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, well above HPS. 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 in range.
Practical notes: LEDs can be hung closer and dimmed, which makes it easy to overshoot PPFD 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" is a Myth. GenetixBay's position: calcium and magnesium demand depends mainly on cultivar, water source (RO vs tap) and medium (coco buffering), not on the light type. See Cal-Mag.
Photon efficacy is how many micromoles of PAR photons a fixture produces per joule of electricity (µmol/J); higher means more light for the same power bill and heat.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.Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.Full spectrum is a marketing term for a grow light with broad output across the visible range, usually a white LED, sometimes with added red, far-red or UV.Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.LED vs HPS is the debate over whether LED or high-pressure sodium lights grow better cannabis; LEDs are clearly more efficient, while final-quality differences remain an open question.Spectrum is the mix of wavelengths a light source emits, from UV through blue, green, red and far-red; it shapes plant form, while intensity drives most yield.
