I would like to measure the intensity of a laser beam after it has passed through a translucent material. However, the sensor that I am using returns values in lux, which reflects the subjective perception of the human eye to light of different wavelengths. Is laser light monochromatic enough that it is possible to use the lux values as a measure of the light intensity?

Additionally, despite (or perhaps beacuse of) reading the following pages (1,2), I am still rather confused about what the term "intensity" means in the context of laser light. The unit lux is equivalent to lumen per square meter, yet intensity units seem to be given in terms of solid angle. However, I would think that this must be different for lasers, given that the beam is collimated.

I have seem the following units given for intensity:

  • W/sr
  • cd (lm/sr)
  • W/$\mathrm{m^2}$
  • W/(sr $\mathrm{m^2}$)

Which of these is correct, and am I justified in using lux?


Any unit involving solid angle (steradians) is usable but not helpful here since they are tailored to characterise radiation emitters, meanwhile you are just trying to measure received radiation some arbitrary distance away from a light source.

You can stick with Lux for your purposes. There is no problem with describing received monochromatic light in terms of lumens (well, as long as the light is in the visible range). If you want to convert your Lux measurements into W/m² you can use a luminous efficacy table.

Table of Luminous Efficacy


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