Here is a good response on the UVB Meter Owners forum:
https://groups.yahoo.com/neo/groups/UVB_Meter_Owners/conversations/topics/8476?reverse=1
"The Solarmeter 6.2 is still a very valuable tool - I wouldn't be without mine!
These are really good meters, solid, reliable and worth their weight in gold.
What I now say to people is this - If you don't already have one, I'd suggest buying a 6.5 instead because the numbers are so much easier to understand, and because you can directly compare sunlight with lamp light. (OK, physicists will tell you it is only an approximation, and that's true, but for all practical purposes it seems perfectly reasonable to approximate...)
But if you already have a 6.2, keep using it!
Use it to monitor decay in your lamps, by routinely, maybe once a month, holding it at a set distance from each lamp in its vivarium, and make a note of the reading.
Use it to set up new lamps, when you know they are good quality from a reliable brand, and you are replacing like for like - you know what sort of readings are "good" for your reptile in his basking zone...
But don't compare sunlight readings with lamp readings - or different lamp types with each other - because that's where the weakness lies, in using broadband "total UVB" meters.
Some UVB- emitting mercury vapour lamps are particularly problematical because unlike sunlight, they tend to have a higher percentage of their UVB in the shorter wavelengths (290 - 313nm) than the longer ones (314-320nm) and the shorter wavelengths are far more powerful at producing vitamin D3 and burning skin, than the longer ones.
This means that a Solarmeter 6.2 can give a fairly low reading from some MVBs which is equivalent, in terms of sunburning strength, to a much higher reading from real sunlight.
I know what you're all going to ask me now.... So how can we find out what's a "good" reading from Lamp X, compared to sunlight..... Sigh.
At the level of technology we have right now, and given the big natural variation between individual lamps even from the same batch; slight inevitable differences between sensor responses in different Solarmeters; even if folks all publish their sets of readings from both meters, everything will be an approximation.
But having said that.... see if I've got the lamp tests posted in our files section, for the lamps you're interested in. I publish tables of readings for both meters, at all distances measured, so you can get some idea of the UV Index represented by any given Solarmeter 6.2 reading, by comparing across the tables.
Hope this helps. (But chances are, it just muddies the water a bit more
)"