Black-white sensor sensitivity to IR radiation of different wavelength essentially differs. Between image sensors of different types, there is also a difference in spectral sensitivity.
Color cameras are practically insensitive to infra-red illuminators.
VideoCAD considers this difference through in advance calculated and experimentally verified spectral efficiency factors of IR illuminators of different wavelengths.
The spectral efficiency factor for IR radiation (lumen/watt) of the specified wavelength and specified type of image sensor is equal to the light flux (lumen) of a halogen incandescent lamp with color temperature Tc=3100K having equal efficiency with IR radiation of the specified wavelength having power of 1 watt for the specified type of image sensor
Through the spectral efficiency factor the power of IR radiation is converted to photometric equivalent. This is necessary, as camera sensitivity is measured in photometric units (lux).
Cameras' sensitivity in VideoCAD is given to the light from halogen incandescent lamp, according to standard CEA 639 ' Consumer Camcorder or Video Camera Low Light Performance '.
Spectral efficiency factors are given for five basic types of image sensors:
•
Standard black-white CCD and CMOS
sensors;
•
Sony ExView HAD™CCD sensors
, having increased relative spectral sensitivity to infra-red radiation;
•
Standard color sensors
of day/night cameras with color filters (without IR filter).
Color filters weaken sensitivity in visible light range more strongly, than in infra-red range. As a result, image sensors of day/night cameras have lower sensitivity in visible light range, or, that is the same, the increased relative sensitivity in infra-red range.
•
Sony ExView HAD ™ sensors
of day/night cameras with color filters (without IR filter).
Effects from decreasing sensitivity in visible light range by color filters and from increasing sensitivity in infra-red range due to Sony ExView HAD ™ technology are added. Such image sensors have the highest relative sensitivity in infra-red range.
•
Standard color sensors
with color filters and IR filter. Color cameras are practically insensitive in infra-red range.
During modeling images, VideoCAD automatically calculates efficiency of each illuminator depending on image sensor type of the active camera.
Reflecting ability of different objects differs in light of incandescent lamps and in IR light. Spectral efficiency factors are obtained from tests with white paper and a human body.
Example 1: Scene is illuminated by halogen incandescent lamp with color temperature Tc=3100K, the luxmeter shows illumination of 0.1 lux.
To reach exactly the same image from the camera at illumination from IR illuminator with the spectral efficiency factor of 50 lumen/watt, IR illumination should be 0.1/50=0.02 watt/m2.
As color cameras are insensitive to IR radiation, for them the spectral efficiency factor of IR radiation is equal to zero.
During design it is not necessary to change these factors.
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