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