# Metal halide

> A metal halide (MH) lamp is a gas-discharge grow light with a bluish-white spectrum, traditionally used in the vegetative stage before switching to HPS.

- Canonical: https://genetixbay.com/knowledge/glossary/metal-halide.html
- Also written: MH, MH lamp, metal-halide lamp, halide, MH bulb
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

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](https://genetixbay.com/knowledge/glossary/hps.md). 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](https://genetixbay.com/knowledge/glossary/internode.md) from the extra blue) and HPS for flower (more output per watt and more red-orange). [Efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) of MH is lower than HPS, typically around 1.2–1.5 µmol/J, 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.

## Related terms

[HPS](https://genetixbay.com/knowledge/glossary/hps.md), [CMH](https://genetixbay.com/knowledge/glossary/cmh.md), [LED](https://genetixbay.com/knowledge/glossary/led.md), [Photon efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md)

## Read more

- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)

## Sources

1. Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7