What are T8 & T12 mean in lighting?

All your answers to your research project is here:

Search Results | Lighting Research Center (http://www.googlesyndicatedsearch.com/u/lightingresearchcenter?q=T12&sa=Search&domains=lrc.rpi.edu&domains=lrc.rpi.edu&sitesearch=lrc.rpi.edu)

Also search for T8

Then you can summarize your answers.

what are T8 & T12 mean in lighting??
T means tubular; T8 means the lamp's diameter is 1 inch(2.54 cm), T12 =1.25 inches

The 8 means 8*1/8" and 12 means 12 * 1/8", hence the diameters of 1" and 1.25"