Television surveillance in bright sunlight.

#Special Equipment

Television surveillance in bright sunlight..

Kulikov Alexander Nikolaevich

TELEVISION OBSERVATION IN BRIGHT SUNLIGHT

Source: magazine «Special Equipment»

Instead of an introduction.

In ten years of work in the field of security television, I regularly observe the same picture. In gloomy autumn, a team of installers returns from the site.

The cameras are installed, the system is functioning, the commission has accepted the work. I remember the comments of the head of security, who was pleased with how well the cameras “show” at night, and how high the image clarity is during the day.

But spring comes, and the phones start ringing… It turns out that in bright sunlight, some cameras “go white”, others work with distortions, “white pillars”, loss of image over a significant area of ​​the photoreceiver, and so on. Then there are complaints, business trips, replacement of cameras and lenses, loss of time and money.

When building television systems, special attention is paid to night observation. Cameras with sensitive photoreceivers, high-aperture aspherical lenses are selected, and a system of artificial illumination of objects and territories is used.

At the same time, people often forget about the peculiarities of daytime surveillance, believing that if there is a lot of light, then everything will be visible anyway. However, it is in bright sunlight that situations arise when not only large areas, but also the entire image may be lost in the image formed by a security camera. This article discusses the peculiarities of surveillance in bright sunlight.

 1. Absolute image contrast.

The main reason leading to deterioration in the quality of video surveillance in daytime conditions is high absolute image contrast, that is, the ratio of the illumination of the brightest and darkest of the observed objects.

At night, the absolute contrast can be less than 100, when objects are illuminated by the diffuse light of the night sky.

Fig. 1. Illustration of the increase in image contrast with increasing illumination.

During the day, the absolute contrast increases to tens of thousands, and when the Sun enters the field of view of a television camera, to a million times.

Such an increase in contrast is caused by the illumination conditions of a complex surface from a single light source — the Sun.

The illumination of objects in the shade can be reduced to 100 lux or less, while the illumination of light surfaces under direct sunlight is more than 100,000 lux. The illumination of glare from shiny surfaces and water can reach 10 6 lux, and the equivalent illumination of the solar disk, according to some estimates, reaches 108 lux, that is, 100 million lux.

No television camera is capable of simultaneously observing (in one field of view) objects that differ in illumination by tens and hundreds of thousands of times. In such situations, losses of video information in some areas of the image are inevitable. The designer's task is to minimize the losses that occur when the television system operates under conditions of light overload.

2. Differences between natural and television observations.

The range of illumination perceived by the eye approaches a billion. However, during the day we do not see stars in the sky, although the absolute contrast between the sky and the stars is no more than ten thousand.

The fact is that the contrast sensitivity of the human eye is only 2% [ 1 ], so the discernible absolute contrast does not exceed 50.

The eye can only examine individual sections of the billionth range in turn, adapting to each section of illumination. When observing the terrain, a person alternately shifts his gaze from one object to another.

If the object is bright, the person squints. When looking closely at an object in the shadow, the observer protects his eyes from the blinding Sun with his palm.

Observation of a wide range of illumination by the human eye is possible only by shifting the gaze from dark to bright objects and back.

A television camera is usually fixed in place. Therefore, objects with high absolute contrast can fall into its field of view simultaneously. The television system operator observes the image on a small video monitor.

As a result, the «TV camera — monitor — eye» system does not have the advantages that arise from natural observation due to shifting the gaze and alternately examining bright and dark objects.

Next we will discuss the possibility of television observation of objects with different illumination simultaneously, that is, «in one television field».

In this case, the loss of video information in bright and dark areas of the scene is inevitable. Additional narrowing of the observed contrast occurs due to insufficient brightness of the monitor screen and artificial lighting inside the room. The equivalent illumination of the monitor screen is less than 500 lux, which worsens the contrast sensitivity of the eye, the maximum only in the area of ​​several thousand lux.

When observing images on color, and especially computer video monitors (the equivalent illumination of the screen of the latter is less than 100 lux), the range of illumination observed by the eye is reduced even more.

Therefore, when television observation of areas and objects illuminated by the Sun, it is necessary to use monitors with maximum screen brightness.

A high-contrast monitor with a large screen size will expand the range of observable illumination and reduce the likelihood of losing part of the image under difficult lighting conditions.

3. Limitations of image contrast in one field of a television camera.

The light image is projected by the lens onto the photosensitive elements of the CCD matrix. The dynamic range of the elements determines the range of working illumination of the television camera in one field. Photons of light are converted into photoelectrons, entering the photosensitive cells, therefore, when calculating the signal and noise characteristics, it is convenient to use the unit of charge measurement — electron.

3.1 Dependence of the maximum contrast on the area of ​​the photosensitive element.

The maximum contrast is determined by the ratio of the maximum and minimum distinguishable charge levels in the elements.

The maximum charge level is called the control capability of the CCD [ 2 ], which is proportional to the geometric area and depth of the potential well of the element. In CCD matrices, charge packets, moving to the output device, pass through several charge transfer sections. The smallest potential wells are possessed by elements of the accumulation and storage sections, which primarily limit the charge level.

In modern CCD matrices of 1/6 — 1/2 inch formats with a volumetric charge transfer channel, the control capability of the element is in the range from 12,000 to 300,000 electrons.

The minimum number of electrons is determined by the root-mean-square value of the CCD matrix readout noise and is 20–40 electrons, depending on the gate capacity of the first transistor of the output device. Consequently, the dynamic range of modern CCD cameras is between 600 and 7500.

To obtain the maximum contrast values, these values ​​should be divided by 10, since only starting with such a signal/noise ratio can objects in the image be distinguished. Substituting the area of ​​the photosensitive elements from the reference data, you can find the maximum contrast for CCD matrices of different formats and resolutions.

Table 1. Maximum contrast realized in standard CCD matrices from SONY depending on the area of ​​the photosensitive element*.

• For EXWAWEHAD series matrices, the maximum contrast values ​​should be multiplied by 1.3.

Table 1 shows that TV cameras with 1/2-inch photodetectors provide the maximum range of operating illumination, i.e., they provide minimal loss of information when observing high-contrast images on sunny days.

However, the high cost of cameras with half-inch CCD matrices does not allow their use in most security systems.

With limited resources, it is optimal to use cameras with 1/3-inch standard-resolution CCD matrices, the best of which in terms of the range of working illumination is currently the ICX255AL from SONY.

3.2 Contrast limitation in electronic shutter mode. «Blur» and spreading of the charge image in the CCD matrix.

When using lenses with a constant aperture, the electronic shutter mode is used to adapt the camera to the illumination level. In this mode, as the illumination increases, the charge accumulation time in the CCD matrix automatically decreases, and therefore the sensitivity.

Modern cameras provide a minimum exposure time of 1/10000 to 1/100000 seconds. But even the latter value is not enough for reliable observation of objects illuminated by sunlight.

When installing a standard small-sized lens with an M12 thread and a relative aperture of F 1.8 in the camera, with an exposure of 1/100,000, the CCD matrix stops seeing when the illumination on the object is more than 30,000 lux, which is insufficient for observation in sunlight. When observing, images of white walls of buildings, snow, clouds, and especially objects shining in the Sun will be lost. It would seem that the accumulation time can be reduced to one millionth of a second or less, which is not difficult from a circuit standpoint.

But the reduction of the accumulation time in standard CCD matrices is hampered by the «smear» of the image. The «Smear» parameter of 0.005% for standard CCD matrices is usually ignored as insignificant. However, such a small value of «smear» is obtained only with a total accumulation time of 20 milliseconds.

At an exposure of 1/100000 of a second, the «smear» signal increases by 2000 times and becomes equal to 10%, which is manifested in the form of clearly visible «white stripes» at the top and bottom of bright objects in the image. If the illumination of an object is more than 10 times higher than the maximum illumination (a lamp filament, the Sun), then the «smear» value exceeds 100% and the effect of «charge spreading over the surface of the CCD matrix» — blooming — occurs.

In 1999, SONY launched the production of a new generation of CCD matrices under the EXWAVEHAD trademark. In Russia, the WAT902H television cameras from WATEC and the VNC-703 from EVS are well known, in which the new generation of SONY matrices are installed.

The advertising for these cameras focused on the improved sensitivity of the EXWAVEHAD series CCD matrices. However, another advantage of the new matrices was not noted – a 30 times lower level of “blurring” when observing bright objects.

a) standard CCD matrix ICX055BL	b) EXWAVEHAD CCD matrix ICX255AL

Fig. 2 Illustration of «smearing» of the image and the effect of charge spreading when observing the filament in a TV camera on CCD matrices from SONY.

Significantly lower blurring from bright objects improves image quality in TV cameras with new SONY matrices when working during the day in conditions of light overload.

However, it should be noted that most new cameras do not implement all the advantages of EXWAVEHAD series matrices. This is explained by the fact that other components of TV cameras (sync generators, drivers, amplifiers) are designed to operate in standard modes corresponding to conventional CCD matrices.

Figure 3. Dependence of the integral signal of the «blur» image for TV cameras on a standard ICX055BL CCD matrix and EXWAVEHAD ICX255AL CCD on the accumulation time at a field frequency of 50 Hz.

It is evident from Figure 3 that the EXWAVEHAD CCD can reduce the minimum exposure time in the electronic shutter mode by an order of magnitude, compared to standard CCDs, which will expand the range of working illuminations in a camera with a lens with a constant aperture to 100,000 lux. This value is sufficient for reliable observation of objects in sunlight.

3.3 Effect of the CCD matrix mode on resistance to light overloads.

The performance of a television camera under strong light overloads (the Sun or a spotlight in the field of view) depends not only on the size of the photosensitive cell (the format and number of elements of the CCD matrix) and the type of lens.

To a large extent, the ability to withstand overloads is determined by the adjustment method and the circuit of the television camera.

Many television camera manufacturers, in pursuit of low cost, simplify the circuits, excluding the adjustment elements. As a result, due to the spread of CCD matrix parameters, cameras of the same model differ significantly from each other in overload capacity.

Fig. 4 Illustration of the reduction in the overload capacity of a TV camera on a CCD matrix with incorrectly set Xsub and RZ modes.

In addition to the optimal mode settings, the CCD matrix control circuit has a noticeable effect on the quality of observation in bright light. When overloaded, the current in the secondary power supply circuits increases several times, so the accuracy of maintaining the mode, and therefore the degree of manifestation of the charge spreading effect, depends on their power and stability.

It should be noted that, as a rule, the modes that ensure optimal observation at night and during the day are different. As a result, camera developers choose a compromise mode, which leads to additional image loss during overloads.

For example, to improve the accuracy of the black level reference circuit in low light, in cameras on SONY, Samsung, SHARP matrices, the level is fixed both by the front and rear idler elements of the CCD.

During light overloads, the spreading charge gets into the rear idler elements, which leads to distortion of the fixation circuit, up to complete loss of the image, in cases where the image of a bright object is projected onto the right edge of the CCD matrix.

To expand the range of working illumination of television cameras, it is necessary to change the operating mode of the CCD matrix during the day and at night.

The maximum gain from switching the CCD matrix modes in night and day conditions is achieved in cameras with EXWAVEHAD series CCD matrices.

For example, in the VBP-551 television camera manufactured by the Russian company EVS, when using lenses with a constant aperture, it is possible to observe objects with an illumination of 100,000 lux and is resistant to light overloads.

Such characteristics are ensured by a minimum exposure time in the electronic shutter mode of 1/1,000,000 seconds and adaptive switching of the CCD modes day — night.

a)	b)

Fig.5 Reduction of the spreading charge area in the VBP-551 camera with adaptive CCD mode – a), compared to the standard WAT-902H camera – b). The same lens with a constant aperture of F(1.8) was installed in both cameras. The equivalent illumination of the filament of a 75 W incandescent lamp is about 106 lux.

3.4 Contrast Limitation in Lenses. Light Scattering in Lenses, Glare, and Distortion.

The most important element of a television camera, determining the quality of the image in bright sunlight, is the lens. The differences in the quality of lenses, even within the same class, are very large.

It should be noted that for effective work during the day in conditions of light overload, some parameters become important that are not regulated in the passport data for most lenses available on the market.

The minimum relative aperture of the lens diaphragm is usually indicated in the passport data and is within Fmin. = (32…..360).

The range of illumination control using the aperture is equal to the square of the ratio of the minimum and maximum relative apertures. For standard lenses with a fully open aperture, Fmax is usually 1.2.

Considering that the maximum working illumination, recalculated for the object with an accumulation time of 20 ms (electronic shutter off), is approximately 20 lux, we can determine the maximum permissible illumination provided by this lens.

Table 2. Illumination control range and maximum observed illumination on the object depending on the minimum relative aperture of the ARD lens.

The table shows that simple ARD lenses with minimum relative apertures F(32) and F(64) are unsuitable for use in bright sunlight.

It should be noted that for reliable operation of the camera in conditions of light overloads, not only a wide range of illumination control in the lens is necessary, but also linearity of control, especially in the final section, when the lens diaphragm is almost closed.

With insufficient linearity, self-excitation (image flickering) is possible in the camera-lens system at maximum illumination levels.

Unfortunately, the adjustment characteristic is usually not given in the passport data for lenses. The best in terms of linearity are wide-range lenses with miniature film filters installed on the lens aperture sections.

Glare and distortion caused by the lens aperture.

If the lens design is poor, glare is formed due to the reflection of light from its internal surfaces and, first of all, from the aperture.

As a rule, lenses with a minimum aperture control range have the maximum level of glare.

Fig. 6 Observation of a bright light source through a lens with strong glare.

At certain angles between the lens axis and the axis directed at a bright object, the level of glare may become unacceptable and lead to partial loss of image when a bright source enters the field of view of the television camera.

Unfortunately, no parameters concerning lens glare are given in the passport data, therefore, it is necessary to conduct an independent statistical selection of lenses with minimal glare.

Scattering and re-reflection of light in lenses and inside the lens.

An additional limitation on the ability to observe maximum contrast in one field is imposed by light scattering in lenses and light re-reflection from the walls and other internal elements of the lens.

The situation is worsened by the fact that modern CCD matrices are sensitive in the near IR range. Therefore, the internal surfaces of the lens, which are black and matte at first glance, may turn out to be «white» in the infrared region of the spectrum and increase the harmful effect.

Light scattering in lenses and light reflections inside the lens appear as additional, even illumination, reducing image contrast. At first glance, this may seem useful, as a natural way to reduce contrast.

In fact, light scattering leads to two negative aspects:

• Noise in dark areas of the image increases, since the significantly larger photon noise of parasitic illumination is added to the readout noise of the output device, while dark details of the image are irretrievably lost.
• There is a noticeable «expansion» of the boundaries of bright objects, while the expanded boundaries mask and do not allow observation of nearby dark objects.

Fig. 7. Illustration of the expansion of the boundaries of a bright object in lenses with significant light scattering.

Unfortunately, light scattering characteristics are also not provided in the passport data for lenses, therefore, it is also necessary to keep statistical records of this parameter independently. It should be noted that light scattering in lenses with plastic lenses is noticeably greater than in glass ones. Therefore, for TV cameras operating in bright sunlight, it is advisable to use lenses only with glass lenses.

Less light scattering is also found in lenses with special interference films applied to them, weakening the infrared component of the spectrum. However, the use of such lenses is not always acceptable, since they worsen the sensitivity of black-and-white cameras at night by 2-3 times.

4. Modes for expanding the maximum observable contrast.

4.1 Gamma correction.

Gamma correction is a mandatory element of any television camera. With the help of this type of nonlinear signal processing, the logarithmic law of illumination perception by the human eye is matched with the linear dependence of the light-signal characteristics of the television camera and video monitor.

In simple terms, gamma correction consists of additional amplification of weak signal levels. Television cameras use various degrees of gamma correction from 0.7 to 0.45.

Fig. 8 Amplitude characteristics of the gamma correction unit in the CXA1310AQ (SONY) microcircuit, which is used in many modern black-and-white television cameras [ 3 ].

When the camera is operating in sunlight, it is advisable to set the lower of the possible gamma correction values ​​- 0.45, which will allow you to somewhat expand the range of observed illumination from above.

The gamma correction mode creates a comfortable, «correct» visual ratio of illumination, and shifts the lower level of observed illumination upward.

But this advantage is achieved at the cost of the following disadvantages:

• The noise in dark areas of the image increases several times.
• the distinguishability of objects in the middle and upper areas of the illumination range deteriorates.

Therefore, with gamma correction enabled, despite the expansion of the visually observable illumination range, the probability of missing a low-contrast object with average illumination that appears in the field of view increases.

4.2 Back Light Compensation mode.

The «BLC» mode, which appeared several years ago and is actively advertised, is designed for observing objects in difficult conditions — against the light. In terms of circuitry, it is usually implemented in the form of switching the thresholds of the electronic shutter (or the reference level in the ARD lens) and the AGC system so that they become 10 — 20% higher than usual. As a result, the brightest objects (for example, a bright window) are «cut off in the white», and objects of the average level (the face of a person standing in front of the window) are enhanced and become clearly visible.

Thus, the «Back light compensation» mode does not expand the dynamic range, but shifts it for the purpose of better observation of darker objects, at the cost of losing bright objects. There are modifications of the mode in the form of additional switching of «windows» in which automatic control circuits are triggered (cameras from Watec, Sony, Panasonic, etc.).

There is a variant of the BLC mode implementation with the conversion of the upper signal levels into a «negative image» (TV cameras of the JAI company).

The «BLC» mode is useful in a number of cases of television surveillance, but unfortunately, it cannot be used automatically, since the camera «does not know» when the operator is interested in an object in front of a brightly lit surface, and when the image of this surface itself is important.

Currently, remotely controlled TV cameras have appeared in which the operator can quickly turn the «BLC» mode on or off.

4.3 Digital Signal Processing and «Super Dynamic» Cameras.

Undoubtedly, the future belongs to digital signal processing in television cameras. But there are serious obstacles that prevent modern black-and-white cameras with digital signal processing from becoming the undisputed leaders of the television market. First of all, these are limitations in cost, size and power consumption.

If you install a Pentium IV-level processor, 16-bit ADC and DAC, large RAM, etc., in a TV camera, it will become unattainable for 99% of applications. Therefore, simplified specialized DSP processors and ADCs with a small bit depth, usually 8, sometimes 10, are installed in cameras. The result is low efficiency of digital signal processing and the absence of noticeable advantages of digital cameras over analog ones, with the exception of service ones.

Three years ago I was surprised by the low image quality of the WV-BP-510 camera with a digital processor, motion detector, and Sensitivity Enhancer mode. In terms of image quality in daylight conditions, it was significantly inferior to the previous analog model WV-BP310 from the same Panasonic.

The reason is the small number of quantization levels in the ADC and DAC in this camera, which was visually observed as a coarse quantized image with characteristic «square-nested» noise. Another example of the insufficiently high efficiency of digital signal processing is the famous «Super dynamic» set — a CCD matrix and a DSP processor from the same company, used in the WV-BP-554 camera.

The magnificent idea of ​​obtaining two signals in one field, the total dynamic range of which is 40 times greater than the standard, effectively depicted in advertising brochures, was liked even by non-specialists.

a)

б)

в)

Fig. 9. Illustration of the method for expanding the dynamic range for the Super dynamic series cameras – a), the topology of the standard and Super dynamic CCD matrices – b) and the mechanism for converting signals in the DSP processor – c) from the advertising brochure of Panasonic.

Only later did questions arise: how does this happen, with 10-bit ADCs and DACs? Doesn't the scattering of light in the lenses, etc., interfere with the processing? In addition, the dynamic range of each element of the Super dynamic matrix must be at least 2 times smaller than the standard and correspond to matrices of the 1/5 inch format.

The latter is due to the fact that the signals of two fields are simultaneously stored in one 1/3-inch CCD matrix (Fig. 9 b).

After testing the famous camera, it turned out that only through lengthy adjustment could one obtain a dynamic range equal to that of conventional 1/3-inch cameras. Cameras on 1/2-inch matrices clearly surpassed the «Super dynamic» in all parameters, despite the interesting idea and all the intricacies of digital processing methods.

It's a pity, I really wanted a miracle… I remember the old joke that was loved by the masters of tube television in the 60s: «The good thing about gamma correction is that you can turn it off.» Unfortunately, this saying is also true for the BLC and Super Dynamic modes.

5. Additional ways to protect against light overloads.

5.1 Tips for installing the camera and choosing the angle of view. Protective visors, hoods and filters.

It is important not only to choose the right television camera and lens, but also to install it in the best possible way.

Here are some practical rules that provide the best protection against light overload:

• The angle of view of the lens should be as small as possible.
• A sun visor with a dark matte inner surface should be installed on the camera. Its length should be as long as possible, so that its upper edge is slightly visible in the image.
• The camera should be installed as high as possible, so that it looks down from above, and the area of ​​the sky in the camera's field of view is minimal.
• For very narrow angles of view (less than 10 angular degrees), a light-protective hood with a matte dark inner surface should be placed directly on the lens. The hood significantly reduces the scattering of light in the lenses of narrow-angle lenses.
• If there is sky in the camera's field of view, and the Sun may appear at certain moments, it is advisable to attach a neutral filter with a 5-10x attenuation to the upper edge of the protective visor so that it covers the sky in the image, or at least the zone of possible passage of the Sun.
• In cameras with microcircuit sets from SONY, Samsung, Sharp, the right edge of the image (location of the rear idle CCD elements) should be covered with opaque material.
• Before installing the camera on the object with the lens installed, it is necessary to check its stability when observing the direct Sun, bright clouds and the filament of an incandescent lamp observed «at point-blank range». In case of self-excitation of the lens-camera system, it is necessary to increase the threshold of the lens diaphragm, which will guarantee the stability of its operation at the cost of some deterioration in image quality.

5.2 Remote control of television cameras.

Automatic adjustments and adaptation modes embedded in television cameras do not always work optimally when observing in conditions of light overload.

Therefore, television cameras with remotely controlled parameters have recently begun to appear.

The most common are cameras controlled via the RS-485 protocol, which is widely used in computer applications.

The advantages of this remote control option are:

• Long control range, exceeding 1 km,
• Low cost of control cable, the ability to use twisted pair.
• The ability to connect several dozen television cameras to one cable without additional extenders.
• The ability to control a television camera system from a special remote control or from a computer.
• A single control protocol standard that allows installation of cameras from different manufacturers in one system.

Modern cameras with RS-485 control have the ability to adjust a large number of parameters, as well as telemetry modes that allow remote diagnostics of the camera, determining the ambient temperature, supply voltage at the camera input, etc.

When observing in sunlight, the greatest effect will be provided by remote adjustments of the lens aperture and exposure time, gain control, switching of gamma correction modes and backlight observation modes.

Computerized television systems have a new opportunity for software control of television camera parameters depending on the time of day and year. It will not only improve the quality of observation, but also reduce possible operator errors in the most difficult observation conditions.

Another useful feature may be software self-tuning and self-diagnostics of a television system with controlled television cameras, which can be performed periodically according to a given algorithm without the previously required routine work of installers and operators.

Table 3. Television CCD cameras with remote adjustment of parameters via the RS-485 interface.

Conclusions.

To ensure reliable television surveillance in conditions of sunlight and light overloads, you should:

1. Use lenses with automatic diaphragm, choosing models with a minimum relative aperture value of no worse than F(360), with low light scattering and glare.
2. Use TV cameras with CCD matrices of formats no less than 1/2 — 1/3 inch of the EXWAVEHAD series from SONY, which have the least «blurring» of images from bright objects. Consider that standard-resolution matrices are one and a half times superior to high-resolution matrices in terms of maximum observable contrast.
3. If it is necessary to install lenses with a constant aperture, you should choose cameras with an electronic shutter that implement a minimum exposure of 1/1000000 second and have an automatic CCD mode switching system «night — day». Such cameras will ensure minimal loss of information when observing in conditions of light overload.
4. Use sun-protective visors or hoods of the maximum possible length with a dark matte inner coating.
5. Cameras should be installed on the ground as high as possible so that the area of ​​the sky in the camera’s field of view is minimal.
6. In the most difficult observation conditions, it is advisable to use television cameras with remotely adjustable parameters, which will allow operators to quickly and optimally adjust camera modes to changing observation conditions.
7. To expand the visually observable range of illumination, large black-and-white video monitors with maximum screen brightness should be selected.

LISTING.

1. Television: Textbook. Manual for Universities/R.E. Bykov, V.M. Sigalov, G.A. Eisenhardt; Ed. by R.E. Bykov – Moscow: Higher. School, 1988. – 248 p.: ill.
2. Seken K., Tompseth M. Charge Transfer Devices/Transl. from English Ed. V.V. Pospelova, R.A. Suris. – M.: Mir. 1978. – 327 pp.
3. Single Chip Processing for CCD Monochrome Camera CXA1310AQ. Data book «CCD Area Image Sensor», SONY Corporation Semiconductor Company, 1996. P 1200 — 1212.