All - The Solarmeter is the way to go, but there is a much less expensive option that has worked well for me. I use it in addition to the Solarmeter 6.5. There is a product called Smart UV Checker. It is a personal UV Index meter for your iPhone. The sensor is a very small unit that plugs into your headphone jack on your phone, then there is a free app you download. You can find these for anywhere between $25 and $35. I have had no problem with it at all. Since I use the same light fixtures on each of my enclosures, I made a rack that holds my phone and hangs on the light fixture in the exact same place every time. This way I can very accurately measure bulbs on a regular basis and monitor the degradation. I would absolutely suggest that everyone use a meter of some type as the bulbs I use (Reptisun 10.0 HO T5) can vary drastically in initial output. I find duds regularly. Here is a link to the Smart UV sensor on Amazon. https://www.amazon.com/dp/B00ZF332XY/?tag=exoticpetnetw-20