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Example 9 Determining spectral efficiency of light source |
  
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Example 9 Determining spectral efficiency of light source |
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See before: What is the spectral efficiency.
Problem
There is a lamp and a camera. It is necessary to measure spectral efficiency of this lamp for this camera in relation to the halogen incandescent lamp.
The matter of measuring Spectral efficiency is the achievement of equal average brightness of the same pixel on the image, when the scene is illuminated by the test lamp and a tungsten halogen lamp. When the brightness of the same pixel on the image is the same, a Luxmeter will show different readings. The Spectral efficiency factor equals to the ratio of these two readings.
Equipment:
| • | Luxmeter (a device for measuring illumination). Almost any kind of such a device with standard CIE spectral response will suit. |
| • | Any PC-based video capture system, TV-tuner with video input, etc. The System should allow to display live video on the computer screen. |
| • | Tungsten halogen lamp of approximately the same power consumption, as testing lamp. |
| • | Manual iris lens, focal length 4-8mm. |
| • | Stand. It is recommended for convenience and accuracy. |
Measuring should be performed in a dark room.
Order of work
1. Switch off all image processing options and AGC in the camera.
AGC switching off increases accuracy and allows working with illumination of 5..10lx, even if the camera has sensitivity of 0.2..1lx.
2. Mount the manual iris lens on the camera, mount the camera on stand, direct it towards sheet of paper, connect to computer, display image on the screen.
3. On the sheet of paper, in the camera field-of-view place the luxmeter sensor.
4. Close to the sensor window directly on the sensor, fix a small piece of even white paper.
5. Launch Video Analyzer. Switch to the One pixel test tab. Mark Cycle box, click Start. After that move fast the mouse cursor to the small piece of even white paper on the screen, close to the sensor window.
6. Switch off common light. Switch on the testing lamp and fix it at a middle distance from luxmeter sensor.
There should be a possibility to approach and move away the lamp.
Be sure, that the luxmeter sensor window and the small piece of paper on the sensor have visually equal illumination.
7. By adjusting aperture, obtain that the Average brightness measured by the Video analyzer would be equal to 0.22-0.25 after finishing measuring cycle by 100 frames.
The objects on the image should be recognized.
You must not change the lamp supply voltage. It should be equal to nominal lamp supply voltage. Change Illumination only be means of distance variation between the lamp and the sheet of paper.
8. Write down luxmeter reading and the Average brightness value according to the Video analyzer reading.
9. Switch off the testing lamp. Switch on the halogen lamp.
10. By changing the distance from the lamp to the sheet of paper, obtain equal with step 8 value of Average brightness after finishing measuring cycle by 100 frames.
You must not change the lamp supply voltage. It should be equal to nominal lamp supply voltage. Change Illumination only be means of distance variation between the lamp and the sheet of paper.
Be sure, that the luxmeter sensor window and the small piece of paper on the sensor have visually equal illumination.
11. Write down the luxmeter reading.
12. Spectral efficiency factor of the tested lamp=illumination from tungsten halogen lamp (step11)/illumination from the tested lamp (step 8).
You can insert the obtained factor in the Illuminator calculation box in VideoCAD.
See also: What is the spectral efficiency, Spectrum analyzer.