Matters of different cameras’ sensitivity in visible spectrum band and IR band, impact of CCTV different elements on image quality, application conditions, IR illuminators characteristics and many other issues will not be mentioned in this article. Here we will deal with the issues connected with visibility and invisibility of IR to the naked eye, that is to an independent observer whose sector of vision comprises an IR illuminator and objects illuminated by it.
Curiously enough but IR illumination application for image acquisition in CCTV during nighttime has become so usual that we even don’t always think about why it is IR that we use. The first thing that comes to mind is the fact that the black-and-white camera has high sensitivity in IR band.
This is really so but it is enough to look at the spectral characteristic of CCD array sensitivity to make sure that the camera sensitivity in the visible light is significantly higher than in IR band.
The answer is simple – the main advantage of IR illumination is its invisibility to the naked eye.
And the thing here is not only about the fact that it is necessary to make the illumination invisible thus misleading the intruder. Today environmentalists speak more and more often about light pollution and its negative impact on human beings. In some countries they adopt legislative acts on "light ecology". There also exist quite obvious cases when IR illumination can not be substituted by illumination in the visible spectrum when the source of such light can blind humans. These are the cases with motorways, airfields, railways ets.
Sensitivity of a human eye falls as the wavelength grows. The illustration presents a standardized visibility function of a human eye at the so called “night vision” when illumination is 10-4 cd/m2.
A human eye has maximal sensitivity at about 510 nm. At the wavelength of 720 nm. the sensitivity is equal to 0,000004780, at the wavelength of 780 nm. - 0,000000139 from maximal one.
However at sufficient power of IR illuminator with the wavelength less than 900-920 nm. its illumination can be seen by a human eye. At the same time the illumination which falls on the surrounding objects at the wavelength more than 800 nm. can not be seen but if to look at the illuminator itself one can see faint red glow.
One should remember that the infrared diode spectrum may have a complex structure: aside from a main lobe there might be phantom lobes as well. This means that the diode can emit not only within the specified wavelength but also produce a less intensive radiation, at wavelengths substantially different from the main range. In particular the diode can create spurious emission in the red.
The device must be tested before using IR illuminators in particularly critical cases when it is necessary to ensure complete invisibility of its illumination. For this purpose it is enough to switch one the illuminator in complete darkness and make sure that its illumination can not be seen by the human eye from the appropriate distance.
It is necessary to take into account that the longer the Illuminator wavelength the lower its efficiency because camera’s sensitivity decreases with the wavelength growth. That is why when choosing the wavelength of IR illuminator it is necessary to take into account the application of CCTV with IR illumination.