A metal halide (MH) lamp makes light from an arc through mercury vapour and metal halide salts in a quartz arc tube. The salts broaden the output into a bluish-white spectrum with more blue than HPS. MH lamps run on ballasts, often the same ones that drive HPS lamps of matching wattage (check compatibility; many digital ballasts are rated for both).
In practice
The classic HID setup was MH for veg (compact, tight internodes from the extra blue) and HPS for flower (more output per watt and more red-orange). Efficacy of MH is lower than HPS, typically around 1.2–1.5 µmol/Jbeing reviewed, and output falls faster with age. Today MH is mostly found in older rooms or used as a supplementary spectrum.
Like HPS, MH lamps emit substantial radiant heat and contain mercury. Quartz-tube MH lamps also emit some UV; use the protective glass the fixture was designed for, and do not run a lamp with a cracked outer bulb.
Don't confuse with
- CMH (ceramic metal halide): uses a ceramic arc tube; more efficient and broader spectrum.
- HPS: orange-yellow, higher efficacy, the old flower standard.
- LED: far more efficient, with a spectrum that can match any of these.
