Introduction to the Knowledge of Image CCD






What is a CCD?

CCD is a charge coupled device. It uses charge meter to express signal size and transmits signal by coupling mode. It has the advantages of self-scanning, wide sensing spectrum range, small distortion, small volume, light weight, low system noise, low power consumption, long service life and high reliability, and can be made into a series of highly integrated devices. Components.



What is the role of CCD?

CCD image sensor can directly convert optical signal into analog current signal. The current signal is amplified and converted to analog-to-digital, which can realize image acquisition, storage, transmission, processing and reproduction.





CCD related technical parameters

1. Pixels: This is a common parameter. In the case of chip determination, the higher the pixel is, the lower the sensitivity is, and the two are inverse ratio relations; therefore, the higher the pixel is, the better the sensitivity is. It is suggested that the sensitivity should be ensured as first as possible when the pixel is enough; as for when the pixel is enough, the resolution required is about 1139 * 85 for 100 * (n.a.1.40) objective lens and 0.63 * interface. 8, that is, 1 million pixels;


2. Dynamic range: In fact, this parameter depends on the other two parameters. The higher the dynamic range = 20 * log10 (full well electron / total noise) is, the higher the sensitivity of CCD is.


3. Full-well electrons: From the calculation of dynamic range, it can be seen that the larger the number of full-well electrons, the better.


4. Noise: Simply understood as clutter signal, there are readout noise and dark noise. The extra noise of camera electronic components in image processing is related to electronic efficiency. It has fast reading speed, high noise, high temperature and dark noise.


5. Refrigeration: when CCD works, the temperature will rise, which will produce noise, especially long exposure time (if fluorescence photography and other conditions require a longer exposure time). If the temperature drops, this kind of noise can be reduced. So we can see that there are many refrigeration methods, such as fan, semiconductor refrigeration, water cycle system. Cold and liquid nitrogen refrigeration, the lower the refrigeration, the better noise reduction, but the higher the cost;


6. Grey scale: usually write how many bits, this value is better, so that in some levels of more or difficult to distinguish on the photography of the map will help, the common is the hospital blood department blood smear photography: red blood cells are very thin and many, often observed under the microscope will find that there are many overlapping, the human eye is also more than It is better to distinguish the overlapping parts, but if you switch to the ccd, it needs more than 12 bits, preferably 14 bits. For gray scale analysis or fluorescence quantitative analysis, the gray scale is better than the high point.


7. Chip size: Because of the inverse relationship between pixels and sensitivity, the chip size is naturally larger.


8. Speed: The faster the better, but pay attention to distinguish: some manufacturers are writing and reading speed, some are writing preview speed, some are writing acquisition speed; reading speed is not necessarily high you preview, acquisition speed, because it is also affected by the back interface, computer and so on; when preview speed is compared, pay attention to resolution. How much, the acquisition speed is relatively good, because his changes are basically only affected by the level of computer configuration;


9. Interface: An important factor affecting speed, generally speaking, the speed of USB is the slowest, followed by 1394, and the serial port is fast; however, the compatibility is quite the opposite.


10. Bining: This is a common way to improve the preview and acquisition of ccd. The higher the binning support, the faster the speed will be, but at the expense of resolution - in fact, it calculates several pixels as one pixel, such as 2 x 2, or four pixels as one pixel.


11. Exposure time: My personal habit is to see how long the exposure time is, and the longer the support time is, the better when shooting weak light. As for the minimum exposure time, it can reflect the sensitivity of CCD side by side in principle, but there are many conditions to refer to, so I seldom pay attention to this parameter.


12. gain: The larger the gain, the shorter the exposure time required, but the corresponding noise will increase.







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