Well since this light is maintained by IREA, Yes it IS Mercury vapor, that is all that IREA uses. Is Mercury Vapor for everything.
I live in IREA territory, and I see lots of these bucket lights all over the place, In Woodland Park, Here in Bailey, also in Castle Rock. Everything is Mercury Vapor. IREA loves to use MV M-400's for their higher-wattage applications. And for lower wattage applications is always a yardblaster or power bracket.
Here they hate NEMA heads. Everything is cobraheads. In small rural areas in southeastern massachusetts though, 50W HPS NEMA heads are everywhere as well as L-150 and L-250 on the bigger roads.
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I live in IREA territory, and I see lots of these bucket lights all over the place, In Woodland Park, Here in Bailey, also in Castle Rock. Everything is Mercury Vapor. IREA loves to use MV M-400's for their higher-wattage applications. And for lower wattage applications is always a yardblaster or power bracket.