The T-5 bulb is the part which needs to be at 35*C or 100*F, not the ballast which is usually remotely mounted. Ballasts are almost always electronic as opposed to magnetic... the electronic use switching transistors, which regulate the power precisely going to the lamps... The ballasts are also potted (encased in goo) usually which encapsulates heat instead of radiating it. But the key is that the bulbs not the ballasts have an operating temp.
They deliberately made the T-5 bulb's efficiency peak at 100*F (35*C) because the size is more compact, and all Normal T-5s are actually High Output (as opposed to normal output). This makes the bulbs hotter, and because most luminaries are enclosed, they expected internal temps to be high...
It was just a design decision as this chart shows:
from here:
The Light Edge, Inc. - T5HO Technology : T5 temperature performance
The improvements of T-5 over T-8, which were not previously possible (i.e. you could have had a HO T-8)
Smaller diameter means 5% more efficiency that T-8
rated for 35*C (100*F)
Lower Mercury content 3mg vs. 15mg per 4' (1.2m) tube
Higher color rendering Index Standarized @ 85+
lower lumen depreciation (because of lower mercury)
Longer life span (because of lower mercury)
I suspect the gas pressure and content is the the key in having efficiency tuned for higher temperatures. Also the lower mercury content makes the bulb function differently... Mercury functions as a vital low pressure gas which allows ignition of the bulb... Without mercury, none of this would be possible (at low pressures). They have gone to great lengths in HIDs (MH and HPS) to eliminate Mercury, and as you can see in the link below (if interested) how they compensate for this lower mercury level. Many HIDs still do have Mercury.
T-5 are not as much of a radical departure, as a refinement of T-8 lamps, the smaller diameter is not the main focus, even though they look very cool!
The T5 Fluorescent Lamp: Coming on Strong
"One result of enclosing a smaller lamp with a relatively high output in a smaller fixture is heat — and lots of it. For that reason, the T5 lamp provides peak light output at 35°C (95°F) air temperature, whereas the T8 and the T12 lamp provide peak light output at a 25°C (77°F) ambient air temperature."
And my favorite site about T-5 lights is below:
Introduction | What are T5 Lamps? | T5 Fluorescent Systems | Lighting Answers | NLPIP
Do your t-5 bulbs get hot?
Linear Mode
