Technical indicators and test methods for surveillance cameras

Camera classification

Cameras on the market today can be roughly classified in the following ways:

According to the color of the image: Colour ( Color ), Black and White ( B/W ), Color automatically turn black and white (automatically converted according to the size of the ambient light);

According to the sensor points: CCD , CMOS ;

According to the size of the sensor: 1" , 2/3" , 1/2" , 1/3" , 1/4" ;

According to the power supply: DC 12V , AC 24V , DC/AC water account 24V , AC 220V ;

According to the sensitivity (minimum illuminance) points: conventional (general) camera color 1 ~ 3lux (F1.2 , F1.4) , black and white 0.05 ~ 0.1lux (F1.2 , F1.4) ; lower illumination camera color 0.5 ~1lux(F1.2) , black and white 0.01~0.05lux(F1.2) ; low illumination camera is black and white 0.005~0.01lux(F1.2) ; Starlight camera is black and white 0.0001~0.005lux(F1.2) ;

According to the synchronization mode: random synchronization (internal synchronization), external synchronization, power synchronization, VD2 (pulse) synchronization;

According to the lens interface points: C format, CS format.

The main technical indicators of the camera

There are two technical standards currently used in camera measurement: one is the national standard GB12338-90 "black and white universal application TV camera measurement method", the other is the international standard IEC1146-1-1994 "color television camera ( PAL/SECAM/NTSC ) measurement method 》 The main technical indicators of the camera include:

polarity

There are positive and negative polarity points. In the positive polarity, the bright area level is higher than the dark area level and the sync head is a negative level; in the negative polarity state, the bright area level is lower than the dark area level and the sync head is a positive level.

Output amplitude

Refers to the peak-to-peak signal of the highest white level to the top of the sync pulse at a 75 Ω load (standard amplitude is 1Vp-p , where the video level is 0.7Vp-p and the sync head is 0.3Vp-p ).

Minimum illumination

When the amplitude of the video signal at the output of the camera is reduced to “a specified value”, the illumination on the subject is held. This “a certain value” standard is different from that of each manufacturer. Please pay attention to the reader. In addition, the minimum illuminance of the test is affected by many factors, including:

1. The defined value of the useful signal (black level to white level) varies from manufacturer to manufacturer. The camera with the same minimum illuminance has different effects due to the different amplitudes. For the chooser, this is to compare against the same conditions;

2. The same camera in different apertures, the minimum illumination is not the same. The larger the aperture, the larger the representative aperture, the lower the illumination will be reduced (that is, the sensitivity will increase);

3. Minimum illumination color temperature is also affected, such as: GB12338-90 standard is 28560K, EIA standard is 3100K;

4. The minimum illumination is also related to the reflectance (target reflection and background);

5. The minimum illumination is also related to the size of the AGC , the shutter status, and so on.

Active pixels (pixels)

For CCD and CMOS cameras, the effective primitives and sharpness are directly related, and generally they are linear. Taking a black-and-white camera as an example, the pixel is 752 × 582 (CCIR) and the horizontal resolution is 570TVL . If the pixel is 795 × 596 , the horizontal resolution is generally 580TVL . The camera circuit has a certain degree of improvement and reduction.

Resolution (clarity)

It refers to the ability to discriminate the minimum width of a black-and-white line from the same line of black and white lines of a frame, and the maximum number of lines per frame height identified on a common test chart. Resolving power is divided into horizontal resolution (H) and vertical resolution (V) ; there are also central resolution and edge resolution.

Brightness discrimination level

Also known as gray scale, it is the brightness level that can be distinguished from the blackest to the whitest in a television image.

Letter to credit ratio

It is the ratio of the peak-to-peak amplitude of the useful signal amplitude in the video signal to the value of the random clutter, expressed in decibels (dB) . The SNR is divided into unweighted SNR and weighted SNR. The weighted SNR refers to the addition of a 6 MHz low-pass filter and a 100 kHz high-pass filter between the camera output and the video clutter measurement instrument . In addition, there is a distinction between the luminance signal-to-noise ratio and the chroma signal-to-noise ratio.

scanning frequency

The number of scanning electron beam repetitions per unit of time, usually expressed in Hertz (Hz) .

Synchronously

Divided into external synchronization, internal synchronization (random synchronization), power synchronization, VD2 pulse synchronization. Generally better cameras will have external synchronization, internal synchronization, power synchronization, and even VD2 pulse synchronization (such as Panasonic products). In general, DC-powered cameras have internal synchronization (random synchronization). The advantages are simple, and the disadvantage is poor synchronization. Especially in systems with video switchers and video matrices, the image will jump and not appear during the switching process. Steady phenomenon.

Camera main technical index measurement method

The main technical indicators of the camera measurement methods, mainly " IEC1146-1-1994 standard" and " GB12338-90 standard." " IEC1146-1-1994 standard" measurement device includes: (1) test card: optional definition card, multi-function card. (2) Light source: 3100K , two side light (here is the reflection card). (3) Oscilloscope: Sorting line oscilloscope, sagittal oscilloscope, and video integrated tester. (4) Black-and-white monitor: A monitor with a horizontal resolution of 1000 lines and a screen-down function.

The " GB12338-90 standard" measurement of basic devices include: (1) test card: sub-transmission TV integrated test chart, transmissive TV linear test chart, transmissive TV comprehensive test chart B , transmissive TV gray test chart A type. (2) Light source: A type light source, color temperature 2856K . (3) TV test signal generator. (4) Oscilloscope (5) Monitor: B/W ≥ 800 lines

Measurement of polarity

Including the measurement of the polarity output amplitude, both can be measured with an oscilloscope and terminated with a 75 Ω load.

Minimum illuminance measurement

First, there should be a light source that can adjust the illuminance. The reflective test card is the incident light source, or the transmissive test card is a transmission light source for adjustment. It should be noted that the outdoor illuminance from 1 × 10-4 - 2.5 × 104lux, 1 × 10-4 refers to the night light pollution (Star-class), 2.5 × 104lux means under strong sunlight. Light meter can use Japan Minolta low light meter.

Measurement of effective pixels (pixels)

The TER company's video integrated measuring instrument VM-700A can be used , plus the accessories of the CCD camera. This not only can measure the effective picture element (pixel), but also can measure the number of defects of the CCD device (poor number of pixels).

Sharpness measurement

Black-and-white cameras can use the GB12338-90 test equipment and methods of the national standard , ingest the test chart of GB6996.1 with a video camera , observe the maximum number of lines that can be distinguished on the wedge of the center of the image on the monitor. The color camera can use the test equipment and method of the international standard " IEC1146-1-1994 Color Camera ( PAL/SECAM/NTSC ) Measurement Method". Taking a color camera (the horizontal resolution nominal value is 330TVL ) as an example, it has been proved through experiments that the requirement of the visual measurement method and the waveform measurement method is the highest. If the visual measurement method is 330TVL , the waveform measurement method is followed by 300TVL . This is because the human eye has a delay effect, which generates a certain error; using the waveform analyzer (selected oscilloscope) as a line selection measurement, you can move from the high TVL to the low TVL , to the critical waveform stability setting, read the line The corresponding value of TVL is more scientific.

In addition, there are differences between black and white monitors and color monitors. If a black-and-white monitor ( ≥1000TVL ) is used, the camera's horizontal resolution is measured as 330TVL ; if a color monitor ( ≥500TVL ) is used and the camera is horizontally deterministic, 300TVL is measured .

Brightness discrimination level measurement

Also known as the gray scale measurement method, refers to the camera ingesting the GB699.12 gray scale test card, which uses the visual method to measure the maximum distinguishable brightness level on the monitor. It should be noted that: (1) Other grayscale cards are also available, typically more than ten cards. (2) The brightness and contrast knobs of the monitor can be adjusted to measure at the maximum resolution.

Signal to noise ratio measurement

GB12338-90 standard measurement method

The measurement method is that the camera ingests a grayscale test chart of GB6996.12R so that the amplitude of the video signal at the output of the camera is 0.7VP-P . After covering the mirror cover, the signal to noise ratio is read on the video clutter measuring instrument. It should be noted that the automatic control ( AGC ) and correction circuit of the camera should be turned on during the test, and a 6MHZ low-pass filter and a 100KHZ high-pass filter should also be connected .

Half video amplitude measurement

Position the camera at the grayscale card and adjust the aperture so that the video output level is 350mVp-p . At this point to disconnect the camera's AGC and correction circuit, and access 6MHZ low-pass filter and 100KHZ high-pass filter. It should be noted that the noise at white level ( 700mVp-p ), medium white level ( 350mVp-p ), and black level ( 0.0mVp-p ) is different. The TEK-VM700A can also measure the video noise spectrum.

Scan frequency measurement

The video integrated tester TEK-VM700A can also be used to measure with a digital time scale oscilloscope. In general random synchronization (internal synchronization) cameras ( PAL ) , the error of the frame frequency and line frequency and 24Hz and 15625Hz is relatively large, and the cameras of the same type also have deviations from each other. The power-synchronized camera frame rate and line frequency are 25 Hz and 15625 Hz .

The information in this article comes from the Internet and was reorganized and edited by China Rescue Equipment Network.