yeah there's nothing lightweight about glass refractors that's for sure! They'll certainly outlast plastic though as long as they're not subject to abuse.
Only polycarbonate lenses yellow. Acrylic ones don't yellow. Polycarbonate is more vandal resistant than acrylic but it's more expensive and yellows over time. Acrylic is less vandal resistant but it's cheaper and doesn't yellow which is why acrylic is the most commonly used lens when using plastic. I think acrylic is a little newer than polycarbonate which is why when older fixtures have plastic lenses, they're typically polycarbonate and that's why they yellow. Glass is the most expensive but it's the only one that can resist high heat without damage so that's the only reason it's used nowadays. Of course glass is offered for all wattage cobraheads but plastic is usually only offered up to 150 or 175W lamps in small fixtures and 250W (HPS only) for medium. I personally prefer glass lenses but i like plastic lenses too, mostly becuase there's more of a variety of plastic lenses than there is glass. for lower wattage lights acrylic is the most economical in most situations.
@streetlight98, the glass is "stippled" which was standard for the earlier M400A1 cutoffs. I think this was a way GE used to diffuse the harsh HPS light.
Ahh interesting. Joe Maurath has a 1980 50W HPS OV-15 TuDor cutoff light that has a stippled glass too. I wonder why they stopped using the diffused FCO glasses. It really does cut back on the glare from the bare lamp. Even a 50W HPS is harsh to look at without some form of a diffuser.
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Is this glass frosted/fogged or is it just dirty?