I recently purchased a 365 nm UV LED, and set up the proper circuit. When I turn the power on, the LED lights up very dimly. Is this white light (that's the color I saw) being emitted from the LED? If so, are there any methods (preferably inexpensive) for blocking out only visible light?
The specs for the UV LED can be found here: http://www.mouser.com/ds/2/228/LZ1-00UV00-257812.pdf
Figure 5 of the linked datasheet shows there is a small amount of emission in the >380 nm range.
I digitized the spectrum and computed how much relative radiant flux there is greater than 380 nm.
\begin{gather} \Phi_\text{visible} = \frac{\int_\text{380 nm}^{\infty} \Phi_{\lambda} \text{d} \lambda}{\int_{0}^{\infty} \Phi_{\lambda} \text{d} \lambda} \approx 0.087 \end{gather}
So a bit under 10 % of the radiant flux is being emitted in the visible spectrum. I don't know what color this would look like in practice, though I suspect it might be have blue-violet hue since about 88 % of this is in the 380–400 nm range. Assuming 1050 mW total radiant flux, this means there's about 92 mW emitted in the visible spectrum.
You can find "reasonably priced" (~30 USD) shortpass and bandpass filters which transmit UV while blocking visible wavelengths, for example this one. This would reduce the visible radiant flux to about 10–30 mW (eyeball estimated). If you have more money available, there are of course better filters.
