What is a cmos camera? Difference between CCD and CMOS matrices. Scope of application of CMOS matrices
The matrix is the basis of any photo or video device. It determines the quality and size of the resulting image. Today, two different technological principles are used in the manufacture of matrices - CCD and CMOS. You can often hear the question: “Which matrix to choose: CCD or CMOS?” There are heated debates about this among fans of photo and video equipment. In this article we will review these two types and try to figure out which matrix is better - CCD or CMOS.
general information
The matrices are designed to digitize the parameters of light rays on their surface. It is not possible to talk about a clear advantage of one of the technologies. You can make comparisons based on specific parameters and identify a leader in one aspect or another. As for user preferences, often the main criterion for them is the cost of the product, even if it is inferior in quality or technical characteristics to its competitor.
So, let's understand what both types of devices are. A CCD matrix is a microcircuit that consists of light-sensitive photodiodes; it is created on a silicon basis. The peculiarity of its operation lies in the principle of operation of a charge-coupled device. A CMOS matrix is a device created on the basis of semiconductors with an insulated gate with channels of different conductivity.
Principle of operation
Let's move on to identifying the differences that will help you make your choice: what is better - a CMOS or CCD matrix? The main difference between these two technologies is the principle of their operation. CCD devices convert the charge from the pixels into an electrical potential, which is amplified outside the light sensors. The result is an analog image. After this, the entire image is digitized into the ADC. That is, the device consists of two parts - the matrix itself and the converter. CMOS technology is characterized by the fact that it digitizes each pixel individually. The output is a finished digital image. That is, the electric charge in the matrix pixel accumulates in a capacitor, from which the electric potential is removed. It is transmitted to an analog amplifier (built directly into the pixel), after which it is digitized in a converter.
What to choose: CCD or CMOS?
One of the important parameters that determine the choice between these technologies is the number of matrix amplifiers. CMOS devices have a larger number of these devices (at each point), so as the signal passes through, the picture quality decreases slightly. Therefore, CCD matrices are used to create images with a high degree of detail, for example, for medical, research, and industrial purposes. But CMOS technologies are used mainly in household appliances: webcams, smartphones, tablets, laptops, etc.
The next parameter that determines which type is better - CCD or CMOS - is the density of the photodiodes. The higher it is, the fewer photons will be wasted, and accordingly, the image will be better. In this parameter, CCD matrices are ahead of their competitors, since they offer a layout that does not have such gaps, while CMOS has them (transistors are located in them).
However, when the user is faced with a choice: which one - CMOS or CCD - to purchase, the main parameter pops up - the price of the device. CCD technology is much more expensive than its competitor and more energy-consuming. Therefore, it is not advisable to install them where an average quality image is sufficient.
Modern video cameras actively use 2 types of matrices: CMOS and CCD. CMOS sensor (CMOS) built on the basis of CMOS technology, which gave the name to this product (complementary metal-oxide-semiconductor, complementary metal-oxide-semiconductor structure). If in mid-price segment cameras both options are used in approximately equal proportions, then in budget video systems it is CMOS that is more common.
The operating principle of the technology is as follows:
- A reset signal is given;
- Diodes accumulate charge during exposure;
- The parameters are being read.
Main advantages
Key reasons to choose CMOS (CMOS) matrices:- Low cost compared to CCD analogues. As sizes increase, the cost difference continues to grow;
- Low power consumption. An important factor when the camera is running on a battery, an outdated electrical network of the facility, or a significant number of connected devices;
- Possibility of cropped reading - analysis of arbitrary pixels, increasing recording speed. There is no need to read all the information at once, as with a CCD camera. Improves manual focusing quality;
- Used in miniature video cameras.
Flaws
When choosing this type of element, it is worth considering the limitations of CMOS technology:- Increased heating of the device, increased noise;
- Low light sensitivity of the matrix on older camera models. Now the situation has been partially corrected due to a new line of equipment with Exmor technology with an increase in pixel sensitivity;
- Curved image of fast moving objects. Rolling shutter effect.
Scope of application of CMOS matrices
CMOS elements, due to their reliability, low cost and flexible configuration, are widely used in several areas of our life. First of all, in photography, phones and cameras are equipped with precisely these matrices, satisfying the needs of the user. Second place – video surveillance:- When protecting apartments;
- Airport surveillance;
- Construction site control;
- In the office;
- In the mall;
- In stock;
- For other objects with different operating conditions.
Matrices can be found in the road (monitoring the behavior of road users), science, medicine, and industry.
CCD is a charge-coupled device. This type of matrix was initially considered to be of higher quality, but also more expensive and energy-consuming. If you imagine the basic principle of operation of a CCD matrix in a nutshell, then they collect the whole picture in an analog version, and only then digitize it.
Unlike CCD matrices, CMOS matrices (complementary metal-oxide-semiconductor, complementary logic on metal-oxide-semiconductor transistors, CMOS) digitize each pixel in place. CMOS matrices were initially less power-consuming and cheaper, especially in the production of large matrices, but were inferior to CCD matrices in quality.
The advantages of CCD matrices include:
- Low noise level.
- High pixel fill factor (about 100%).
- High efficiency (the ratio of the number of registered photons to their total number incident on the photosensitive area of the matrix, for CCD - 95%).
- High dynamic range (sensitivity).
The disadvantages of CCD matrices include:
- The principle of signal reading, and therefore the technology, is complex.
- High level of energy consumption (up to 2-5W).
- More expensive to produce.
Advantages of CMOS matrices:
- High performance (up to 500 fps).
- Low power consumption (almost 100 times compared to CCD).
- Cheaper and easier to produce.
- The prospects of the technology (on the same chip, in principle, it costs nothing to implement all the necessary additional circuits: analog-to-digital converters, processor, memory, thus obtaining a complete digital camera on one chip. By the way, the creation of such a device has been carried out jointly since 2002 Samsung Electronics and Mitsubishi Electric).
The disadvantages of CMOS matrices include
- Low pixel fill factor, which reduces sensitivity (effective pixel surface ~75%, the rest is occupied by transistors).
- A high noise level (it is caused by the so-called tempo currents - even in the absence of lighting, a fairly significant current flows through the photodiode), the fight against which complicates and increases the cost of the technology.
- Low dynamic range.
Introduction to Image Sensors
When an image is captured by a video camera lens, light passes through the lens and hits the image sensor. The image sensor, or matrix, consists of many elements, also called pixels, that record the amount of light falling on them. The pixels convert the resulting amount of light into the corresponding number of electrons. The more light that hits a pixel, the more electrons it will generate. The electrons are converted into voltage and then converted into numbers according to the ADC (Analog to Digital Converter) values. The signal made up of such numbers is processed by electronic circuits inside the video camera.
Currently, there are two main technologies that can be used to create an image sensor in a camera, these are CCD (Charge-Coupled Device) and CMOS (Complimentary Metal-Oxide Semiconductor). . Their characteristics, advantages and disadvantages will be discussed in this article. The picture below shows CCD (top) and CMOS (bottom) image sensors.
Color filtering. As described above, image sensors record the amount of light falling on them, from light to dark, but without color information. Since CMOS and CCD image sensors are “color blind,” a filter is placed in front of each sensor to assign a color tone to each pixel in the sensor. The two main color registration methods are RGB (Red-Greed-Blue) and CMYG (Cyan-Magenta-Yellow-Green). Red, green and blue are the primary colors, various combinations of which can make up the majority of the colors perceived by the human eye.
The Bayer filter (or Bayer array), consisting of alternating rows of red-green and blue-green filters, is the most common RGB color filter (see Fig. 2). The Bayer filter contains twice the number of green “cells” because The human eye is more sensitive to green rather than red or blue. This also means that with this ratio of colors in the filter, the human eye will see more detail than if three colors were used in equal proportions in the filter.
Another way to filter (or register) color is to use the complementary colors cyan, magenta, and yellow. The complementary color filter is usually combined with a green color filter in the form of a CMYG-color array, as shown in Figure 2 (right). A CMYG color filter usually offers a higher pixel signal because... has a wider spectral bandwidth. However, the signal must be converted to RGB to be used in the final image, which entails additional processing and introduces noise. The consequence of this is a decrease in the signal-to-noise ratio, which is why CMYG systems are generally not as good at rendering colors.
The CMYG color filter is typically used in interlaced scan image sensors, while RGB systems are primarily used in progressive scan image sensors.
The photosensitive matrix is the most important element of the camera. It is she who converts the light falling on her through the lens into electrical signals. The matrix consists of pixels - individual light-sensitive elements. On modern matrices, the total number of photosensitive elements reaches 10 million for amateur devices and 17 million for professional ones. An N megapixel matrix contains N million pixels. The more pixels on the matrix, the more detailed the photo turns out.
Each photosensitive element is a capacitor that charges when exposed to light. The capacitor charges more strongly the brighter the light falling on it, or the longer it is exposed to light. The trouble is that the charge of the capacitor can change not only under the influence of light, but also from the thermal movement of electrons in the matrix material. Some pixels receive more thermal electrons, while others receive fewer. The result is digital noise. If you take a picture of a blue sky, for example, in the picture it may look like it consists of pixels of slightly different colors, and a picture taken with a closed lens will consist of more than just black dots. The smaller the geometric size of the matrix with the same number of megapixels, the higher its noise, the worse the image quality.
For compact digital devices, the matrix size is usually indicated as a fraction and measured in inches. What’s interesting is that if you try to calculate this fraction and convert it from inches to millimeters, the resulting value will not coincide with the actual dimensions of the matrix. This contradiction arose historically when the size of a television transmitting device (vidicon) was indicated in a similar way. For digital SLR cameras, the matrix size is either directly indicated in millimeters, or indicated as a crop factor - a number indicating how many times this size is smaller than the frame of a standard 24x36 mm film.
Another important feature of matrices is that a matrix with N megapixels actually contains N megapixels, and moreover, the image from this matrix also consists of N megapixels. You say, what is strange here? But the strange thing is that in the image each pixel consists of three colors, red, green and blue. It would seem that on the matrix, each pixel should consist of three light-sensitive elements, respectively red, green and blue. However, in reality this is not the case. Each pixel consists of only one element. Where does color come from then? In fact, a light filter is applied to each pixel in such a way that each pixel perceives only one of the colors. The filters alternate - the first pixel perceives only red, the second - only green, the third - only blue. After reading information from the matrix, the color for each pixel is calculated based on the colors of this pixel and its neighbors. Of course, this method somewhat distorts the image, but the color calculation algorithm is designed in such a way that the color of small details can be distorted, but not their brightness. And for the human eye looking at the picture, it is the brightness rather than the color of these details that is more important, so these distortions are almost invisible. This structure is called the Bayer pattern, named after the Kodak engineer who patented this filter structure.
Most modern photosensitive matrices used in compact digital cameras have two or three operating modes. The main mode is used for photography and allows you to read an image of maximum resolution from the matrix. This mode requires the absence of any illumination of the matrix during frame reading, which in turn requires the presence of a mechanical shutter. Another, high-speed mode allows you to read the entire image from the matrix at a frequency of 30 times per second, but at a reduced resolution. This mode does not require a mechanical shutter and is used for previews and video shooting. The third mode allows you to read the image twice as fast, but not from the entire area of the matrix. This mode is used for autofocus operation. The matrices used in SLR digital cameras do not have high-speed modes.
But not all photosensitive matrices are designed this way. The Sigma company produces Foveon matrices, in which each pixel actually consists of three light-sensitive elements. These matrices have significantly fewer megapixels than their competitors, but the image quality from these matrices is practically not inferior to their multi-megapixel competitors.
Fuji's SuperCCD matrices have another interesting feature. The pixels in these matrices are hexagonal in shape and arranged like a honeycomb. On the one hand, in this case the sensitivity increases due to the larger pixel area, and on the other hand, with the help of a special interpolation algorithm, better image detail can be obtained.
In this case, interpolation really allows you to improve the detail of the image, unlike devices from other manufacturers, where the image from a matrix with a conventional pixel arrangement is interpolated. The fundamental difference between these matrices is that the pixel spacing is half as large as the pixels themselves. This allows you to increase image detail along vertical and horizontal lines. At the same time, conventional matrices have better diagonal detail, but in real photographs there are usually fewer diagonal lines than vertical or horizontal ones.
Interpolation– algorithm for calculating missing values from neighboring values. If we know that at 8 am the temperature outside was +16 degrees, and at 10 it rose to +20, we will not be much mistaken if we assume that at 9 am the temperature was about +18.
In a CCD sensor, light (charge) incident on a sensor pixel is transmitted from the chip through one output node, or through just a few output nodes. The charges are converted to a voltage level, accumulated and sent out as an analog signal. This signal is then summed and converted into numbers by an analog-to-digital converter, outside the sensor (see Figure 3).
CCD technology was invented specifically for use in video cameras, and CCD sensors have been in use for 30 years. Traditionally, CCD sensors have a number of advantages over CMOS sensors, namely better light sensitivity and low noise. Lately, however, the differences are barely noticeable.
The disadvantages of CCD sensors are that they are analog components, which requires more electronics "near" the sensor, they are more expensive to manufacture, and can consume up to 100 times more power than CMOS sensors. Increased power consumption can also lead to higher temperatures in the camera itself, which not only negatively affects image quality and increases the cost of the final product, but also the environmental impact.
CCD sensors also require higher data transmission speeds, because... all data passes through just one or more output amplifiers. Compare Figures 4 and 6 showing boards with a CCD sensor and a CMOS sensor, respectively.
At an early stage, conventional CMOS chips were used for display, but the picture quality was poor due to the low light sensitivity of CMOS elements. Modern CMOS sensors are manufactured using more specialized technology, which has led to rapid growth in image quality and light sensitivity in recent years.
CMOS chips have a number of advantages. Unlike CCD sensors, CMOS sensors contain amplifiers and analog-to-digital converters, which significantly reduces the cost of the final product, because it already contains all the necessary elements to obtain the image. Each CMOS pixel contains electronic converters. Compared to CCD sensors, CMOS sensors have greater functionality and greater integration capabilities. Other advantages include faster readout, lower power consumption, high noise immunity and smaller system size.
However, having electronic circuitry inside the chip does introduce the risk of more structured noise, such as streaking. Calibration of CMOS sensors during production is also more complex than with CCD sensors. Fortunately, modern technology allows the production of self-calibrating CMOS sensors.
In CMOS sensors, it is possible to read an image from individual pixels, which allows you to “window” the image, i.e. read the readings of not the entire sensor, but only a certain area of it. Thus, it is possible to obtain a higher frame rate from part of the sensor for subsequent digital PTZ (eng. pan/tilt/zoom, panorama/tilt/zoom) processing. In addition, this makes it possible to transmit multiple video streams from one CMOS sensor, simulating several “virtual cameras”
HDTV and megapixel cameras
Megapixel sensors and high-definition television allow digital IP cameras to provide higher image resolution than analogue CCTV cameras, i.e. they provide a greater ability to discern details and identify people and objects - a key factor in video surveillance. A megapixel IP camera has at least twice the resolution of an analog CCTV camera. Megapixel sensors are key to high definition television, megapixel and multi-megapixel cameras. And can be used to provide extremely high image detail and multi-stream video.
Megapixel CMOS sensors are more widely used and much cheaper than megapixel CCD sensors, although there are also quite expensive CMOS sensors.
It is difficult to manufacture a fast megapixel CCD sensor, which is of course a disadvantage, and therefore it is difficult to manufacture a multi-megapixel camera using CCD technology.
Most sensors in megapixel cameras are generally similar in image size to VGA sensors, with a resolution of 640x480 pixels. However, a megapixel sensor contains more pixels than a VGA sensor, so the size of each pixel in a megapixel sensor is smaller than the size of a pixel in a VGA sensor. The consequence of this is that each pixel in a megapixel sensor is less sensitive to light.
One way or another, progress does not stand still. Megapixel sensors are rapidly developing, and their light sensitivity is constantly increasing.
Main differences between CMOS and CCD
CMOS sensors contain amplifiers, A/D converters, and often additional processing chips, whereas in a CCD camera, most of the signal processing is done outside the sensor. CMOS sensors consume less power than CCD sensors, which means the camera can be kept at a lower temperature inside. Increased temperature of CCD sensors may increase interference. On the other hand, CMOS sensors can suffer from structured noise (banding, etc.).
CMOS sensors support image windowing and multi-stream video, which is not possible with CCD sensors. CCD sensors usually have one A/D converter, while in CMOS sensors each pixel has one. Faster reading in CMOS sensors allows them to be used in the manufacture of multi-megapixel cameras.
Modern technological advances are erasing the difference in light sensitivity between CCD and CMOS sensors.
Conclusion
CCD and CMOS sensors have different advantages and disadvantages, but technology is advancing rapidly and the situation is constantly changing. The question of whether to choose a camera with a CCD sensor or a CMOS sensor becomes irrelevant. This choice depends only on the client’s requirements for the image quality of the video surveillance system.
As you know, cameras are divided into two large categories - analog and digital - based on the light-sensitive surface that captures the image. In an analog camera, this surface was photographic film - a simple thing with a certain photosensitivity, a certain number of single-use frames, from which, after chemical processing, it was possible to obtain an imprint of the image on paper.
In digital cameras, this fundamental role is taken on by the matrix. Matrix— a device whose main function is to digitize certain parameters of light falling on its surface. This process is shown in detail and clearly in an excellent video from Discovery in our article ““, if you haven’t watched it yet, be sure to do so!
There are two main, most popular and at the same time competing matrix technologies - these are CCD And CMOS. Let's figure out today what difference between CCD And CMOS matrices?
We will try to understand their differences without diving into the details of physics, just to have an idea not only of how the camera works, but also of what matrix is currently on your camera. I think this will be enough for a novice photographer, but those who are interested in details can delve further on their own.
CCD matrix, source: Wikipedia
So, CCD- This charge-coupled device (CCD - device with feedback charging). This type of matrix was initially considered to be of higher quality, but also more expensive and energy-consuming. If you imagine the basic principle of operation of a CCD matrix in a nutshell, then they collect the whole picture in an analog version, and only then digitize it.
Unlike CCD matrices, CMOS matrix (complementary metal-oxide-semiconductor, complementary logic on metal-oxide-semiconductor transistors, CMOS), digitize each pixel in situ. CMOS matrices were initially less power-consuming and cheaper, especially in the production of large matrices, but were inferior to CCD matrices in quality.
CMOS sensor, source: Wikipedia
CCD matrices have higher image quality and still remain popular in the fields of medicine, industry, and science, where image quality is critical. Recently, CCD matrices have reduced energy consumption and cost, and CMOS matrices have significantly improved image quality, especially after a technological revolution in the production of CMOS sensors, when, using Active Pixel Sensors (APS) technology, a transistor amplifier was added to each pixel for reading, which made it possible to convert charge to voltage right in the pixel. This provided a breakthrough for CMOS technology; by 2008 it had become practically an alternative to CCD matrices. Moreover, CMOS technology made it possible to shoot video and introduce this function into modern cameras, and most modern digital cameras are equipped with CMOS matrices.
We wrote about matrices about choosing a video camera for a family. There we touched on this issue easily, but today we will try to describe both technologies in more detail.
What is the matrix in a video camera? This is a microcircuit that converts a light signal into an electrical signal. Today there are 2 technologies, that is 2 types of matrices – CCD (CCD) and CMOS (CMOS). They are different from each other, each has its own pros and cons. It is impossible to say for sure which one is better and which one is worse. They develop in parallel. We will not go into technical details, because... they will be tritely incomprehensible, but in general terms we will define their main pros and cons.
CMOS technology (CMOS)
CMOS matrices First of all, they boast about low power consumption, which is a plus. A video camera with this technology will work a little longer (depending on the battery capacity). But these are minor things.
The main difference and advantage is the random reading of cells (in CCD reading is carried out simultaneously), which eliminates smearing of the picture. Have you ever seen “vertical pillars of light” from point-like bright objects? So CMOS matrices exclude the possibility of their appearance. And cameras based on them are cheaper.
There are also disadvantages. The first of them is the small size of the photosensitive element (in relation to the pixel size). Here, most of the pixel area is occupied by electronics, so the area of the photosensitive element is reduced. Consequently, the sensitivity of the matrix decreases.
Because Since electronic processing is carried out on the pixel, the amount of noise in the picture increases. This is also a disadvantage, as is the low scanning time. Because of this, a “rolling shutter” effect occurs: when the operator moves, the object in the frame may be distorted.
CCD technology
Video cameras with CCD matrices allow you to obtain high-quality images. Visually, it is easy to notice less noise in video captured with a CCD-based camcorder compared to video captured with a CMOS camera. This is the very first and most important advantage. And one more thing: the efficiency of CCD matrices is simply amazing: the fill factor is approaching 100%, the ratio of registered photons is 95%. Take the ordinary human eye - here the ratio is approximately 1%.
High price and high energy consumption are the disadvantages of these matrices. The thing is, the recording process here is incredibly difficult. Image capture is carried out thanks to many additional mechanisms that are not found in CMOS matrices, which is why CCD technology is significantly more expensive.
CCD matrices are used in devices that require color and high-quality images, and which may be used to shoot dynamic scenes. These are mostly professional video cameras, although there are household ones too. These are also surveillance systems, digital cameras, etc.
CMOS matrices are used where there are no particularly high requirements for picture quality: motion sensors, inexpensive smartphones... However, this was the case before. Modern CMOS matrices have different modifications, which makes them very high quality and worthy from the point of view of competing with CCD matrices.
Now it is difficult to judge which technology is better, because both demonstrate excellent results. Therefore, setting the type of matrix as the only selection criterion is, at a minimum, stupid. It is important to take many characteristics into account.
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