HPS...

[Wilty]

here's info. from a green house.

"Halide systems produce an intense blue light that is excellent for vegetative plant growth. The blue light spectrum closely matches spring light and promotes healthy, compact growth in most plants. Choose a metal halide system if you garden indoors with no additional light source or if you need a light system to propagate plants"

"It is if you need supplemental lighting to encourage your plants to fruit and flower. If you garden in a greenhouse and can rely on the sun to provide necessary blue light, an HPS system would be the proper choice for you. The rich red hue of the sodium bulb recreates the autumn sun, stimulating the plant to advance and finish its growing cycle. Flowers and vegetables finished off under HPS light will show tighter, stouter blossoms with increased yields. But during a vegetative cycle of growth, using an HPS light alone may cause plants to grow long and thin, or leggy. Look into a metal halide system for vegetative growth"

There was mention of a "conversion bulb" that is a HPS bulb in the spectrum of a MH.

"Because the conversion process itself takes some energy to accomplish, conversion bulbs aren’t quite as efficient as regular metal halide or high pressure sodium bulbs, but they will give you the best of both worlds with just one light system. "