I know that the T in T4 stands for Tubular.  What does the 4 stand
for? (in the case of a T4 lamp) or the 5 in the case of a T5 lamp?

Thank you

Howdy besparkle-ga,

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
pages.

"T2, T4, T5 , T8, T12"
http://www.lampco.co.uk/glossary_t2t4t5t8t12.asp

"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!
http://www.lrc.rpi.edu/programs/nlpip/lightingAnswers/lat5/pc1.asp

"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.


Search strategy:

Google search on: T4 T5 lamps
://www.google.com/search?q=T4+T5+lamps

Looking Forward, denco-ga - Google Answers Researcher

T4 seems to stand for 1/2" diameter:
http://www.budgetlighting.com/store/agora.cgi?page=microfluorescent_under_cabinet_ultra-thin_fluorescent_fixtures.html