Converting camera feed into relative intensity in mW

asked 2019-10-20 20:34:48 -0500

Vizier87 gravatar image

Hi guys, general question here. I think this might have been questioned many times but my searches always end up in some rabbit hole.

As per the title, is there an objective way to establish the power intensity in mW of an ROI in a camera feed?

I've been using the method to convert the readings from my camera using the mean of the ROI (the code is something like follows:)

mean =  cv::mean(ROI);

But the intensity, as far as I understood it, is relative or in arbitrary units (a.u.).

Is the measure of intensity, if need be in mW, the sum of all the pixel intensities in an array ROI?

My golden standard is if possible, we can make the measurement comparable to a photodiode or a power meter.

How do we link the scalar value with an absolute unit of power?

Thanks. Vizier87

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light intensity can be expressed in lux or lumen, not milliwatts (you know, there is a subtle difference between electrons and photons...)

kbarni gravatar imagekbarni ( 2019-10-21 10:07:51 -0500 )edit

Ok my bad.. so if I wanted to express it in mW, is it possible?

Vizier87 gravatar imageVizier87 ( 2019-10-21 20:11:29 -0500 )edit

Light cannot be expressed in mW; and the power is not a "relative measure".

It' another question that photons cannot be measured directly, so they are transformed in electricity using the photoelectric effect which can be measured.

However cameras aren't conceived to measure light intensity. They have a relatively low dynamic range, non-linear response and they adapt themselves to the luminosity of the scene. The color filters filter out a large part of the light too (with different intensities for each filter).

To measure the light intensity it's much better to use a specialized tool as a light meter (or luxmeter). Much simpler, cheaper, and more reliable.

Or take a photodiode and hook it up to a voltmeter (or Arduino) for a DIY solution.

kbarni gravatar imagekbarni ( 2019-10-22 05:20:10 -0500 )edit