1. Classification of cameras According to photoelectric conversion devices, it can be divided into photoconductive camera tubes, solid photoelectric sensors CCD, and complementary metal oxide semiconductor CMOS.
2. CCD camera
1. Definition
CCD is the abbreviation of Charge Coupled Device (Charge Coupled Device). It can convert light into electric charge and store and transfer the charge. It can also take out the stored charge and change the voltage, so it is an ideal camera element. The CCD camera composed of it is widely used because of its small size, light weight, unaffected by magnetic field, and anti-vibration and impact characteristics.
2. Classification
(1) Divided by imaging color
u Color camera: suitable for distinguishing the details of the scenery, such as distinguishing the color of clothes or scenery. The amount of information increases due to the color, which is generally considered to be 10 times that of a black and white camera.
u Black and white camera: used in areas with insufficient light and areas where lighting equipment cannot be installed at night. When only monitoring the position or movement of the scene, you can choose a black and white camera with a resolution usually higher than that of a color camera.
(2) Divided by resolution
u Low-end type: about 250,000 pixels, color resolution of 330 lines, black and white resolution of about 400 lines.
u Mid-range type: between 250,000 and 380,000 pixels, color resolution is 420 lines, black and white resolution is 500 lines and below.
u High-end type: above 380,000 pixels, color resolution greater than or equal to 480 lines, black and white resolution above 600 lines.
(3) Divided by sensitivity
u Ordinary type: The illumination required for normal operation is 1～3 LUX.
u Moonlight type: The illumination required for normal operation is about 0.1 LUX.
u Starlight type: The illumination required for normal operation is below 0.01 LUX.
u Infrared illumination type: In principle, it can be zero illumination and use infrared light source for imaging.

Summer sun	100,000 Lux
Cloudy outdoor	10,000 Lux
TV studio	1,000 Lux
Dusk indoor	10 Lux
Indoor fluorescent lamp	100 Lux
Street lights at night	0.1 Lux

(4) Divided by the size of the CCD target surface of the imaging element

The size of the CCD target surface of the imaging element is divided into 1 inch, 2/3 inch, 1/2 inch, 1/3 inch, 1/4 inch, etc. Among them, 1/3 inch and 1/2 inch are the most common ones, and 1/5 CCD with a target surface of 2 inches is under development.

CCDsize	Level（mm）	vertical（mm）	diagonal（mm）
1inch	12.7	9.6	16
2/3inch	8.8	6.6	11
1/2inch	6.4	4.8	8
1/3inch	4.8	3.6	6
1/4inch	3.2	2.4	4

3. Main technical performance

(1) Clarity

The definition number is an important parameter to measure the quality of a camera. It refers to the maximum number of lines that can be seen on the monitor when the camera captures black and white stripes arranged at equal intervals. When this number of lines is exceeded, only a gray patch can be seen on the screen and black and white lines can no longer be distinguished.

The resolution of industrial surveillance cameras is usually between 380 and 460 lines.

The resolution is determined by the number of pixels of the camera device. The more pixels of the camera device, the higher the resolution and the higher the camera grade.

(2) Minimum illumination

The minimum illuminance is the brightness value of the scene when the brightness of the subject is low to a certain level and the video signal level output by the camera is low to a certain value.

Generally, the minimum illuminance of a color camera is 2～3 Lux, and the measurement of illuminance is based on a certain lens aperture coefficient. Therefore, you cannot just look at the minimum illuminance specified in the camera manual. The lower the minimum illumination, the higher the camera grade.

Compared with color cameras, black-and-white cameras are only sensitive to light intensity (brightness) signals because they do not have chroma processing. Therefore, the illuminance of black-and-white cameras is lower than that of color cameras, and can generally achieve 0.1 Lux at F1.4 , As for the low light camera, it is even lower.

(3) Signal to noise ratio

 The signal-to-noise ratio is also an important performance index of the camera. When the camera captures a bright scene, the monitor display is usually brighter, and it is difficult for the observer to see the interference noise in the screen; when the camera captures a dark scene, the monitor display is relatively dim, and the observer It is easy to see the snowflake-like noise in the picture. The strength of the interference noise is directly related to the quality of the camera's signal-to-noise ratio, that is, the higher the camera's signal-to-noise ratio, the less the interference noise has on the picture.

The signal-to-noise ratio is the ratio of the signal voltage to the noise voltage, and is usually represented by the symbol S/N. Because under normal circumstances, the signal voltage is much higher than the noise voltage, and the ratio is very large. Therefore, the actual calculation of the camera signal-to-noise ratio is usually based on the logarithm of the mean square signal voltage to the mean square noise voltage, multiplied by a coefficient of 20, expressed in dB.

Generally, the signal-to-noise ratio value given by the camera is the value when the automatic gain control is turned off, because when the gain is turned on, the small signal will be increased, so that the noise level will also be increased accordingly. The typical value of CCD camera signal-to-noise ratio is generally 45-55dB. When measuring the signal-to-noise ratio parameters, a video clutter meter should be used to directly connect to the video output terminal of the camera.

(4) Automatic gain control (AGC)

All cameras have a video amplifier that amplifies the signal from the CCD to a usable level. The amount of amplification is the gain, which is equivalent to a higher sensitivity, which can make it sensitive in low light, but in bright light. The amplifier will be overloaded in the environment, distorting the video signal. For this reason, it is necessary to use the camera's AGC circuit to detect the level of the video signal, and to switch AGC in a timely manner, so that the camera can work in a larger illumination range, which is the dynamic range, that is, automatically under low illumination. Increase the sensitivity of the camera, thereby increasing the intensity of the image signal to obtain a clear image. Cameras with AGC function will have higher sensitivity at low illumination, but the noise will be more obvious at this time, because the signal and noise are amplified at the same time.

(5) Backlight compensation (BLC)

Also known as backlight compensation or backlight compensation, it can effectively compensate for the darkness of the main body of the picture when the camera is shooting in a backlight environment. Usually, the camera's AGC working point is determined by averaging the content of the entire field of view, but if the field of view contains a very bright background area and a very dark foreground target, the AGC work determined at this time The point may not be suitable for the foreground target, and backlight compensation may improve the display condition of the foreground target.

When the backlight compensation function is introduced, the camera only detects a sub-area of the entire field of view, and determines the operating point of the AGC circuit by calculating the average signal level of this area. Since the average level of the sub-region is very low, the AGC amplifier will have a higher gain, which will increase the amplitude of the output video signal, thereby making the main picture on the monitor clear. At this time, the background picture will be brighter, but the subjective brightness difference between it and the main picture will be greatly reduced, and the visibility of the entire field of view will be improved.

When the backlight compensation is turned on, the camera only averages a sub-region of the entire field of view to determine its AGC operating point. At this time, if the foreground target is located in this sub-region, the visibility of the foreground target is expected to improve.

(6) Electronic shutter (ES)

Electronic shutter is a term that compares the mechanical shutter function of a camera, which is equivalent to controlling the light-sensitive time of a CCD image sensor. Since the essence of CCD light-sensing is the accumulation of signal charge, the longer the light-sensing time, the longer the accumulation time of signal charge and the greater the amplitude of the output signal current. By adjusting the accumulation time of the light signal charge (that is, adjusting the width of the clock pulse), the function of controlling the CCD photosensitive time can be realized.

When the electronic shutter is closed, for NTSC cameras, the CCD accumulation time is 1/60 second; for PAL cameras, it is 1/50 second. When the electronic shutter is open, for NTSC cameras, the electronic shutter covers the range from 1/60 second to 1/10000 seconds in 261 steps; for PAL cameras, the electronic shutter covers from 1/50 second to 1/10000 seconds in 311 steps The range of seconds.

When the electronic shutter speed increases, the light focused on the CCD decreases within the time allowed by each video field, which will reduce the sensitivity of the camera. However, higher shutter speeds will produce a "pause action" for viewing moving images. Effect, this will greatly increase the dynamic resolution of the camera.

(7) White balance (WB)

White balance is only used in color cameras. Its purpose is to realize that the camera image can accurately reflect the situation of the scene. There are two ways of manual white balance and automatic white balance.

Automatic white balance is divided into continuous white balance and automatic white balance control.

Continuous white balance is also called automatic tracking white balance, which is continuously adjusted as the color temperature of the scene changes, and the range is 2800～6000K. This method is most suitable for occasions where the color temperature of the scene is constantly changing during shooting, to make the color appear natural, but when there is little or no white in the scene, such as the scene is mostly blue sky, white clouds or sunset and other high color temperature objects and When the scene is dark, continuous white balance cannot produce the best color effect.

To automatically control the white balance, you need to first aim the camera at a white reference target such as a white wall, white paper, etc., and then change the menu or switch setting from manual to automatic, and keep it at this position for a few seconds or until the image appears white After the white balance is executed, turn the automatic mode switch back to the manual position to lock the white balance setting. At this time, the white balance setting will remain in the camera's memory until the execution is changed again, the range is 2300 ~10000K, during this period, the setting will not be lost even if the camera is powered off.

(8) Synchronization method

Camera synchronization methods generally include internal synchronization, power synchronization and external synchronization. Internal synchronization (INT) uses the crystal oscillator circuit inside the camera to generate a synchronization signal to complete the operation. Power synchronization (LL), also called linear lock or line lock, uses the AC power of the camera to complete the vertical drive synchronization, that is, the camera is synchronized with the power zero line. External synchronization (EXT) uses a synchronization signal generated by an external synchronization signal generator to send to the camera's external synchronization input to achieve synchronization.

The synchronization signal can be a color composite video or black burst signal (VBS), a black and white composite video or a composite synchronization signal (VS), and it can also be a multiplexed vertical drive signal (VD2) and composite video output signal of an external device such as a matrix.

4. Technology development trend

(1) The image size of the CCD sensor is developing towards integration, light weight, high pixels, and multiple formats

The silicon chips and processing costs for manufacturing CCD sensors are high. Due to the advancement of lithography machines, the size of the CCD sensor is developing towards 1/2 inch, 1/3 inch, 1/4 inch, and 1/5 inch while still maintaining the characteristics of high sensitivity.

The image size of various CCD sensors is decreasing, but the number of pixels is increasing. It has evolved from the early 512 (H) × 596 (V) to 795 (H) × 596 (V), and even more than one million pixels CCD sensor. In order to improve the resolution in the horizontal and vertical directions, the usual interlaced scan has been developed to progressive scan format.

(2) Reduce the working voltage of the CCD sensor and reduce power consumption

The CCD cameras developed at the initial stage include +24V, +22V, +17V and +5V, etc. The current general-purpose one is +12V. In order to cooperate with the application of PC camera and network image transmission, two working voltages of +12V and +5V are gradually adopted.

(3) Improve the manufacturing efficiency of CCD cameras

In order to reduce the manufacturing cost of CCD cameras and realize high-speed automated production, manufacturers pursue compact structures and are committed to miniaturization of CCD cameras. So far, multi-chip integrated modular manufacturing technology (MCM) has been implemented for multilayer boards.

(4) Digitization of CCD camera

In the manufacture of CCD cameras, DSP digital processing is gradually realized from the previous Analog simulation system. Electronic computers and special software systems can be used to realize the quantitative adjustment of various parameters of CCD cameras, especially color CCD cameras, to ensure the performance of CCD cameras. Optimization consistency of indicators and quantitative modification of parameters under special conditions of use.

Three, CMOS camera

The CMOS sensor is a sensor that usually has a sensitivity 10 times lower than that of a CCD sensor. Because the human eye can see targets below 1 Lux (night of a full moon), the CCD sensor can usually see 0.1 to 3 Lux better than the human eye, which is 3 to 10 times the sensitivity of the CMOS sensor. The sensitivity of CMOS sensors is generally in the range of 6 to 15 Lux. CMOS sensors have a fixed noise that is 10 times higher than that of CCD sensors. The fixed pattern noise always stays on the screen as if it is a pattern. Because CMOS sensors are basically useless below 10 Lux, they are generally only used for very low-end home security.

The CMOS sensor does not require complicated processing, and directly converts the electrons generated by the image semiconductor into a voltage signal, which is 10 to 100 times faster than the CCD sensor. Therefore, the CMOS sensor is very useful for high frame cameras, and the high frame rate can reach 400-2000 frames/sec. This advantage is very useful for viewing high-speed moving objects. But because there is no high-speed digital signal processor, there are only a few high-speed cameras on the market, and the prices are generally very high.

CMOS sensor can put all the logic and control loop on the same silicon chip block, which can make the camera simple and easy to carry, so the CMOS camera can be made very small.

Although CMOS cameras consume the same or higher energy consumption than CCD cameras, CMOS sensors use very few rings such as CDS, TG and DSP rings, so the total energy consumption of the same size is reduced by 1/2 to 1/4 compared to CCD cameras.

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