What is a gun camera? What are the characteristics?

1. The number of pixels

The number of pixels refers to the maximum number of pixels of the gun camera CCD sensor, and some of them give the number of pixels in the horizontal and vertical directions, such as 500H× 582V, and some give the waited product values ​​of the first two, such as 300,000 pixels. For a CCD chip of a certain size, the larger the number of pixels, the smaller the area per pixel unit, and the higher the resolution of a gun camera composed of the chip.

2. Resolution

Resolution is an important parameter to measure the advantages and disadvantages of black and white gun cameras. It means that when a gun camera takes equally spaced black and white stripes, it can be seen on a monitor (which should have a higher resolution than the camera). When the number of lines exceeds this number, only a grayed-out screen can be seen and black and white lines can no longer be distinguished.

The resolution test usually uses a gun camera to shoot the resolution test card shown in FIG. 1 and read the data through the waveform monitor. The specific method is as follows:

(1) Arrange the test environment. Place the resolution test card on the standard test light box, about 3m away from the camera. The video output of the gun camera is connected to a waveform monitor and the output of the waveform monitor is connected to a high resolution black and white monitor.

(2) The illuminance of scenery is set to 2000lx, and the color temperature of the light source is set to 3200K.

(3) Adjust the lens focal length (or select the appropriate fixed-focus lens and move the camera forward or backward slightly) to fill the monitor screen with the image of the resolution card and sharpen the image with precise focus.

(4) Observe the resolution card shown in Figure 1 with an optional oscilloscope to obtain the waveform shown in Figure 3.

(5) Adjust the lens aperture so that the signal white level reaches 100% (70mV). At this time, the number of TV lines whose modulation is deeper than 5% is the limit resolution.

3. Minimum illumination

The minimum illumination is also an important parameter to measure the merits of the gun camera. Sometimes the word “lowest” is omitted and the “illumination” is abbreviated. The minimum illuminance is a luminosity value of the scene when the illuminance of the subject to be shot is low to a certain extent and the level of the video signal output by the gun camera is as low as a predetermined value. When measuring this parameter, the maximum relative aperture of the lens should also be specified. For example, with the F1.2 lens, when the brightness value of the subject is as low as 0.04LX, the amplitude of the video signal output by the camera is 50% of the maximum amplitude, ie, 350mV (the maximum amplitude of the standard video signal is 700 mV), the minimum illumination of this gun camera is called 0.04LUX/F1.2. The brightness value of the subject is no more low, and the amplitude of the video signal output by the gun camera cannot reach 350 mV. This is reflected on the screen of the monitor and it will be a gray image with difficulty in distinguishing levels on one screen.

At present, due to the fact that the market's specifications are not yet uniform, the low illumination characteristics of some gun cameras with low illumination indices may not be as good as the low illumination characteristics of gun cameras with slightly higher illumination indexes. The reason for this is that the standards used in the measurement of low-lightness indicators are different. Still taking the aforementioned example as an example, if the video signal amplitude output by the camera is reduced to 30% of the maximum amplitude (that is, 210 mV) as a reference, the brightness value of the imaged object can be further lowered, such as 0.03LX. If the optical lens is further replaced with an F1.0 lens, the luminous brightness of the subject can be further reduced due to the increase of the luminous flux, such as 0.02LX.

It can be seen from the above analysis that it is impossible to accurately determine which camera has low illuminance characteristics by putting aside the measurement standard and simply comparing the illuminance nominal value of a certain brand of gun camera with the illuminance nominal value of another brand of gun camera. Good conclusions, because the relative aperture of the lens they use and the specified value of the output video signal may not be the same. Therefore, the minimum illuminance index can only be used for designing and purchasing equipment.

4. Signal to noise ratio

The SNR is also a major parameter of the gun camera. When a gun camera captures bright scenes, the screen displayed by the monitor is usually brighter, and observers cannot easily see the interference noise in the screen. When the camera captures a dark scene, the screen displayed by the monitor is darker. At this time, it is easy to see snow-like noise in the picture. The strength of the interference noise (that is, the degree of influence of the interference noise on the picture) is directly related to the quality of the gun-type camera signal-to-noise ratio. That is, the higher the signal-to-noise ratio of the gun camera, the greater the influence of the interference noise on the picture. small.

The so-called 'signal-to-noise ratio' refers to the ratio of the signal voltage to the noise voltage, which is usually represented by the symbol S/N. Since in general the signal voltage is much higher than the noise voltage, the ratio is very large. Therefore, the actual calculation of the camera signal-to-noise ratio is usually based on the ratio of the mean squared signal voltage to the mean square noise voltage. The number is multiplied by a factor of 20, that is, the unit of equation (2-3) is expressed in dB.

The signal-to-noise ratio given by the general gun type camera is a value when the AGC (automatic gain control) is turned off, because when the AGC is turned on, the small signal is boosted, so that the noise level is also increased accordingly. The typical value of the CCD gun camera signal to noise ratio is generally 45dB ~ 55dB. When measuring the SNR parameter, a video clutter measuring instrument should be used to connect directly to the video output terminal of the gun camera.