IR illumination in video surveillance.

There are several known methods of additional illumination of objects when building video surveillance systems: high-sensitivity cameras are used, including day-night cameras, infrared illumination, thermal imaging equipment. All these methods, in addition to advantages, also have disadvantages.
For example, achieving high sensitivity often leads to deterioration of other parameters of video cameras. Some cameras have an algorithm that reads a combined signal from 8 pixels in low light conditions. According to the developers, this leads to an increase in sensitivity by another 10 times. At the same time, the developers admit an 8-fold decrease in resolution. In order to increase sensitivity, the so-called charge accumulation mode is often used, i.e., in the twilight, the camera operates with a long-open aperture or with a long-open electronic shutter. The range of the electronic shutter is specified in seconds and currently ranges from 1/50 to 1/100,000 of a second. This does indeed increase sensitivity, but the camera's performance deteriorates significantly.
Thermal imaging equipment is very expensive and produces a very unique image corresponding to the distribution of thermal fields.
Modern infrared illumination used for video surveillance systems is a kind of intermediate link between thermal imaging technology and high-sensitivity cameras and represents very interesting devices.
Infrared radiation used for video surveillance systems belongs to the so-called near infrared spectrum. Visible radiation, often called light, is electromagnetic radiation perceived by the human eye. This radiation is characterized by wavelengths in the range from 380 nm with an energy of 3.1 eV to 760 nm with an energy of 1.6 eV. That is, the longer the wavelength of the radiation, the lower its energy.

The maximum continuous spectrum of solar radiation is located in the «green» region of 555 nm, which accounts for the maximum sensitivity of the eye (Fig. 1). Infrared (invisible to the human eye) or thermal radiation is divided into short-wave with a wavelength of λ = 0.76–2.5 μm (near IR spectrum), medium-wave λ = 2.5–50 μm and long-wave λ = 50–2000 μm (far IR spectrum).
As everyone understands, the organization of IR illumination consists of the competent selection of infrared radiation sources and receivers capable of registering this radiation, so that the video surveillance system performs its functions even under normal lighting.

Registration of IR illumination radiation
In semiconductors, electrons are generated when they absorb electromagnetic radiation (including optical radiation). In the internal photoelectric effect, for intrinsic absorption, the photon energy must be no less than the width of the so-called forbidden band of the semiconductor (Eg), i.e. for intrinsic absorption of photons with the formation of electron-hole pairs, the following condition must be met: h Eg , where h is the photon energy
 is the radiation frequency (λ = c/ν)
h is Planck's constant.
The long-wavelength limit of photoconductivity is determined by the relationship:
λ = hc/Eg=1.24/Eg(eV) (1)
This is the maximum wavelength of radiation that will be absorbed by a semiconductor with a given band gap with the formation of electron-hole pairs.
From the same relationship it follows that the human eye is sensitive only to radiation with an energy of heV (λμm).

Based on these considerations, silicon (Si) was chosen to register visible radiation. It is one of the most common materials on earth, the width of the forbidden zone of which is 1.1 eV and the maximum of the spectral characteristic of which is located at λ = 0.85 μm — in the near IR region. Photosensitive matrices based on it are used in video surveillance cameras. It is the possibility of using silicon matrices as a photosensitive element of video surveillance cameras that made the prices for this equipment affordable for widespread use.
Fig. 2 shows the spectral characteristics of a conventional Sony matrix and a Sony HAD matrix. It is evident from the characteristics that the shift in the maximum sensitivity by 50–60 nm led to a relative increase in sensitivity at the main wavelengths of IR illuminators: 880 nm from 13–15 to 23–25% and 940–950 nm from 7–8 to 10–12% of the maximum. When choosing video cameras that are supposed to operate using IR illumination, preference should be given to cameras with a SONY ExView HAD CCD matrix, not because of the sensitivity in general, but because of the increased sensitivity in the IR range. Given the overall increase in sensitivity, these cameras are more effective when using IR illumination.

Another important detail needs to be illustrated. To work with IR illumination, it is necessary to use black and white cameras. In case of using day-night cameras, it is necessary to choose cameras with a mechanical IR filter. In conventional color cameras, designed for daytime operation at high temperatures, IR filters are installed to protect the sensitive element from overexposure. The results of using a daytime sensor to work with IR illumination are shown in Fig. 3. The photo was taken with a Mobotix camera, which has two sensitive elements, in a completely dark corridor with only IR illumination. On the left is a night sensor, on the right is a daytime sensor.

Lenses for systems with IR illumination
The focal lengths for IR radiation and visible light are slightly different, since the wavelength of IR radiation is longer than the wavelength of visible radiation. Therefore, IR radiation has a lower refractive index, the plane of the focused image is usually located behind the matrix plane. If the image is sharp during the day, then at night, when using IR illumination, objects at the same distance will be out of focus. That is, some parts of the images obtained in infrared and visible light may be defocused. All other things being equal, this effect is more noticeable when using cheap plastic optics. To minimize this effect, special lenses with IR correction are used in black-and-white cameras and day-night cameras, especially when using IR illumination. These are expensive lenses. The operating principle of a lens with IR correction is shown in Fig. 3a.

A more practical solution: adjust the back focus of the video camera lens at night under infrared light, in which case the depth of field will be minimal, and all objects will be in focus. During the day, the depth of field will increase the focus area to a larger range, compensating for the difference between the focus under infrared and normal light. In the case of using a black and white TV camera with some reserve in sensitivity, it is possible to use a filter to isolate only the IR radiation and focus on it.
To compensate for these distortions, you can use manual or automatic change of lens focus for different modes.

Infrared emitters for illumination
Halogen illuminators, LEDs and lasers are used as infrared emitters. It should be noted that with a high power of halogen illuminator 300-500 W, its service life is short: 1000-2000 hours. IR LEDs and laser IR diodes have a significantly longer service life. In general, the service life of solid-state emitting diodes is significantly less than that of the rest of the element base — this is one of the few unsolved problems of modern semiconductor electronics. And this point, of course, must be taken into account when choosing a camera.

Semiconductor LEDs are often used for IR illumination, having a maximum spectral characteristic at a wavelength of 0.7 μm and a maximum at a wavelength of 0.9 μm. Figure 4 shows the relative emission spectra of some typical LEDs at room temperature. Semiconductors used to create LEDs of a certain range have a band gap greater than this value. Currently, the most common LEDs are based on gallium arsenide (GaAs), since GaAs is the most technologically mastered semiconductor.
In the professional lexicon of installers, illumination with a maximum at a wavelength of 0.7 µm is called «visible», namely red. This is not because the human eye suddenly saw infrared radiation, but because it is impossible to create an emitter with a monochromatic radiation spectrum. The spectral characteristic of any emitter includes several wavelengths. Radiation with a wavelength of 0.7 µm is recorded by the human eye as red, and the working infrared radiation is the region of the spectral characteristic with a wavelength of more than 0.76 µm.
Let us explain in more detail the already mentioned fact about some blurring of LED radiation, or more precisely, about the presence of wavelengths of a certain range in the LED radiation spectrum. This effect, as is known, is absent in semiconductor lasers.

Laser radiation is highly monochromatic and creates a strictly directed beam of light. Laser radiation occurs directly under the action of current flowing through a forward-biased diode, the so-called pump current. Fig. 5 shows the radiation spectra of a diode laser at pump currents below the threshold (LED mode), near the threshold and above the threshold values, illustrating the effect of narrowing the radiation band when switching to the laser generation mode

Therefore, if the installer cares about the concealment and invisibility of IR backlights, then it is necessary to choose invisible backlights with a maximum at a wavelength of 0.9 µm. If this is not so important, then it is possible to use IR backlights with a maximum spectral characteristic at a wavelength of 0.7 µm. As in any technical problem, there is another side to the issue. According to the first part of the well-known relationship (1), the longer the wavelength of radiation, the lower its energy. This means that radiation with a longer wavelength, under the same equal conditions, will act at a shorter distance, cover a smaller angle than radiation with a shorter wavelength.
Thus, infrared illumination with a shorter wavelength is preferable in terms of illumination efficiency. At the same time, LEDs (emitters) with a wavelength of less than 0.9 μm are visible to the naked eye. All this means that when using IR illumination at short distances (up to 10-15 m) in order to hide the illumination, IR illuminators with a wavelength of 0.7 μm are not always suitable, and a compromise must be made between efficiency (0.7 μm) and stealth (0.9 μm). When using infrared illuminators at long distances and at small radiation angles, efficiency is more important, since it is difficult to visually find such illuminators. Black and white video cameras compatible with IR illumination see in the IR spectrum, but somewhat worse than in the visible spectrum, and their sensitivity decreases with increasing wavelength. Thus, infrared illumination with a shorter wavelength is preferable (in terms of illumination efficiency).

At the same time, LEDs (emitters) with a wavelength of 0.7 microns are visible to the naked eye.
At the same price level, the illumination that acts at a greater distance covers a smaller angle. The illumination with a shorter wavelength has a greater radius of action or covers a larger angle than the illumination with a longer wavelength for equipment of the same price range. This is explained by the design features of the LEDs. Fig. 6 shows the design of three LEDs (a — hemisphere, b — truncated sphere, c — paraboloid). Fig. 7 shows the radiation pattern of the LEDs (a — flat geometry, b — hemispherical geometry, c — parabolic geometry). Obviously, the required radiation pattern can be obtained by changing the geometry of the device. Studying the shapes of the radiation pattern, you can easily understand why IR illuminations provide either a long range or a large illumination angle. The radiation pattern of LEDs of different designs has either an elongated narrow shape or a flat wide shape.

(Continuation of the article and review of the built-in and remote IR illuminators presented on the market — «IR illumination»)