Matrix for CCTV cameras. What to pay attention to?

The image quality of a video camera largely depends on the photosensitive sensor (matrix) used in it. After all, put at least the best processor for digitizing video - if a bad image is received on the matrix, it will not become a good one. I will try to popularly explain what you should pay attention to in the characteristics of the sensor of a video surveillance camera, so that later it would not be painfully painful when you look at the image ...

Matrix type

On the Internet, you will surely find information that CCD (CCD, charge-coupled device) and CMOS (CMOS, complementary metal-oxide-semiconductor) photosensitive arrays are used in CCTV cameras. Forget it! It has long been only CMOS, only hardcore.
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CCD matrixes, with all their merits (better photosensitivity and color rendition, lower noise level) are practically not used in video surveillance. Because the very principle of their operation of CCD matrices - sequential reading of charge across cells is too slow to satisfy the demands of fast, high-resolution video cameras. Well, the most important thing is that CCD is more expensive in production, and in today's highly competitive environment every penny of profit is on the account. That is why all the key manufacturers focused on the release of CMOS matrices.

It remains to producers, by the way, not so much. The largest, as of the beginning of 2017, are the companies: ON Semiconductor Corporation (which once absorbed the well-known profile company Aptina), Omnivision Technologies Inc., Samsung Electronics and Sony Corporation. In addition, the matrix for their own needs produces, for example, the company Canon, Hikvision.

Competition to old brands is attempting to create young, full of enthusiasm and money Chinese chip makers of the “second echelon”, like SOI (Silicon Optronics, Inc.) and others. It is difficult to say whether young growth will survive when the CMOS sensors come to saturation and become too closely . But in any case, in this segment the emergence of new players and the aggravation of the struggle are not excluded, after all, it is not too difficult to modernize the production of CMOS sensors.

Large global brands such as Hikvision or Dahua usually prefer to work with first-tier or own manufacturers of matrices. Local behave differently. For example, Tecsar even in low-cost cameras uses matrices with a good reputation from ON Semiconductor, Omnivision, and Sony. In the assortment of other “popular” brands, for example Berger , SOI sensors, etc. are widely represented.


How to make a matrix of digital cameras

CMOS Leadership

CMOS technology provides for the placement of electronic components (capacitors, transistors) directly in each pixel of the photosensitive matrix.



Pixel structure and CMOS matrix

This reduces the effective area of ​​the photosensitive element and reduces the sensitivity, plus active elements increase the level of intrinsic noise of the matrix. But the technology allows to convert the charge of a photosensitive element into an electrical signal directly in the matrix and form a digital image signal much faster, which is critical for video cameras. That is why CMOS is better suited for CCTV cameras that require quick frame changes.


The principle of operation of CCD and CMOS matrices

Plus, the ability to randomly read CMOS matrix cells makes it possible to literally “on the fly” change the quality and bitrate of the resulting video, which is impossible for CCD. And the power consumption of CMOS solutions is lower, which is also important for compact surveillance cameras.

Let there be color

To obtain a color image, the matrix decomposes the light flux into its constituent colors: red, green, and blue. To do this, use the appropriate filters. Different manufacturers vary the placement and number of light-sensitive elements of different colors, but the essence does not change.

The principle of image formation on the photosensitive matrix:


P - photosensitive element
T - electronic components



How the CMOS camera sensor works and works can also be viewed on this Canon video:


CMOS matrices of all manufacturers are based on the above described general principles, differing only in the details of the implementation on silicon. For example, in pursuit of cheapness and super-profit, chip makers are trying to produce the matrix as small as possible. Payment for this is inevitable ...

Why big is good

The size (or in other words, format) of a matrix is ​​usually measured diagonally in inches and indicated as a fraction, for example, 1/4 ", 1/3", 2/3 ", 1/2 inch, etc.

The first rule for choosing the best matrix is ​​quite simple: with the same number of pixels (resolution), the larger the physical dimensions of the sensor, the better. The larger matrix has larger pixels, which means it captures more light. The pixels of the larger matrix are less closely spaced, which means less influence of mutual interference and a lower level of spurious noise, which directly affects the quality of the resulting image. Finally, a larger matrix allows for greater viewing angles when using a lens with the same focal length!


Photosensitive matrix produced by ON Semicondactor for video surveillance cameras



Photosensitive matrix mounted on the camcorder board

Alas, large-format matrixes in mass CCTV cameras are now practically not used due to the high cost of both the matrices and the lenses for them, which should have larger lenses and, accordingly, size and cost. Today, cameras are set mainly in matrix size 1/2 "- 1/4" (this is the smallest). Choosing a camera, you need to clearly understand that buying an ultra-cheap model with a 1/4 "SOI production matrix and a tiny lens with plastic lenses of dubious transparency, you will not be able to create a video control system of acceptable quality that would clearly distinguish small details of the events captured. when shooting in low light conditions.

Choosing a camera with a Sony matrix size 1/8 ”you get a much better video quality a priori, the camera with such a matrix can already be used in a professional video surveillance system. And the sensitivity of such a camera will certainly be higher, which will allow better shooting in weak illumination: in bad weather, at dusk, in a darkened room, etc. With an increase in resolution at the same matrix size, the photosensitivity decreases, and this also needs to be taken into account when choosing. At the back door, it makes sense to choose a matrix with a lower resolution and higher sensitivity than an ultra-high resolution camera with a low sensitivity matrix on which, due to noise, nothing can be clearly distinguished.

Light sensitivity

The photosensitivity of the matrix determines the possibility of its work in low ambient lighting conditions. From the point of view of physics, this looks quite trite: the less light energy is enough to obtain an image by the matrix, the higher its photosensitivity. But! Let's be frank, it’s not worth pursuing high sensitivity. The fact is that modern video surveillance cameras successfully switch to day / night modes, while reducing the illumination, transferring the matrix to a black and white image mode with higher sensitivity. Plus, the automatic activation of the infrared illumination gives the cameras the ability to shoot perfectly even in complete darkness. For example, in a closed room with no windows and with the lights turned off, when there is not even a word about the level of some ambient light. Light sensitivity remains critical for cameras lacking IR illumination, but using such in modern video surveillance is almost a moveton. Although non-backlight hull models are still on sale, of course.


Comparison of matrices from different manufacturers

In general, the rule is this: the higher the illumination, the better will remove the matrix and, accordingly, the camera. Therefore, it is not recommended to put cameras on half-dark nooks, even if they have good sensitivity. Keep in mind that the specification of the matrix cameras usually indicates the minimum level of illumination, when you can capture at least some image. But no one promises that this image will be at least of acceptable quality! It will be disgusting in 100% of cases, it will be difficult to disassemble anything on it. To achieve at least a satisfactory result, it is recommended to shoot at least at illumination at least 10-20 times greater than the minimum allowable for the matrix.

Manufacturers have come up with a number of technical solutions to improve the sensitivity of CMOS arrays and reduce the loss of light in the process of fixing the image. One principle is basically used for this: take out the photosensitive element as close as possible to the microlens of the matrix collecting the light. First, Sony proposed its Exmor technology, which shortened the light path in the matrix:



Then, progressive manufacturers jointly switched to using matrices with reverse illumination, which allows not only reducing the path of light through the matrix, but also making the useful area of ​​the photosensitive layer larger by placing it above other electronic elements in the cell:



Back light technology gives the camera maximum sensitivity. Hence the conclusion - “with other things being equal,” it is better to purchase a camera using a matrix with reverse illumination than without it.

In order to improve the image in low-light conditions for low-sensitivity, low-cost arrays, camera manufacturers can use various tricks. For example, the “slow shutter” mode, and more simply, the slow shutter mode. However, the “smearing” of the contours of moving objects already at the stage of fixing the image with a matrix in this mode does not allow talking about a little bit of high-quality video shooting, so this approach is completely unacceptable in security surveillance where details are important.

The appearance of Starlight technology, which first appeared in Bosch cameras in 2012, was a definite breakthrough in image quality. This technology, thanks to the combination of the enormous sensitivity of the matrix (of the order of 0.0001 - 0.001 lux) and a very effective noise reduction technology, made it possible to obtain very high-quality color images from video cameras in low-light conditions and even at night.



While the traditional way of overcoming low light - using IR illumination - provides an opportunity to get a clear image only in monochrome mode (shades of gray), cameras with Starlight technology allow you to get a color image with much more informative. In particular, in low light, the video surveillance system with Starlight technology can easily distinguish the colors of cars, clothes and other important features.

Here is a demonstration of Starlight technology in action:



Results

When choosing a surveillance camera, be sure to pay attention to the characteristics of the matrix, and not just its resolution. After all, the quality of the image, and hence the usefulness of the camera, will largely depend on this. First of all, one should pay attention to a reliable brand, size and resolution of the matrix, the photosensitivity is of principle only for cameras lacking IR illumination.

I highly recommend taking a camera with a matrix, on which you can find a sane datasheet with detailed information, and not to buy a cat in a bag. For example, you can easily find specifications for matrices manufactured by ON Semiconductor, Omnivision, or Sony. But a little bit of the detailed characteristics of the SOI matrix can not be found during the day with a lantern. There is a suspicion that the manufacturer has something to hide ...

And the bottom line is this: CMOS matrices have unconditionally won in video surveillance devices and in the near future are not going to give up any competing technology.