Sensitivity of different types of image sensor to radiation of different wavelengths (spectral sensitivity) essentially differs from human eye spectral sensitivity (and luxmeter sensitivity coinciding with it).
Radiation spectrums of different lamp types can be different.
For example, luminescent and Incandescent lamps produce equal scene illumination according to luxmeter reading. But the image from a black-white camera directed on this scene will not be equal at scene illumination produced by different lamps. The difference in the equivalent illumination can reach four or more times. Color cameras are practically insensitive to IR illuminators, and black-white cameras have sensitivity depending on maximum wavelength of the IR illuminators.
Spectral efficiency factor for specified lamp and camera types is equal to light flux (lumen) produced by halogen incandescent lamp with color temperature Tc=3100K. At that, this light flux is equivalent for the specified camera type to one lumen of the light flux from the specified lamp type.
In other words:
Spectral efficiency factor for specified lamp and camera types shows for how much efficiency of light produced by this lamp for this camera higher or lower than efficiency of light produced by halogen incandescent lamp with color temperature Tc=3100K, in case when both lamps create equal illumination according to luxmeter reading.
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 image sensors with color filters and permanent IR filter.|
Spectral sensitivity of color sensors with permanent IR filter is similar to human eye spectral sensitivity and a luxmeter spectral sensitivity, therefore the Spectral efficiency factor is closed to one.
During modeling images, VideoCAD automatically calculates efficiency of each illuminator depending on image sensor type of the loaded camera.
For color cameras and visible light sources the Spectral efficiency factors are closed to one (0.8..1.2), but for black-white cameras the factors can be from 0.25 to 1.2.
For special light sources the Spectral efficiency factors can be in wide range.
Example 1: Black/white camera sensitivity, given in specification, is equal to 0.1 lux at scene illumination produced by halogen incandescent lamp.
Sensitivity of the same camera to the mercury lamp light with spectral efficiency factor 0.45 will be equal to 0.1*0.45=0.22 lux.
Example 2: The scene is illuminated by standard incandescent lamp with spectral efficiency factor 1.15, the luxmeter shows scene illumination 0.1 lux.
If obtain the same image from camera at illumination produced by sodium lamp with spectral efficiency factor 0.35, the luxmeter will show 0.1*1.1/0.35=0.31 lux.
Example 3: Efficiency of any lamp for a human eye is equal to the light efficiency. Relative efficiency of any lamp for CCTV is equal to the product of light efficiency multiplied by spectral efficiency. Spectral efficiency of halogen incandescent lamp (Tc=3100C) is considered to be one.