The sensitivity of cameras depends on the exposure time. Within the validity of the Reciprocity principle, the camera sensitivity is directly proportional to the exposure time.
However, increasing exposure time leads to blurring of moving objects that is often unexpected. Usually, when measuring sensitivity of cameras and while accepting built CCTV system a customer looks only at static images, but moving targets are of interest.
The greater the exposure time and the greater the velocity of an object on the screen, the more the blur.
Automation of camera (AESC) constantly adjusts the exposure time (shutter speed) depending on the illumination on the image sensor. The more light is, the less exposure time is set. But from a certain threshold light AESC sets the maximum exposure time and further reducing light doesn't increase the exposure time. Knowing the maximum exposure time and the threshold illumination is very useful in designing CCTV systems.
VideoCAD7 offers a tool for modeling blur of moving objects in dependence of the exposure time. But to use this tool, you need to know the exposure time.
Knowing the maximum exposure time is also needed correct interpretation of results of measuring sensitivity and sensitivity values in the camera specification. With a minimum illumination the camera sets the maximum exposure time. Unless otherwise indicated, the sensitivity value in the camera specification can be achieved at the maximum exposure time.
The maximum exposure time of standard analog cameras without light accumulation is 20ms for PAL and 16,5 ms for NTSC. IP cameras and analog cameras with accumulation of light can use exposure time up to 200ms and more. With latent increasing the maximum exposure time, many manufacturers increase the certified sensitivity of their cameras.
Many cameras allow to modify the maximum exposure time, switching the "night", "supersensitive" modes, etc. But these settings don't report the real value of the exposure time. If the camera has several "night" modes, for their proper use it is necessary to know the maximum exposure time for each of them.
There is a camera. It is necessary to measure its maximum exposure time and the threshold illumination at which the camera switches to the maximum exposure time.
|•||Oscilloscope. Any analog oscilloscope will suit.|
|•||Luxmeter (a device for measuring illumination). Almost any kind of such a device with standard CIE spectral response will suit.|
|•||In case of testing analog cameras, we need any PC-based video capture system, TV-tuner with video input, etc. The System should allow to display live video on the computer screen and separate frames. For testing IP cameras it is enough to have possibility to display video on the screen and separate frames.|
|•||Compact tungsten halogen lamp 12V, 10W.|
|•||Stabilized power supply unit for the lamp. 12V DC, without considerable ripples of output voltage.|
|•||Lens with F1.2 aperture, parameters of which are reliable, focal length is 4-8mm. If the camera allows to mount only mini-lenses (M12), the mini-lens with a known aperture (F2.0) is needed.|
|•||Stand. It is recommended for convenience and accuracy.|
If you want to measure only the maximum exposure time, and measuring the threshold illumination is not required, then luxmeter, incandescent lamp and power supply are not needed. Instead, it is required a manual iris lens.
1. Set the lens on the camera, install the camera on the stand, point the camera at the screen of the oscilloscope, connect camera to computer, display image from the camera on the computer screen.
2. Place the luxmeter sensor near to the oscilloscope screen.
3. Enable a "night" mode in the camera setting.
4. Set the oscilloscope sweep time - 50ms/div. Scan - automatic (If your oscilloscope has such settings).
On the oscilloscope screen a flying spot must be visible. On the computer screen instead of the spot, flickering segments will be seen.
5. Turn on the incandescent lamp. Turn off the ambient light.
If you need only to measure the maximum exposure time, then instead of changing light simple close iris on the lens.
6. Set the minimum brightness of the oscilloscope spot, to make the segments on the computer screen barely visible. Light of the spot should not affect operation of camera's automatics.
7. By moving the lamp away, reducing the illumination of the oscilloscope screen, detect a moment when the length of the segments on the computer screen becomes the maximum and no longer increases with further decreasing light until the appearance of strong noise on the screen.
At the moment when the length of the segments becomes maximum, luxmeter shows the threshold illumination.
As a result of our measurements, the threshold illumination was about 50lux. That is, when light is less than 50lux, the testing IP camera with 2 megapixel 1/3" CMOS sensor began to use the maximum exposure time.
8. Save and view a frame in which a segment of the maximum length gets wholly. If you can not get such frame, increase the oscilloscope sweep to 100ms/div.
The length of the segment in the frame in the scale of oscilloscope screen equals to the camera's exposure time.
9. If necessary, by the method described above you can take a function of the exposure time in dependence of the light.