How to see the quality of a camera from its parameters?

Abstract: CCD size of CCD color camera is also the camera target surface. Generally speaking, the larger the size, the more pixels it contains, the higher the definition and the better the performance. The CCD pixel of CCD color camera is the main performance index of CCD, which determines the clarity of displayed image. The higher the resolution, the better the image details. Today, understanding TV Bian Xiao will teach you how to look at camera parameters. Come and find out. Detailed explanation of camera parameters How to see the quality of a camera from the parameters?

1 and main technical indexes of CCD color camera

(1)CCD size, that is, the camera target surface.

Generally speaking, the larger the size, the more pixels it contains, the higher the definition and the better the performance. With the same number of pixels, the larger the size, the richer the displayed image level.

(2)CCD pixels

It is the main performance index of CCD, which determines the clarity of the displayed image. The higher the resolution, the better the image details. CCD consists of array photosensitive elements, and each element is called a pixel. The more pixels, the clearer the image. At present, most of the markets are demarcated by 250,000 and 380,000 pixels, and more than 380,000 pixels are high-definition cameras.

(3) Horizontal resolution.

The typical resolution of color camera is between 320 and 500 TV lines, mainly including 330 lines, 380 lines, 420 lines, 460 lines and 500 lines. The resolution is expressed by TVLINES, and the resolution of color camera is between 330-500 lines. Resolution is related to CCD and lens, and also directly related to the frequency band width of camera circuit channel. Usually, the bandwidth of 1MHz is equivalent to the resolution of 80 lines. The wider the frequency band, the clearer the image and the larger the line value. Resolution is the number of horizontal lines multiplied by 0.75. So the highest vertical resolution is: NTSC:525 x 0.75 = 393；; Pal: 625x0.75 = 470。

Horizontal resolution measurement method:

A. Inspection (analysis) chart: The camera directly shoots the inspection chart, and the vertical and horizontal resolution is directly read on the monitor. When testing multiple cameras, you should use the same lens (fixed focus and two variable lenses are recommended), and clearly and accurately count the given scale lines *** 10 vertical lines and 10 horizontal lines according to the central circle of the test card appearing on the left and right sides of the monitor screen. Represents vertical definition and horizontal definition respectively, and gives the corresponding number of lines. For example, 350 vertical lines and 800 horizontal lines. At this time, it is best to use a high-speed black and white display. When testing, you can focus on the distant scene, or you can focus while measuring. It's better to use both, and you can see the difference (convergence) of this camera.

B. Bandwidth measurement: measure the bandwidth of the image signal read by the camera with an oscilloscope, and multiply the measured bandwidth by 80 to get the resolution of the camera. For example, if the measured bandwidth is 5MHZ, 5X80=400.

(4) Lighting

Also called sensitivity. F: lux = 1: 10

A. minimum illumination. Minimum illumination is also an important parameter to measure the quality of a camera. Sometimes, the word "minimum" is omitted and referred to as "illuminance" for short. The minimum illumination is the scene brightness value when the brightness of the shooting scene is low enough to make the video signal level output by the camera low to a specified value. When measuring this parameter, it is also related to the lens, and the maximum relative aperture of the lens should be specially indicated. For example, if the lens of F 1.2 is used, when the brightness value of the shooting scene is as low as 0.04Lux, the amplitude of the video signal output by the camera is 50% of the maximum amplitude, that is, it reaches 350mV (the maximum amplitude of the standard video signal is 700mV), then the minimum illumination of this camera is said to be 0.04LUX/F 1.2. If the brightness value of the shot scene is low, and the video signal output by the camera is less than 350mV, it will be reflected on the monitor screen, so it is difficult to distinguish a distinct and gloomy image. It is the sensitivity of CCD to ambient light, or the darkest light that CCD needs for normal imaging. The unit of illumination is lux. The smaller the value, the less light is needed and the more sensitive the camera is. When measuring the minimum illuminance, put the camera in a dark room, with movable 220V incandescent lamps in front and behind the dark room. The voltage regulator can be adjusted from 0V to 250V to adjust the brightness of the light in the dark room, and the light can also be adjusted from the brightest to the darkest. During the test, record a minimum illuminance value (dim the movable lamp with the voltage regulator until the built-in test chart in the darkroom can't be seen clearly), and then record the next minimum illuminance value with the smallest aperture. When the amplitude (black level to white level) of the useful video signal detected on the oscilloscope drops to 70MV, measure the illuminance value on the test chart with an illuminance meter. Generally, cameras with sensitivity above 0. 1lux are ordinary; Cameras below 0. 1lux are high-sensitivity cameras, also known as electronic intensifying cameras or night vision cameras. Generally, the minimum ambient lighting requirements are used to indicate the camera sensitivity. The sensitivity of black-and-white cameras is about 0.02-0.5Lux (lux), and most color cameras are above 1Lux. 0. 1Lux camera is used in common monitoring occasions; It is recommended to use a 0.02Lux camera at night or in poor ambient light. When used with near infrared lamp, you must also use a low-light camera.

B. Maximum illumination. When measuring the highest illuminance, put the camera in a dark room, with movable 220V incandescent lamps in front and behind the dark room. The voltage regulator can be adjusted from 0V to 250V, and the light can also be adjusted from the darkest to the brightest. During the test, open the aperture of the camera to the maximum measurement test chart, and adjust the voltage of the voltage regulator. When the amplitude of the video signal detected on the oscilloscope does not exceed 150% of the rated value, measure the illuminance value on the test chart with an illuminance meter. Record the highest illumination value.

(5) scanning system.

There are PAL system and NTSC system.

(6) Signal-to-noise ratio.

Signal-to-noise ratio is also a main parameter of the camera. The so-called "signal-to-noise ratio" refers to the ratio of signal voltage to noise voltage, which is usually represented by the symbol S/N, where S represents the image signal value of the camera under the hypothesis of meta-noise, and N represents the noise value (such as thermal noise) generated by the camera itself. The ratio of the two is the signal-to-noise ratio, expressed in decibels (dB). The higher the signal-to-noise ratio, the better, with a typical value of 46dB. Generally speaking, the signal voltage is much higher than the noise voltage, and the proportion is very large. Therefore, the actual calculation of the signal-to-noise ratio of the camera is usually based on 10, and the logarithm of the ratio of the mean square value of the signal voltage to the mean square value of the noise voltage is multiplied by the coefficient 20, and the unit is expressed in dB. The typical value is 46db. If it is 50db, the image is a little noisy, but the quality is good. If it is 60db, the picture quality is excellent and there is no noise. When the camera shoots a bright scene, the picture displayed by the monitor is usually bright, and it is difficult for observers to see the interference noise in the picture; When the camera shoots a dark scene, the picture displayed by the monitor is dim, and the observer can easily see the snowflake-like interference noise in the picture at this time. The intensity of interference noise (that is, the influence of interference noise on the picture) is directly related to the signal-to-noise ratio of the camera, that is, the higher the signal-to-noise ratio of the camera, the smaller the influence of interference noise on the picture.

Generally, the signal-to-noise ratio (SNR) values given by cameras are the values when AGC (Automatic Gain Control) is turned off, because when AGC is turned on, small signals will be enhanced, so the noise level will be improved accordingly. The typical signal-to-noise ratio of CCD cameras is generally 42dB-56dB. When measuring the signal-to-noise ratio parameters, the video clutter meter should be directly connected to the video output end of the camera. The noise of the camera is related to the choice of gain. When in the gain enhancement position, the noise will naturally increase. Conversely, in order to clearly see the effect of noise, we can observe it in the state of increasing gain. In the same state, compare different cameras to judge the advantages and disadvantages. Generally speaking, the gain selector switch of the camera should be set at the ON(0DB) position for observation or measurement.

Noise is also related to contour correction (edge hooking). Contour correction not only enhances the detailed contour of the image, but also enhances the contour of noise and increases the particles of noise. During the noise test, the contour correction switch should usually be turned off. The so-called contour correction is to enhance the detail components in the image. Make the image appear clearer and more transparent. If the contour correction is removed, the image will appear blurred. In the early days, contour correction was only carried out in the horizontal direction. Now digital contour correction is used to correct the horizontal and vertical directions, so its effect is more perfect. However, contour correction can only reach an appropriate level. If the contour correction amount is too large, the image will appear stiff. In addition, the result of contour correction will make people's facial spots more prominent

When measuring, use the 6MHZ low-pass filter and 100MHZ high-pass filter of the video clutter meter. When testing, open the camera aperture to the maximum measurement test chart, and turn off AGC (automatic gain control) and contour correction (trimming). When the amplitude of the video signal detected on the oscilloscope is 0.7Vp-p, cover the lens, and the video signal detected on the video clutter meter should be about 0.4 VP-p .. Then calculate the signal-to-noise ratio.

(7) video output.

The video signal output by the camera consists of brightness signal, chroma signal, synchronization signal, color synchronization signal and blanking signal. When testing, open the camera aperture to the maximum measurement test chart, but the image should not be too white and overloaded, and then adjust the focal length to make the close-up image or test chart clear. Measure its value with a waveform oscilloscope. Most of them are from 1Vp-p to1.2vp-p75Ω, using BNC connectors.

These include:

A. output brightness amplitude measurement. Open the lens aperture as large as possible to avoid image overload, and then adjust the focal length to make the close-up image clear. Measure its value with a waveform oscilloscope.

B. output synchronous amplitude measurement. Open the lens aperture as large as possible to avoid image overload, and then adjust the focal length to make the close-up image clear. Measure its value with a waveform oscilloscope.

C. measure the output color burst amplitude. Measure the duration of color burst signal with waveform oscilloscope, or read out the number of cycles of subcarrier. Measure the output signal on the vector oscilloscope. Measure the R. amplitude and phase of YL and CY. When measuring the amplitude, the color burst amplitude is 100%. When measuring the phase, make the color burst vectors consistent.

D, outputting color subcarrier amplitude measurement. Cover and cover the lens, measure the distance from the center of the bright cluster to the coordinate origin of the vector oscilloscope, and then convert it into voltage.

E, measuring the amplitude of the output blanking signal. Open the lens aperture as large as possible to avoid image overload, and then adjust the focal length to make the close-up image clear. Measure its value with a waveform oscilloscope.

(8) Test of color reducibility.

The color monitor that should be selected when testing this parameter. First observe people and clothes from a distance to see if there is color distortion, compare brightly colored objects and see the sensitivity of the camera. Put the color test chart in front of the camera to see how clear the picture is, whether it is too light or too thick. Then, shoot the moving color object again to see if there is any color difference tailing, delay and blur.

(9) Backlight compensation.

There are two ways to test this parameter: one is to turn on the voltage-regulating lamp in front of the camera in the darkroom and turn it to the brightest, then put a picture or text under the lamp, take a picture with the camera facing the light to see if the picture and text can be seen clearly, and adjust the on-off switch to see if there is any change, which is the best. The other is to shoot the camera out of the window in sunny conditions to see if you can see the images and words clearly at this time.

(10) distortion.

Seeing distortion means putting the test card in front of the camera to make the whole color bar appear color distortion, whether the straight line is straight, whether it is bent, and whether the corners and borders are bent.

(1 1) power consumption.

Measure the current with a multimeter and adjust the voltage with a small regulator.

Supplementary explanation:

Many users want to monitor in the dark at night. Please note: because CCD camera also relies on light reflection to image, if there is no light, its image will only be dark and a lot of snowflakes. How to get an image? One way is to increase visible light illumination, such as street lamps and searchlights; One is to add infrared light (especially when a visible light source cannot be installed). For color CCD cameras, the response to infrared light is not enough, and some day and night color cameras will automatically switch to black and white mode at night. So your monitoring system requires night use, and you must use a black and white CCD camera.

Infrared lights can be divided into indoor and outdoor, short distance and long distance. Indoor infrared lamps in the range of10 ~ 20m are commonly used. Due to the reflection of the wall, the image effect is not bad. The infrared lamp used outdoors for a long distance is not ideal and expensive, so it is generally not used until it is absolutely necessary.