A high-pressure sodium (HPS) lamp produces light by passing an arc through sodium 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/Jbeing reviewed, well below good LEDs. 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, 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 cyclesbeing reviewed. 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, a broader-spectrum discharge lamp.
- LED: solid-state, more efficient, less radiant heat at the canopy.
Myth check
LED vs HPS for final quality is an open question at GenetixBay: an open Bay Trial. See Mythbusters.
