Digital knowledge: the principle of CCD

When we talk about the size of CCD , we are actually referring to the area of ​​the photosensitive device, which includes CCD and CMOS. The larger the area of ​​the photosensitive device, the more photons it captures, the better the photosensitivity, and the lower the signal-to-noise ratio. CCD/CMOS is a component used by digital cameras for photosensitivity and imaging, which is equivalent to the film in traditional optical cameras.

The surface of the photosensitive components on the CCD has the ability to store electric charges and is arranged in a matrix. When the surface senses light, the electric charges will be reflected on the components. The signals generated by all the photosensitive components on the entire CCD form a complete picture.

If you decompose CCD, you will find that its structure consists of three layers: the first layer is the "micro lens", the second layer is the "color separation filter" and the third layer is the "photosensitive layer".

The first layer of "micro lens"

We know that the key to digital camera imaging lies in its photosensitive layer. In order to expand the light collection rate of CCD, the light receiving area of ​​a single pixel must be expanded. However, the method of increasing the light collection rate is also likely to reduce the image quality. This layer of "micro lens" is equivalent to adding a pair of glasses in front of the photosensitive layer. Therefore, the photosensitive area is no longer determined by the opening area of ​​the sensor, but by the surface area of ​​the micro lens.

The second layer is the "color separation filter"

The second layer of CCD is the "color separation filter". There are currently two color separation methods, one is the RGB primary color separation method, and the other is the CMYK complementary color separation method. These two methods have their own advantages and disadvantages. First of all, let's understand the concepts of the two color separation methods. RGB is the three primary color separation method. Almost all colors that can be recognized by human eyes can be composed of red, green and blue. The three letters RGB are Red, Green and Blue, which means that the RGB color separation method is adjusted through the colors of these three channels. Let's talk about CMYK, which is composed of the colors of four channels, namely cyan (C), magenta (M), yellow (Y), and black (K). In the printing industry, CMYK is more applicable, but the colors it adjusts are not as many as RGB.

The advantage of primary color CCD is sharp image quality and true color, but the disadvantage is noise problem. Therefore, you can note that the ISO sensitivity of digital cameras using primary color CCD will not exceed 400. In contrast, complementary color CCD has an additional Y yellow filter, which is more detailed in color resolution, but it sacrifices some image resolution. In terms of ISO value, complementary color CCD can tolerate higher sensitivity, which can generally be set above 800.

The third layer: photosensitive layer

The third layer of CCD is the "photosensitive film", which is mainly responsible for converting the light source passing through the color filter into electronic signals, and transmitting the signals to the image processing chip to restore the image.

The size of traditional camera film is 35mm, 35mm is the diagonal length, and the photosensitive area of ​​35mm film is 36 x 24mm. When converted to digital cameras, the closer the diagonal length is to 35mm, the larger the CCD/CMOS size. Many SLR digital cameras have a CCD/CMOS size close to 35mm, such as Nikon D100, the CCD/CMOS size area reaches 23.7 x 15.6, which is much larger than consumer-grade digital cameras, while the CMOS size of Canon EOS-1Ds is 36 x 24mm, reaching the area of ​​35mm, so the imaging is relatively good.

Currently, there are four types of consumer digital cameras on the market: 2/3 inch, 1/1.8 inch, 1/2.7 inch, and 1/3.2 inch. The larger the size of the CCD/CMOS, the larger the photosensitive area and the better the imaging effect. A 1/1.8-inch 3-megapixel camera is usually better than a 1/2.7-inch 4-megapixel camera (the latter has a photosensitive area of ​​only 55% of the former). While increasing the pixels of the same size CCD/CMOS is certainly a good thing, it will also lead to a reduction in the photosensitive area of ​​a single pixel, which may result in underexposure. However, if you want to maintain the existing image quality while increasing the CCD/CMOS pixels, you must increase the total area of ​​the CCD/CMOS while at least maintaining the area of ​​a single pixel. At present, it is difficult to manufacture larger CCD/CMOS, and the cost is also very high. Therefore, digital cameras with larger CCD/CMOS sizes are also more expensive. The size of the photosensitive device directly affects the volume and weight of the digital camera. Ultra-thin and ultra-light digital cameras generally have small CCD/CMOS sizes, and the more professional the digital camera, the larger the CCD/CMOS size.