State-of-the-art CCTV can not be imagined without IR illuminator. 8-10 years ago these were nearly exclusive expensive devices solely from famous foreign manufactures. The bulk of them were represented by illuminators on the basis of halogen lamps with power from tens to hundreds watts. There IR illumination is formed by dispersion filters from IR glass which absorb the visible spectrum. Sometimes lamp bulb plays the role of such a filter. The limited operation time can be considered as the main drawback of IR illuminators on the basis of lamps, especially taking into account great voltage instability of Russian network systems. The maintenance charges on illumination system increase as a result of frequent change of expensive lamps. But even today such illuminators find their application.
At that time LED IR illuminators were extensively used only in door answering units as elements of hidden illumination. Only few outside manufacturers produced LED illuminators. Russian manufactures of IR illumination for CCTV then only made first steps in this direction. At that time they created and patented (patent RF #2134906) the first models of now popular masked IR illuminators: IR Plate illuminator, IR Bolt illuminator and IR Rod illuminator.
Nowadays one can not imagine covert night video surveillance without LED IR illuminators. For the last few years their assortment has been significantly extended owing to the devices with different entrance angles and illuminating ranges. There even appeared devices with remotely changing entrance angle and therefore with changing illuminating range which could be used together with video cameras equipped with transfocators. Development of color-video cameras with night/day mode made it possible to use IR illumination in color CCTV. Nominally LED IR illuminators can be divided into illuminators themselves and relatively low-power IR illumination devices which as a rule have masked design and are used for hidden local illumination.
Modern IR illuminators are equipped with LEDs with generation lines of 850-880, 920-930 and 940-950 nm. In any case it is the most efficient way to use illuminators with shorter wavelength at which the sensitivity of various CCD arrays is 15%-25% of the maximum value as compared to 7%-12% for illuminators with longer wavelength. This significantly increases range of illumination action. Besides at the more short-wave illumination, image sharpness do not decrease that much because light can penetrate deeper into the CCD array and lens defocusing does not manifest itself greatly due to change of refraction coefficient. On the other hand red illumination (which can be seen by the human eye) of diodes’ chips with the wavelength of 850-880 nm. can reduce to nothing all the measures for the secrecy of surveillance. Generation line shift to 940-950 nm. can lead to decrease of glow intensity of chips together with simultaneous shift of emission color to dark-cherry that is to the color near to the border of eye sensitivity. Certain decrease of image sharpness and contrast (when CCD array works in the close IR band) is conditioned together with already mentioned reasons by the growth of focal spot and illumination monochromaticism. Besides the already mentioned reasons the specificity of image received by the camera during IR illumination is conditioned by leveling of reflectivity factor of different materials and surfaces. For example, dark fabrics and camouflage painting look like nearly even light materials. But in the number of cases such an effect increases the possibility of surveillance object detection.
The most simple way to carry out the absolute secrecy requirement of illumination especially at illuminating the near-field region of surveillance is the diffuse reflection from the ceiling, walls or any other shields. Even if one manages to get maximal diffuse reflection practically in hemisphere, the most efficient radiator in this case is the wide-angle illuminator with exposure angle of 70-90 angular degrees. Another solution which is also efficient enough is the application of illuminators with milky-white or matt lenses made in the form of ceiling panels and identical in design with glow lamps.
It is necessary to point out that as a result of high LED spectral brightness, IR light filters have merely decorative function and do not practically participate in forming the radiation spectrum. Besides with the help of dispersion filters which are nearly always used it is practically impossible to effectively reduce spurious emission without significant loss. Appearing only at limiting currents, the visible component of IR LEDs radiation has equally narrow band as the main one and is located at 600 and 700 nm.
The power supply of IR illuminators which are not equipped with built-in current stabilizer should be generated from constant-voltage power supply. This is conditioned by the usage of LED diodes in the modes close to limiting behavior for high efficiency achievement. Besides, current pulsation at bad filtering can lead to illumination pulsation and possible fluctuation of image brightness as a result of beating with vertical sweep of a video camera.
A responsible manufacturer of LED IR illuminator will indicate the following:
If one can not find qualitative characteristics of the image received (attainable resolution or signal/noise relations) it is considered that the minimal quality of image with distinct recognition of fixed border of black and white fields of big size is provided.
When comparing several illuminators in order to choose the most suitable device and the power consumption and directional characteristic are comparable it is reasonable to be guided by minimal declared illuminating range supposing that special understating of characteristics by manufacturer is hardly probable.
Radiated power (coefficient of efficiency) ŋ can be estimated with accuracy enough for practice using power consumption of illuminator with due account for modern LED which does not exceed 20-25%.
It is known that with the increase of range, illumination decreases on quadratic law. If to consider dependence of illumination from illuminator power as linear, quadratic dependence of required power with illumination range is obvious. In the first approximation at exposure angle up to 65-70 angular degrees, decrease of illumination with extension of illuminator directional characteristic can also be supposed as quadratic. For example, to double the illumination range one needs to increase power 4 times or decrease angle 2 times. These simplest correlations enable to compare different models of IR illuminators.
In conclusion we may say that despite certain principal drawbacks, covert video surveillance in darkness or in low-light conditions using IR illumination is practically the only simple and affordable way to get visual information.
Nikolay Chura ("The Security Algorithm" magazine - No. 4, 2004)