Grow Light Metrics: PPFD, DLI, and Why Watts Don't Matter

Quick Glossary

• PAR — photosynthetically active radiation (400–700 nm)
• PPFD — photosynthetic photon flux density at the leaf surface (µmol/m²/s); intensity at a specific point
• DLI — daily light integral (mol/m²/day); the cumulative PPFD over the full photoperiod

Why Watts Don't Measure Efficiency

Photon efficacy (PPE):

• Modern LED: 2.5–3.5 µmol/J
• Quality HPS: 1.7–2.0 µmol/J
• Older fluorescent: 0.8–1.2 µmol/J

Distance from the fixture — PPFD drops proportionally to the square of the distance (twice the distance = four times less light).

Compare fixtures by PPE and actual PPFD, not by wattage.

PPFD Targets by Crop

DLI: Calculation and Targets

Formula: PPFD × 3600 × hours / 1,000,000 = mol/m²/day

Uniformity

Good uniformity (80%+) is achieved by raising fixture height, using a wide beam angle, or adding multiple light sources.

Three Critical Mistakes

1. Choosing by wattage without checking PPE
2. "It looks bright" — the human eye is sensitive to green; plants respond to blue and red
3. Increasing PPFD without CO₂ enrichment — at 400 ppm, plants saturate at 600–800 µmol/m²/s; above that, photoinhibition occurs

Signs of a Correct Setup

• PPFD measured with a quantum meter matches target ranges
• DLI within the target range for the crop
• Uniformity ≥75%
• Plants develop evenly across the entire growing area