Light Meters?

[ALToronto]

What you're seeing is not UV light itself, but the luminescence of another material (in this case, paper) during exposure to UV light. Most organic materials will luminesce, and some will fluoresce (emit their own light). About half of all diamonds fluoresce under UV.

Different wavelengths will cause different colour luminescence.

Your regular cool temp LEDs provide some UV already, you don't need more.