7, white balance (AWB)
Color camera to restore the color of the object, you must keep the white balance normal. |
