# LED

> An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.

- Canonical: https://genetixbay.com/knowledge/glossary/led.html
- Also written: LEDs, light-emitting diode, LED grow light, LED fixture, quantum board, LED bars
- 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 light-emitting diode (LED) is a semiconductor that emits light when current passes through it. Horticultural LED fixtures combine many diodes, usually phosphor-coated white chips plus deep red (around 660 nm) and sometimes far-red or UV, on boards or bars driven by a power supply. The spectrum is set by the diode mix and can be tuned freely.

## In practice

Kusuma, Pattison and Bugbee (2020) placed the best horticultural LEDs near 3 [µmol/J](https://genetixbay.com/knowledge/glossary/photon-efficacy.md), well above [HPS](https://genetixbay.com/knowledge/glossary/hps.md). More of the electricity becomes light and less becomes heat, and the heat that remains leaves through heatsinks rather than as radiant heat on the canopy. Leaves under LED tend to run cooler than under HPS at the same air temperature (Nelson and Bugbee 2015), so LED rooms are often run 1–3 °C (2–5 °F) warmer in air temperature to keep [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) in range.

Practical notes: LEDs can be hung closer and dimmed, which makes it easy to overshoot [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md) and bleach tops. Use a quantum meter, not the "equivalent watts" on the box. Diodes slowly lose output over thousands of hours; drivers are the usual failure point.

## Don't confuse with

- **Blurple**: older red and blue-only LEDs.
- **Full spectrum**: a marketing label, not a guarantee of efficacy.

## Myth check

"LEDs need more [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md)" is a *Myth*. GenetixBay's position: calcium and magnesium demand depends mainly on **cultivar**, water source ([RO](https://genetixbay.com/knowledge/glossary/ro-water.md) vs tap) and medium ([coco](https://genetixbay.com/knowledge/glossary/coco-coir.md) buffering), not on the light type. See [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md).

## Related terms

[Photon efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md), [HPS](https://genetixbay.com/knowledge/glossary/hps.md), [LED vs HPS](https://genetixbay.com/knowledge/glossary/led-vs-hps.md), [Spectrum](https://genetixbay.com/knowledge/glossary/spectrum.md), [Full spectrum](https://genetixbay.com/knowledge/glossary/full-spectrum.md)

## Read more

- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)
- [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.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
2. 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/
3. Westmoreland FM, Kusuma P, Bugbee B, 2021. Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLoS ONE 16(3): e0248988. https://doi.org/10.1371/journal.pone.0248988