One problem with some cameras is that they might have a gamma (non-linear response). If you can vary the laser power, then you can make your own calibration curve of power vs. measured brightness. This will have the gamma curve, or if you have a linear system (gamma of 1) it will be a straight line. So basically with that intensity calibration curve, you can use it as a formula to get out the "real" power if you feed it a gray level.
It's whole other discussion if you want to get into units about exactly what intensity means, as opposed to power, illuminance, irradiance, lux, brightness, etc. The kinds and names and meanings of all these optical measurements can make your head spin. It does mine, and I have a Ph.D. in optics. See the attached short paper that my late, former professor wrote on the topic.
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