These are designed specifically for indoor horticulture, and have a more balanced color spectrum. HPS bulbs are made out of a translucent ceramic arc tube containing a mixture of sodium, mercury and xenon gas. HPS bulbs range in power from 35w to 1000w, with the 250w, 400w, 600w and 1000w being the most popular for indoor horticulture. HPS bulbs should be changed out after about 24 months. The bulbs slowly lose their brightness over time and to maintain good light quality the bulbs must be changed before the end of their rated life.
Sulfur lighting is the new kid on the block.
I have had no dealings with this type of lighting. At this point in time sulfur lighting is extremely expensive when compared to H.I.D. lighting. There have been a lot of fantastic claims about the efficiency and brightness of these lights, however from what I have heard lately the claims were greatly exaggerated. There are companies working on a marketable sulfur lighting system for the hobby gardener and I will keep you posted when I learn more. Grow lights are obviously less intense and don't cover a very big area.