Myths and Reality of Night CCTV

#night video surveillance

Myths and reality of night video surveillance

CHURA Nikolay Iosifovich

MYTHS AND REALITY OF NIGHT VIDEO SURVEILLANCE

The fairly frequent use of infrared illumination in video surveillance in security systems and in night video surveillance mode in household television cameras has created a myth that night surveillance in the near IR range is more effective.

A number of customers of video surveillance systems have formed a persistent opinion about some special “vigilance” of television cameras in this mode.

A significant contribution to the creation of this misconception was made by manufacturers of IR illuminators, declaring long ranges with negligible emitter power, and without specifying the required sensitivity of television cameras.

The development by SONY of the EXview CCD matrix manufacturing technology and their wide application in both special and household video equipment gave rise to a number of publications and advertising promises about the “phenomenal” abilities of such equipment in night mode to see in almost complete darkness with poor transparency of the environment (atmosphere) and almost through clothing and other obstacles.

And all this thanks to the increased sensitivity of 6 — 8 dB in the near IR region.

Functionally, such TV cameras in night conditions are almost equivalent to thermal imaging systems.

In this case, there is confusion in the principles of obtaining an image.

In the near IR range, the TV camera uses radiation reflected by the object and the background.

While thermal imaging is based on recording the environment and objects' own radiation, caused by their temperature. It is implemented in the ranges of 3-5 and 8-13 microns due to the presence of effective receiving devices and transparency windows in the atmosphere.

However, in the near IR range, at the border of which EXview TV cameras are relatively effective, there is almost no own radiation of objects located at a temperature not exceeding several hundred degrees.

For this reason, observing objects by recording their own radiation in this range, and especially behind some obstacles, is absolutely impossible.

The appearance of EXview cameras has given new impetus to promises of amazing results in terms of observation range, image clarity and contrast using IR illumination.

Such statements are explained by advertising purposes and are objectively understandable, despite the dubiousness of such a practice.

Apparently, on this basis, some consumers have formed an opinion about the preference for the widespread use of IR illuminators for video surveillance at night.

Until now, black and white cameras have been used mainly in domestic video surveillance.

This was explained by the significantly higher cost of all equipment for obtaining color images.

The ever-wider distribution of computer and digital systems for processing and recording, working everywhere with color video signals, will most likely lead in the near future to a reorientation to color television cameras, despite their lower sensitivity and resolution.

For accessible and inexpensive computer systems, the presence of color information somewhat compensates for their traditional image deficiencies in resolution and contrast.

In turn, this same process is stimulated by the appearance of highly sensitive color television cameras at affordable prices and even modifications with a day/night mode.

Compared to standard cameras, EXview CCD cameras have higher sensitivity and greater resistance to “blurring” and “flooding” of the image from maximum brightness sources in their field of view.

Their higher sensitivity is achieved, among other things, due to the shift of the spectral maximum of the characteristic towards the spectral maximum (0.9 – 1.0 µm) of the matrix material – silicon.

Fig. 1 shows the spectral characteristics of the high-resolution CCD matrices ICX409AL of Super HAD (I) technology and ICX259AL of Exview HAD (II) technology manufactured by SONY.

The absolute values ​​of the sensitivity maxima are reduced to a single scale.

The characteristics show that the shift of the sensitivity maximum by 50–60 nm led to a relative increase in sensitivity at the main wavelengths of IR illuminators: 880 nm from 13–15 to 23–25% and 940–950 nm from 7–8 to 10–12% of the maximum. Given the overall increase in sensitivity, these cameras are certainly more effective when using IR illumination.

But at the same time, there is no reason to expect any “different” visibility under natural lighting in twilight and night conditions.

Moreover, the shift of the maximum spectral sensitivity in the direction of orange and red, for the black-and-white version of the TV camera, is fraught with some change in the relative brightness of the color components and a slight decrease in the overall contrast of the image even under white lighting.

To illustrate this effect, photo 1 shows black-and-white images of the color scale of the PAL standard measurement table, obtained using black-and-white TV cameras with Super HAD (1) and Exview HAD (2) matrices.

The decrease in contrast is especially noticeable at the boundaries between the white-yellow and red-purple fields.

The differences in the equivalent brightness of the color fragments provide some increase in the contrast of the black-and-white image under visible “white” lighting due to the unevenness of the typical spectral characteristics of black-and-white TV cameras, which is practically identical to the eye visibility curve shown in Fig. 2.

Fig. 1.

Fig. 2.

Photo 1.

In general, natural illumination in night and twilight conditions is formed by solar radiation scattered in the atmosphere, reflected and proper radiation of the moon, planets, and proper radiation of the atmosphere, earth's surface and stars.

The spectral distribution of solar radiation outside the atmosphere is almost identical to the distribution of radiation from a black body with a temperature of 6000 K.

About 50% is emitted in the infrared region of the spectrum, 40% in the visible region, and 10% in the ultraviolet and X-ray regions.

When radiation passes through the atmosphere, it is absorbed and scattered by its components, as a result of which the spectral range of radiation narrows to 0.3 — 3.0 μm.

The power and spectral composition of the transmitted radiation depend heavily on the altitude of the sun and the state of the atmosphere.

Fig. 3 shows the spectral composition of radiation from a black body with a temperature of 6000 K (1) and solar radiation outside the atmosphere (2) and at the earth's surface (3).

With a decrease in the sun's altitude and an increase in the translucent thickness of the atmosphere and the scattering component, the proportion of IR radiation increases from 50 to 79%.

Due to the change in spectral composition in the evening light and early twilight, the actual illumination for TV cameras with an extended IR boundary can be expected to be somewhat higher in the first approximation than for a matrix with a typical spectral characteristic.

Fig. 3

The Moon's radiation consists of its own and reflected solar radiation.

The Moon radiates as an absolutely black body heated to a temperature of 400 K with a spectral maximum in the region of 7.2 μm.

The spectral reflectivity of the lunar surface increases with increasing wavelength, which somewhat shifts the spectral maximum of reflected radiation to the long-wave region.

It is generally accepted that the maximum of the total radiation density of the Moon corresponds to a wavelength of 0.64 μm.

Changes in the integral illumination of the earth's surface at night in the absence of clouds at different phases of the moon are given in Table 1.

Table 1

Taking into account cloudiness, illumination can decrease by more than an order of magnitude, but without a significant change in the spectral composition. Cloudiness similarly affects such natural sources as stars and planets.

The spectral composition of the radiation reflected by the moon, despite a slight shift to the red region, does not provide a significant improvement in illumination in the near IR region, including for EXview technology matrices.

In the presence of the moon, there is no point in considering other sources due to their insignificant contribution.

Illumination of the Earth's surface on moonless nights consists of the intrinsic and reflected radiation of the planets of the solar system with spectral irradiance maxima from 5 μm (Jupiter) to 20 μm (Saturn) for intrinsic radiation and 0.5 μm for reflected solar radiation.

The maximum spectral density of radiation of most of the brightest stars falls in the range of 0.5 — 1.0 μm. However, with an integral illumination of no more than 5×10-5 lux, this component can be ignored for video surveillance.

The maximum spectral density of solar radiation scattered by the atmosphere is observed in the region of 0.5 μm, with intrinsic radiation in the region of 10 μm.

That is, the sky in solid clouds radiates as an absolutely black body (ABB) with a temperature equal to the ambient temperature with an accuracy of several degrees.

With natural light on moonless nights, television cameras are practically inoperable, with the exception of special highly sensitive systems with accumulation or electron-optical converters.

From the above, we can conclude that the advantages of using Exview matrix TV cameras in night conditions with natural light are their higher integral sensitivity.

The shift of the spectral maximum of sensitivity towards the near IR range provides slightly greater efficiency only with artificial lighting by incandescent lamps and IR illuminators.

It is noteworthy that incandescent lamps have a maximum radiated power at a wavelength of about 1 μm.

Moreover, for any TV camera, “white” visible lighting is preferable.

This ensures maximum sensitivity and maximum image contrast.

This is especially true when using color high-sensitivity TV cameras with day/night mode.

In this case, with sufficient illumination, there is a potential opportunity to switch to color mode.

Especially when using color cameras with Exview matrices.

When performing night video surveillance in complete darkness using IR illumination, it is necessary to take into account some features caused by the operation of the camera in a fairly narrow spectral region of the near IR range, practically determined by the spectral band of the illuminator.

The reflective characteristics of various materials in the near IR region and in the visible range are very close, therefore the general character of the image practically repeats the image of the visible range.

Moreover, there is no reason to expect a special “penetrating vision” through media and materials. On the other hand, there is a general decrease in image contrast as with any monochrome illumination.

IR illuminators for this case can be considered very monochrome sources, especially on semiconductor emitters. A similar effect of decreasing contrast with monochrome lighting is also characteristic of the eye. Any viewer who has been to a show event using colored lighting is familiar with it.

When working simultaneously in the visible and near IR ranges, some reduction in image clarity is expected due to changes in the lens focus at different wavelengths of the radiation used.

All other things being equal, this effect is more noticeable when using cheap plastic optics. To compensate for these distortions, you can use manual or automatic change of lens focus for different modes. In case of using a black-and-white TV camera with some reserve in sensitivity, it is possible to isolate only IR radiation with a filter and focus on it.

This problem can be radically solved by using broadband corrected or mirror optics. However, this is too expensive.

Another reason for the deterioration of image clarity in the IR range, even with IR illumination at night, is due to the comparability of the geometric dimensions of the matrix resolution element with the diffraction limit of the optics used for this wavelength.

The main format of matrices used at present is 1/3 inch.

The dimensions of one sensitive element (pixel) in black-and-white matrices manufactured by SONY are 9.8×6.3 μm for typical resolution matrices (500×582 pix) and 6.5×6.25 μm for high-resolution matrices (752×582 pix).

Typical lenses used for surveillance cameras have relatively small input apertures. This is especially true for built-in lenses of miniature cameras.

In this case, even the theoretical dimensions of the focal spot at the first diffraction minimum (Aerie circle) are comparable to the pixel size, and for long-focus lenses they can even exceed them.

It should be taken into account that the actual dimensions of the focal spot, depending on the quality of the optics, can be 2-3 times greater than the calculated values.

For example, for a built-in lens with a focal length of 16 mm, the calculated value of the focal spot in the visible range (0.5 μm) will be about 4 μm.

When using an IR illuminator with a wavelength of 940 nm for its minimum visibility to the eye, the calculated value of the focal spot will already be about 7.4 μm, i.e. comparable to the pixel size even for a normal-resolution TV camera.

The situation will be even worse when using TV cameras with ? inch matrices, where the pixel size is already 4.85 x 4.65 µm.

In this regard, for covert video surveillance in the near IR range, it is advisable to use the most powerful optics.

This will not only achieve maximum sensitivity, but also ensure minimal focal spot sizes that do not limit the resolution of the TV camera even for work in the near IR region of the spectrum.

In conclusion, it can be stated that for conventional video surveillance, both in natural and artificial lighting, the near IR range has no advantages over the visible spectrum.

Moreover, the achievable sensitivity, clarity and contrast are significantly inferior to similar characteristics in the visible range.

Video surveillance in the IR range is inevitable only for covert surveillance at night and other cases where the use of visible lighting is undesirable.

In this case, of course, it is necessary to use IR illuminators, one of the main requirements for which is the invisibility to the eye of both the radiation and the emitter itself.

With the spread of color television cameras with day/night mode, the possibility of covert night video surveillance appeared in color imaging systems.

In any case, the use of the IR range for video surveillance is advisable only for solving special problems.

Only in this case, the generic shortcomings of the method, some of which are listed above, will not lead to disappointment.

Literature

1. Kulikov A.N. Television surveillance in difficult conditions.//“Special equipment”, 2000, No. 5, p. 13 – 19.
2. Kriksunov L.Z. Handbook of the Fundamentals of Infrared Technology. M.1978.
3. SONY Company Website, Sony
4. Chura N.I. IR Illumination in Television Surveillance.//“Special Technology” 2000, No. 1, pp. 35 – 39.