CHURA Nikolay Iosifovich.
SOME ASPECTS OF USING IR ILLUMINATION IN VIDEO SURVEILLANCE
Covered video surveillance in low light conditions can no longer be imagined without the use of IR illumination.
IR illuminators using LED emitters are increasingly used.
In recent years, the efficiency and unit power of these emitters have increased significantly, which allows them to successfully compete with illuminators on incandescent lamps with a halogen cycle.
Modern IR illuminators use LEDs with generation lines of 870 — 880 and 940 — 950 nm. Considering the spectral sensitivity characteristics of typical CCD matrices, shown in Fig. 1, it is most effective to use emitters with a minimum wavelength. In this case, the reduction in the equivalent sensitivity of the TV camera is minimal, and this allows for an increase in the illumination range.
In addition, the effect of image defocusing as a result of a change in the refractive index of the optics, and with it the shift of the focal plane of the lens, is also minimal.
However, the distinct glow of the light-emitting areas of the light-red LEDs can negate all measures to conceal surveillance.
The shift of the generation line to the region of 940 – 950 nm leads to a decrease in the intensity of the visible glow of the emitter areas with a simultaneous shift in the glow color to dark cherry.
Apparently, the lower visibility of the glow is primarily due to the approach of the visible component to the limit of eye sensitivity (750 nm).
This is also confirmed by the glow color, by which it is possible to estimate the visible component as close to 600 nm and 700 nm, respectively, for emitters with a wavelength of 870 – 880 nm and 940 – 950 nm.
For a long time, many specialists tried to explain the phenomenon of the visibility of IR diodes by the fact that the high-frequency part of the main radiation spectrum falls into the eye's sensitivity area.
According to the operating principle, the LED is a fairly monochrome source, the spectrum width of which at the 0.5 level does not exceed 30 — 40 nm. At the same time, one cannot count on extended «tails» of the spectrum that could fall into the eye's sensitivity area.
Moreover, the color perception of any qualified observer contradicted these assumptions.
At the same time, the spectrograms supplied by the manufacturers did not contain extraneous emission lines.
Fig. 1.
Special measurements of the spectrum of emitters based on the IK-6 matrix LED with a generation wavelength of 880 nm, carried out on a modernized spectrometer, clearly recorded a second maximum in the 600 nm region, which is about 0.0074 of the maximum intensity of the main radiation at a wavelength of 870 — 880 nm.
Radiation with this wavelength has a bright red color. Apparently, the mechanism for the emergence of the second maximum for emitters at 940 and 950 nm is similar. This is indirectly confirmed by a shift in the visible component of the glow to a cherry color.
Fig. 1 shows the relative position of the spectral characteristics of the CCD matrix (I), the main spectra of the IR emitters with a wavelength of 880 nm (II) and 950 nm (III), and the spectra of the parasitic visible component (II’) and (III’) for each emitter.
When using IR illuminators, it is quite difficult to determine the required backlight power to create the required illumination at the observation object.
The manufacturer usually standardizes the power consumption, the illumination range, and the directional pattern of the IR illuminator.
The angle of the beam pattern is usually standardized at 1/2 of the maximum power. The given illumination range assumes simultaneous indication of the sensitivity of the camera, resolution and signal-to-noise ratio of the image obtained.
The criterion for minimum image quality is a clear distinction between the fixed boundary of the black and white fields at the noise level.
Difficulties in standardizing IR illumination, insufficiency of the specified characteristics, as well as frequent cases of discrepancy between the actual characteristics and the declared ones have led to the widespread use of the experimental method of selecting IR illuminators in real conditions directly at the installation site.
The lack of data on the radiation power does not allow determining the power density at the site.
Direct measurement of IR radiation power is difficult due to the low availability of optical power meters. But even if they are available, direct measurements are problematic due to the inconsistency of large apertures of light beams of illuminators and input windows of measuring photodetectors. With sufficient accuracy for practice, it is possible to estimate the emitted power from the consumed power of the illuminator, taking into account h (efficiency) of modern LEDs, which does not exceed 20 — 25%.
The directional pattern of LED IR illuminators, with rare exceptions, is formed by the built-in focons of the LEDs themselves and has the shape of a cone. The value of the opening angle of the characteristic is standardized, as a rule, at a level of 1/2 relative to the maximum located along the axis of the light beam. Examples of typical directional characteristics with angles of 40 and 80 angular degrees are shown in Fig. 2 and 3. Within the boundaries of the 1/2 level, practically 65 to 80% of the total power is emitted, depending on the focon design, the presence of an additional reflector and the opening angle of the characteristic.
Fig. 2.
Fig. 3
The sensitivity of television cameras, as well as other optical-electronic devices operating in the visible range of light, is standardized by illumination (lx) or luminous flux (lm) — photometric quantities characterizing the effect of visible light on the human eye. Luminous flux is the power of light radiation, estimated by its effect on the eye.
In general, the luminous flux of a light source with a uniform spectral density in the range l 1 — l 2 is equal to:
where: k = 683; Рl is the radiation power at a wavelength of l; y is the visibility function of the eye.
For monochromatic radiation at the maximum sensitivity of the eye (l = 555 nm) for a radiation power of 1 W, the luminous flux will be 683 lm.
Surface illumination (E) is the ratio of the luminous flux (Ф) incident on it to its area (S). For a sufficiently distant source, the light wave can be considered flat. In this case, when light falls on the surface at an angle j, the expression for the average illumination value is:
Considering the above relationship for monochromatic radiation at maximum sensitivity, we write the expression for illumination through the power of light radiation:
When using IR illumination for video surveillance, the infrared source can be considered fairly narrow-band and monochrome. That is, with sufficient accuracy for practice, it is possible to consider the entire power of the emitter concentrated in the maximum of its spectral characteristic. By analogy with the interpretation of the interaction of monochrome and white light with the human eye, we will estimate the creation of equivalent illumination by IR illumination for the CCD matrix of a television camera, taking into account its sensitivity in the spectral region of the illumination. The average typical spectral characteristic of the sensitivity of the CCD matrix (I) is shown in Fig. 1.
The graph shows that the sensitivity of television cameras for the generation lines of common LED IR emitters with a wavelength of 880 nm (II) and 950 nm (III) is about 14 and 5% of the maximum, respectively.
The area of the light spot on the object is calculated in a simplified way using the known expression for the area of the base of the cone, in this case, the light cone, taking into account the illumination range L and the flat angle of the beam pattern a .
For simplicity, we assume that the axis of the light beam and the plane of the illuminated object are orthogonal. The average equivalent illumination in the light spot of IR radiation within the power density of 0.5 from the maximum for different wavelengths can be estimated from the following expression:
where:
Рпотре – power consumption of the IR illuminator, W;
h – efficiency of the illuminator (» 0.2);
Кl – for 880 nm » 0.14; for 950 nm » 0.05;
Ка – from 0.65 to 0.8;
L – range, m;
a – flat angle of the beam pattern, ang. grad
Of course, the resulting estimate will be very approximate, but even it will allow us to identify obvious discrepancies in the declared parameters, which are often found on the IR illuminators market.
In conclusion, we can state that IR illumination devices are an effective means of providing covert video surveillance in low light conditions using standard television cameras. Domestic manufacturers of these products, if not in terms of production volume, then in terms of design solutions, achieved technical parameters and, naturally, prices, can already now seriously compete with the largest foreign firms.