MH (Metal Halide)

MH (Metal Halide) lights were the standard vegetative-stage indoor cannabis lighting from the 1980s through the 2010s, paired with HPS lights for the flowering stage in two-bulb HID (High-Intensity Discharge) cultivation systems. The technology: MH bulbs use an electrical discharge through mercury vapor with metal halide additives (typically dysprosium iodide or scandium triiodide) inside a quartz arc tube, producing intense light heavily concentrated in the blue and violet spectrum (400-500nm). The blue-rich spectrum closely mimics natural midsummer sun and promotes compact vegetative growth — short internodal spacing, dense foliage, robust root development. Standard MH sizes: 250W, 400W, 600W, 1000W bulbs are most common; 400W is the sweet spot for residential vegetative cultivation. MH operating characteristics: light output 80-115 lumens per watt (less efficient than HPS); spectrum heavily weighted toward blue with weaker red output (excellent for vegetative growth, poor for flowering); operating temperature very high — bulbs reach 800°F+ surface temperature, requiring 18-24" minimum distance from canopy; significant heat output requiring cooling; bulb life 10,000-15,000 hours; bulbs degrade significantly after 50% of life expectancy. The traditional HID cultivation cycle: vegetative stage — MH bulb providing blue spectrum optimal for compact leafy growth (4-6 weeks); flowering stage — switch to HPS bulb in same fixture for yellow-orange spectrum optimal for bud production (8-12 weeks). Some commercial operations used "dual arc" bulbs that combined MH and HPS technology in a single bulb, but these compromised the optimization of either spectrum. Universal HID ballasts can run both MH and HPS bulbs in the same fixture, allowing simple bulb-swap between vegetative and flowering stages without buying a separate fixture. CMH (Ceramic Metal Halide) is the modern evolution of MH technology — using ceramic arc tubes for higher efficiency, better spectrum balance, and longer bulb life; CMH largely replaced standard MH for serious growers throughout the 2010s before being itself replaced by full-spectrum LED. MH remains popular for budget vegetative-only setups, but commercial cultivation has overwhelmingly migrated to full-spectrum LED that performs both vegetative and flowering with a single fixture.