# HPS

> HPS (high-pressure sodium) is a gas-discharge lamp with orange-yellow light peaking near 589 nm; it was the standard flowering light before LEDs.

- Canonical: https://genetixbay.com/knowledge/glossary/hps.html
- Also written: high-pressure sodium, high pressure sodium, sodium lamp, DE HPS, double-ended HPS, SE HPS
- 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 high-pressure sodium (HPS) lamp produces light by passing an arc through [sodium](https://genetixbay.com/knowledge/glossary/sodium.md) vapour (with mercury and xenon) inside a ceramic tube. Its spectrum is concentrated in yellow and orange around 589 nm, with little blue. It needs a ballast to start and run. Single-ended (SE) lamps screw into a socket; double-ended (DE) lamps are held at both ends and are more efficient.

## In practice

For decades a 600 W or 1,000 W HPS over a 1.2 × 1.2 m (4 × 4 ft) area was the flowering standard. Modern DE HPS fixtures reach about 1.7–2.1 [µmol/J](https://genetixbay.com/knowledge/glossary/photon-efficacy.md), well below good [LEDs](https://genetixbay.com/knowledge/glossary/led.md). HPS also emits a lot of radiant (infrared) heat that warms leaf surfaces directly. Nelson and Bugbee (2015) showed leaves under HPS run warmer than under LED at the same air temperature and [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), which changes ideal room settings: HPS rooms are often run cooler in air temperature.

Bulbs lose output with age; many growers replace them every 2–4 cycles. Handle with care: lamps contain mercury and get extremely hot.

## Don't confuse with

- **Metal halide (MH)**: a related discharge lamp with bluer light, traditionally for veg.
- **CMH**: [ceramic metal halide](https://genetixbay.com/knowledge/glossary/cmh.md), a broader-spectrum discharge lamp.
- **LED**: solid-state, more efficient, less radiant heat at the canopy.

## Myth check

[LED vs HPS](https://genetixbay.com/knowledge/glossary/led-vs-hps.md) for final quality is an open question at GenetixBay: an open [Bay Trial](https://genetixbay.com/knowledge/glossary/bay-trials.md). See [Mythbusters](https://genetixbay.com/knowledge/mythbusters.md).

## Related terms

[LED](https://genetixbay.com/knowledge/glossary/led.md), [Metal halide](https://genetixbay.com/knowledge/glossary/metal-halide.md), [CMH](https://genetixbay.com/knowledge/glossary/cmh.md), [LED vs HPS](https://genetixbay.com/knowledge/glossary/led-vs-hps.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)
- [Heat and light stress: taco leaves, bleaching and foxtails](https://genetixbay.com/knowledge/environment/heat-light-stress.md)

## Sources

1. Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS ONE 10(10): e0138930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598144/
2. 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