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.