CCD camera instruction manual introduction

Technically speaking, the minimum illuminance should be at the standard TV frame rate, and the output level of the CCD camera is the required screen illuminance at a certain amplitude, whose unit is 1ux. Of course, this includes the camera at maximum gain and maximum aperture (the so-called maximum aperture, that is, the smallest f-number. For a 1 / 2-inch ccd camera, the typical value is f1.4, and for a 2 / 3-inch ccd camera, the typical value is f1. 7); In addition, the reflectivity of the screen should also be 89.9%, the color temperature is 3,200k, γ is 0.45, the automatic turning point is turned off and so on.

1. Test conditions for minimum illumination

1. Maximum gain

Usually + 18db, but + 24db and + 30db cameras appear one after another. Due to the difference in the maximum gain of the camera, the minimum illumination will also be different, which will inevitably cause confusion.

2. Output level

A major factor in determining the minimum illuminance is the output level of the camera, which is 100% (0.7v) or 70% (0.49v) of the standard level. What is more reasonable. By Robert? According to the information in the article "Evaluation of Low Illumination of Cameras" (see Table 1), it can be seen that the output level requirements of cameras of the same type, different manufacturers, or the same manufacturer and different types are different.

It can be seen from Table 1 that for broadcast-grade cameras, the average output power requirement is 100%, which is relatively uniform; but for the latter three cameras, the requirements are different and there is no unified standard. It can be seen that the claimed minimum illumination is not Derived under uniform standards.

3. γ curve

As mentioned above, one of the measurement conditions for the lowest illuminance is γ = 0.45, but this is not all true. There are several other situations that affect the gamma curve:

a. Black compression. This is widely used in cameras, and its main purpose is to reduce noise at low illumination, which obviously affects the γ curve.

b. Automatic knee point circuit. This is also widely used in cameras. The purpose is to compress the dynamic range under high illuminance, so that the curve deviates significantly from 0.45, which is why the function should be turned off when measuring the minimum illuminance.

2. Minimum illumination and s / n

If high gain is used in low illumination, s / n will inevitably deteriorate. It can be seen from the following calculation that if the camera gain is 0db, s / n is 60db when γ = 1. If the gain is changed to + 18db, the s / n is 42db; when the gain is + 18db, γ = 0.45, the s / n is 36db. In order not to make s / n excessively deteriorate under low illumination, CCD cameras usually adopt the following measures:

1. Chip CCD

a. Using large CCD chip

Generally speaking, a larger CCD chip has a larger pixel area. The larger the area of ​​the received light, the more the output charge of the pixel and the higher the sensitivity. It is easy to improve the overall quality of the camera. For example, 2 / 3-inch chips are widely used in broadcast grades, 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 volume and weight of the camera lens and dichroic prism, which is also a reason for the small chip size used in household models. In addition, it should be noted that even using a larger chip may not make s / n significantly change. It stands to reason that the pixel area of ​​the 2 / 3-inch CCD chip is twice that of the 1 / 2-inch CCD chip, and the pixel area of ​​the 1 / 2-inch CCD chip is also larger than the pixel area of ​​the 1 / 3-inch CCD chip. Double, but not double the corresponding s / n. Because the current chip technology is constantly improving, especially in response to consumer demand, the structure of 1/2, 1 / 3-inch CCD chips is made more complicated, which reduces the area occupied by shift registers, transmission gates, control lines, and channel suspension. The light receiving area of ​​the pixels is increased from the surface, and as a result, the sensitivity of these chiplets 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 light tracking simulation technology, the photoelectric conversion efficiency is greatly improved.

c.backthinning technology

It can increase the quantum efficiency by 1 to 2 times. This technology is used in professional-grade CCD, which can better improve the performance of low illumination.

d. Cooling CCD

Although CCD dark current noise, or thermal noise and fixed pattern noise are not important at room temperature, as the temperature increases, its impact cannot be ignored. Usually the temperature increases by 10 ℃, and the dark current doubles. Using a unique electronic cooler on the CCD chip can reduce the working temperature of the chip by about 15 ℃. When the temperature of the chip exceeds 35 ℃, this electronic cooler starts to work, so that the s / n remains unchanged, which is very effective for eng operating in harsh outdoor environments.

1. CCD signal readout

Using dual-pixel readout technology, dpr (doublepixelreadout) adds the charge (level) of two CCD pixels together, thereby outputting a double-level image signal. In this way, the image signal + 6db can be improved without increasing the noise level.

2. Circuit processing

a. Black compression

We know that the gamma coefficient has a great influence on the signal (0.1 ~ 0.3v) under low illumination, especially noise. Using black compression can reduce the γ value, which relatively reduces noise.

b. Black cut

By cutting the black level at low illumination, the noise can be significantly reduced.

c. reduce bandwidth

Low gain is generally accompanied by high gain. In order to reduce noise, reduce bandwidth, or reduce clarity is often used. The commonly used method is to use digital noise reduction technology; comparing the video field, the difference between them is not large, and the average can increase s / n; the disadvantage is that it produces lag. It is also possible to compare the average of pixels in a field, but this reduces the resolution.

As mentioned above, the minimum illuminance value is subject to multiple conditions. Under general conditions, the minimum illuminance value can be derived from the camera sensitivity: if the sensitivity of a camera is f8, 2,000 lux (0db), it is equivalent to f1.4 , 62.5lux (0db), or f1.4, 7.8lux (+ 18db). The principle is that 2,000 lux is doubled every time the aperture is reduced; 2,000 lux is doubled every time the gain is increased by + 6db. Therefore, in the case of 100% signal output, the minimum illumination of the camera is 7.8lux, and in the case of 70% signal amplitude output, the minimum illumination is 3.2lux.

Estimation process: f8 → f1.4, after 5 stops (f8, f5.6, f4, f2.8, f1.4), then 2,000lux should also be divided by 25, that is, 2,000lux / 25 = 62.5lux .

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 decreases + 18db to 42db (γoff). But the minimum illuminance was measured under the condition of γ = 0.45, so the s / n dropped to 36db at this time. If no noise reduction measures are taken, when the gain = + 30db, the s / n will only be 24db, which seriously affects the image quality, which is why the corresponding noise reduction measures must be taken to increase the s / n at high gain.

Taking into account the above measures currently taken to improve s / n under low illuminance, we list the minimum illuminance in this case and the estimated value of s / n:

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

Among them: fs is the number of sensitivity apertures measured in the standard state, fmin is the number of apertures under the lowest illuminance (that is, the maximum aperture of the camera), g is the gain value under the lowest illuminance, nbin represents the number of lines and reading when the CCD is read, nf Several fields of integration (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 ≦ is taken is that considering photon noise, it should generally be reduced by 3 ~ 5db. If g is 30db when calculating the minimum illuminance, and other conditions are as described above, then the calculation result is 1lux.

As mentioned above, since there is no international standard for minimum illumination, camera manufacturers often give some additional conditions in the description of the minimum illumination index of cameras, such as dxc-637 produced by sony: the minimum illumination is 1lux, the additional condition is + 30db, dpron, f1.7; z-1800 produced by Hitachi, with a minimum illuminance of 1.5 lux, an additional condition of f1.8, a gain of + 24db, and ultragain (super gain) on.