Some HPS bulbs are rated (intended) to be used in open light fixtures. They have some Uv block built into the glass bulb. Somewhere on the package it should say "open fixture rated"

Other types of HPS bulbs are rated (intended) to be used inside glass covered fixtures. They don't have Uv blockers built in the bulb. The glass acts as a Uv block, and usually needs to be air cooled; like what growbe said. It should say somewhere on the package "rated for inclosed fixtures".

The same goes for Metal Halide bulbs: which have even more Uv ray.

p.s....wear your sunglasses....