Fluorescent tube ratings

T stands for Tubular and the number is the diameter of the tube in 1/8"

T5 will be 0.625" in diameter or 15.9mm

T8 are 1" diameter.

 

Is it that strange? It is fairly logical and there were/are: T2, T4, T5, T8, T12 and possibly a few others. How else would you name them?

 

They are not strange units,they are sized in proper imperial units and were so named before we were made to go over to metric measurement, against the will of the majority of the population.
A T5 tube is 5/8" in diameter and the metric sizes are the nearest equivalent to the proper size of the tubes.

 

The T rating been in use 50 years or so, can't recall exact year.
You also have a numeric number code for the tube colour, like 927 warm white & 941 cool white for example.

https://en.wikipedia.org/wiki/Fluorescent-lamp_formats

 

Not quite ...

the 9 or 8 refers to the phosphor and the 27, 30, 35, 40, 60 are a shortened version of the colour temp - 2700 &c

 

I'm well aware of that, thought it easier to give a linky explaining better than I can, regardless of the tube type, the last two digits usually give the K rating (kelvin number).