Technically speaking, the minimum illuminance should be the standard illuminance of the CCD camera at a certain amplitude in the standard TV frame ratio, in units of 1 ux. Of course, this includes the camera at maximum gain and maximum aperture (the so-called maximum aperture, the smallest f-number, for a 1/2-inch ccd camera, typically f1.4, and for a 2/3-inch ccd camera, a typical value is f1. 7); In addition, the reflectivity of the screen should also be 89.9%, color temperature 3,200k, γ is 0.45, automatic inflection point is turned off, and so on. First, the minimum illumination test conditions 1, the maximum gain Usually +18db, but +24db and +30db cameras appear one after another. Due to the difference in maximum gain of the camera, the minimum illumination will be different, which will inevitably lead to confusion. 2, the output level One of the main factors determining the minimum illumination is the output level of the camera, which is 100% (0.7v) or 70% (0.49v) of the standard level. By Robert? As shown in the article "Camera Low Illumination Assessment" (see Table 1), it can be seen that the output levels of the same type, different manufacturers, or the same manufacturer and different types of cameras have different output levels. It can be seen from Table 1 that for broadcast cameras, the average output requirement is 100%, which is relatively uniform; but for the latter three cameras, the requirements are different and there is no uniform standard. It can be seen that the claimed minimum illumination is not Drawn under the unified standard. 3, γ curve As stated above, one measurement condition of the minimum illuminance is γ = 0.45, but not all. There are several other situations that affect the gamma curve: a. Black compression. This is widely used in cameras, the main purpose of which is to reduce noise in low illumination, which obviously affects the gamma curve. b. Automatic knee circuit. This is also widely used in cameras, the purpose of which is to compress the dynamic range under high illumination so that the curve deviates significantly from 0.45, which is why the function is turned off when measuring the minimum illumination. Second, the minimum illumination and s / n Using high gains in low illumination tends to make s/n worse. From the calculations that follow, if the camera gain is 0 db, s = n is 60 db when γ = 1. If the gain is changed to +18db, then s/n is 42db; when the gain is +18db, γ=0.45, s/n is 36db. In order to not excessively deteriorate s/n under low illumination, CCD cameras usually adopt the following measures: 1, chip CCD a. Use a large CCD chip In general, a large CCD chip has a correspondingly large pixel area, and the area of ​​the received light is increased, which inevitably increases the pixel output charge and increases the sensitivity, which tends to improve the overall quality of the camera. For example, 2/3-inch chips are widely used in broadcast, 1/2-inch chips are widely used in professional grades, and 1/3-1/4 inches are widely used in household grades. However, the size of the chip will inevitably increase the size and weight of the camera lens and the dichroic prism, which is one reason for the small chip size of the household model. In addition, it should be noted that even with a larger chip, it may not be possible to make a significant change in s/n. It stands to reason that the pixel area of ​​a 2/3-inch CCD chip is twice that of a 1/2-inch CCD chip, and the pixel area of ​​a 1/2-inch CCD chip is larger than that of a 1/3-inch CCD chip. Doubled, but not twice as high as the corresponding s/n. Because the current chip technology continues to improve, especially the demand for consumption, the 1/2- and 1/3-inch CCD chip structure is more complicated, and the area occupied by the shift register, the transmission gate, the control line, and the channel suspension is reduced. The light-receiving area of ​​the pixel is increased from the surface, and as a result, the sensitivity of these small chips is improved, or s/n is improved. b. Microlens technology A new type of hemispherical microlens is added to each CCD pixel chip, and combined with optical tracking simulation technology, the photoelectric conversion efficiency is greatly improved. C.backthinning technology It can increase quantum efficiency by 1~2 times. This technology is used in professional CCD to improve low illumination performance. d. Refrigeration CCD Although CCD dark current noise or thermal noise and fixed pattern noise are not important at normal temperature, the effect cannot be ignored as the temperature increases. Usually the temperature rises by 10 ° C and the dark current doubles. The use of a unique electronic cooler on the CCD chip can reduce the operating temperature of the chip by about 15 °C. When the temperature of the chip exceeds 35 ° C, the electronic cooler starts to work, so that s / n remains unchanged, which is very effective for the operation of outdoor harsh environments. 1, CCD signal readout Using a two-pixel readout technique, dpr (doublepixelreadout) adds the charge (level) of two CCD pixels together to output a double-level image signal. In this way, the image signal +6db can be improved without increasing the noise level. 2, circuit processing Black compression We know that the gamma coefficient has a great influence on the signal (0.1~0.3v) under low illumination, especially noise. With black compression, the gamma value can be reduced, and the noise is relatively reduced. b. Black cutting By cutting the black level at low illumination, noise can be significantly reduced. c. Reduce bandwidth Low illumination is generally accompanied by high gain. In order to reduce noise, reducing the bandwidth, or reducing the resolution is often used. A common method is to use digital noise reduction technology; comparing video fields, the difference between them is small, and averaging can improve s/n; the disadvantage is that lag is generated. It is also possible to compare averages between pixels in one field, but the resolution is reduced. As mentioned above, the minimum illuminance value is subject to multiple conditions. Under normal conditions, the minimum illuminance value can be derived from camera sensitivity: if the sensitivity of a camera is f8, 2,000 lux (0 db), it is equivalent to f1.4. , 62.5 lux (0 db), or f1.4, 7.8 lux (+18 db). The principle is that for every aperture reduction, 2,000 lux is doubled; for every +6 db gain, 2,000 lux is also doubled. Therefore, the minimum illumination of the camera is 7.8 lux at 100% signal output and 3.2 lux at 70% signal amplitude output. The calculation process: f8 → f1.4, after 5 stops (f8, f5.6, f4, f2.8, f1.4), then 2,000 lux should also be divided by 25, ie 2,000 lux / 25 = 62.5 lux . Gain 0db → 18db (6db & TImes; 3) illumination 62.5lux / 23 = 7.8lux. If the camera s/n is 60db (condition: gain 0db, γoff), then at a gain of +18db, s/n is reduced by +18db, which is 42db(γoff). However, the minimum illuminance is measured at γ = 0.45, so s/n drops to 36 db. If no noise reduction measures are taken, the s/n will only be 24db when the gain is +30db, which seriously affects the image quality. This is why the corresponding noise reduction measures must be taken at high gain to improve the s/n. Taking into account the above measures taken to improve s/n in low illumination situations, we list the minimum illumination and the estimated s/n in this case: Minimum illumination = 2,000/2 [fs/fmin] & TImes; 2 [g/6] & TImes; nbin & TImes; nflux s / n (minimum illumination) ≦ s / n (0db) - g + 10 × log10 (nbin × nf) db Where: fs is the number of apertures measured in the standard state, fmin is the number of apertures under the minimum illumination (ie, the maximum aperture of the camera), g is the gain value under the lowest illumination, and nbin is the number of readings when the CCD is read and the reading is nf A few points (noise reduction). If fs=f8, fmin=f1.4, s/n(0db)=60db, g=24db, nbin=2, nf=1, then: Minimum illumination = 2lux s/n (minimum illumination) ≦39db The reason why it is taken because of photon noise is generally reduced by 3~5db. If g is 30 db when calculating the minimum illuminance, and other conditions are as described above, the calculation result is 1 lux. As mentioned above, since the minimum illumination has not yet been internationally standard, camera manufacturers often give some additional conditions in the description of the minimum illumination of the camera, such as dxc-637 produced by Sony Corporation: the minimum illumination is 1 lux, and the additional condition is +30 db. Dpron, f1.7; z-1800 produced by Hitachi, with a minimum illumination of 1.5 lux, with an additional condition of f1.8, a gain of +24 db, and an ultragain (super gain) on. Interchangeable Power Adapter,5W Interchangeable Power Adapter,20W Interchangeable Power Adapter,Interchangeable Power Adaptor Adapter Guangdong Mingxin Power Technologies Co.,Ltd. , https://www.mxpowersupply.com