If you’ve shopped for a grow light recently, you’ve seen the acronyms: PPF, PPFD, DLI. They’re not interchangeable. They measure different things, and confusing them is the single most common way buyers overpay for a light that doesn’t fit their space.
This guide breaks down what each one actually means, when each one matters, and how to use them when comparing fixtures.
Why “Lumens” and “Watts” Don’t Work for Plants
Lumens measure light as the human eye perceives it. Plants don’t see light the way humans do — they’re driven primarily by photons in the 400-700 nm range (Photosynthetically Active Radiation, or PAR). Lumens overweight green and yellow wavelengths your eye is most sensitive to, and underweight the red and blue wavelengths that drive most plant growth.
Watts measure electrical power consumption — the input side, not the light output. Two fixtures pulling 600 W from the wall can output dramatically different amounts of usable photosynthetic light depending on diode quality, driver efficiency, and thermal management. Watts tell you what the light costs to run, not what it produces.
The horticultural lighting industry uses three plant-specific units instead.
PPF — Photosynthetic Photon Flux
Unit: μmol/s (micromoles of photons per second)
What it measures: The total photosynthetically active photons emitted by the fixture per second.
PPF is the single most important fixture-level specification. It’s the total output of the light, summed across the full 400-700 nm PAR range. A fixture rated at 1,500 μmol/s emits 1,500 micromoles of usable photons every second, regardless of how that light is distributed across the footprint.
Think of PPF like the total water output from a sprinkler. It tells you the total flow rate but says nothing about how evenly the water is distributed.
How it’s measured: Inside an integrating sphere — a calibrated reflective chamber that captures every photon emitted from the fixture and counts them. This is the same method used to measure lumens, but with a quantum sensor that responds equally to all PAR wavelengths (rather than mimicking human eye response).
What to watch for:
- “Diode-level” vs. “fixture-level” PPF. Some manufacturers quote PPF based on the sum of diode datasheet values rather than measured fixture output. The fixture PPF is always lower because of optical losses, driver inefficiency, and thermal derating. Always insist on fixture-level PPF.
- PPF/J vs. PPF. Efficacy (PPF per joule of input energy) is reported separately as μmol/J. Don’t confuse a 2.7 μmol/J efficacy figure with a 2.7 μmol/s output figure — they’re completely different.
PPFD — Photosynthetic Photon Flux Density
Unit: μmol/m²/s (micromoles of photons per square meter per second)
What it measures: The intensity of PAR light landing on a specific square meter at a specific point in the canopy.
If PPF is the total water output of the sprinkler, PPFD is how wet a specific patch of grass actually gets. PPFD varies across the footprint — directly under the fixture it’s high, and at the edges of the coverage area it’s lower.
PPFD is what the plant actually receives. The closer the fixture, the higher the PPFD under it (and the more uneven the distribution). The farther away, the more uniform but lower the PPFD.
Target PPFD ranges (general guidance — varies by crop):
- Seedlings and clones: 100-300 μmol/m²/s
- Vegetative stage: 300-600 μmol/m²/s
- Flowering / fruiting (light-loving crops like cannabis or tomatoes): 600-1,000 μmol/m²/s
- High-intensity flower (with CO₂ supplementation): 1,000-1,500 μmol/m²/s
PPFD above approximately 1,500 μmol/m²/s under ambient CO₂ (~400 ppm) generally hits diminishing returns or causes light stress in most crops — additional photons can’t be photosynthesized without more CO₂.
How it’s reported: Manufacturers typically publish a PPFD map — a grid showing PPFD at multiple points across the coverage area, at a stated mounting height. The most useful maps show an average PPFD and the min/max range so you can see how uniform the distribution actually is. A PPFD map that shows only the center value is hiding edge fall-off.
How AGL verifies PPFD: We don’t rely on the manufacturer’s claimed PPFD figures. We use the manufacturer’s IES file to compute PPFD ourselves at any mounting height — based on the actual candela distribution the fixture emits. More on IES files here.
DLI — Daily Light Integral
Unit: mol/m²/day (moles of photons per square meter per day)
What it measures: The total PAR light delivered to a specific spot over a 24-hour period.
DLI is what the plant accumulates day after day. It’s the metric that ultimately determines biomass and yield — within the photosynthetic range, more total daily light (up to a crop-specific saturation point) produces more growth.
The formula:
DLI = PPFD × photoperiod_hours × 3,600 ÷ 1,000,000
So at 600 μmol/m²/s with an 18-hour photoperiod:
DLI = 600 × 18 × 3,600 ÷ 1,000,000 = 38.9 mol/m²/day
Target DLI ranges (general — varies by crop):
- Leafy greens, herbs: 12-20 mol/m²/day
- Strawberries, tomatoes (greenhouse): 20-30 mol/m²/day
- Cannabis veg: 30-40 mol/m²/day
- Cannabis flower: 40-65 mol/m²/day (commercial setups)
How They Connect
The relationship between the three is straightforward once you know the math:
- PPF is fixture output — the total photon production.
- PPFD is what hits a specific point — depends on PPF, fixture distribution, distance from canopy, and footprint size.
- DLI is daily accumulation — PPFD × hours of light.
You can have a high-PPF fixture that produces poor PPFD if it’s mounted too high or has uneven distribution. You can have an excellent DLI from a modest fixture if the photoperiod is long enough. None of the three metrics alone is enough to evaluate a grow light — you need all three in context.
How to Use These When Comparing Lights
When you’re comparing two fixtures, the apples-to-apples comparison looks like this:
- Start with PPF. Total fixture output. More PPF = more total photons available.
- Check PPF/W (efficacy). Higher = more photons per dollar of electricity. Above 2.7 μmol/J is considered good in 2026; above 3.0 μmol/J is excellent. More on efficacy here.
- Look at the PPFD map. Is the distribution uniform? Average and edge PPFD matter more than peak center PPFD.
- Calculate the DLI you’ll achieve at your target PPFD and photoperiod. Make sure it hits the range for your crop.
- Verify coverage area. The PPFD map should match the fixture’s claimed footprint at the claimed mounting height.
Bottom Line
Lumens and watts are wrong tools for choosing a grow light. PPF tells you the fixture’s total output, PPFD tells you what reaches the plant, and DLI tells you the daily dose. Use all three together, never one in isolation, and always check that the manufacturer’s numbers are backed by an IES file or independent lab data.
When in doubt, look for the Verified or Third-Party Lab Verified badge on a listing in our directory — those numbers have been audited.
Sources and further reading:
- DesignLights Consortium (DLC) Horticultural Technical Requirements
- ASABE EP642 — Photometric Test Methods for Horticultural Lighting
- Bruce Bugbee, Utah State University Crop Physiology Lab — published research on PPFD and plant response