Using a Power Meter to detect unwanted fluorescence in a filter

Absorptive glass filters have varying degrees of non-trivial fluorescence, which can be problematic in some applications. Here’s a fairly simple way to check if a filter is fluorescing:

  • Place the filter in a collimated beam with a power meter behind it, located very close to the filter and measure the signal.
  • Increase the distance (>> 10x the beam diameter) from the filter to the power meter and measure again the signal.

Any fluorescent light will be lambertian, so the amount measured will be inversely proportional to the square of the distance.

The measured power of the collimated light will be independent of the distance (within the limits of the collimation).