http://www.stevezimages.com/infrared-filter/
Light and Infrared Irradiation in Cctv Systems
Over the past few years we have witnessed the appearance of day night CCTV cameras, all featuring a “miraculous” capability to amplify by many hundred folds the camera “minimum illumination” by removal of the IR Cut Filter and by turning off the color circuit. Usually, the day mode sensitivity ranges from 1 lux to 0.25 lux and leaps to 0.01 ~ 0.0001 lux for the camera’s night (B/W) mode. This “miraculous” increase is not real; it is a measurement trick by exposing the CCD (Charged Coupled Device) to high power infrared emission and specifying it inadequately in a very low lux value.
Lux and Infrared power are not the same. Lux is a photometric unit that measures point to point illumination (dependent on the quantity of irradiated light and the distance between the light source and the illuminated object) while Infrared power is a radiometric unit – measured in Watts . Illumination makes it possible for us to see while infrared irradiation is invisible to the human eye.
The distinction amongst illumination and infrared irradiation is in the frequencies, or the wavelength. The wavelength of the visible spectrum ranges from 400nm (violet) to 700nm (red) which is divided into a rainbow of colors from red to orange to yellow to green to blue and to violet.
CIE (Commission International de l’Eclairage) also known as International Commission of Illumination, has clearly stated the measurement of light, known as photometry, applies only to the visible portion of the optical spectrum and includes units such as nit, lumen, candela and lux. Measurements of human’s eye sensing ability have shown that the efficiency is highest to green color (100% at around 555 nm) and it declines to 50% near orange (around 610 nm) and blue (around 510 nm) and less than 10% near red ( around 650 nm) and violet ( around 470 nm). Humans do not sense radiated waves in the ultraviolet (below 400 nm) or the infrared (above 700 nm) regions.
CIE standards are quite complex but it will be sufficient to comprehend that the value for a green light source(555nm) with 1W of luminous flux, is equal to 683 lumen, which for a point to point measurement equals 683 Candela . In contrast, 1W of red (650 nm) luminous flux equals only 68 candela and 68 lumen and 1W of infrared (over 700 nm) radiant flux equals 0 lumen, 0 candela and 0 lux.
The problem is that few people know what lux and infrared radiation are, or how illumination and radiation are measured. One of the devices for measuring illumination in lux is the illumination meter, also known as a light meter. The light meter includes a filter and is calibrated to measure only light units within the visual optical spectrum as defined by CIE. This filter is similar to the IR cut filter preventing all infrared radiation from passing in order to keep the measuring accurate. That is why all the known exposure meters, light meters and other illumination measuring equipment will read 0 lux or some residual value when exposed to infrared radiation.
In CCTV light meters are used to measure the scene illumination observed by a security camera, but should never be used to measure infrared irradiation, because its reading will be 0 lux, or a residual reading like 0.001 lux, even though the scene may be exposed to considerable quantity of IR radiation.
In order to understand very well the significant impact that these measurements have in designing a CCTV system we must first understand how a CCD (Charged Coupled Device) works.
The CCD sensor is designed to resemble the human eye, with similar color sensing efficiency. That is why CCD sensors are most sensitive to the green (550nm) region of the spectrum and drop to 0 sensitivity in the ultraviolet (under 400nm) region. CCD sensors however do sense infrared radiation over 700nm and up to 1000nm (some are more sensitive and can sense up to 1200nm).
The CCD sensitivity is much lower in the IR region and drops to only 5-10% efficiency or less, as shown in a typical CCD efficiency graph.
Even though the sensor’s sensing efficiency in the IR region is very low, at day time infrared radiation which is heavily radiated by the sun, by hot objects, by human body and by incandescent or halogen light bulbs must be prevented from reaching the color CCD. If such IR is radiated into the optical path of a color CCD, it will cause heavy disturbances in the processing of color pictures. That is why every color CCD (or CMOS) sensor must be fitted with an IR cut filter that blocks the infrared radiation from reaching it. As explained above, sensors are far less sensitive to infrared radiation than to visual spectrum. Therefore the concept that the removal of the IR cut filter increases the camera sensitivity a hundred or even a thousand times is a simple case of distortion, induced by the improper use of a light meter for measuring infrared irradiation.
The common inaccurate test for the night mode of a day night camera is performed by introducing a powerful infrared radiator (incorrectly named IR illuminator) for irradiating infrared onto a dark scene. The test is carried out by connecting a Cctv Camera (with its IR cut filter removed) to a waveform analyzer for measuring the output signal. The day/night camera is placed to observe an IR irradiated scene. After that the light meter is positioned into the scene to measure the infrared irradiation in lux.
This is a misleading test that generates false results. We will get an IR induced B/W signal into the waveform analyzer with a value like 20 IRE, while the light meters will read 0 lux or a residual value, such as 0.01~0.0001 lux, which is used in the camera’s technical chart to define sensitivity or its minimum illumination. You should be aware that IRE is a unit of signal level or strength and is not associated to infrared radiation in any way.
The correct but more “difficult” lab test of a CCTV camera’s sensitivity or minimum illumination is performed by placing a standard 2000 lux source with a test chart in front of the camera (connected to a waveform analyzer) with its IR cut filter removed. After the initial are inserted in front of the light source until the video signal measured drops to 20 IRE. The minimum illumination will than be calculated on a basis of the neutral Density Filter reduction factor. Neutral density filters are available in different transmittance factors and reduce the luminous intensity by ratios such as ½, 1/16, 1/50, 1/100 etc. The neutral density filters are very similar to those used in auto iris smart lenses.
This is why whenever you see a day/night camera with a minimum illumination values like 0.001 lux or even 0.001 lux, try to keep in mind one thing – marketing strategies sometimes weight more than common sense.
About the Author
Lucian Maftei
Born in 1978 Lucian Maftei is one of the top romanian CCTV experts. At the age of 29 he has published 3 books
Supraveghere video – 2006
Camere de supraveghere video – 2006
Placa de captura – 2007
Maftei is also the CEO of one of the biggest romanian CCTV equipment importers, installers and distributors.
How to take infrared photos with a digital camera – Week 67
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