# Spectrum

> 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.

- Canonical: https://genetixbay.com/knowledge/glossary/spectrum.html
- Also written: light spectrum, spectral distribution, SPD, spectral power distribution, light colour, light color
- 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's spectrum (spectral power distribution) is how its output is spread across wavelengths, usually shown as a graph from about 300 to 800 nm. Plants read spectrum through photoreceptors: phytochromes respond to red and [far-red](https://genetixbay.com/knowledge/glossary/far-red.md), cryptochromes and phototropins to blue and [UV-A](https://genetixbay.com/knowledge/glossary/uv-a.md), UVR8 to [UV-B](https://genetixbay.com/knowledge/glossary/uv-b.md). Spectrum steers shape, [stretch](https://genetixbay.com/knowledge/glossary/stretch.md), leaf thickness and timing; the total number of photons does most of the work on yield.

## In practice

Growers talk about "veg spectrum" (more blue, compact plants) and "bloom spectrum" (more red). With modern white [LEDs](https://genetixbay.com/knowledge/glossary/led.md), the differences between fixtures are smaller than the marketing suggests. Westmoreland and colleagues (2021) found that lowering the blue fraction modestly raised cannabis yield, but that [photon efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) mattered more for cost. *Established*.

Practical rules: a broad white spectrum with extra red is a sound default for both stages. Low [red to far-red ratio](https://genetixbay.com/knowledge/glossary/red-far-red-ratio.md) promotes stretch. Extra UV-B produced little visible change in controlled trials.

## Units and formula

Bands commonly cited: UV-B 280–315 nm, UV-A 315–400 nm, blue 400–500 nm, green 500–600 nm, red 600–700 nm, far-red 700–750 nm (some definitions to 800 nm).

## Don't confuse with

- **Colour temperature (K)**: a single number describing how warm or cool white light looks; two lamps with the same kelvin can have very different spectra.
- **Intensity (PPFD)**: how much light, not which wavelengths.

## Related terms

[Full spectrum](https://genetixbay.com/knowledge/glossary/full-spectrum.md), [Blue light](https://genetixbay.com/knowledge/glossary/blue-light.md), [Far-red](https://genetixbay.com/knowledge/glossary/far-red.md), [Red:far-red ratio](https://genetixbay.com/knowledge/glossary/red-far-red-ratio.md), [PAR](https://genetixbay.com/knowledge/glossary/par.md)

## Read more

- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)

## Sources

1. 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
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