Digital Camera Secret Series--CCD and CMOS

The birth of digital cameras not only created new photography experiences and equipment, but also directly or indirectly created many new terms with the sudden increase in the application and knowledge of electronic components. For those who often use digital cameras, these terms may already be familiar. However, it may not be that easy to explain them completely clearly. So I specially organized a few groups of commonly used nouns to make it easier for everyone to understand digital cameras. Please pay attention to our series. In the first issue, I will start with the CCD that replaces traditional camera film. In fact, this is also the most important point of digital cameras.

1. Traditional CCD:

I believe many friends know the importance of CCD and that it is an important component that determines the performance of digital cameras, but we really need to understand CCD. It’s not just one or two sentences.

1. Understanding CCD

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Figure 1

CCD (Charge Coupled Device) is a semiconductor in a digital camera that can record changes in light (as shown in Figure 1), usually measured in megapixels. The number of megapixels in digital camera specifications refers to the resolution of the CCD, which also refers to the number of photosensitive components on the CCD of this digital camera.

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Figure 2

Photosensitive component on CCD The surface has the ability to store charges and is arranged in a matrix (as shown in Figure 2). When the surface senses light, the charge will be reflected on the component. The signals generated by all the photosensitive components on the entire CCD constitute a complete picture. Therefore, CCD is usually used in digital cameras and scanners as photosensitive components.

2. The "sandwich" structure of CCD

If you dissect the CCD, you will find that the structure of the CCD is like a sandwich. The first layer is the "micro lens", and the second layer is It is the "color separation filter" and the third layer "photosensitive layer". You must be wondering why the "lens" is directly mounted on the CCD?

The first layer of "micro lens"

Actually, this is a typo in English translation: "ON-CHIP MICRO LENS", which was developed by SONY in the early 1980s. The technology comes out. This is to effectively increase the total pixels of the CCD, but also to ensure that a single pixel continues to shrink to maintain the standard area of ??the CCD. Therefore, the light-receiving area of ??a single pixel must be expanded. However, increasing the aperture ratio (lighting rate) to increase the light-receiving area actually worsens the image quality. Therefore, the aperture rate can only be increased to a certain limit, otherwise the CCD will become inferior. To improve this problem, SONY took the lead in installing tiny lenses on each photodiode (single pixel). This design is like putting on glasses for a CCD. The photosensitive area is no longer determined by the opening area of ??the sensor, but by the surface area of ??the micro lens. In this way, the size of a single pixel can be taken into account, and the aperture ratio can be increased in terms of specifications, so that the sensitivity can be greatly improved (Figure 3).

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Figure 3

The second layer is "Color separation filter"

The second layer of CCD is the "color separation filter". There are currently two color separation methods, one is RGB primary color separation method, and the other is CMYG complementary color color separation method. Both methods have advantages and disadvantages. However, in terms of output, the output ratio of primary color and complementary color CCD is about 2:1.

The advantage of primary color CCD is sharp image quality and true colors, but the disadvantage is noise problem. Therefore, everyone can note that the ISO sensitivity of digital cameras that generally use primary color CCD will not exceed 400. In contrast, the complementary color CCD has an extra Y-yellow color filter, which is more detailed in color resolution, but it sacrifices part of the image resolution. In terms of ISO value, the complementary color CCD can tolerate higher sensitivity, and generally can Set above 800 (Figure 4, Figure 5, Figure 6, Figure 7).

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Figure 4

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Figure 5

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Figure 6

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Figure 7

The third layer: photosensitive layer

The third layer of CCD is the "photosensitive sheet". This layer is mainly responsible for converting the light source passing through the color filter layer into electronic signal and transmit the signal to the image processing chip to restore the image.

3. CCD arrangement

Traditional CCDs are arranged in a matrix. However, this approach limits the ability to further improve the resolution in the effective area (based on current technology, 1.8 inches The ideal value of CCD is about six million pixels, and it is a reasonable value to correct to four million pixels in consideration of cost and manufacturing yield). Therefore, some manufacturers have cleverly come up with the idea of ??changing the order of the CCDs, hoping to enhance the resolution. FUJI Fine Pix 4700 adopts this approach. The technology developed by FUJIFILM is called "SUPER CCD". This technology rotates the CCD pixel body at a 45-degree angle and arranges it in a honeycomb pattern (as shown in Figure 8). The result is that the wiring between the PHOTO diodes is eliminated. realize its greater potential. Because the difference between the shape and the vertical direction of the pixel is small, it becomes approximately octagonal, making the light-receiving part larger. Achieves high sensitivity equivalent to ISO 800. The S/N of SUPER CCD is approximately 2 times higher than before, and color reproduction has also been greatly improved. As a result, the tonal reproducibility of the high light and shadow portions in particular is greatly improved, balancing the resolution and tones for smoother images.

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Figure 8

What needs to be pointed out here Yes, FUJI claims that the original 2.4 million pixels in the 1.7-inch screen have been upgraded to 4.3 million! Although the effect is so amazing, it is still necessary to see the actual test results to determine how much resolution is enhanced by this effect.

2. CMOS (Complementary Metal-Oxide Semiconductor, complementary oxide metal semiconductor)

CMOS, like CCD, can record changes in light in digital cameras Semiconductor. The manufacturing technology of CMOS is no different from that of general computer chips. It mainly uses semiconductors made of two elements: silicon and germanium, so that there are N (positively charged) and P on the CMOS. (negatively charged) level semiconductor, the current generated by these two complementary effects can be recorded and interpreted as an image by the processing chip. However, the disadvantage of CMOS is that it is too prone to noise. This is mainly because the early design made CMOS When processing rapidly changing images, overheating will occur due to too frequent changes in current.

So what are the advantages of CMOS over CCD? In fact, the advantages of CMOS over CCD are low cost and low power consumption. , easy to manufacture, and can be placed on the same chip as the image processing circuit. However, due to the above shortcomings, CMOS can only survive in the economical digital camera market.

Canon D30 initially chose CMOS as the photosensitive component. This has caused many experts to "break their glasses" because it is very rare to use CMOS in high-end digital cameras. However, according to the measured photos of CANON D30 BETA recently seen on DPREVIEW, it seems that CMOS has exceeded the previous level. However, it is still unclear whether CANON D30 has improved the original CMOS design or made revolutionary improvements in the chip that interprets images. It is undeniable that CMOS is only one-third of CCD. The power consumption is about 1%. For digital cameras with increasingly demanding battery performance, developing towards CMOS may be the solution for developing new models in the future.

If you are still unclear about CMOS. The following website has a detailed description of CMOS in English:

http://tech-www.informatik.uni-hamburg.de/applets/cmos/cmosdemo.html

Three , New generation of CCD technology innovation

Times are changing, and traditional CCD technology can no longer meet the needs of current users for digital cameras. Below, we will introduce two new models launched in 2002. Two representative CCD technologies allow everyone to have a deeper understanding of CCD:

Fuji released the third generation Super CCD technology (Figure 9 and Figure 10)

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Figure 9

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Figure 10

In 1999, Japan's Fujifilm developed the first generation SuperCCD, which was used on FinePix 4700z because it could improve pixels and resolution. Therefore, it was very popular. Then in 2001, Fuji corrected all the noise shortcomings of the first-generation Super CCD and upgraded the effective pixels to a higher resolution of 3.1 million and a maximum pixel of 6.02 million. These CCDs were installed on the FinePix 6800z/6900z, which became Fujifilm’s best-selling digital camera that year.

The new generation of SuperCCD III combines the above advantages and adds:

Signal processing capability - the creativity of this technology is to use the camera's built-in signal processing processor to integrate the results obtained from the first photo (2832X2128) photo, specifically based on RGB as the standard, and every 4 pixels of the three colors as a calculation basis, the performance of the photo at a high sensitivity of ISO 1600 is integrated. Using the principles of calculation, the color of photos under low light can be improved and corrected, and the noise ratio increased by electronic interference can be avoided. But the disadvantage is that the original high-pixel photos will be reduced to a (1280X960) size ratio.

CCD horizontal/vertical pixel mixed operation - This is another special skill of SuperCCD III, and it is the first time in the world that CCD adopts horizontal/vertical pixel mixed operation technology. This method can allow a CCD with an effective pixel of 3 million to surpass the threshold of 15fps that is generally limited to QVGA animation recording (resolution 320×240) - because no matter how fast the speed goes, the processing speed of the digital camera is not enough and the picture is easy to Darker. This time, through the algorithm, multiple pixels are integrated into one, allowing the digital camera to relax the shutter limit of animation, so SuperCCD can achieve a maximum recording capacity of 30fps at VGA resolution (640×480). And can effectively increase the sensitivity by more than 4 times. In other words, the animation shot with SuperCCD III is at the level of VCD.

Foveon Company of the United States released multi-layer color-sensitive CCD technology

Before Foven Company released X3 technology, the general CCD structure was a honeycomb-like color filter plate (Figure 11), with a pad underneath The photoreceptor is used to determine which of the three RGB primary colors the incident light is.

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Figure 11

However, cellular technology (Also known as mosaic technology in the United States) The disadvantages are: the resolution cannot be improved, the color discrimination ability is poor, and the production cost is high. For this reason, the production of high-end CCDs has been monopolized by Japan over the years. The new X3 technology allows electronic technology to successfully imitate the color sensing principle of "real film" (Figure 12), "sensing color layer by layer" according to the absorption wavelength of light! , corresponding to the disadvantage of cellular technology that one pixel can only sense one color, the same pixel of X3 can sense three different colors, which greatly improves the image quality and color performance.

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Figure 12

Supports more powerful CCD computing technology VPS (Variable Pixel Aize); in addition, X3 also has a feature that is very similar to the SuperCCD III horizontal and vertical computing integration method we introduced previously, also through the combination of "group pixels" (Figure 13). X3 can also achieve ultra-high ISO values ??(resolution must be reduced) and high-speed VGA recording rates. What's more powerful than SuperCCD is that each pixel of X3 can sense three color values. In theory, X3's animation shooting may be more refined than SuperCCD III under the same speed conditions.