Most growers buy an LED, hang it at the manufacturer's suggested height, and never touch it again. That gets decent results. This guide is about getting exceptional results — understanding the light well enough to push your plants to their photosynthetic ceiling without burning them.
PPFD: the number that actually matters
Lumens, lux, and watts are marketing numbers. PPFD (Photosynthetic Photon Flux Density, measured in µmol/m²/s) is the only number that tells you how much usable light your plants are actually receiving.
- Lumens/lux measure brightness as the human eye perceives it — weighted toward green light, which plants use least efficiently
- PPFD measures photons in the 400–700nm range (PAR), weighted equally across the spectrum — what the plant actually uses for photosynthesis
A 100W light and a 300W light can deliver the same PPFD at the canopy if the 100W has better optics and efficiency. Wattage tells you about your electric bill, not your plant's food supply.
PPFD targets by growth stage
| Stage | PPFD target (µmol/m²/s) | DLI (mol/m²/day) | Notes |
|---|---|---|---|
| Seedling | 100–300 | 5–15 | More causes light stress on fragile leaves |
| Early veg | 300–600 | 19–39 | Ramp up as the plant develops |
| Late veg | 600–900 | 39–58 | Push toward 900 only with strong genetics and good VPD |
| Flower (no CO₂) | 600–900 | 26–39 | At 12/12, DLI drops — compensate with higher PPFD |
| Flower (with CO₂) | 900–1500 | 39–65 | CO₂ above 800ppm raises the light saturation point |
The key insight: in flower, you switch from 18/6 to 12/12. That's a 33% reduction in light hours. If you keep the same PPFD, your DLI drops by a third. Either increase PPFD at the flip or accept lower total light delivery.
Reading a PPFD map
Every serious LED manufacturer publishes a PPFD map: a grid showing light intensity at different points beneath the fixture at a specific hanging height.
What to look for
Uniformity matters more than peak center PPFD. A light that hits 1200 in the center and 400 at the edges creates hotspots and shaded zones. A light that delivers 800 uniformly across the entire canopy will outperform it in actual yield.
The edge drop-off is where cheap lights fail. Budget panels use fewer, wider-angle diodes to cut costs. The center looks fine, but PPFD drops 50–70% at the edges. Premium panels (Samsung LM301B/H diodes, high-quality drivers) maintain 80%+ uniformity across the footprint.
Hanging height changes everything. PPFD maps are measured at one specific height — usually 12", 18", or 24". Moving the light 6 inches changes the distribution dramatically. Closer = higher center PPFD but worse uniformity. Further = lower peak but more even spread.