Although not a popular topic, this is important to growing plants under artificial light and it behooves you to know and study it. Basically, it is a synergistic photosynthesis effect between visible light and far red to near infrared light (FR-IR). Alone, FR-IR is nearly invisible and produces negligible photosynthesis.

Incandescent (especially Halogen) light bulbs and HPS (especially extra high pressure sodium) are good sources of FR-IR. Typical metal halide lamps are deficient (I suspect some special metal halide bulbs might be good). That is why you should supplement regular metal halide lighting with FR-IR.

Sylvania Growlux Wide Spectrum fluorescent lamps had a phosphor that emitted just far red light, but, this phosphor deteriorated much faster than the other phosphors. Thus, these lamps were noticeably less effective after about two weeks of use, even though there was no visible change in light intensity. Too bad, because these lamps were exceptionally effective for the first week.