# T5 fluorescent

> T5 fluorescent lights are slim 16 mm tubes giving low-intensity, low-heat light; once the standard for clones and seedlings, now largely replaced by LED.

- Canonical: https://genetixbay.com/knowledge/glossary/t5-fluorescent.html
- Also written: T5, T5HO, T5 HO, fluorescent, fluorescent tubes, CFL, compact fluorescent
- 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"

Fluorescent lamps make light by exciting mercury vapour to emit UV, which a phosphor coating converts to visible light. "T5" means the tube is 5/8 inch (about 16 mm) in diameter. T5HO (high output) tubes are the grow-room version, typically 54 W for a 4 ft (1.2 m) tube. CFLs (compact fluorescents) are the same technology folded into a bulb.

## In practice

T5 banks were the classic clone and [seedling](https://genetixbay.com/knowledge/glossary/seedling.md) light: gentle, even, cool enough to hang 10–20 cm (4–8 in) above plants, and cheap. A 4-tube T5HO fixture gives roughly 200–400 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) close underneath, suitable for propagation and early veg but too weak for flower.

[Efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) is around 1.0–1.2 µmol/J, well below [LED](https://genetixbay.com/knowledge/glossary/led.md). Tubes lose output with age, and many growers replace them yearly. Broken tubes release mercury; clean up following local hazardous-waste guidance. Today, low-power LED bars do the same job with better efficiency and no mercury.

## Don't confuse with

- **T8 and T12**: wider, older, lower-output fluorescent tubes.
- **LED tubes**: LED replacements in a fluorescent form factor.
- **Germicidal UV-C tubes**: look similar but emit harmful UV-C; never use them on plants or people.

## Related terms

[LED](https://genetixbay.com/knowledge/glossary/led.md), [Photon efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md), [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [Cloning](https://genetixbay.com/knowledge/glossary/cloning.md)

## Read more

- [Seedling care: the first three weeks](https://genetixbay.com/knowledge/germination/seedling-care.md)
- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)