The comment by stanmartin1952-ga is absolutely correct in that a T4 lamp
is 1/2" in diameter. But, what if it is a T5 or T8? The website of the
UK based Lamp Company provides the answer for us on one of their Glossary
"T2, T4, T5 , T8, T12"
"Refers to the diameter of a fluorescent tube (in 1/8th of an inch) ..."
So, a T5 lamp should be 5/8" in diameter, and the "Lighting Research Center"
website from Rensselaer Polytechnic Institute's National Lighting Product
Information Program verifies that, and provides illustrations and metric
conversions as well!
"T5 lamps have a diameter equal to 5 times an eighth of an inch, or 5/8"."
If you need any clarification, please feel free to ask.
Google search on: T4 T5 lamps
Looking Forward, denco-ga - Google Answers Researcher