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 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 close underneathbeing reviewed, suitable for propagation and early veg but too weak for flower.
Efficacy is around 1.0–1.2 µmol/Jbeing reviewed, well below LED. 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.
