Camera's ability to create images with a necessary quality in low light conditions has an important value in many of outdoor video surveillance systems.
Unfortunately, there is no unified standard of measuring sensitivity in CCTV industry. Cameras' producers give values of sensitivity measured in there own methods. The values of sensitivity given in specifications of many producers correspond to the image on which on the strong noise background it is hardly possible to distinguish only silhouettes of large high-contrast objects.
Few producers give values of the sensitivity measured according to open standards.
As a result, cameras with actually equal sensitivity, but produced by different manufactures, have values that can differ in ten times in the specifications.
However for design calculation or modeling the cameras' sensitivity should be specified unequivocally.
In VideoCAD sensitivity is defined through the Minimum scene illumination (lux) of a scene with reflection factor = 0.75, at which the image from the camera with mounted lens with a known aperture (F number) has known signal/noise ratio (dB) and video signal amplitude (IRE).
The scene is lit by a tungsten halogen lamp with colour temperature Tc=3100 +-200K.
For the unequivocal description of sensitivity the following parameters should be specified:
|•||Minimum illumination (lux);|
|•||Lens aperture (F);|
|•||Signal/noise ratio (dB);|
|•||Video signal amplitude (IRE);|
The definition is based on standard CEA 639 ' Consumer Camcorder or Video Camera Low Light Performance'.
If precise modeling is required, it is recommended to measure sensitivity of camera models in use.
The measurement results can be inserted to parameters of camera's model in VideoCAD. After that to obtain the image from the camera's model in any illumination conditions it is enough to assign necessary model to the loaded camera.
Sensitivity of cameras' models, which are available in the Table of camera models, have been measured in accordance with detailed procedure.
The method is based on standard CEA 639 ' Consumer Camcorder or Video Camera Low Light Performance '.
At comparison of measured parameters in the Table of camera models with data of producers it is possible to see, that the measured sensitivity of camera Pelco MC3710H-7X exact matches to the value given in the specification. Cameras by JVC have the close sensitivity.
Sensitivity of other tested cameras is essentially lower the values given in specifications.
Correspondence of sensitivity measuring method to the standard CEA 639 is declared also by Axis Communications Inc.
If an installation firm does not have technical conditions to measure sensitivity of cameras practically, at a choice of sensitivity value for camera models in VideoCAD it is necessary to be guided by following practical rules for the defect-free, competently designed cameras without extended dynamic range.
1. Sensitivity of black-white cameras with high resolution CCD image sensors (752x582 effective pixels) 1.3" IT CCD, Sony Super HAD ™ CCD is 0.06-0.15lx at signal/noise ratio =17dB, lens aperture F1.2.
2. Sensitivity of black-white cameras with high resolution image sensor (752x582 effective pixels) 1.3" Sony ExView HAD ™ CCD is 0.04lx at signal/noise ratio =17dB, lens aperture F1.2.
3. Sensitivity of high resolution color cameras (752x582 effective pixels) is approximately 1lx at signal/noise ratio =17dB, lens aperture F1.2.
4. Sensitivity of cameras with standard resolution CCD image sensor (500x582 effective pixels) is better than ones with high resolution CCD image sensors (752x582 effective pixels) approximately by 15-20 %.
5. Sensitivity of day/night cameras (removable IR filter) in black-white mode is approximately twice worse than the sensitivity of black-white cameras with the same image sensors.
6. Sensitivity of easy day/night cameras (permanent IR filter) in black-white mode is approximately 1.5 times better than the sensitivity of color cameras with the same image sensors.
7. To obtain camera sensitivity with a lens with aperture different from F1.2 it is necessary to multiply sensitivity with aperture F1.2 by the ratio of squared lens aperture by 1.44 (1.2 squared).
For example, a camera with 1.3" Sony ExView HAD ™ CCD and a lens with aperture F2.0 (mini, M12) has sensitivity equal 0.04 * (2*2/1.44) =0.11lx. Where 0.04 lx is sensitivity at the aperture F1.2.
8. Real sensitivity of cameras with extended dynamic range and complex digital image processing can be insufficient despite good values in the specification. It is recommended to test such cameras before usage in a project. During testing pay attention to possible strong reducing of resolution in low light conditions.
Sometimes the high sensitivity is attained due to reducing of noises as a result of digital image processing. Measuring shows good signal/noise ratio up to low illumination. However at the same time the resolution decreases substantially.
Reducing of resolution and increasing exposure time at decreasing illumination are typical for cameras with night vision modes and some IP cameras as well.
Use Sharpness parameter for modeling of resolution reducing in VideoCAD.
9. IRE value at sensitivity measuring for different cameras can differ substantially and equal from 20-50 for normal AGC gain (Normal AGC, Standard AGC) up to 90-100 for the high AGC gain (HI AGC, Super AGC).
Use Max. AGC gain parameter for modeling boosted contrast without signal/noise ratio enhancement.
Recall that the maximum exposure time of analog cameras is 20ms for PAL and 16.5 ms for NTSC. Exactly for this exposure time the sensitivity values of simple analog cameras are shown above.
IP cameras in low light conditions automatically use the exposure time greater in several times. This is not noticeable on static scenes, but leads to blurring of moving objects.
Sensitivity of IP cameras can depend on the size of each pixel of the image sensor. The higher the number of pixels is, the smaller pixel size - the worse the sensitivity.
Correctly measuring sensitivity of modern IP cameras is not an easy task. When light is decreased, IP camera automatically turns on the noise reduction, merges neighboring pixels, reduces frame rate, multiplies exposure time, disables color. The black level rises and the noise is lost in black together with dark image details. Meanwhile turning off this automation is impossible with many IP cameras. When the noise reduction system can not be disabled, the signal/noise ratio becomes not good criterion of image quality.
In our tests, we got from an IP camera with 1/4" CMOS image sensor with 640 * 480 pixels and F2.0 lens, an image with SNR 17dB at 1,3 lux scene illumination, when the camera sets the exposure time = 40ms. Increasing exposure time up to 160ms increases sensitivity to 0.3 lux.
2-megapixel IP camera with 1/3" CMOS sensor with resolution 1600 * 1200 pix and F1.2 lens, gave out an image with SNR 17dB at illumination of 1.0 lux, with exposure time of 80ms. Increasing the exposure time up to 240ms increases the sensitivity to 0,3 lux.
The exposure time was checked by our own technique.
See also: Illumination and camera sensitivity in CCTV.