Covert video surveillance in low-light-level conditions can not be imagined without IR illumination nowadays. IR illuminators with light-emitting diodes are finding ever-growing use. For the last few years the efficiency and unit capacity of these illuminators have grown significantly. This enables their successful competition with illuminators on the basis of halogen lamps.
In modern IR illuminators they use LEDs with generation lines of 870-880 and 940-950 nm. Taking into account spectral sensitivity of standard CCD array, presented on figure 1, illuminators with minimal wavelength are the most efficient ones. In this case decrease of equivalent sensitivity of TV camera is minimal and this enables to increase illumination range. Besides image misfocusing effect as a result of refraction coefficient change together with focal plane of the lens shift is minimal as well. However light-red distinct glow of diodes’ chips can reduce to nothing all the measures for the secrecy of surveillance. Generation line shift to 940-950 nm. leads to decrease of glow intensity of chips together with simultaneous shift of emission color to dark-cherry. In all probability, less visibility of glow is connected first of all with approaching of visible component to sensitivity border of an eye (750 nm.). It is also confirmed by the color of the glow thanks to which we can assess the visible component as close to 600 nm. and 700 nm. respectively for emitters with the wavelength of 870-880 nm. and 940-950 nm. Specialists have long explained the IR LED luminous efficiency as the high-frequency edge of the emission spectrum being sensed by the human eye. LED based on its operation principle is a relatively monochrome source, its spectrum width not exceeding 30-40 nm at 0.5 level. Plus one can not expect any long "tail" areas in the spectrum that could be caught by the human's eye. Besides color sensation of any qualified observer contradicted these suppositions. At the same time spectrograms, attached by manufactures, did not contain extraneous emission lines.
Special spectrum measurements of emitters on the basis of array LED IR-6 with the generation wavelength of 880 nm. which were carried out on a modernized spectrometer definitely registered the second maximum in the region of 600 nm. comprising 0,0074 from the maximum value of primary emission power at the wavelength of 870-880 nm. Emission with such wavelength has bright red color. In all probability the appearance mechanism of the second maximum for the emitters for 940 and 950 nm. is the same. This is indirectly confirmed by the shift of glow visible component to cherry color. On figure 1 one can see the relative position of spectral characteristic of CCD array (I) of main spectrums of IR-illuminators with the wavelength of 880 nm. (II) and 950 nm. (III) and spectrums of spurious visible component (II’) and (III’) for every emitter.
When using IR-illuminators it is quite hard to determine the necessary illumination power to create the necessary illumination on the surveillance object. As a rule, manufactures normalize power consumption, illumination range and directional characteristic of IR-illuminator. At that the directional characteristic opening angle is normalized most often on the S level from the power maximum value. The given illumination range supposes that at the same time they also indicate camera sensitivity, resolution and signal/noise relation of an image received. The criterion of image minimal quality is the distinct recognition of immovable border of black and white fields on the noise level. The difficulties of IR-illumination normalization is the insufficiency of the given characteristics as well as it is also quite common when the real characteristics do not correspond to those that are indicated. That led to the expansion of experimental method when they tried IR-illuminators in real conditions straight during the installation process.
Lack of data on radiation power does not allow to determine power density on the object. Direct measurement of IR-illumination power is difficult in view of rare availability of optical power meters. But even if they are available the direct measurements are problematic because of the mismatch between the large apertures of the light beam and the size of the photodetector windows. Radiation power can be assessed with accuracy enough for practice on the basis of illuminator power consumption with regard to modern LEDs coefficient of efficiency which does not exceed 20-25%.
LED IR-illuminator directional characteristic is nearly always formed by built-in LED focons and has a cone form. The directional characteristic opening angle value is normalized as a rule on the S level relative to the maximum located on the light beam axis. The examples of standard directional characteristics with 40 and 80 angle degrees are given on fig. 2 and fig. 3. Within the borders of S level from 65 to 80% of nearly all the power is radiated, depending on focon construction, additional reflector availability and directional characteristic opening angle.
Camera sensitivity, as well as the sensitivity of other optical-electrical devices operating in the visible spectrum, is normalized by illumination (lux) or luminous flux (lumen) – photometric quantities which characterize the influence of visible light on the human’s eye. Luminous flux is the luminous radiation power assessed on the basis of its influence on the human’s eye. In general case luminous flux of a light source with uniform spectral concentration is equal to:
Luminous flux for monochromatic radiation in the eye sensitivity maximum (555 nm.) for radiation power of 1 Wt. is equal to 683 lumens.
The illuminance of a surface (Å) is the ratio of the incident flux (Ô) to its area (S). A light wave for quite a remote source can be considered as flat. In this case at light incidence on the surface angularly the expression for the illumination mean value is of the form:
Taking into account the above-mentioned correlation for monochromatic radiation in the sensitivity maximum, we will write down the illumination expression through luminous radiation power:
When using IR-illumination for video surveillance one may consider an infrared source as quite narrow-band and monochrome. That is with accuracy enough for practice one may calculate all the emitter power concentrated in the maximum of its spectral characteristic. As in the case with interpretation of monochrome and white light interaction with the human eye we can assess the creation of equivalent IR illumination for CCD array with regard to its sensitivity in the illumination spectral region. Averaged standard spectral characteristic of CCD array sensitivity (I) is given on fig. 1. The diagram shows that the camera sensitivity for generation lines of widespread LED IR-illuminators with the wavelength of 880 nm. (II) and 950 nm. (III) is about 14 and 5% from the maximal value respectively.
The area of the light spot on the object is approximately calculated by the known formula for the area of a base of cone, in our case a light one, with regard to illumination range L and flat opening angle of directional characteristic.
For simplicity, we assume that location of light beam axis and illumination object plane is orthogonal. Average equivalent illumination in a light spot of IR-radiation in the range of power density of 0,5 from the maximum value for different wavelengths can be assesses from the following expression:
where:
P cons – power consumption of IR-illuminator, Wt;
ŋ – illimination coefficient of efficiency (≈ 0,2);
K λ – for 880 nm. ≈ 0,14; for 950 nm. ≈ 0,05;
K α – from 0,65 up to 0,8
L- range, m;
α – directional characteristic opening angle, angle degrees
Undoubtedly the received estimation is going to be highly approximate, however even this estimation enables to find out evident discrepancy of the stated parameters which is quite often can be found on IR-illuminators’ market.
In conclusion we may state that devices for IR-illumination are the efficient means for covert video surveillance in low-light conditions using standard cameras. Domestic manufactures of these devices already now can seriously compete with major foreign companies if not in production volume, than in construction solutions, achieved technical parameters and, of course, prices.
Nikolay Chura ("The Special Equipment" magazine - No. 3, 2002)