Infrared illumination in television surveillance.

#Infrared illumination, #Special Equipment

Infrared illumination in television surveillance.

Chura Nikolay Iosifovich

INFRARED ILLUMINATION IN TELEVISION SURVEILLANCE

Source: «Special Equipment» magazine

Television surveillance in the interests of ensuring security and safety involves obtaining a satisfactory image of an object under any lighting conditions.

This applies to both closed spaces with switched off or emergency lighting, and to outdoor objects, regardless of the location of the observation zone, time of day and weather conditions.

When using standard television equipment, the use of artificial lighting is inevitable in this case.

But even for special and relatively expensive high-sensitivity night vision systems that allow television surveillance at illumination levels of about 0.001 lux, even a small amount of illumination can significantly increase the contrast and resolution of the image.

Almost all modern television cameras are based on light-sensitive CCD matrices. X

The spectral sensitivity characteristic of the matrix, optimized according to the eye sensitivity curve, is shown in Fig. 1, which allows for fairly correct reproduction of the relative brightness of color fragments of the image in black and white.

Fig. 2 shows a family of typical characteristics of spectral sensitivity of some CCD matrices from SONY.

In many cases, the use of conventional artificial lighting in the visible range is optimal. This is preferable at least because it allows the TV camera to operate at its maximum sensitivity (555 nm).

But there are a number of tasks of covert video surveillance and, naturally, in this case, the use of lighting invisible to the eye is necessary.

Moreover, this is often not associated with any special operations. Hidden illumination simply does not attract attention to the hidden camera, which allows you to more successfully resist or rather not be seen by modern “intellectual vandals”.

Fig. 1. Characteristic of the matrix spectral sensitivity, optimized according to the eye sensitivity curve.

Fig. 2. Family of typical characteristics of spectral sensitivity of some CCD matrices from SONY.

It is necessary to distinguish two cases of application of infrared (IR) illumination.

In the first case, the invisibility of the scattered or diffusely reflected light flux is required, as a maximum, but the unmasking glow of the radiation sources themselves is acceptable. In this case, it is possible to use emitters with a wavelength of 920, 880 and even 850 nm.

In the second — the absolute invisibility of the emitter itself is required, even with direct visual observation from a close distance.

For this, emitters with a wavelength of 940-950 nm are used

It should be noted that, despite the sensitivity limit of the eye of 700-750 nm, any observer after 5-10 minutes in complete darkness can clearly distinguish luminous emitters even with a wavelength of 920-940 nm with a power of 20-40 mW with angular dimensions of up to 1.0 arc minutes.

The mechanism of this phenomenon is not entirely clear and is apparently due to the fantastically high sensitivity of the adapted eye. Naturally, the visibility of the emitters depends on the power density of the radiation entering the observer's eye.

All infrared light sources for video surveillance can be divided into two groups, differing in purpose, and, consequently, in characteristics and design.

Long-known and widespread sources include various IR spotlights, headlights and lanterns designed to illuminate objects of observation, both outdoors and indoors.

The concealment of the illumination is ensured only in dark conditions, and even then, for a number of emitters — only at a fairly large distance, due to the significant glow in the red region of the spectrum. In addition, their appearance is clearly associated with the lighting device. As an option for completely hidden illumination using illuminators, it is possible to offer the creation of a diffused light flux from the ceiling or special screens with diffuse reflection.

For these cases, wide-angle illuminators with radiation angles of up to 80-90° are most effective. Illuminators are located behind cornices, beams and other elements that hide them from the eyes of the observer.

The most well-known of such devices are illuminators with incandescent lamps. Their efficiency is quite high due to the spectral maximum in the region of 1.0 μm for a tungsten emitter with a temperature of 2800-3000°C.

Figure 3 shows a typical graph of the spectral density of a tungsten incandescent lamp. They mainly use lamps with a halogen cycle, which have a separate or built-in reflector.

Floodlights usually have a moisture-proof case with cooling fins and simple brackets for mounting and aiming at the angle of elevation. They are available with a supply voltage of 220, 110, 24 or 12 V.

To isolate the IR region and suppress the visible part of the radiation spectrum, dispersion filters based on IR glasses (IRGs) are used.

Figure 4 shows the spectral characteristics of a number of IRGs. In rare cases, interference filters can be used to solve special problems, but their high cost, limited durability, and low efficiency in diverging beams of thermal incoherent sources significantly limit their use.

Fig. 3. Typical spectral density graph of a tungsten incandescent lamp.

Fig. 4. Spectral characteristics of a series of IR lamps.

As an example, Table 1 presents the technical characteristics of the VEDEOSYS INTERNATIONAL iN5-1000 Hi-Harp IR series floodlights.

Table 1

With the advent of powerful and efficient LEDs, semiconductor IR illuminators are increasingly used.

Their main advantages in comparison with incandescent lamps include greater spectral brightness at the operating wavelength, a significantly longer service life, reaching 100 thousand hours, and lower cost (taking into account operating costs).

The main technical problem for semiconductor IR illuminators is to ensure effective heat dissipation from the LED pad.

The permissible current and luminous flux of a single emitter depend on this, and, consequently, the required total number of LEDs and, ultimately, the dimensions and cost of the entire floodlight.

For example, Tables 2, 3 and 4 present the characteristics of some foreign and domestic IR floodlights.

For Russian products, the radiation angles are determined at a level of 0.5 of the maximum power density.

The illumination range is estimated as the threshold for recognizing a white field on a black field for a camera with a sensitivity of 0.1 lux on an object.

Table 2

Table 3

The floodlights of the “Luna” series are of interest due to the presence of a built-in current stabilizer, which ensures a low dependence of the luminous flux on the supply voltage, line length (drive resistance) and the number of operating emitters (in case of their partial failure).

Table 4

The IKP-40 and IKP-100 models are equipped with a current stabilizer and, therefore, have a fixed luminous flux in the range of operating voltages and temperatures.

The wavelength range of 850-960 nm for the products listed in Table 4 means that the manufacturer uses LEDs in specific samples for one of the wavelengths of this range.

When using semiconductor IR illuminators, it is necessary to take into account the actual wavelength value, i.e. the effect of changing the sensitivity of the TV camera and the resulting illumination range. Fig. 5 shows the spectral characteristics of a typical CCD matrix and IR diodes.

Fig. 5. Spectral characteristics of a typical CCD matrix (I) and IR diodes (II, III).

The domestic manufacturer, as a rule, provides a range of maximum distances. On the one hand, this is due to the uncertainty of the standardization methodology, on the other hand — a significant dependence of the obtained illumination on the reflective characteristics of the object of observation and the surrounding landscape at the operating wavelength.

In any case, IR illumination has some features. This is a noticeable blurriness of the image due to a change in focal length in the IR range when using standard lenses and a decrease in contrast as a result of leveling the reflection and absorption coefficients of various materials with monochrome narrow-band illumination.

In the simplest cases, to eliminate the defocus effect with IR illumination, you can limit the spectral range of the camera to the IR region by installing an IR filter in front of the lens.

However, to maintain the required characteristics in the daytime, the camera must have a sensitivity reserve of almost ten times. Naturally, it is more effective to use special broadband optics.

In addition to spotlights based on discrete elements, small-sized emitters based on six-element LED matrices with a supply voltage of 12 V have become widespread. The LEDs of the matrix are welded onto a single metal base, connected in series and equipped with a built-in ballast resistor.

A whole range of emitters is produced in various designs and with radiation angles of 160, 120, 40 and 20°.

Equipped with a radiator, the emitters are used as miniature spotlights or are built into structural elements of buildings or equipment for hidden lighting. The technical characteristics of the emitters are summarized in Table 5, the appearance of the products is shown in Photo 1.

Table 5

Photo 1. External appearance of emitters, based on six-element LED matrices.

In recent years, specialized IR illumination devices with a camouflaged appearance (RU Patent No. 2134906) have been increasingly used in our country.

The main reason for this, in our opinion, is the fight against domestic vandalism and, in turn, the requirements for the secrecy of surveillance itself.

In the practice of developed countries, such devices have not found wide application.

Apparently, the general law-abidingness of citizens, and perhaps the lack of technical education, are at play.

How else can one explain the name “hidden IR illumination” for clearly glowing IR LEDs behind the transparent plastic window of a video intercom?

One of the options for a “vandal-proof” hidden illumination device for a television camera is a flat panel in which the emitters are hidden behind an opaque infrared filter in the visible area. An inscription, pictogram, or apartment or office number is applied to this panel.

Elements of the inscription, pictograms, numbers or signs are located in such a way as not to block the radiation. The panel with the IR filter is black, which somewhat limits its use. On light and white surfaces, a plate with a milky light filter can be used, but in this case, radiation losses can reach 30-50%.

Another version of a camouflaged lighting device is a bolt and threaded stud design with emitters installed at the ends.

Photo 2 shows IR illuminators in the form of an apartment number, a bolt and a stud. Photo 3 shows their television image when turned on.

Photo 2. IR illuminators in the form of an apartment number, a bolt and a stud.

Photo 3. TV image of IR illuminators when turned on.

Technical characteristics of camouflaged emitters are summarized in Table 6.

Table 6

In conclusion, it can be stated that IR illumination is an important factor in increasing the efficiency of covert video surveillance in low light conditions, and also as a means of combating backlighting.

Semiconductor LED illuminators are increasingly used as radiation sources, especially for hidden illumination in the near zone.

In such situations, only specialized sources are applicable in camouflage design, operating at a wavelength of no more than 940 — 950 nm.