PARDOXES OF BUILT-IN IR ILLUMINATION.

PARDOXES OF BUILT-IN IR ILLUMINATION.

PARDOXES OF BUILT-IN IR ILLUMINATION

In the last few years, TV cameras equipped with built-in IR illumination have become quite “fashionable”. Almost every one of the many manufacturers in Taiwan and Korea produces several such models for indoor and outdoor installation. New manufacturers appearing in abundance almost completely duplicate known and popular design solutions. Naturally, several miniature LEDs installed in the body of a cheap small-sized TV camera around the lens do not cause any fundamental objections (photo 1). Such illumination can be considered as a kind of auxiliary means, and sometimes an imitation of an illuminator. As a rule, these are 4-6 LEDs with a glow wavelength of 880-920 nm and a unit radiated power not exceeding one milliwatt. The axisymmetric radiation pattern is formed by the LED’s own phacon and is 20-30 angular degrees. Such TV cameras are equipped with standard lenses with a focal length of 3.6 mm and have a field of view from 74×54 to 45×33 angular degrees for sensitive sensors of 1/3 and 1/4 inches, respectively. Due to the insignificance of the emitted power and the inconsistency of the directional pattern, such illumination is relatively effective at a distance of no more than 1 — 1.5 m and only for the central part of the frame. Its effectiveness is even less for TV cameras with CMOS technology sensors due to their lower sensitivity. For color TV cameras, as has been repeatedly noted, IR illumination is fundamentally inapplicable, unless, of course, one of the “day-night” technologies is used in the TV camera. Despite this, similar examples of “video equipment” are also found on the market. In case of vandalism, ceiling versions of TV cameras with a built-in lens and IR illumination can be used (photo 2). Sometimes even cameras with a replaceable lens are equipped with low-power built-in IR illumination (photo 3). Which is an even more controversial solution for cameras of this class and with a practically variable field of view.

Photo 1.

Photo 2.

Photo 3.

Recently, small-sized hermetic cameras without heating have been increasingly used for outdoor video surveillance. In addition to a sun visor and bracket, they are most often equipped with a built-in IR illuminator (photo 4). The illuminator emitters are placed around the camera lens behind a common porthole. As a rule, these are from six to several dozen LEDs. The design of the cameras often resembles classic thermal housings or hermetic boxes (photo 5). Additional positive factors include the heat emitted by the LEDs, which reduces the likelihood of the porthole fogging. However, recently, photo sensors have been increasingly used to turn on the backlight when the illumination decreases. Emitters installed in front of the protective glass, common to the camera lens, are not so harmless. In this case, it is important that part of the IR radiation always gets into the camera lens as a result of reflection and re-reflection in the material of the porthole. In optical systems, such reflections are eliminated by applying antireflective coatings. For the TV cameras in question, this technology is rarely used due to its high cost and low efficiency. In addition, the portholes are made of plastic with a non-uniform structure of non-optical quality. Only their small thickness, not exceeding 0.7 — 0.5 mm, somewhat minimizes the impact of these defects. In order to eliminate the first and most powerful glare from the inner surface of the porthole, the lens frame is structurally moved as close as possible to this surface or a soft hood is used. These measures allow us to obtain acceptable image quality for normal-sensitivity TV cameras (0.5 – 0.1 lux) in low-light conditions with only a slight “veil” and reduced contrast.

Photo 4.

Photo 5.

Significantly greater problems arise for high-sensitivity TV cameras on EX-wave HAD matrices. In this case, with built-in illumination, it is not always possible to fully realize their high sensitivity. Comparative tests conducted with the SK-2020X TV camera (photo 6), equipped with a lens with a focal length of 8 mm and built-in IR illumination of 8 LEDs confirmed the influence of the above factors.

Photo 6.

On the indoor route, the SK-2020X camera provides a threshold observation range (recognition of the boundaries of white and black fields) of about 30 m along the sighting axis. When the illuminator is moved outside the camera, the threshold range increases to 35 m. The range of normal image quality increased from 19-22 m to 22-25 m with a 2-3-fold increase in image contrast. That is, when using a separate illuminator with similar light characteristics, the actual observation range increased by 13-15%. Of course, the increase in the overall range was affected by the reflection of IR radiation from the walls of the corridor in which the route passed. Apparently, for open areas, the maximum range, as well as the range of normal image quality, may be somewhat less, but the overall influence of the illuminator location will naturally remain. With an increase in the backlight power, the effect of parasitic illumination increases.

Nowadays, there are even exotic designs on the market with several dozen built-in LEDs in dome-camera-type housings (photo 7). In this case, it is almost impossible to ensure tight contact between the lens frame and the hemispherical lampshade to reduce direct illumination into the lens. Reflection and scattering in a fairly thick layer of curved plastic, on the inhomogeneities of the material that is far from optical quality with low transparency lead to even greater illumination and a serious limitation of the real sensitivity of the TV camera, as well as a significant decrease in image contrast. Dust settling on the lampshades during operation not only reduces their transparency, but also significantly enhances all the described effects.

Even greater problems arise during operation of the above-mentioned outdoor sealed television cameras with built-in IR illumination, which after just a few days in low light conditions confidently demonstrate the glow of dirty protective “glass” instead of an image of the object. Naturally, a similar effect is observed when using visible illumination. The desire to place a larger number of LEDs leads to their unacceptable proximity to the boundaries of the lens aperture, which only exacerbates the problem.

Photo 7.

Another negative factor of the built-in illuminator is the interfering backscattering of the environment with low transparency (dust, smog, snow or rain). The simplest method of minimizing this interfering factor is the use of distributed or side illumination, i.e. separate illuminators. More complex special methods for solving this problem (strobing by range, spectral and polarization selection) are complex and expensive, and therefore are used only for solving special problems. The above disadvantages are also relevant for the increasingly popular color day-night cameras, which also widely use built-in IR illumination (photo 8).

Photo 8.

In conclusion, we can conclude that, despite all the ease of use, built-in IR illumination for TV cameras can only be recommended for solving the simplest and not very important tasks at a practically household level. In professional systems, the most optimal use is individual illuminators with directional patterns matched to the field of view of the camera. To minimize backscattering of the environment, it is preferable to use distributed lighting or lateral placement of directional sources.