# Emerson effect

> The Emerson effect is the jump in photosynthesis when far-red light (above 680 nm) is combined with shorter red light, exceeding the sum of each alone.

- Canonical: https://genetixbay.com/knowledge/glossary/emerson-effect.html
- Also written: Emerson enhancement effect, Emerson enhancement, red drop, Emerson red drop
- 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"

The Emerson effect, described by Robert Emerson in the 1950s, has two parts. The "red drop" is the sharp fall in [photosynthetic](https://genetixbay.com/knowledge/glossary/photosynthesis.md) efficiency for light beyond about 680 nm on its own. The "enhancement effect" is the finding that adding shorter-wavelength light restores and boosts that efficiency, so the combined rate exceeds the sum of the two lights separately. It was key evidence that photosynthesis runs two photosystems in series: photosystem I absorbs [far-red](https://genetixbay.com/knowledge/glossary/far-red.md) well, photosystem II does not.

## In practice

For growers, the Emerson effect is the reason far-red photons are not wasted when a fixture also delivers plenty of 400–700 nm light. Zhen and Bugbee (2020) measured far-red photons up to about 750 nm contributing to canopy photosynthesis at near-equal efficiency when mixed with [PAR](https://genetixbay.com/knowledge/glossary/par.md). This supports counting ePAR (400–750 nm) for whole-canopy work. *Established* in model crops; cannabis-specific canopy measurements remain few.

It does not mean you can replace PAR with far-red. A far-red-only source gives very little photosynthesis and a lot of [stretch](https://genetixbay.com/knowledge/glossary/stretch.md). Typical fixtures with far-red keep it to a minority of total photons, often under 10–15%.

## Don't confuse with

- **Red:far-red ratio**: a developmental signal via [phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md), separate from the photosynthetic enhancement.
- **Far-red**: the waveband; the Emerson effect is how it interacts with other light.

## Related terms

[Far-red](https://genetixbay.com/knowledge/glossary/far-red.md), [PAR](https://genetixbay.com/knowledge/glossary/par.md), [Red:far-red ratio](https://genetixbay.com/knowledge/glossary/red-far-red-ratio.md), [Photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md)

## Read more

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

## Sources

1. Zhen S, Bugbee B, 2020. Far-red photons have equivalent efficiency to traditional photosynthetic photons: implications for redefining photosynthetically active radiation. Plant, Cell & Environment 43(5):1259–1272. https://doi.org/10.1111/pce.13730