GrowShop editorial guide
What are grow LEDs, PPFD, and DLI?
Watts describe electricity use, not the light a plant receives. Distribution, distance, photoperiod, and growing area must be considered together.
Core terms
| Term | What it describes | Sufficient on its own? |
|---|---|---|
| Watt | Fixture electricity use | No |
| PPFD | Photosynthetic photon density reaching a surface each second | Needs a map and test conditions |
| Photoperiod | Daily time the light is on | Assess with plant type and growth stage |
| DLI | Accumulated photosynthetic light over a day | Requires PPFD and duration |
When comparing products
One centre-point PPFD value does not describe distribution across the whole growing area.
- Is measurement height and coverage stated?
- Was the PPFD map made under comparable conditions?
- Are heat management and safety documents provided?
- Are hanging distance and photoperiod recommendations separated by plant type?
- Are warranty and driver details written down?
Distance and uniformity
Moving a fixture closer can increase intensity at some points while changing coverage and uniformity. Treat the manufacturer's distance guidance as a starting point and check plant response with an appropriate light measurement.
Calculate daily light from a constant PPFD
DLI (mol/m²/day) = PPFD (µmol/m²/s) × lighting hours × 0.0036. At a constant 300 µmol/m²/s for 16 hours, DLI is 17.28 mol/m²/day. This is an arithmetic example, not a crop target. A single sunlight reading cannot represent a whole day; varying light must be integrated over time.
References: Apogee Instruments
Compare lighting on the same basis
- Use the same mounting height and growing area when comparing PPFD maps.
- Check edge readings as well as the centre; a single peak value does not describe uniformity.
- Ask for the measured power draw, test conditions and manufacturer warranty.
- Separate a calculated light total from evidence that a particular crop needs it.
References: University of Minnesota Extension
DLI calculator
For constant artificial light. Enter the PPFD at canopy level and daily lighting hours.
Formula: PPFD × hours × 0.0036. This estimates light exposure, not a crop recommendation. For changing sunlight, use measurements integrated over time.
Enter both values to calculate.
Sources
Technical explanations on this page are independent summaries based on the institutional sources below.
Editorial method
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