CCD Camera
What is CCD camera?
Charge-coupled device (CCD) is a solid-state electronic device used to detect light on a silicon wafer, and a clock pulse voltage is used to generate and control the change in the semiconductor potential well to store and transfer charge information. Invented by W.S. Boyle and G.E. Smith of Bell Labs in 1969, charge-coupled devices consist of a regularly arranged array of metal-oxide-semiconductor (MOS) capacitors and input and output circuits. The charge-coupled device uses the amount of charge to represent the different states of the dynamic shift register, which is more sensitive to detect changes in light than a conventional negative. A CCD camera is a camera that uses a CCD sensor.
Working principle of CCD camera
The image of the subject is focused on the CCD chip through the lens, and the CCD accumulates the corresponding proportion of charge according to the intensity of light, the charge accumulated in each pixel under the control of the video timing, point by point outward shift, after filtering, amplification process, the formation of video signal output. The video signal is connected to the video input of a monitor or TV set to see the same video image as the original image. It is a semiconductor toise, to simplify the power supply CCD camera, generally from the external input only one power supply (12V), and various other voltage values of the power supply are obtained from the power conversion.
A. Resolution selection
Evaluation of the camera resolution is the horizontal resolution, the unit for the line pair, that is, the number of black and white line pairs can be distinguished after imaging. Commonly used black and white camera resolution is generally 380-600, and color 380-480, the larger the value the clearer the imaging. General surveillance occasions, with about 400 lines of black and white cameras can meet the requirements. For medical, image processing, and other special occasions, 600 lines of cameras can get a clearer image.
B. Imaging sensitivity
The minimum ambient illumination requirements are usually used to indicate the camera sensitivity. The sensitivity of black and white cameras is about 0.02and -0.5 Lux (lux), color cameras more than 1 Lux. 0.1 Lux cameras are used for general surveillance occasions; at night use or when the ambient light is low, the recommended use of 0.02 Lux cameras. With the use of near-infrared lamps, you must also use a low-light camera. In addition, the sensitivity of the camera is also related to the lens, 0.97 Lux / F0.75 is equivalent to 2.5 Lux / F1.2 is equivalent to 3.4 Lux / F1.
Reference environmental illumination: 100000 Lux in the summer sun, 10000 Lux outdoors on cloudy days, 1000 Lux in TV studios, 300 Lux from 60W desk lamp 60cm desktop, 100 Lux indoor fluorescent light, 10 Lux indoors at dusk, 10-15 Lux by candlelight at 20cm, 0.1 Lux by street light at night.
C. Electronic shutter
Electronic shutter time is between 1/50-1/100000 seconds, the camera’s electronic shutter is generally set to automatic electronic shutter mode, according to the environment of light and dark automatically adjusts the shutter time to get a clear image. Some cameras allow users to manually adjust the shutter time to adapt to certain special applications.
D. External synchronization and external triggering
External synchronization refers to the same synchronization signal between different video devices to ensure the synchronization of the video signal, it can ensure that the video signal output from different devices has the same frame, and line starts time. To achieve external synchronization, you need to input a composite synchronization signal (C-sync) or composite video signal to the camera. External synchronization does not guarantee that the user from the specified moment gets a complete continuous frame image, to achieve this function, you must use some special external trigger function of the camera.
CCD devices are made of silicon material and are more sensitive to the near-infrared, the spectral response can be extended to about 1.0um. Its response peaks at a green light (550nm) with a distribution curve. When covert surveillance at night, you can use near-infrared light illumination, the human eye cannot see the environment, but it can be imaged. As the CCD sensor surface has a layer of UV-absorbing transparent electrodes, so the CCD is not sensitive to UV. Color camera imaging unit on the red, green, and blue filter strip, so the color camera is not sensitive to infrared, or ultraviolet.
E. Size of CCD chip
CCD imaging size is commonly used as 1/2″, 1/3″, etc., the smaller the imaging size of the camera volume can be made smaller. Under the same optical lens, the larger the imaging size, the larger the field of view. Chip specifications Imaging surface size (width X height) diagonal 1/2 6.4×4.8mm 1/3 4.8×3.6mm 6mm camera lens selection guide in the TV surveillance system how to be monitored according to the scene environment, the correct choice of a camera lens is very important, because it directly affects the composition of the system at the end of the system monitor to see the effect of the monitored surface painting can meet System design requirements (in terms of screen range or image details), so the correct choice of the camera lens can make the system to get the most optimal design and can obtain good monitoring results.
Lens selection of CCD camera
CCD camera lens in terms of aperture can be divided into manual aperture lens and auto iris lens two, and in terms of focal length can be divided into the fixed-focus lens and zoom lens two. The following use different environments to talk about how to correctly select the camera lens.
CCD video camera
A. Manual, automatic aperture lens selection
Manual, automatic iris lens selection depends on the use of the environment and whether the illumination is constant. For the case of constant illumination in the environment, such as an elevator car, closed corridor, or no direct sunlight in the room, can choose a manual aperture lens, so that the system can be installed in the initial commissioning according to the actual illumination of the environment, a one-time adjustment of the lens aperture size, to obtain satisfactory brightness screen. For ambient illumination in the case of frequent changes, such as with the time of daylight and illumination changes in the foyer, window, lobby, etc., are required to use automatic iris lens (must be equipped with an automatic iris lens socket camera), so that you can achieve automatic adjustment of the screen brightness, to obtain a good more constant brightness of the surveillance screen. For auto iris lens control signals can be divided into DC and VI DEO control of two kinds, namely DC voltage control and video signal control. This is the type of auto iris lens selection, the camera auto iris lens socket connection, as well as the choice of auto iris lens drive mode switch.
B. Fixed focus, zoom lens selection
Fixed focus and zoom lens selection depend on the size of the monitored scene range, as well as the required clarity of the monitored scene picture. Lens specifications (lens specifications are generally divided into 1/3″, 1/2″ and 2/3″, etc.) under certain circumstances, the relationship between the lens focal length and the lens field of view: the longer the focal length of the lens, the smaller the field of view of the lens; in the case of a certain lens focal length, the relationship between the lens specifications and the lens field of view: the larger the lens specifications, the larger the field of view of the lens. So the above relationship can be seen: in the case of a certain lens object distance, with the lens focal length becomes larger, the smaller the range of the monitored scene seen on the end of the system monitor, but the picture details are becoming clearer; and with the increase in lens specifications, the range of the monitored scene seen on the end of the system monitor is increased, but its picture details are becoming blurred. In the lens specifications and lens focal length under the premise of a certain, CS-type interface lens field of view will be greater than the C-type interface lens field of view. Commonly used lens models are 4mm, 8mm, 12mm, and 16mm.
The lens field of view can be divided into image horizontal field of view as well as image vertical field of view, and the image horizontal field of view is greater than the image vertical field of view. Field of view generally refers to the horizontal field of view of the image of the lens.
C. Theoretical calculation of the correct choice of lens focal length
The field of view of the lens of the scene is an extremely important parameter, the lens field of view with the focal length of the lens and the size of the camera specifications and change (its changing relationship as described previously), covering the focal length of the scene lens can be calculated using the following formula: (1) f = u * D / U; (2) f = h * D / Hf: lens focal length, U: the actual height of the scene, H: the actual width of the scene, D: the lens to the scene measured distance, u: the image height, h: image width. For example: when electing a 1/2″ lens, image size u = 4.8mm, h = 6.4mm, the lens to the scene distance D = 3500mm, the actual height of the scene U = 2500mm (the actual width of the scene can be calculated by the following formula H = 1.333-U, this relationship is determined by the camera viewfinder CCD film). Substitute the above parameters into the formula (1), you can get f = 4.8-3500/2500 = 6.72mm, so the choice of 6mm fixed-focus lens can be.
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