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Measuring low illumination

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Measuring low illumination

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Measuring low illumination

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Not expensive luxmeters which are presented on the market have the lower-range value about 0.1..0.5 lux, but minimum scene illumination for modern cameras reaches 0.04 lux. Therefore when measuring sensitivity of such cameras it is necessary to attenuate sensitivity in known number of times (it is convenient to attenuate in 50-200 times).

 

It is possible to attenuate sensitivity in the following ways:

By limiting exposure time of electronic shutter;
By calibrated manual iris lens;
By calibrated attenuation filter.

 

       Setting limit of electronic shutter

 

If camera allows setting exposure fixed value of the electronic shutter (ES, AES, AESC, ESC) on 1/1000-1/10000s, this way will be the easiest way of sensitivity attenuation.

For example, when setting exposure time on 1/10000s, the CCIR/PAL camera sensitivity is attenuated in 10000/50=200 times. Thus, the sensitivity of 0.05 lx will correspond to scene illumination of 0.05x200=10lx.

 

For EIA/NTSC cameras the attenuation will make 10000/60=167 times.

 

Although the Reciprocity principle is generally correct in digital photography, some cameras can have a difference between the shutter time from the settings and the actual exposure time. Additionally the Reciprocity failure can take place in some situation. Therefore, this method may be inaccurate.

 

We recommend to check real exposure time of your camera.

 

       Calibrated manual iris lens

 

 

Sensitivity measuring is made with the lens with narrowed aperture.

Camera sensitivity with such lens should be attenuated in known number of times. It is possible to use standard lens with manual iris after calibration as the lens with narrowed aperture.

 

Equipment:

 

For calibrating lens in addition to the equipment listed above the following is necessary:

 

Lens with manually adjustable iris. Focal length must be the same as in lens with aperture F1.2.
Camera with attenuated sensitivity or color camera with sensitivity 1..3lx preferably with the possibility to switch off AGC.
The Video Analyzer utility is included in CCTVCAD Lab Toolkit.
Projector with halogen incandescent lamp 500-1000watt, 220V.
Sticky tape

 

Order of calibration

 

1. Switch off all image processing and AGC in the camera.

 

AGC switching off increases calibration accuracy and allows working with illumination of 5..10lx, even if the camera has sensitivity of 0.2..1lx.

 

2. Mount lens F1.2, on the camera, mount the camera on stand, direct it towards sheet of paper, connect to computer, display image on the screen.

 

3. 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. Run Video Analyzer. 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 on tungsten halogen lamp. Switch off common light.

 

7. By changing distance from the lamp to the sheet of paper, 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.

Be sure, that the luxmeter sensor window and the small piece of paper on the sensor have visually equal illumination.

 

8. Write down luxmeter reading and the Average brightness value according to the Video analyzer reading.

 

9. Substitute the lens F1.2 to the lens with manual iris.

 

10. Using the projector with halogen lamp set illumination exactly 100 times more than the illumination registered in step 8. Fix the projector.

 

11. By adjusting lens iris obtain the Average brightness value from the same point on the image according to the Video analyzer reading, equal to the value written down in step 8.

 

12. Fix the iris adjustment ring by the sticky tape.

 

13. Verify the Average brightness value.

 

14. Remove and mount again the lens. Make sure that the Average brightness value is changed only insignificantly.

 

The calibrated lens, with which the camera sensitivity is attenuated in 100 times, is obtained.

With this lens the sensitivity of 0.05lx will correspond to the scene illumination 0.05x100=5lx.

 

It is recommended to verify lens calibration after measuring sensitivity of each camera.

 

       Using attenuation filters

 

It is possible to measure sensitivity of camera with mini-lens (M12) using attenuation filters. Such cameras, as a rule, do not have possibility to fix electronic shutter exposure value. It is difficult to find mini-lens with manual iris.

 

 

Spectral transmission characteristic of standard neutral density optical filters is not standardized in sensitivity range of CCD sensors. It is standardized only in eye sensitivity range. Therefore it is possible to use filter only after measuring its transmission of the light produced by halogen lamp for the same CCD sensor as in the tested camera.

When using filters for sensitivity measurement without preliminary transmission measurement, significant errors in measurement results are possible.

Transmission coefficient value of filters for black/white and color cameras can differ. It is necessary to make calibration separately for black/white and color cameras.

 

As standard filters of neutral density have unknown transmission coefficients in the sensitivity range of CCD sensors and need their measurement, with the same result it is possible to use self-made filters, made from floppy disk, with unknown transmission coefficient also.

 

Measuring filter transmission coefficient

 

Equipment:

 

For measuring in addition to the equipment listed above the following is necessary:

 

Camera with attenuated sensitivity or color camera with sensitivity 1..3lx preferably with the possibility to switch off AGC.
The Video Analyzer utility is included in CCTVCAD Lab Toolkit.
Projector with halogen incandescent lamp 500-1000watt, 220V.

 

Order of measurement

 

1. Switch off all image processing and AGC in the camera.

 

AGC switching off increases calibration accuracy and allows working with illumination of 5..10lx, even if the camera has sensitivity of 0.2..1lx.

 

2. Mount lens F1.2, on the camera, mount the camera on stand, direct it towards sheet of paper, connect to computer, display image on the screen.

 

3. 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. Run Video analyzer. 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 on tungsten halogen lamp. Switch off common light.

 

7. By changing distance from the lamp to the sheet of paper, 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.

Be sure, that the luxmeter sensor window and the small piece of paper on the sensor have visually equal illumination.

 

8. Write down luxmeter reading and the Average brightness value according to the Video analyzer reading.

 

9. Mount measuring filter on the lens.

 

When using filters from a floppy disk it is convenient to use lens cover with cut out hole as a mount.

 

10. Switch on the projector with halogen lamp 500-1000 watt, 220V.

 

11. By changing the distance from the projector 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.

 

12. Write down the luxmeter reading.

 

13. Filter transmission coefficient = illumination without filter/illumination with filter.

 

Calibrated filter is obtained with which the camera sensitivity is attenuated in a known transmission coefficient. For example, if the filter transmission coefficient is 0.0125, the camera sensitivity of 0.05lx with the filter will correspond to the scene illumination 0.05/0.0125=4lx.

 

 

See also: About camera sensitivity, Measuring camera sensitivity, Sensitivity and Resolution>Camera.

External link: "The principles of CCTV design in VideoCAD. Part 4. Illumination and camera sensitivity in CCTV"(*.pdf) .