ahh they seel these two packs in clear and diffused? I'll have to check if they have them here. As of two weeks ago they didn't.

Does the UV excite something in the glass? And what brand/vintage is this yardlight it's lit in?

I think all or most clear mercs are like that. Both my Westy 175w clear and my Caster 175w has the came glowing glass effect.

I noticed that these have shorter thinner electrodes than good old zesty westies or sylvania, pre China of course. I heard that these would run the ballasts hotter than a "good" merc and last a mere 10k hours versus cheepese sylvania (somehow) lasting ~24k. Btw, these are made by feit, known for labeling diffuse coat lamps /DX.