Wow. Thank you for that info.Geisman bulbs are (or were) designed specifically for dimmable fixtures. Since my fixture isn't dimmable, it was a coin toss decision between ATI and Geisman bulbs. ATI won the toss. I may go with Geisman when it's time to change them out, as they're a little cheaper.
If you are using D-D Geisman bulbs, I hope you're aware that D-D and Geisman are no longer working together. D-D is the company that developed the special phosphors that Geisman uses in their bulbs, and the development for use in dimmable fixtures. They will continue making these bulbs, but it will be under their own brand. You can read about it here: http://www.advancedaquarist.com/blog/d-d-issues-statement-clarifying-the-recent-giesemann-powerchrome-t5-announcement