So I picked up a pack of these today to play around with.
After some testing and discussion with Dr. Baines, they are mostly useless and quite misleading.
The card themselves are nicely made, the size and shape of a credit card. The reactive paint (?) is a matte texture and has a "low, moderate, high" gauge. The labeling uses the word "accurately" wich I found questionable. It also cleverly makes no mention of "UVB" just general "UV".
It was a bit hard to get photos showing the color change, I hadn't thought of covering part of the reactive paint at the time I made the first few photos.
Some of the cheep "UV" meters use UVA - these are often labeled as "not suitable for artificial lighting." I mentioned these cards in the lighting group and the response was that they are probably similar, if even more crude. So, my first test was a closed window. UVB is filtered 100% by glass and plastic, UVA is not. According to the cards I'm getting adequate UV for a bearded dragon through my closed window. According to my solarmeter I'm getting (accurately) 0.
Next, about 1-2" from my reptisun 10 T5 the UVI is 13.7. According to the card, it's about the same as through the closed window.
Since it was a sunny day, I popped them out into the sun. The card got the darkest under this lighting meanwhile the UVI is about 6.5+....
They even respond very weakly to the halogen and daylight florescent bulbs.
So... Basically.... Like the cheep sensors these cards are likely responding to UVA not to UVB and are not a meaningful tool for judging the health or strength of our bulbs. They aren't even good for measuring decay, since UVB loses its potency much faster than UVA. Maybe they would be a bit useful in gauging the UVA in our enclosures, but that's about it.