Every lamp manufacturer advertises with watts. "300W LED for the perfect grow." The watt rating describes how much electricity the lamp consumes — not how much light it actually delivers to the plant. These are two completely different things.
Plants care about exactly one metric: how many photons per second per square metre arrive in the photosynthetically relevant wavelength range. That is PPFD. And how many of those accumulate over the entire light day — that is DLI.
PPFD — the fundamental measure for light intensity
PPFD stands for Photosynthetic Photon Flux Density. The unit is µmol/m²/s — micromoles of photons per square metre per second. The relevant wavelength range: 400–700 nm (PAR). Infrared and UV are not counted.
PPFD target values by growth phase
| Phase | PPFD Target | Light Distance | Note |
|---|---|---|---|
| Germination / Cutting | 100–300 µmol/m²/s | 30–50 cm | Too much light inhibits root development |
| Vegetative (early) | 300–500 µmol/m²/s | 25–40 cm | Gentle build-up |
| Vegetative (strong) | 500–800 µmol/m²/s | 20–35 cm | Maximum growth rate |
| Early Flower | 600–900 µmol/m²/s | 18–30 cm | Promote bud formation |
| Flower | 800–1100 µmol/m²/s | 15–25 cm | Dense buds, resin production |
| Late Flower / Ripening | 600–900 µmol/m²/s | 20–30 cm | Reduce stress |
The inverse square law
Light intensity decreases with the square of the distance. A lamp at 20 cm delivering 900 µmol/m²/s does not deliver 450 at 40 cm — but approximately 225.
Practical consequence: measure PPFD at canopy height — not from the manufacturer's diagram. Actively adjust the distance to the growth phase. The height-adjustable mounting of the Growix Core is designed precisely for this.
DLI — the decisive daily dose
PPFD describes intensity at a single moment. DLI — Daily Light Integral — describes the cumulative light quantity over the entire light day. Unit: mol/m²/day.
Example: 800 µmol/m²/s × 18h × 3600 / 1,000,000 = 51.8 mol/m²/day
| Phase | DLI Target | Typical Light Day |
|---|---|---|
| Germination | 6–12 mol/m²/day | 18–20h |
| Vegetative | 20–35 mol/m²/day | 18–20h |
| Flower | 35–50 mol/m²/day | 12h |
| Autoflowering | 25–40 mol/m²/day | 20h |
For an interactive DLI calculator with a full PPFD × hours combination table: DLI Calculator — Daily Light Integral for Every Growth Stage.
A short light day with high intensity can deliver the same DLI as a long light day with low intensity. For autoflowering strains under 20h of light, PPFD is therefore more important than under 12/12.
Measuring PPFD correctly — quantum sensor vs. lux meter
A normal lux meter measures brightness according to the human perception curve — heavily weighted towards green. Plants preferentially use red and blue. A lux meter under a red-blue-dominant LED systematically delivers values that are too low.
For PPFD you need a quantum sensor with flat spectral sensitivity in the PAR range (400–700 nm).
| Device | Price | Accuracy | Recommendation |
|---|---|---|---|
| Apogee SQ-500 | ≈ €320 | ± 5% | Professional standard |
| LI-COR LI-190R | ≈ €500 | ± 5% | Research quality |
| Samsara Sciences | ≈ €65 | ± 5–8% | Budget option, acceptable |
| Lux meter (converted) | €10–30 | ± 20–40% | Not suitable |
The Growix Light Matrix System — 85W, 1300 µmol/m²/s peak
- Peak PPFD of 1300 µmol/m²/s at minimum distance — upper end of the flower target range
- Passive aluminium core heatsink — no fan, no noise
- Full-spectrum LED, tuned to the PAR optimum (400–700 nm)
- Light cycle control via Growix OS — second-accurate, configurable per phase
- Electricity costs: ≈ €17 / month at 85W and 18h light day