When cameras need to see events at night, sensors with removed infra-red cut filter and infra red illuminators should be used. Cameras that have no infrared cut-filter (IRC) are the B/W (monochrome) models, but color cameras with removable IRC can also be used. The latter ones are typically referred to as Day/Night (D/N) cameras. Not long ago, in CCTV there were B/W and color cameras and one would choose based on whether the main usage would be at night or day-time. Today, all cameras come as color, but some have the D/N capability by automatically removing the IRC filter for improved low light vision and even better with infra red light.
Infrared light is used because the native sensitivity of the Silicon (CCD and CMOS sensors) has very good sensitivity in and near the infrared region. These are the wavelengths longer than 700 nm. As mentioned at the beginning of this book, the human eye can see up to 780 nm, with the sensitivity above 700 nm being very weak, so in general we say that the human eye only sees up to 700 nm.
Sensor without IRC filter see much better in the infrared portion of the spectrum. The reason for this is the nature of the photo-effect itself. Longer wavelength photons (which are usually blocked by the ORC filter in a color camera) penetrate the Silicon structure more deeply. The infrared response is especially high with B/W CCD chips, or color ones without an IRC filter.
A couple of infrared light wavelengths are common to CCTV infrared viewing. Which one is to be used and in what case depends first on the camera’s spectral sensitivity (various manufacturers have different spectral sensitivity sensors) and, second, on the purpose of the system.
The two typical infrared wavelengths used with halogen lamp illuminators are: one starting from around 715 nm and the other from around 830 nm.
If the idea is to have infrared lights that will be visible to the public, the 715 nm wavelength is the better choice. If night-time hidden surveillance is wanted, the 830 nm wavelength (which is invisible to the human eye) should be used.
The halogen lamp IR light come in two versions: 300 W and 500 W. The principle of operation is very simple: a halogen lamp produces light (with a similar spectrum as the black body radiation), which then goes through an optical high-pass filter, blocking the wavelengths shorter than 715 nm (or 830 nm). This is why we say wavelengths starting from 715 nm or starting from 830 nm. The infrared radiation is not one frequency only but a continuous spectrum starting from the nominated wavelength.
The energy contained in the wavelengths that do not pass the filter is reflected back and accumulated inside the infrared illuminator. There are heat sinks on the IR light itself that help cool down the unit, but still, the biggest reason for the short MTBF (1000–2000 hr) of the halogen lamp is the excessive heat trapped inside the IR light.
The same description applies to the 830 nm illuminators; only in this case we have infrared frequencies invisible to the human eye. As mentioned earlier, 715 nm is still visible to many.
These infrared illuminators may pose a certain danger, especially for installers and maintenance people. The reason for this is that the human eye’s iris stays open since it does not see any light, so blindness could result. This can happen only when one is very close to the illuminator at night, which is when the human eye’s iris is fully opened. The best way to check that the IR works is to feel the temperature radiation with your hand; human skin senses heat very accurately. Remember, heat is nothing but infrared radiation.
The halogen infrared illuminators are mains operated, and photo cells are used to turn them on when light falls below certain lux level.
Both types of halogen infrared illuminators mentioned come with various types of dispersion lenses, and it is desirable to know what angle of coverage is best for a situation. If the infrared beam is concentrated to a narrow angle, the camera can see farther, provided a corresponding narrow angle lens is used (or a zoom lens is zoomed in).
Halogen lamp infrared lights offer the best illumination possible for night surveillance, but their short lifespan has initiated new technologies, one of which is the solid-state infrared LEDs (Light-Emitting Diodes) mounted in the form of a matrix. This type of infrared is made with high-luminosity infrared LEDs, which have a much higher efficiency than standard diodes and radiate a considerable amount of light, yet require much less electrical power. Such infrared lights come with a few different power ratings: 7 W, 15 W, and 50 W. They are not as powerful as the halogen ones, but they are smaller, and their MTTF over 100,000 hr. There are IP HD cameras today which come with built-in high efficiency IR LEDs, able to be powered over a PoE switch and illuminate areas up to 25 m.
How far you can see with such infrareds depends on the camera in use and its spectral characteristics. It is always advisable to conduct a site test at night for the best understanding of distances. The angle of dispersion is limited to the LED’s arrangement, and this usually ranges between about 30° and 40°, if no additional optics are placed in front of the LED matrix.
Another type of IR used in applications is an infrared LASER diode (LASER=light amplification by stimulated emission of radiation). Perhaps not as powerful as the LEDs, but with a laser source, the wavelength is very clean and coherent. A typical LASER diode radiates light in a very narrow angle, so a little lens is used to disperse the beam (usually up to about 30°). Lasers use very little power.
One last technical note is about the focusing point of a projected infrared image on a sensor with IRC filter. Since the infrared wavelength are longer than the visible light, when IRC filter is removed, the focusing point of the infrared wavelengths falls behind the sensor pixel plane. The image may look slightly blurry. In order to fix this, either the lens needs to be re-focused for the night view or infrared corrected lens needs to be used.
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