Cameras with built-in IR illumination have been quite popular in the last few years. Nearly every manufacturer of cameras in Taiwan and Korea produces several cameras of this kind for indoor and outdoor application. New manufactures are appearing in abundant who nearly copy famous and popular construction solutions. It is natural that a few tiny LEDs placed inside the casing of small cheap TV camera around the lens do not draw principal objections (photo 1).
Such illumination can be considered as supplement and sometimes – as illuminator imitation. As a rule these are 4-6 LEDs with wavelength of 880-920 nm. and unit radiation power which does not exceed a milliwatt. Axisymmetric directional characteristic is formed by LED and makes up 20-30 angular degrees. Such TV cameras are equipped by a standard lens with 3,6 mm. focal distance and their field of view is from 74х54 up to 45х33 angular degrees for sensitive sensors of 1/3 and 1/4 inches respectively. By reason of radiation power insignificance and directional characteristic maladjustment, such illumination is relatively effective at a distance not more than 1-1,5 m. and only for central part of the frame. Its efficiency is even lower for TV cameras with CMOS sensors as a result of their lesser sensitivity. As we have already mentioned several times IR illumination for color video cameras is principally inapplicable provided the camera does not have a night/day mode. Despite this such examples of “video equipment” can also be found on the market. If there exists the danger of vandalism one can apply TV cameras which are fixed on the ceiling and have built-in lens and IR illumination (photo 2).
Sometimes even TV cameras with interchangeable lens are equipped with low-power built-in IR illumination (photo 3).
But it is even a more arguable solution for such TV cameras which have practically variable field of view.
In the last few years small sealed TV cameras without heating are getting wider application in outdoor video surveillance. Besides a sun visor and a blacket they are usually equipped with built-in IR illuminator (photo 4).
Illuminator emitters are placed around the camera’s lens behind a single protective glass. As a rule these are from 6 to tens of LEDs. TV camera construction often resembles classical thermo-casings or sealed boxes (photo 5).
Additional positive factor is the heat released by LEDs which decreases the possibility of protective glass weeping. However lately more and more often they have been using photosensors to switch on illumination at light decrease. Emitters placed before the protective glass which is common for a camera lens are not that harmless. In this case the part of IR illumination in the result of reflection and re-reflection in the material of protective glass always gets into the camera lens. In optical systems such reflection is removed by putting antireflecting coating. For the cameras under observation this technology is used quite rarely because it is very expensive and has low efficiency. Besides protective glass is made out of plastic with nonuniform structure of non-optical quality. Only their small thickness which does not exceed 0,7-0,5 mm. minimize a bit the impact of these defects. To remove the first and the most powerful glare from the inner surface of protective glass , the lens carrier constructively moves up as close as possible to this surface or they use soft lens hood. These measures enable to get acceptable quality of images for TV cameras with normal sensitivity (0,5-0,1lux) in the modes of minimal illumination only with insignificant “fog” and contrast decay.
Great problems appear for TV cameras of high sensitivity on the basis of EX-wave HAD arrays. In this case with built-in illumination it is not always possible to fully realize their high sensitivity. Comparison tests of SK-2020Х TV camera (photo 6) equipped with lens with focal distance of 8 mm. and built-in IR illumination made of 8 LEDs confirmed the influence of the above mentioned factors.
Indoors SK-2020Х provides about 30m threshold visibility (contrast between the black and white regions) along the line of sight. If we take the illuminator away, outside the TV camera, the threshold range will increase up to 35 m. Range of image normal quality increased from 19-22 m. up to 22-25 m. at increase in 2-3 times of image contrast. That is when using separate illuminator of analogous light characteristics, the real observer-target range increased 13-15% . Undoubtedly the increase of total range was conditioned by the reflection of IR radiation from the corridor walls where the tests took place. In all likelihood the threshold range for open territory, as well as the range of normal quality image, can be a bit shorter. But the general impact of illuminator location will be naturally preserved. When the illumination power increases so increases the impact of flare light.
Today the market presents even extraordinary devices with tens of built-in LEDs of dome-camera type. In this particular case it is practically impossible to ensure close contact of lens carrier and semi-spherical bowl to reduce direct lens illumination. Re-reflection and dispersion in quite thick layer of plastic of curved shape lead to greater exposure and serious limitation of real camera sensitivity and significant decrease of image contrast on heterogeneities of material far from optic quality with low transparency. Dust which accumulates on lamps during operational process does not simply decrease their transparency but significantly increases all of the above-mentioned effects.
More problems appear during operational process with above-mentioned outdoor sealed TV cameras with built-in IR illumination which already after several days of operating in low-light conditions demonstrate the view of the surveillance object that is obstructed by the screen illumination generated by the dirty protecting "glass". Of course the same effect can be observed when using visible illumination. Tendency to place more LEDs leads to their inadmissible closeness to lens aperture which only worsens the problem.
Another negative factor of built-in illuminator is the spurious backscattering caused by low atmospheric transparence (dust, smog, snow or rain). The simplest method of minimization of this is the application of spread or edge lighting that is - separate illuminators. More complicated special solutions of this problem (range strobing, spectral and polarizing selection) are complex and expensive, that is why they are used only as special solutions. All the above-mentioned drawbacks are urgent for the gaining popularity color TV cameras with Day/Night mode where built-in IR illumination is also widely used.
In conclusion we may say that despite the ease of use, built-in IR illumination for TV cameras can be recommended only for the solution of simple and not very important non-professional tasks. In professional systems the most efficient way is to use separate illuminators with directional characteristics agreed with camera coverage. To minimize backscattering it is preferable to use spread or edge lighting of directional sources.
Nikolay Chura ("The Special Equipment" magazine - No. 6, 2004)