Years ago when I was attending college, I came across clear (no phosphor coated) F15T8 fluorescent light and it was not a germicidal lamp as it stated.  I recalled it was made by ITT.  It does light up similar to clear mercury sign tube.  Has anyone come across this light?

Maybe it has some sort of clear coating on the bulb that doesn't emit UV light..... 

There not need any "clear coating", since just making a regular lamp without phosphor is UV safe.  The germidcal lamps use special glass that lets the UV through.  Some specialized/proprietary germidcal lamps do use quartz.  Normally standard fluorescent sized germidcal lamps would be way too expensive to use quartz.

Sometimes clear lamps are made to allow arc observation for development purpose (both lamp and ballast) and/or education (to show, what happens there in what regime).
Germicidal lamps do mostly use quartz, that's why they are so expensive (beside the small quantity manufacture)

It is known that many linear germidical lamps drop in uv output with usage.  In fact you replace the lamp long before the electrodes fail.  That is not characteristic with quartz.  I find difficult to believe something as large as a f15t8 would be entirely made of quartz.

Due to some reason (i do not know exacly what happen) the fluorescent coating act as getter, when is not there, the atmosphere degrades and cause reduction in radiation output. I think, this would be the primary mechanism, why germicidal lamps degrade to unusable state long before electrodes fail. F15T8 is not as large, i've seen lot of F36T8 (in older hospitals, in fixed installation), they were for sure made of quartz (the tubular part, i'm not sure about filament sterns) and suffer the same aging problem.

It is known that many linear germidical lamps drop in uv output with usage.  In fact you replace the lamp long before the electrodes fail.  That is not characteristic with quartz.  I find difficult to believe something as large as a f15t8 would be entirely made of quartz.