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I hope I am not in the wrong place to ask this kind of question since it is more about image acquisition than photography.

I have to use a Microsoft Lifecam HD webcam to take snapshots of a laser beam and measure its relative intensity. This means that I do not need to compare it to any standard measurement, I just want a behavior that is as close as possible to linearity, that is, if I take two pictures of two different beams and one is twice as powerful than the other, I should be able to measure this 2x factor in the images. It is important to note that the frequency of the incoming light is always (approximately) the same, therefore I should not need to worry about different sensitivities at different frequencies since there are no different frequencies. I have no interest in chromatic fidelity, I only care about intensity. I think I can measure image intensity by simply summing the RGB values from each pixel.

I wonder how to set the camera so that I can obtain a close to linear relation between the image intensity I can measure from the saved image and the actual incoming light intensity. I have access to the following properties:

  • Brightness
  • Contrast
  • White Balance
  • Saturation (remember that I do not care about color, the light I am observing is nominally monochromatic at 632 nm, which is close to red)

Of course these settings have to stay the same between comparable images, what I am asking is whether there are specific values I should keep on all my snapshots to obtain linearity. For example: should I desaturate the image or does that change the intensity (measured as R+G+B) in a nonlinear way? Should I choose a different method to measure image intensity?

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    \$\begingroup\$ I'm voting to close this question as off-topic because it is about using a video camera for laser measurement, rather than photography proper. Perhaps it belongs on Signal Processing.SE? \$\endgroup\$
    – scottbb ♦
    Commented Aug 10, 2016 at 18:42
  • \$\begingroup\$ @scottbb you're probably right, but I think stills from a webcam are poetically on topic (by symmetry given that video from an SLR is off topic)? And image analysis - maybe not. But I'm not sure about signal processing. \$\endgroup\$
    – Chris H
    Commented Aug 11, 2016 at 6:36
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    \$\begingroup\$ @ChrisH The symmetry doesn't circumscribe the cases of topicality. The use of an optical recorder to measure phenomena for scientific purposes does not equate to the art and science of "photography", as commonly understood. Just as recording vibrational modes under 20kHz in materials science applications does not equate to "music production". Still, I could be wrong... that's why it takes multiple "close" votes. \$\endgroup\$
    – scottbb ♦
    Commented Aug 11, 2016 at 12:38
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    \$\begingroup\$ @scottbb I'm always reluctant to close a well-intentioned question; moving is another matter when there's a good home. That says more about me than the Q though. \$\endgroup\$
    – Chris H
    Commented Aug 11, 2016 at 14:57
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    \$\begingroup\$ I'm voting to close this question as off-topic because it is about using a camera as an instrument for a purpose unrelated to photography (scientific or otherwise). \$\endgroup\$
    – mattdm
    Commented Aug 6, 2018 at 23:01

1 Answer 1

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So long as they're fixed it shouldn't matter very much, except for gamma correction which should probably (there are always edge cases) be set to 1. Gain and brightness should probably be low to reduce the dark count. You must fix the exposure time. If that's not an option the experiment won't work.

Having done exactly this I'd urge you to calibrate (which would also be a good test). Get a 10% (OD1) and some 50% (OD0. 3) filters and check that 50% intensity gives you 50% counts. You may need to sum over the spot size. These filters aren't perfectly accurate (though probably more accurate than the webcam) so you still have some error. Varying the laser current won't work, at least at low or high powers (for that laser, and assuming a diode laser) as they're not very linear.

Also check that you avoid saturation. This is hard to do on some cameras, but you'll detect it as constant max value pixels in the red channel first. The max value may not be 255 by the time the software has messed around.

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  • \$\begingroup\$ Thank you. I do not have access to any Gamma correction property, but I should be able to measure its effects with calibration (basically I expect not to find perfect linearity but I can fit an exponential law to find gamma). Exposure is fixed. As for the actual counts and saturation, should I check the red channel only or the 3 channels? As I said the camera software can desaturate the image, but I am not sure how it mixes the 3 channels. \$\endgroup\$
    – gfole
    Commented Aug 10, 2016 at 20:03
  • \$\begingroup\$ When you calibrate you should check both. But for your wavelength, running just the red is probably appropriate as you'd have to figure out a weighting for the other channels' sensitivity to red light. I was using NIR and just added them all. \$\endgroup\$
    – Chris H
    Commented Aug 11, 2016 at 6:33

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