Signal-to-noise ratio (SNR) in the numerical is refers to the ratio of signal to noise voltage, its physical meaning is based on the determination of each primitive signals and the uncertainty of relative size; In applications that require precise metering image, high signal-to-noise ratio is particularly important.

Signal-to-noise ratio sharply decreased with temperature

Generally speaking, the main noise source of the CCD has the following three: first, the photons into device into a photoelectric load generated by noise; Second, photoelectric charge transfer, the noise produced by the charge fluctuation; Third, the photoelectric charge to read when the noise. Thermodynamic law pointed out: the higher the temperature, the greater the average kinetic energy of electrons, the photoelectric conversion, the higher the uncertainty in the process of transfer and output, which means the CCD noise. In addition, the occurrence of Einstein's theory of the photoelectric expounds the photoelectron is associated with incident photon, which has nothing to do with temperature, which means the CCD signal strength does not increase with the increase of temperature. As a result, the signal-to-noise ratio of the CCD sharply decreased with the rise of temperature.

Cooling device can prevent CCD aging

It is interesting to note that the signal-to-noise ratio of the CCD is not only related to temperature, and is associated with the aging degree of CCD device. By contrast, you can see that new CCD device with high signal-to-noise ratio. Common factors of high temperature aging is to speed up the electronic devices, this is the temperature on the CCD imaging quality of another long-term impact.

In order to reduce the influence of environment temperature on CCD SNR, radiator for CCD is effective measures. Figure 2 line A for CCD has the radiator, cooling line B said depend on the chip itself. It can be seen that the same CCD, a radiator signal-to-noise ratio is not about two times that of the radiator.

Dark current of CCD imaging quality is very bad

Dark Current (Dark Current) is caused by thermal excitation produced by electron - hole right. In the real world, all of the image sensor is affected by dark current, the reason is that we are unable to create an environment that is the kelvin's absolute zero temperature. For semiconductor devices, as long as its temperature is not kelvin's absolute zero, the electrons within the components - hole of forever in a generation, migration and the dynamic balance of annihilation. The higher the temperature, electron - hole produced and the rate of migration of the faster, means the larger the dark current. It should be pointed out that the change of temperature and the size of the dark current is non-linear, for example, every 6 ℃ below the temperature, dark current about cut in half.

Dark current measurement unit is electronic/figure yuan/SEC. In normal temperature, the value of dark current is about 250-1000, a figure yuan per second produced 1000 electronic power is not big, but you know, for a 512 x512 CCD, the figure number is about 262000 yuan, per second to read the figure of about $6553600, so it is no exaggeration to "undercurrent".

In the process of production of CCD, the widespread adoption of technology is "photolithography mask + ion penetration". In this process there is no guarantee that the CCD each figure yuan each lattice has exactly the same stability, in other words, each figure yuan under the same conditions produce electronic - hole on the ability of different. The defects of the CCD itself is one of the important reasons that dark current, and the defects of the device itself also makes the generation of dark current is uneven, in CCD in the lighting environment also can produce a fixed pattern. Dark current of the CCD imaging quality is very bad, it also limits the sensitivity and dynamic range of the device.

Cooling device can improve the CCD imaging quality

As far as possible to reduce the temperature of the CCD for increasing a number of technical indicators of CCD is very good. Especially in the hot summer, how to CCD "cool" to become a concern in many engineering applications. In the camera all-weather shield to join the solution of the fan is a mature, but did not directly reduce the temperature of the CCD. It is important to note that the Japanese HARADA (HARADA) company launched this year a few cameras, join the radiator in the machine core directly, in the aspect of improving the quality of the CCD imaging has made the beneficial attempt.