I found this website while searching for facts on HPS vs MH bulbs and there effect on flowering . Just thought it would be nice to share. I know its alot to read but theres a shitload of good info here. Metal Halide (MH) is a highly efficient light source capable of delivering a white light in the range of 2700 to 5500 Kelvin degrees with a typical Color Rendering Index (CRI) ratings in the mid-60s to mid-70s. Some lamp chemistries even obtain CRIs in the 80s. The CRI is an index that measures how closely an artificial light source matches the natural colors of sunlight, with 100 being a perfect score. In contrast, high pressure sodium lamps (below) yield yellow lighting (2200K) and have a very poor color rendering index of 22. White light is a very important attribute of metal halide technology, because it is the closest to the natural sunlight that people prefer. For horticultural purposes, metal halides tend to produce a shorter, more compact plant habit that more closely resembles plants grown outdoors. A plants appearance growing under most metal halide lamps is almost identical to those growing outside, which makes it the preferred lamp for use in living areas. Horticultural metal halide lamps have an enhanced red spectrum, hardly noticeable to the naked eye, which is added for improved fruiting and flowering without sacrificing plant appearance. ----------------------------------------- The orange/red spectrum HPS produces is the spectrum plants use in their reproductive processes, which generally produces larger harvests of higher quality fruits or flowers. A typical HP Sodium (HPS) high output horticultural lamp. The major consideration to growing under high pressure sodium alone is that the plants tend to be taller and leggier with a longer internodal length (spacing between branches) than plants grown under the bluer light of metal halide. Another factor to consider is that plants generally do not appear very healthy growing under HP Sodium light. The operative word being appear, because in fact most plants do grow very well under HPS lighting. As a result of the poor color rendering characteristics of high pressure sodium, the plants tend to look pale, washed out or nitrogen starved. Source: WeGrow.In » Growing Under Artificial Lights Burn one B.