Camera indicator

First, the lens that cannot be underestimated

The lens is the eye of the camera. In order to adapt to different monitoring environments and requirements, it is necessary to configure lenses with different specifications. For example, indoor key monitoring must be equipped with a wide-angle lens to shoot a clear image with a large field of view; In the outdoor parking lot, you should not only see the whole parking lot, but also see the details of the car. At this time, you need a wide-angle and zoom lens, and the monitoring of the border line and the sea defense line needs long image shooting.

1, main lens parameters

Focal length (f): The focal length is the distance between the lens and the photosensitive element. By changing the focal length of the lens, the magnification of the lens can be changed and the size of the shot image can be changed. When the object is far away from the lens, we can use the following formula: the magnification of the lens ≈ focal length/object distance. By increasing the focal length and magnification of the lens, the foreground can be narrowed, the picture range is smaller and the foreground details can be seen more clearly; If the focal length of the lens is reduced, the magnification is reduced and the picture range is expanded, a larger scene can be seen.

Main parameters of lens

Perspective: In engineering practice, we often use horizontal perspective to reflect the shooting range of the picture. The larger the focal length f, the smaller the field of view angle, and the smaller the picture range formed on the photosensitive element; Conversely, the smaller the focal length f, the larger the field of view angle and the larger the picture range formed on the photosensitive element.

Aperture: The aperture is installed behind the lens. The larger the aperture, the greater the amount of light passing through the lens and the higher the definition of the image. The smaller the aperture, the less light passes through the lens and the lower the definition of the image. It is usually expressed by F (luminous flux). F= focal length (f)/ aperture. In the technical indicators of the camera, we can often see such parameters as 6mm/F 1.4, that is to say, the focal length of the lens is 6mm and the luminous flux is 1.4. At this time, we can easily calculate that the aperture is 4.29 mm. Under the same focal length f, the smaller the f value, the larger the aperture, the greater the luminous flux reaching the CCD chip and the better the lens.

2. Classification of lenses

Depending on the size of the viewing angle

Classification by aperture

Second, the fundamental thing to improve the image definition is to improve the photographic ability of the camera.

1, the role of photosensitive elements

At present, the photosensitive elements of mainstream surveillance cameras all use CCD elements, which are actually photoelectric conversion elements. Compared with the previous CMOS photosensitive elements, the sensitivity of CCD is 3 to 10 times that of CMOS, so the CCD chip can receive more optical signals, which are converted into electrical signals by video processing circuits, filtered and amplified to form video signals for output. The stronger the received optical signal, the greater the amplitude of the video signal. When the video signal is connected to the video input of the monitor or TV, you can see the video image. The fundamental thing to improve the image definition is to improve the sensitivity of the camera.

2. Configuration of lens and CCD photosensitive element

In figure 1, we can see that the image formed on the CCD sensor is smaller than the original image, and the size of the image formed on the CCD chip is also different.

The common imaging sizes of CCD are 1/2 inches and 1/3 inches, and the size specifications of CCD determine the specifications of the camera.

Configuration of lens and CCD photosensitive element

The imaging size of CCD, that is, the aspect ratio of camera screen is the same as that of TV screen, usually 4: 3. This ensures that the video image of the camera will not be distorted on the display.

The specifications of the lens are also divided into 1/2 inches, 1/3 inches, etc. 1/2 inch lens can be used for 1/2 inch and 1/3 inch cameras. 1/3-inch lens can only be used for 1/3-inch cameras, not for 1/2-inch cameras. This is because the luminous flux of 1/3-inch lens is only 44% of that of 1/2-inch camera, which cannot meet the luminous flux requirements of 1/2-inch camera.

Let's illustrate the configuration of lens focal length with a picture. Determining the appropriate focal length is an important factor to determine the image quality. F = VD/VF = HD/H. Where f stands for focal length, v stands for the height of CCD imaging size, v stands for the height of the observed object, h stands for the width of CCD imaging size, h stands for the width of the observed object, and d stands for the distance from the object to the lens. Suppose the measured object is 500 mm wide and 400 mm high, and a1/3 "CCD camera is used, and the lens focus is 5000 mm away from the object. From the formula, the focal length f=4.8×5000/500≈48 mm or the focal length f=3.6×5000/400≈45 mm can be calculated. ..

Third, how to shoot a clear image in poor light?

It is required that the surveillance camera can capture clear images in poor lighting conditions or even in the dark environment at night. In the indicator of the camera, we can often see items with low illumination.

1, the concept of illuminance

Illuminance is a unit to measure the sensitivity of a camera, expressed in lux, that is, how dark the image can be taken by the camera. The lower the Lux value, the clearer the image can be captured by the camera in low lighting conditions. We know that the nominal value of the video signal generated by the camera is 1v, and the standard value is 700mv. For example, if a lens with an aperture of F 1.2 is used, when the illumination value of the shooting scene is 0.02Lux/F65438+, the video signal output by the camera is 33%-50% of the standard amplitude of 700mv. When testing the minimum illumination value, we must pay attention to the aperture size of the lens. The smaller the f value, the larger the aperture and the lower the required illumination. The minimum illumination value of different apertures is different.

2. The scheme to realize low-light photography.

We know that CCD cameras are divided into color cameras and black-and-white cameras, and the minimum illumination of ordinary cameras is shown in the following table.

Minimum illumination of ordinary camera

The wavelength range of visible light is 380 nm to 780 nm. Visible light can be divided into red, orange, yellow, green, cyan, blue and purple from long to short. The wavelength shorter than violet light is called ultraviolet light, and the wavelength longer than infrared light is called infrared light. There is a transparent electrode on the surface of CCD sensor, which absorbs ultraviolet light, so CCD cannot accept ultraviolet light. There are red, green and blue filters on the CCD chip of ordinary color cameras, so color cameras can't feel infrared light. Ordinary CCD black-and-white camera has a wide spectral range, and can feel not only visible light but also infrared light.

According to the above principles, engineers often use the following schemes to take clear images in poor light.

(1), ordinary low-illumination CCD black-and-white camera+infrared lamp

An infrared lamp is installed at the monitoring site to radiate "light" and generate infrared light that is invisible to human eyes but can be captured by ordinary cameras. Night shooting can be realized by CCD black-and-white camera.

(2), color black and white camera+infrared light

The so-called color-to-black-and-white camera is a color camera during the day. When the lighting conditions are poor at night, it automatically switches to black-and-white mode by using the high sensitivity of black-and-white images to infrared rays, and shoots with the cooperation of infrared rays. When used with infrared lamp, the camera with low illumination must meet the minimum illumination supported by infrared lamp.

(3), infrared low illumination color camera

The infrared sensitivity of an infrared low-illumination color camera is more than 4 times higher than that of a general camera, and it can work at zero illumination (0Lux).

Infrared low illumination color camera

(4) Low speed shutter camera

Low-speed shutter camera, also known as picture accumulation camera, continuously stores multiple frames (up to 128 frames) with insufficient light through the computer, and accumulates them into clear pictures. With the help of SLOWSHUTTER technology, shooting can be realized at 0.008LUX/F 1.2 illumination. This low-light camera is suitable for museums where red and ultraviolet rays are forbidden, biological activity observation at night, military coastline monitoring at night, etc.

(5), ultra-low illumination camera

The ultra-low illumination camera adopts EXVIEWHAD technology, which greatly improves the sensitivity. The color illumination can reach 0.05LUX, and the black and white illumination can reach 0.003-0.00 1LUX. When equipped with special infrared equipment, high-definition black-and-white images can be obtained, and 0Lux shooting can be realized.

Fourth, camera control.

In order to expand the monitoring range, it is required that the monitoring camera can realize the functions of rotation, zoom, zoom and autofocus. The realization of these functions requires the digital hard disk video recorder to control the camera through the controller.

1, rotation control

Engineers use pan/tilt to install and fix cameras. Pan-tilt is divided into fixed pan-tilt and electric pan-tilt Fixed pan/tilt is suitable for occasions with small monitoring range. After installing the camera on the fixed pan/tilt, adjust the horizontal and pitching angles of the camera, and lock the adjustment mechanism after reaching the best working state. The electric pan/tilt is equipped with a stepping motor, which receives the signal from the controller and drives the camera to rotate, thus realizing accurate positioning and being suitable for large-scale monitoring.

According to the rotation characteristics of the tripod head, the tripod head can be divided into a horizontal tripod head that can only rotate left and right and an omnidirectional tripod head that can rotate left and right and up and down. Generally speaking, the horizontal rotation angle is 0 ~ 350 and the vertical rotation angle is+90. The horizontal rotation speed of constant-speed pan-tilt is generally 3 ~ 10/s and the vertical speed is about 4 /s s. The horizontal rotation speed of variable-speed pan-tilt is generally 0 ~ 32/s and the vertical rotation speed is about 0 ~16/s. In some high-speed camera systems, the horizontal rotation speed of pan-tilt is above 480/s and vertical. ..

2. Realize electric zoom, zoom and autofocus.

(1) The so-called integrated camera integrates the lens, CCD chip, video processing circuit, power supply and housing, and can realize electric zoom, zoom, autofocus and other functions. Whether autofocus can be realized quickly and accurately is the key to evaluate the quality of integrated camera. A good product can focus accurately at one time, while a poor product will go back and forth when focusing, and it needs to set the focus several times. At present, the mainstream of integrated cameras is the magnification of 16, 18, 20, 22, 27 and 32, and the development trend is lower and lower illumination and higher optical magnification. Note that the zoom magnification here refers to the optical zoom magnification.

The key technologies of integrated camera are lens, CCD and DSP processing module. High-end lenses are mainly controlled by Japanese manufacturers, such as Canon, Camputar and Avenir. The CCD chip is mainly Sony of Japan, and SonyCCD can be divided into SuperHAD and Exview, among which Exview is the latest technology, generally adopting the size of 1/4 inches, and the SNR is higher than Super HAD. On the DSP processing chip, Sony's DSP chip can handle the image color very well, making the image look very bright. The DSP of Canon and Nikon is better in grasping light mode and focusing.

(2) Adopt electric zoom lens+ordinary camera.

The electric zoom lens is combined with the ordinary camera, and the automatic control of lens focal length, aperture and focusing is realized by using the video driving principle of the ordinary camera. At present, manufacturers have developed the ultra-high magnification 60x electric zoom lens "D60× 12.5". Its focal length of 750mm (up to 1500mm when using a zoom lens) can clearly identify people 3 kilometers away.

Verb (abbreviation of verb) video image network transmission

1, analog camera+digital hard disk video recorder+computer network system

This is the most widely used network video monitoring system at present. By setting ports, gateways and routes, using digital hard disk video recorders on site as servers, and installing special monitoring software or plug-ins on remote clients' computers, users can see scenes thousands of miles away through the Internet, and realize single-channel and multi-channel video remote monitoring and recording.

2. Analog camera+network video server+computer network system

The signal output by analog camera is analog signal, the signal processed by computer is digital signal, and the digital signal transmitted in network is also digital signal. The network video server converts the analog signal of the analog camera into a digital signal, which is compressed and encoded by an efficient compression chip, and outputs the digital signal that can be transmitted in the computer network, thus realizing the transmission in the form of a digital signal in the computer network. Therefore, the network video server can also be called a video encoder. When one end of the video server is connected with the output signal of the analog camera, and the other end is plugged in with the computer network cable, then a gateway and a route are set on any computer in the Internet, an IE browser is opened, and an IP address or a domain name is input, so that a monitoring screen can be seen on the computer. If the analog camera is equipped with a pan/tilt, we can also control the camera by computer, such as zoom, zoom and rotation. The network video server must be embedded with a real-time operating system, which can be either a Linux version or a Windows version. In terms of stability, the Linux version is better. Using the network video server, different cameras can be selected and equipped, which has greater flexibility.

3. Network camera+computer network system

Network camera is the integration of analog camera and network video server. A/D conversion and video server functions are built into the camera. Like the network video server, it can realize network communication and data transmission according to the network protocol, and can also receive and send alarm signals. This is more convenient, as long as the webcam is installed and plugged in, you can browse.

4.CDMA wireless network video monitoring system

The transmission mentioned above is wired transmission, but it is obviously difficult to use wired transmission in moving vehicles (cars), remote mines and mountainous areas. We can use mature wireless communication technology. The representative product here is China Unicom's Mobile Vision. Mobile video communication CDMA wireless network video monitoring system is a convenient product integrating CDMA data communication function and digital video coding function. It compresses the camera image through the video compression coding module and transmits it to the CDMA network through the intelligent wireless communication terminal, realizing the functions of video data interaction, sending/receiving, encryption and decryption, coding 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 central control terminal of the system through the Internet. The system integrates the advantages of CDMA network and Internet network, and can monitor and manage remotely anytime and anywhere.

Explanation of common technical indexes of intransitive verbs

1, resolution

Image resolution simply refers to the number of points displayed horizontally and vertically on the screen. For example, 1024×728, where "1024" represents the number of points displayed horizontally on the screen and "768" represents the number of points displayed vertically. The higher the resolution, the clearer the image. The higher the resolution, the clearer the image display.

Step 2 be clear

Camera definition is expressed by lines, which are divided into horizontal lines and vertical lines. In practical engineering applications, we often take the horizontal line as the evaluation index of camera definition. The more lines, the higher the definition. The resolution of commonly used black-and-white cameras is generally 450-600, and that of color cameras is generally 330-480. The higher the value, the clearer the image. In general monitoring occasions, a 450-line camera can meet the requirements. For special occasions such as medical treatment and image processing, 600-line camera can get clearer images.

3. Automatic gain control (AGC)

In order to make the camera output standard video signals under different illumination conditions, automatic gain control is introduced into the video processing circuit, and gain feedback control is realized by detecting the average level value of video signals. A camera with AGC function will improve the sensitivity in low illumination, but at the same time it will amplify the interference signal and make the image look messy.

4. Backlight Compensation (BLC)

When the camera shoots in a backlight environment, a black image will appear on the screen. However, the backlight environment is inevitable in safety, and backlight compensation is needed at this time. When the backlight compensation function is introduced, if the camera detects that the video level of a certain area of the captured image is relatively low, the AGC circuit described above can improve and enhance the video level of this area, increase the amplitude of the output video signal, and make the whole image clear and bright. If the theme you want to see darkens because of the bright background, you can set BLC to on state to compensate for the strong backlight.

5. Electronic shutter (EE/AI) switching

We can often see the EE/AI changeover switch at the back end of the camera. EE refers to the electronic shutter mode; AI refers to the automatic aperture lens mode. The electronic shutter of a camera is usually set to the automatic electronic shutter mode. Through the electronic shutter mode, the exposure time of CCD image sensor is adjusted according to the intensity of incident light, so as to obtain a clear image. The electronic shutter time is between1/50-1100000 seconds.

6, signal-to-noise ratio

Refers to the ratio of signal level to clutter level, including power supply clutter, random clutter, single frequency clutter, etc. Usually expressed in decibels (dB). The higher the signal-to-noise ratio, the less clutter and the higher the image signal quality. Signal-to-noise ratio should not be less than 48dB.

7. White balance

In order to restore the color of the object, the color camera must keep the white balance normal.