There was an article on the mega ray site about some faulty 10.0 bulbs causing some pretty severe eye damage a few years ago. I dont know it it has been fixed or not, maybe visit the megaray site. Ive used nothing but a 5.0 compact for the 3 years i have had reggie and he has never had an eye issue. He gets pretty close to the bulb, so if there was a problem, it would have manifested itself by now. I im not mistaken the numbers are just percent of uvb, meaning 5.0% and 10.0%. It could be that because the animal has to be so close the the bulb to get usable uvb, the stronger bulb was causing eye problems.