The term "resolution" is surprisingly difficult to fully understand. If we say that another term of the same meaning, although close, is “resolution”, it is more understandable for most users. But understanding all the nuances of its use in video surveillance is very difficult.

The purpose of this publication is to highlight the following points:

1. What is the traditional understanding of “resolution,” the ability to discern details? What are the limitations of this approach?
2. What does resolution mean in video surveillance, the number of pixels? What limits the use of such a metric?
3. What is the difference between matrix resolution and stream resolution?
4. How much can compression affect resolution?
5. What is the limit on the resolution?

Resolution - the ability to distinguish details

Translated from the traditional in English the word “resolution” is translated as the ability to distinguish details. For example, can you look at the bottom line in a chart used to test vision? How clear an image can the camera show when images from it are viewed through a monitor of adjacent strokes? This is precisely the main indicator of quality, which is focused on results.

It just so happens that this is the approach typically taken in the video surveillance industry. The resolution of the camera was measured by the number of television lines, that is, the number of lines that the camera can provide on the monitor was measured. The more strokes you can see, the more details you can see that the camera captured from the real world - a person’s facial features, car license plates, etc.

What were the limitations of this approach? It's actually quite simple. The fact is that the resolution of cameras, that is, the number of lines on the monitor, was always measured in conditions with good lighting. But it goes without saying that the camera cannot produce the same picture quality if it is illuminated by the sun, or vice versa, there will be no lighting. Then, of course, the quality of the shooting deteriorates significantly. Another additional difficulty is the fact that it is impossible to measure a specific algorithm for changing the quality of shooting; changes will be different for all cameras.

Theoretically, this approach can be used to measure the quality of a camera, but do not forget that it shows the exact result only under ideal conditions, which are almost impossible to achieve in reality.

Resolution - number of pixels

Nowadays, when most video surveillance systems have been transferred to , manufacturers also try to measure the quality of the overall quality of shooting. To do this, simply count the number of physical pixels in the video camera matrix. It is generally accepted that the more pixels (just as they used to rely on the number of television lines) a video camera can produce, the higher the image quality will be.

By analogy with the classic measurement of camera resolution, which was carried out under ideal conditions, manufacturers continue to ignore problems that may affect the quality of shooting.

There are, of course, some exceptions, but in most cases, the worse the lighting, the lower the quality of the shooting will be and the lower the actual resolution of the camera will be. For example, cameras with a relatively small number of pixels, thanks to better image processing technology, can provide much higher quality images, both in bright sunlight and in conditions with a wide dynamic range of lighting.

But despite this, the number of pixels is now considered one of the main characteristics of video surveillance devices. Despite all these limitations, it is always worth remembering that when discussing the topic of resolution, most often even professionals mean not the resolution itself, but the number of pixels. In addition, resolution can manifest itself in other forms.

All things being equal, the higher the camera resolution (number of pixels), the more it will cost. While a camera can have many features, always remember that low light or high dynamic range can make a big difference in image quality.

The table shows examples of the resolution of CCTV cameras most commonly found in the current security market:

Deputy Director for Development Andreev Kuzma.

1) Must select internal and external CCTV cameras. First we need to understand which cameras we want to use, color or black and white? Black and white cameras are more sensitive and cheaper than color cameras; on the other hand, color cameras can capture more information from the object being tracked. When installing color cameras on a staircase in poor lighting conditions, it may turn out that they will not switch to color mode, i.e. they will not have enough light to overcome the switching threshold, in which case there is no need to purchase color video cameras.

• Selecting a video camera matrix (CCD or CMOS). Currently, CCTV cameras use 2 types of matrices: CCD And CMOS. Both CCDs and CMOS sensors use photocells to convert light into electrical charges. The difference between these matrices lies in how the resulting electrical charges are then read.

• Video signal processor present in all analog CCTV cameras. This is the most important module that performs primary video signal processing: adjusts the brightness, color, contrast of the image, and also performs more complex operations. Here are some popular functions performed by a video signal processor:

• Photosensitivity (Lx) today is one of the most important parameters when choosing a camera. When choosing a camera, first of all, you need, of course, to pay attention to sensitivity,
  stated in the specification. The minimum required illumination level is measured in lux. 1 lux means that the camera will show something at dusk, but at night without good artificial lighting it will not “see” anything. Acceptable indicators for a camera that will be installed outdoors are 0.01 lux and below. The table provides some benchmarks for comparison:

However, the number of “luxes” is quite difficult to measure, so the sensitivity indicated in the specification does not always reflect the real capabilities of the camera. In order to get an impression of the sensitivity of the camera, you need to pay attention to the size of the matrix, as well as its type. The larger the size, the better; CCD is better than CMOS; ACS CMOS is better than APS CMOS. But the most reliable and recommended way is, of course, to test by recording several test videos of moving objects in low light conditions.

It is important to understand that in low light conditions the camera's resolution deteriorates significantly. In addition, the effect of “blurring” of moving objects appears due to the increased shutter speed. Typically, when light levels drop below the camera's stated sensitivity, image quality drops to unacceptable levels.

• Focal length or "viewing angle" (F=) – this is the distance from the main point of the lens to the focusing point of the rays. It is important for us to understand that the focal length determines the angle at which the scene is captured. The longer the focal length, the smaller the angle and the greater the apparent approximation. Varifocal lens, or a variable focal length lens, has the ability to change the focal length and, accordingly, the angles. There are a number of paid and free programs, which will help convert millimeters of focal length into angles.

• Resolution of the television camera (TVL). The issue of camera resolution is simple, but often misunderstood. When we're talking about about the resolution of the video surveillance system (TV camera-communication line-recorder-monitor), then the main part of the system will be the input device (that is, in most cases, the resolution of the system will be largely determined by the resolution of the camera). There is vertical resolution and horizontal resolution. These parameters are measured using a test chart.

Vertical resolution- this is the maximum number of horizontal lines that a television camera can transmit. This number is limited by the CCIR/PAL standard to 625 horizontal lines and by the EIA/NTSC standard to 525 lines. The actual vertical resolution (in both cases) is far from these values.

Horizontal resolution- this is the maximum number of vertical lines that a television camera is capable of transmitting (In cases where the documentation only indicates resolution, this should be understood as horizontal resolution). The horizontal resolution of CCD cameras is typically 75% of the horizontal pixels of the CCD. As explained above, this is a result of the 4:3 aspect ratio. Specifically, when counting vertical lines to determine horizontal resolution, we only count the horizontal width equivalent to the monitor's vertical height. The idea behind this is to get lines of equal thickness, both vertically and horizontally. So, if we calculate the total number of vertical lines across the width of the monitor, then they must be multiplied by 3/4 or 0.75. Since this is an unusual calculation, we usually call horizontal resolution TV lines (TBL), and not just lines. The maximum resolution in analog cameras is 600TVL.

Practical experience shows that the human eye has difficulty distinguishing differences in resolution if it is less than 50 lines. This does not mean that the resolution is not important factor In determining the quality of a camera, just a small difference in resolution is barely noticeable, especially if it is less than 10% of the total number of pixels.

Color cameras with a single CCD matrix (used in video surveillance systems) have a lower resolution than black and white cameras due to the division into three color components, despite the fact that the dimensions of these CCD matrices are the same as those of black and white cameras. Three-matrix color cameras used in television broadcasting can have much higher resolution. High-definition television cameras have appeared, where three 1-inch matrices provide a horizontal resolution close to 1000 TV lines.

Signal to noise ratio (S/N). Signal to noise ratio is expressed in decibels (dB). Signal-to-noise ratio measures how good a camera's video signal can be, especially in low-light conditions. Noise cannot be avoided, but it can be minimized. It mainly depends on the quality of the CCD matrix, the electronics and external electromagnetic influences, but also to a large extent on the temperature of the electronics. The metal body of the television camera largely protects from external electromagnetic influences (Strictly speaking, external electromagnetic influences, as a rule, are stationary processes, so they cannot be classified as noise; they are called interference or interference. Ed.). The sources of noise inside the camera are both passive and active components, so the “noise level” depends on their quality, system design and, to a large extent, temperature.

Noise in images is similar in nature to noise in audio recordings. On a screen, a noisy image will appear grainy or snowy, while a color image may have flashes of color. Heavily noisy video signals can be difficult to synchronize and the image may appear blurry and have poor resolution. Noisy image from a television camera it becomes even worse when the illumination of the object decreases, as well as when using AGC with high gain.

The signal-to-noise ratio of a CCD camera is defined as the ratio of the signal to the noise produced by the camera's sensor and electronics. To obtain a camera's true signal-to-noise ratio, all internal circuits (that affect the signal in one way or another) must be turned off, including gamma correction, AGC, electronic shutter, and backlight compensation circuitry. The temperature should be at room temperature.

For CCD cameras in video surveillance, a signal-to-noise ratio of more than 48 dB is considered good. Keep in mind that a 3 dB change in signal-to-noise ratio represents approximately a 30 percent reduction in noise since the video signal level does not change. And when comparing a television camera, in which the signal/noise is equal to 48 dB, with a television camera, in which, for example, this value is equal to 51 dB, the latter will give significantly best picture, which will be especially noticeable at low light levels. When talking about signal-to-noise ratio, we always assume that AGC is disabled. If you do not allow the camera to heat up significantly, the noise will be less.

For comparison, let's give this value: CCD cameras in television broadcast have a signal-to-noise ratio of more than 56 dB, which is extremely good for an analog video signal.

The quality of the image obtained by a digital camera depends on the optical system used and on the light sensitivity of the matrix, which, in turn, is determined by the number of elements of the CCD matrix. The first digital cameras had about 300 thousand of these elements. In modern models, the number of elements reaches 6 million.

The resolving power, or optical resolution, of a digital camera is based on the number of horizontal and vertical image elements it can capture. These image elements are called pixels. How more quantity pixels horizontally and vertically can be captured, the higher the resolution of the camera and, therefore, the clearer the image and the softer the color transitions.

Typically, the resolution of a digital camera corresponds to the number of elements of the CCD matrix. For example, the sensor of a Contax N Digital SLR camera, the frame size of which is the same as the frame size of standard 35 mm film, containing 6 million elements (6 megapixels), provides a resolution of 3040x2008, i.e. 3040 pixels horizontally and 2008 pixels vertically. If you multiply these numbers, you get approximately 6 million. The number of matrix elements is the main characteristic of a digital camera.

Cameras with CCD matrices containing more than 3 million elements allow you to take pictures with a maximum resolution of 2048x1536 pixels. This resolution is considered high and is only necessary for printing. For viewing on the screen, a resolution of 1024x768, and sometimes 640x480, will be sufficient.

But in some cases, the camera can take pictures with a higher resolution than is possible for a given matrix. For example, the AGFA ePhoto 1680 camera with a 1.3 million element CCD matrix can take pictures with a resolution of 1600x1200 pixels.

Multiplying 1600 by 1200 gives 1.92 million pixels. In this case, the possibility of obtaining a resolution higher than optical is provided by software, using special PhotoGenie technology, due to interpolation, i.e. introducing additional points between the actual ones. Their color is determined by neighboring points. The resolution obtained by this method, in contrast to optical, is called interlace.

You should understand: the higher the camera resolution, the larger the print size and the better quality you can get when printing on a printer or using photos in printing.

To obtain the best quality print used in printing, the image resolution should be 1.5 times higher than the halftone screen frequency (lineature) used in printing. High-quality images in printing are printed with a lineature of 150 Ipi (lines per inch) and higher. This means that if you plan to print a picture in a magazine, its resolution must be at least 225 ppi (dots per inch). If you plan to print a photograph with this resolution measuring 18x13 cm, or, what is the same, 7.10x5.12 inches, then by multiplying these values ​​by 225, you will get the camera resolution required for such a photo: 7.10x225=1597; 5.12x225=1152. This means that a camera with a standard optical resolution of 1600x1200 and higher is quite suitable for the task. If you need to print high-quality images larger than 18x13, you will need to use a camera with a higher optical resolution. When using a camera with a maximum resolution of 640x480 pixels, the optimal image size with a resolution of 225 ppi will be 2.84x2.13 inches (640:225=2.84, 480:225=2.13) or 7.2x5.4 cm. Calculations given relate to the preparation of photographs for printing reproduction. For high-quality printing on Epson printers, for example, a resolution of 150 ppi is sufficient, at which most viewers will not be able to see any artifacts. This is achieved by printing technology.

Resolution determines the degree of detail of the image generated by a video surveillance camera, and this parameter is determined by several factors:

• characteristics of the camera matrix,
• lens (its quality, focal length),
• distance to the observed object.

All these points will be discussed below, however, it should be borne in mind that the resolution of the video surveillance system as a whole is also determined by other devices, for example:

• recordings (DVR, video server),
• display (monitor).

Despite the fact that the resolution of a video surveillance camera is determined by the number of pixels in its matrix, for analog video cameras it is indicated in TVL (television lines). This value is determined using a special table; it means how many alternating black and white stripes the video camera can reproduce vertically or horizontally (Fig. 1).

Conventionally, ANALOG CAMERAS can be divided into devices of standard (380-420 TVL, which corresponds to approximately 500 horizontal pixels) and high (560-600 TVL - about 750 pixels) resolution. True, video cameras with a resolution of about 1000 TV lines are now being produced.

The resolution of IP CAMERAS is defined as the product of the number of horizontal and vertical pixels of the matrix (Fig. 2). It is measured in megapixels. The passport data indicates exactly the work. In order to separately determine the horizontal and vertical resolution, it should be taken into account that the aspect ratio of the matrix is ​​3:4.

If we denote the horizontal and vertical resolution, as well as the camera as a whole, respectively, as Xg, Xv, Xk, we get:

Xg=√ Hk/0.75

Хв=0.75*Хг

The next point that affects the detail of the image is the distance to the video surveillance object (Fig. 3).

Objects H1 and H2 are displayed on the matrix with the same size Nm, despite the fact that their actual sizes are different. That is, each of them has the same number of matrix elements. Accordingly, the degree of detail of object H1 will be higher (Fig. 4).

It is worth noting that when organizing a video surveillance system, it is the detail of the image that is of practical interest, which, as has been shown, depends not only on the camera resolution.

By changing the viewing angle of a video surveillance camera, which, by the way, depends on the focal length of the lens, you can obtain the desired degree of detail of objects located at different distances from the video camera.

There are formulas that allow you to make the necessary calculations and corresponding summary tables, however, for convenience, you can use online calculators to calculate the viewing angle and focal length of a video camera.

Since the purpose of this article is to present the very basics regarding the resolution of video cameras, attention was not focused on the fact that horizontal and vertical resolutions are different. In certain situations, this point must be taken into account, but to understand the essence of the issue, the material presented should be sufficient.

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Currently, most image capture systems use CCD (charge-coupled device) matrices as the photosensitive device.

The operating principle of a CCD matrix is ​​as follows: a matrix of photosensitive elements (accumulation section) is created on the basis of silicon. Each photosensitive element has the property of accumulating charges proportional to the number of photons hitting it. Thus, over some time (exposure time) in the accumulation section, a two-dimensional matrix of charges proportional to the brightness of the original image is obtained. The accumulated charges are initially transferred to the storage section, and then line by line and pixel by pixel to the output of the matrix.

The size of the storage section in relation to the accumulation section varies:

• per frame (matrices with frame transfer for progressive scan);
• per half-frame (matrices with frame transfer for interlaced scanning);

There are also matrices in which there is no storage section, and then line transfer is carried out directly through the accumulation section. Obviously, for such matrices to work, an optical shutter is required.

The quality of modern CCD matrices is such that the charge remains virtually unchanged during the transfer process.

Despite the apparent variety of television cameras, the CCD matrices used in them are practically the same, since mass and large-scale production of CCD matrices is carried out by only a few companies. These are SONY, Panasonic, Samsung, Philips, Hitachi Kodak.

The main parameters of CCD matrices are:

• dimension in pixels;
• physical size in inches (2/3, 1/2, 1/3, etc.). Moreover, the numbers themselves do not determine the exact size of the sensitive area, but rather determine the class of the device;
• sensitivity.

Resolution of CCD cameras.

The resolution of CCD cameras is mainly determined by the size of the CCD matrix in pixels and the quality of the lens. To some extent, this can be influenced by the camera’s electronics (if it’s poorly made, it can worsen the resolution, but they rarely do anything frankly bad these days).

It is important to make one note here. In some cases, high-frequency spatial filters are installed in cameras to improve apparent resolution. In this case, an image of an object obtained from a smaller camera may appear even sharper than an image of the same object obtained objectively from a better camera. Of course, this is acceptable when the camera is used in visual surveillance systems, but it is completely unsuitable for constructing measurement systems.

Resolution and format of CCD matrices.

Currently, various companies produce CCD matrices covering a wide range of dimensions from several hundred to several thousand. This is how a matrix with a dimension of 10000x10000 was reported, and this message noted not so much the problem of the cost of this matrix as the problem of storing, processing and transmitting the resulting images. As we know, matrices with dimensions up to 2000x2000 are now more or less widely used.

The most widely, or more precisely, mass-used CCD matrices certainly include matrices with a resolution oriented to the television standard. These are matrices mainly of two formats:

• 512*576;
• 768*576.

Matrices 768*576 (sometimes a little more, sometimes a little less) allow you to get the maximum resolution for a standard television signal. At the same time, unlike matrices of the 512*576 format, they have a grid arrangement of photosensitive elements close to a square, and, therefore, equal horizontal and vertical resolution.

Often, camera manufacturers indicate resolution in television lines. This means that the camera allows you to see N/2 dark vertical strokes on a light background, arranged in a square inscribed in the image field, where N is the declared number of television lines. In relation to a standard television table, this assumes the following: by selecting the distance and focusing the table image, it is necessary to ensure that the upper and lower edges of the table image on the monitor coincide with the outer contour of the table, marked by the vertices of black and white prisms; then, after final subfocusing, the number is read in the place of the vertical wedge where the vertical strokes for the first time cease to be resolved. The last remark is very important because... and in the image of test fields of a table with 600 or more lines, alternating stripes are often visible, which, in fact, are moiré formed by the beating of the spatial frequencies of the lines of the table and the grid of sensitive elements of the CCD matrix. This effect is especially pronounced in cameras with high-frequency spatial filters (see above)!

I would like to note that, all other things being equal (this can mainly be influenced by the lens), the resolution of black-and-white cameras is uniquely determined by the size of the CCD matrix. So a 768*576 format camera will have a resolution of 576 television lines, although in some prospectuses you can find a value of 550, and in others 600.

Lens.

The physical size of the CCD cells is the main parameter that determines the requirement for the resolution of the lens. Another such parameter may be the requirement to ensure the operation of the matrix under light overload conditions, which will be discussed below.

For a 1/2 inch SONY ICX039 matrix, the pixel size is 8.6µm*8.3µm. Therefore, the lens must have a resolution better than:

1/8.3*10e-3= 120 lines (60 pairs of lines per millimeter).

For lenses made for 1/3-inch matrices, this value should be even higher, although this, oddly enough, does not affect the cost and such a parameter as aperture, since these lenses are made taking into account the need to form an image on a smaller light-sensitive field of the matrix. It also follows that lenses for smaller matrices are not suitable for large matrices due to significantly deteriorating characteristics at the edges of large matrices. At the same time, lenses for large sensors can limit the resolution of images obtained from smaller sensors.

Unfortunately, with all the modern abundance of lenses for television cameras, it is very difficult to obtain information on their resolution.

In general, we do not often select lenses, since almost all of our Customers install video systems on existing optics: microscopes, telescopes, etc., so our information about the lens market is in the nature of notes. We can only say that the resolution of simple and cheap lenses is in the range of 50-60 pairs of lines per mm, which is generally not enough.

On the other hand, we have information that special lenses produced by Zeiss with a resolution of 100-120 line pairs per mm cost more than $1000.

So, when purchasing a lens, it is necessary to conduct preliminary testing. I must say that most Moscow sellers provide lenses for testing. Here it is once again appropriate to recall the moire effect, the presence of which, as noted above, can mislead regarding the resolution of the matrix. So, the presence of moire in the image of sections of the table with strokes above 600 television lines in relation to the lens indicates a certain reserve of the latter’s resolution, which, of course, does not hurt.

One more, perhaps important note for those who are interested in geometric measurements. All lenses have distortion to one degree or another (pincushion-shaped distortion of the image geometry), and the shorter the lens, the greater these distortions, as a rule, are. In our opinion, lenses with focal lengths greater than 8-12 mm have acceptable distortion for 1/3" and 1/2" cameras. Although the level of “acceptability”, of course, depends on the tasks that the television camera must solve.

Resolution of image input controllers

The resolution of image input controllers should be understood as the conversion frequency of the analog-to-digital converter (ADC) of the controller, the data of which is then recorded in the controller’s memory. Obviously, there is a reasonable limit to increasing the digitization frequency. For devices that have a continuous structure of the photosensitive layer, for example, vidicons, the optimal digitization frequency is equal to twice the upper frequency of the useful signal of the vidicon.

Unlike such light detectors, CCD matrices have a discrete topology, so the optimal digitization frequency for them is determined as the shift frequency of the output register of the matrix. In this case, it is important that the controller’s ADC operates synchronously with the output register of the CCD matrix. Only in this case can it be achieved best quality transformations both from the point of view of ensuring the “rigid” geometry of the resulting images and from the point of view of minimizing noise from clock pulses and transient processes.

Sensitivity of CCD cameras

Since 1994, we have been using SONY card cameras in our devices based on the ICX039 CCD matrix. The SONY description for this device indicates a sensitivity of 0.25 lux on an object with a lens aperture of 1.4. Several times already, we have come across cameras with similar parameters (size 1/2 inch, resolution 752*576) and with a declared sensitivity of 10 or even 100 times greater than that of “our” SONY.

We checked these numbers several times. In most cases, in cameras from different companies, we found the same ICX039 CCD matrix. Moreover, all the “piping” microcircuits were also SONY-made. And comparative testing showed almost complete identity of all these cameras. So what's the question?

And the whole question is at what signal-to-noise ratio (s/n) the sensitivity is determined. In our case SONY company conscientiously showed sensitivity at s/n = 46 dB, while other companies either did not indicate this or indicated it in such a way that it is unclear under what conditions these measurements were made.

This is, in general, a common scourge of most camera manufacturers - not specifying the conditions for measuring camera parameters.

The fact is that as the requirement for the S/N ratio decreases, the sensitivity of the camera increases in inverse proportion to the square of the required S/N ratio:

Where:
I - sensitivity;
K - conversion factor;
s/n - s/n ratio in linear units,

Therefore, many companies are tempted to indicate camera sensitivity at a low S/N ratio.

We can say that the ability of matrices to “see” better or worse is determined by the number of charges converted from photons incident on its surface and the quality of delivery of these charges to the output. The amount of accumulated charges depends on the area of ​​the photosensitive element and the quantum efficiency of the CCD matrix, and the quality of transportation is determined by many factors, which often come down to one thing - readout noise. The readout noise for modern matrices is on the order of 10-30 electrons or even less!

The areas of the elements of CCD matrices are different, but the typical value for 1/2 inch matrices for television cameras is 8.5 µm * 8.5 µm. An increase in the size of the elements leads to an increase in the size of the matrices themselves, which increases their cost not so much due to the actual increase in the production price, but due to the fact that the serial production of such devices is several orders of magnitude smaller. In addition, the area of ​​the photosensitive zone is affected by the topology of the matrix to the extent that the percentage of the total surface of the crystal is occupied by the sensitive area (fill factor). In some special matrices, the fill factor is stated to be 100%.

Quantum efficiency (how much on average the charge of a sensitive cell in electrons changes when one photon falls on its surface) for modern matrices is 0.4-0.6 (for some matrices without anti-blooming it reaches 0.85).

Thus, it can be seen that the sensitivity of CCD cameras, related to a certain S/N value, has come close to the physical limit. According to our conclusion, typical sensitivity values ​​of cameras for general use at s/w = 46 lie in the range of 0.15-0.25 lux of illumination on the object with a lens aperture of 1.4.

In this regard, we do not recommend blindly trusting the sensitivity figures indicated in the descriptions of television cameras, especially when the conditions for determining this parameter are not given and, if you see in the passport of a camera costing up to $500 a sensitivity of 0.01-0.001 lux in television mode, then before you are an example of, to put it mildly, incorrect information.

About ways to increase the sensitivity of CCD cameras

One way to solve this is to accumulate images over time. The implementation of this method can significantly increase the sensitivity of the CCD. Of course, this method can be applied to stationary objects of observation or in cases where movement can be compensated, as is done in astronomy.

Fig1 Planetary nebula M57.

Telescope: 60 cm, exposure - 20 sec., temperature during exposure - 20 C.
At the center of the nebula there is a stellar object of magnitude 15.
The image was obtained by V. Amirkhanyan at the Special Astrophysical Observatory of the Russian Academy of Sciences.

It can be stated with reasonable accuracy that the sensitivity of CCD cameras is directly proportional to the exposure time.

For example, sensitivity at a shutter speed of 1 second relative to the original 1/50s will increase 50 times, i.e. it will be better - 0.005 lux.

Of course, there are problems along this path, and this is, first of all, the dark current of the matrices, which brings charges that accumulate simultaneously with the useful signal. The dark current is determined, firstly, by the manufacturing technology of the crystal, secondly, by the level of technology and, of course, to a very large extent by the operating temperature of the matrix itself.

Usually, to achieve long accumulation times, on the order of minutes or tens of minutes, the matrices are cooled to minus 20-40 degrees. C. The problem of cooling the matrices to such temperatures has been solved, but it is simply impossible to say that this cannot be done, since there are always design and operational problems associated with fogging of protective glasses and heat release from the hot junction of a thermoelectric refrigerator.

At the same time, technological progress in the production of CCD matrices has also affected such a parameter as dark current. Here the achievements are very significant and the dark current of some good modern matrices is very small. In our experience, cameras without cooling allow making exposures at room temperature within tens of seconds, and with dark background compensation up to several minutes. As an example, here is a photograph of the planetary nebula M57, obtained with the VS-a-tandem-56/2 video system without cooling with an exposure of 20 s.

The second way to increase sensitivity is the use of electron-optical converters (EOC). Image intensifiers are devices that enhance the luminous flux. Modern image intensifiers can have very large gain values, however, without going into details, we can say that the use of image intensifiers can only improve the threshold sensitivity of the camera, and therefore its gain should not be made too large.

Spectral sensitivity of CCD cameras

For some applications, the spectral sensitivity of the CCD is an important factor. Since all CCDs are made on the basis of silicon, in their “bare” form the spectral sensitivity of the CCD corresponds to this parameter of silicon (see Fig. 2).

As you can see, with all the variety of characteristics, CCD matrices have maximum sensitivity in the red and near-infrared (IR) range and see absolutely nothing in the blue-violet part of the spectrum. The near-IR sensitivity of CCDs is used in covert surveillance systems illuminated by IR light sources, as well as when measuring thermal fields of high-temperature objects.

SONY produces all its black-and-white matrices with the following spectral characteristics (see Fig. 3). As you can see from this figure, the sensitivity of the CCD in the near IR is significantly reduced, but the matrix began to perceive the blue region of the spectrum.

For various special purposes, matrices sensitive in the ultraviolet and even X-ray range are being developed. Usually these devices are unique and their price is quite high.

About progressive and interlaced scanning

The standard television signal was developed for a broadcast television system, and from the point of view of modern image input and processing systems, it has one big drawback. Although the TV signal contains 625 lines (of which about 576 contain video information), 2 half-frames are displayed sequentially, consisting of even lines (even half-frame) and odd lines (odd half-frame). This leads to the fact that if a moving image is input, then the analysis cannot use a Y resolution of more than the number of lines in one half-frame (288). In addition, in modern systems, when the image is visualized on a computer monitor (which has progressive scan), the image input from the interlaced camera when the object is moving causes an unpleasant visual effect of doubling.

All methods to combat this shortcoming lead to a deterioration in vertical resolution. The only way to overcome this disadvantage and achieve resolution that matches the resolution of the CCD is to switch to progressive scanning in the CCD. CCD manufacturers produce such matrices, but due to the low production volume, the price of such matrices and cameras is much higher than that of conventional ones. For example, the price of a SONY matrix with progressive scan ICX074 is 3 times higher than ICX039 (interlace scan).

Other camera options

These others include such a parameter as “blooming”, i.e. spreading of the charge over the surface of the matrix when its individual elements are overexposed. In practice, such a case may occur, for example, when observing objects with glare. This is a rather unpleasant effect of CCDs, since a few bright spots can distort the entire image. Fortunately, many modern matrices contain anti-blooming devices. So, in the descriptions of some of the latest SONY matrices, we found 2000, which characterizes the permissible light overload of individual cells, which does not yet lead to charge spreading. This is a fairly high value, especially since this result can be achieved, as our experience has shown, only with special adjustment of the drivers that directly control the matrix and the video signal pre-amplification channel. In addition, the lens also makes its contribution to the “spreading” of bright points, since with such large light overloads, even small scattering beyond the main spot provides a noticeable light support for neighboring elements.

It is also necessary to note here that according to some data, which we have not verified ourselves, matrices with anti-blooming have a 2-fold lower quantum efficiency than matrices without anti-blooming. In this regard, in systems that require very high sensitivity, it may make sense to use matrices without anti-blooming (usually these are special tasks such as astronomical ones).

About color cameras

The materials in this section somewhat go beyond the scope of consideration of measuring systems that we have established, however, the widespread use of color cameras (even more than black and white) forces us to clarify this issue, especially since Customers often try to use black and white cameras with our cameras. color television cameras with white frame grabbers, and they are very surprised when they find some stains in the resulting images, and the resolution of the images turns out to be insufficient. Let's explain what's going on here.

There are 2 ways to generate a color signal:

• 1. use of a single matrix camera.
• 2. use of a system of 3 CCD matrices with a color separation head to obtain R, G, B components of the color signal on these matrices.

The second way provides the best quality and is the only way to obtain measurement systems; however, cameras operating on this principle are quite expensive (more than $3000).

In most cases, single-chip CCD cameras are used. Let's look at their operating principle.

As is clear from the fairly wide spectral characteristics of the CCD matrix, it cannot determine the “color” of a photon hitting the surface. Therefore, in order to enter a color image, a light filter is installed in front of each element of the CCD matrix. In this case, the total number of matrix elements remains the same. SONY, for example, produces exactly the same CCD matrices for black-and-white and color versions, which differ only in the presence of a grid of light filters in the color matrix, applied directly to the sensitive areas. There are several matrix coloring schemes. Here is one of them.

Here 4 different filters are used (see Fig. 4 and Fig. 5).

Figure 4. Distribution of filters on CCD matrix elements

Figure 5. Spectral sensitivity of CCD elements with various filters.

Y=(Cy+G)+(Ye+Mg)

In line A1 the "red" color difference signal is obtained as:

R-Y=(Mg+Ye)-(G+Cy)

and in line A2 a “blue” color difference signal is obtained:

-(B-Y)=(G+Ye)-(Mg+Cy)

It is clear from this that the spatial resolution of a color CCD matrix, compared to the same black and white one, is usually 1.3-1.5 times worse horizontally and vertically. Due to the use of filters, the sensitivity of a color CCD is also worse than that of a black and white one. Thus, we can say that if you have a single-matrix receiver 1000 * 800, then you can actually get about 700 * 550 for the brightness signal and 500 * 400 (700 * 400 is possible) for the color signal.

Leaving aside technical issues, I would like to note that for advertising purposes, many manufacturers of electronic cameras report completely incomprehensible data on their equipment. For example, the Kodak company announces the resolution of its DC120 electronic camera as 1200*1000 with a matrix of 850x984 pixels. But gentlemen, information does not appear out of nowhere, although visually it looks good!

The spatial resolution of a color signal (a signal that carries information about the color of the image) can be said to be at least 2 times worse than the resolution of a black-and-white signal. In addition, the “calculated” color of the output pixel is not the color of the corresponding element of the source image, but only the result of processing the brightness of various elements of the source image. Roughly speaking, due to the sharp difference in brightness of neighboring elements of an object, a color that is not there at all can be calculated, while a slight camera shift will lead to a sharp change in the output color. For example: the border of a dark and light field gray will look like it consists of multi-colored squares.

All these considerations relate only to the physical principle of obtaining information on color CCD matrices, while it must be taken into account that usually the video signal at the output of color cameras is presented in one of the standard formats PAL, NTSC, or less often S-video.

The PAL and NTSC formats are good because they can be immediately reproduced on standard monitors with a video input, but we must remember that these standards provide a significantly narrower band for the color signal, so it is more correct to talk about a colored image rather than a color one. Another unpleasant feature of cameras with video signals that carry a color component is the appearance of the above-mentioned streaks in the image obtained by black-and-white frame grabbers. And the point here is that the chrominance signal is located almost in the middle of the video signal band, creating interference when entering an image frame. We do not see this interference on a television monitor because the phase of this “interference” is reversed after four frames and averaged by the eye. Hence the bewilderment of the Customer, who receives an image with interference that he does not see.

It follows from this that if you need to carry out some measurements or decipher objects by color, then this issue must be approached taking into account both the above and other features of your task.

About CMOS matrices

In the world of electronics, everything is changing very quickly, and although the field of photodetectors is one of the most conservative, new technologies have been approaching here recently. First of all, this relates to the emergence of CMOS television matrices.

Indeed, silicon is a light-sensitive element and any semiconductor product can be used as a sensor. The use of CMOS technology provides several obvious advantages over traditional technology.

Firstly, CMOS technology is well mastered and allows the production of elements with a high yield of useful products.

Secondly, CMOS technology allows you to place on the matrix, in addition to the photosensitive area, various framing devices (up to the ADC), which were previously installed “outside”. This makes it possible to produce cameras with digital output “on a single chip.”

Thanks to these advantages, it becomes possible to produce significantly cheaper television cameras. In addition, the range of companies producing matrices is expanding significantly.

At the moment, the production of television matrices and cameras using CMOS technology is just getting started. Information about the parameters of such devices is very scarce. We can only note that the parameters of these matrices do not exceed what is currently achieved; as for the price, their advantages are undeniable.

Let me give as an example a single-chip color camera from Photobit PB-159. The camera is made on a single chip and has the following technical parameters:

• resolution - 512*384;
• pixel size - 7.9µm*7.9µm;
• sensitivity - 1 lux;
• output - digital 8-bit SRGB;
• body - 44 PLCC legs.

Thus, the camera loses four times in sensitivity, in addition, from information on another camera it is clear that this technology has problems with a relatively large dark current.

About digital cameras

Recently, a new market segment has emerged and is rapidly growing, using CCD and CMOS matrices - digital cameras. Moreover, at the present moment there is a sharp increase in the quality of these products simultaneously with a sharp decrease in price. Indeed, just 2 years ago, a matrix with a resolution of 1024*1024 alone cost about $3000-7000, but now cameras with such matrices and a bunch of bells and whistles (LCD screen, memory, vari-lens, convenient body, etc.) can be bought for less than $1000 . This can only be explained by the transition to large-scale production of matrices.

Since these cameras are based on CCD and CMOS matrices, all the discussions in this article about sensitivity and the principles of color signal formation are valid for them.

Instead of a conclusion

The practical experience we have accumulated allows us to draw the following conclusions:

• The production technology of CCD matrices in terms of sensitivity and noise is very close to physical limits;
• on the television camera market you can find cameras of acceptable quality, although adjustments may be required to achieve higher parameters;
• Do not be fooled by the high sensitivity figures given in camera brochures;
• and also, prices for cameras that are absolutely identical in quality and even for simply identical cameras different sellers may differ by more than twice!

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