# CMH

> CMH (ceramic metal halide) is a discharge lamp with a ceramic arc tube giving a broad, sun-like spectrum and better efficacy than standard metal halide.

- Canonical: https://genetixbay.com/knowledge/glossary/cmh.html
- Also written: ceramic metal halide, LEC, light emitting ceramic, CDM, ceramic MH, 315 W CMH
- 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"

Ceramic metal halide (CMH) lamps use a ceramic arc tube, like [HPS](https://genetixbay.com/knowledge/glossary/hps.md), filled with [metal halide](https://genetixbay.com/knowledge/glossary/metal-halide.md) salts, like MH. The ceramic tolerates higher temperatures, so the lamp runs hotter and more efficiently with a broader and more stable spectrum than quartz MH. "LEC" (light emitting ceramic) is a marketing name for the same lamp class. The common grow sizes are 315 W and 630 W (two lamps), on low-frequency square-wave ballasts.

## In practice

CMH was popular in the 2010s as a single lamp for both veg and flower, valued for its [broad spectrum](https://genetixbay.com/knowledge/glossary/full-spectrum.md), good colour rendering for inspecting plants and a small amount of UV. [Efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) is roughly 1.5–1.9 µmol/J, better than MH and similar to or below DE HPS, and well below modern [LEDs](https://genetixbay.com/knowledge/glossary/led.md).

A 315 W CMH typically covers about 0.9 × 0.9 m (3 × 3 ft) for flower. Like other HID lamps, it emits radiant heat and contains mercury. Some CMH lamps emit enough UV that the fixture glass matters; follow the lamp's rated fixture type.

## Don't confuse with

- **Metal halide (MH)**: quartz arc tube, bluer, less efficient.
- **HPS**: ceramic tube too, but [sodium](https://genetixbay.com/knowledge/glossary/sodium.md) fill and orange-yellow light.
- **LED**: solid-state; higher efficacy and dimmable over a wide range.

## Related terms

[Metal halide](https://genetixbay.com/knowledge/glossary/metal-halide.md), [HPS](https://genetixbay.com/knowledge/glossary/hps.md), [LED](https://genetixbay.com/knowledge/glossary/led.md), [UV-A](https://genetixbay.com/knowledge/glossary/uv-a.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