What DLI does lettuce need?

12–17 mol/m²/day. Below 10 it stretches and stays pale; above 20 it tipburns and bolts faster. The middle of the range gives best yield per electricity cost.

How do I measure PPFD without a quantum meter?

Use a smartphone PAR meter app (Photone, Lux Light Meter) for ±15% accuracy — enough to catch a 50% underdose. Borrow a real Apogee MQ-500 for ±5% accuracy if precision matters. Lumens and lux are not PAR — don't convert.

What's the relationship between watts and PPFD?

Not direct. Watts × efficacy (µmol/J) = total PPF (µmol/s). PPF divided by lit area = PPFD. A 100W lamp at 2.7 µmol/J over 0.4 m² delivers ~675 PPFD before losses. Same wattage at 1.2 µmol/J delivers 300 PPFD.

How do I increase DLI without buying a bigger lamp?

Lower the lamp (PPFD scales inversely with distance²) or extend the photoperiod. Both come with limits — too close burns plants; too long triggers bolting in lettuce and other long-day-sensitive crops.

Is more light always better?

No. Plants saturate. Lettuce above DLI 20 doesn't grow faster — it tipburns. Tomato above DLI 40 sees diminishing returns and increased water demand. Match DLI to crop, not lamp capacity.

DOC №126SEC: LIGHTINGREV: 2026-05-19AUTHORED

PPFD, DLI, and PAR — Hydroponic Lighting Explained

PPFD is light intensity, DLI is daily total, PAR is the wavelength range plants use. This guide turns confusing acronyms into actionable numbers for every crop.

BY ROOTLESS FARM

Quick answer

Three lighting terms that decide whether your hydroponic grow succeeds:

PPFD (Photosynthetic Photon Flux Density, µmol/m²/s) — how many usable photons hit a square meter per second. The "intensity" you're feeding the plant at any given moment.
DLI (Daily Light Integral, mol/m²/day) — total photons per square meter over the full daily photoperiod. The "dose" the plant receives per day.
PAR (Photosynthetically Active Radiation, 400–700 nm) — the wavelength range plants actually use for photosynthesis. Most grow light marketing is about PAR coverage.

The formula linking them: DLI = PPFD × photoperiod hours × 3600 ÷ 1,000,000.

Example: 300 PPFD × 16h × 3600 ÷ 1,000,000 = 17.3 DLI — perfect for lettuce.

Why these acronyms matter

Three reasons:

Watts mislead. A 100W LED at 2.7 µmol/J delivers 270 µmol/s. A 100W LED at 1.2 µmol/J delivers 120 µmol/s. Same wattage, more than 2× the photosynthesis. Watts alone tell you nothing about plant growth.
Lumens are wrong unit. Lumens measure human-eye response. Plants use a different spectrum (more blue + red, less green). A 5000 lumen warm-white shop LED can match a 5000 lumen "grow LED" for human eyes and lose by 40% for plants.
Distance squares. PPFD drops with the square of distance. A lamp at 24" delivers ~4× the PPFD of the same lamp at 48". Position matters as much as wattage.

DLI targets by crop

Crop	DLI (mol/m²/day)	PPFD at 14h photoperiod
Microgreens	9–14	180–280
Lettuce, leafy greens	12–17	240–340
Basil, herbs	18–22	360–440
Strawberry	17–22	340–440
Cucumber	22–30	440–600
Tomato (veg)	22–30	440–600
Tomato (fruit)	30–40	600–800 (at 14h)
Pepper	25–35	500–700

These ranges assume a balanced spectrum (modern white-LED grow light, ~2.5 µmol/J efficacy). For old HPS or low-efficacy LEDs, plan for the upper end of each range.

How to measure PPFD

With a quantum sensor (best)

A real PAR meter (Apogee MQ-500, MQ-200, ~$300–500) measures PPFD directly with ±5% accuracy. The gold standard for serious growers.

With a smartphone app (acceptable)

Apps like Photone (iOS) and Lux Light Meter Pro use the phone's camera as a rough PAR detector. Accuracy ±15% if the app is calibrated for your specific LED type — enough to catch large errors.

With lumens (wrong but possible)

If you only have a lumens reading, convert to approximate PPFD:

White LED (5000K): lumens × 0.015 ≈ PPFD.
Warm white LED (3000K): lumens × 0.013 ≈ PPFD.
HPS: lumens × 0.012 ≈ PPFD.

These are rough — within ±30%. Real measurement beats conversion every time.

How to measure DLI

DLI is calculated, not measured directly. Two paths:

From PPFD + photoperiod (the formula)

DLI = PPFD × hours × 3600 ÷ 1,000,000

For 300 PPFD running 16 hours:

300 × 16 × 3600 = 17,280,000
17,280,000 ÷ 1,000,000 = 17.3 DLI

With a DLI meter (data logger)

A datalogger like the Apogee SQ-110 measures PPFD continuously over 24 hours and outputs the integrated DLI directly. Useful for variable-light setups (windowsill + grow light combinations).

For a quick calculation use the DLI calculator.

How PPFD varies across the canopy

A single grow lamp doesn't produce uniform PPFD across the canopy. Center is brightest; edges are dimmer.

Practical implications:

Center plants get more DLI than edge plants. If your canopy is uneven (some plants 50%+ brighter than others), expect uneven growth.
Rotate plants weekly in multi-plant setups for even exposure.
Buy lamps with published PAR maps. Reputable brands publish PPFD heatmaps at multiple hanging heights. No heatmap = no data = guesswork.

DLI through the day (uneven sun and the natural reference)

Outdoor full sun delivers PPFD ~2000 µmol/m²/s at midday — far higher than any indoor lamp. But the daily average is much lower because of dawn, dusk, and clouds.

Typical outdoor DLI for comparison:

Tropical, summer: 50+ mol/m²/day.
Mid-latitude, summer: 30–45 mol/m²/day.
Mid-latitude, winter: 5–15 mol/m²/day.

Indoor lettuce production at DLI 14 actually beats most winter sunlight conditions — and matches early summer outdoor lettuce growing. This is why hydroponic lettuce is grown indoors year-round even in sunny climates.

Common mistakes

Trusting "equivalent wattage" labels. A "1000W LED" that pulls 200W from the wall is a 200W lamp, period. Use a Kill-A-Watt meter to verify draw.
Underestimating distance effect. Raising a lamp from 24" to 36" drops PPFD by 56% (not 33%). PPFD scales with distance squared.
Running too-long photoperiods. 24-hour lighting on lettuce wastes electricity and accelerates bolting. 14h is the right balance.
Photoperiod creep from timer drift. Daylight savings shifts and timer battery failures cause schedules to drift. Verify monthly.
Confusing lumens with PAR. A cheap 6500K bedroom LED has lumens but poor PAR efficacy. Buy lamps labeled with PPF and µmol/J, not lumens.

Calculate your DLI

PPFD (μmol/m²/s)Photoperiod (h)

DLI

20.2 mol/m²/day

// RECENT RUNS (LOCAL ONLY)

// NO SAVED RUNS YET.

FAQ

5 entries

Q01What DLI does lettuce need?: 12–17 mol/m²/day. Below 10 it stretches and stays pale; above 20 it tipburns and bolts faster. The middle of the range gives best yield per electricity cost.
Q02How do I measure PPFD without a quantum meter?: Use a smartphone PAR meter app (Photone, Lux Light Meter) for ±15% accuracy — enough to catch a 50% underdose. Borrow a real Apogee MQ-500 for ±5% accuracy if precision matters. Lumens and lux are not PAR — don't convert.
Q03What's the relationship between watts and PPFD?: Not direct. Watts × efficacy (µmol/J) = total PPF (µmol/s). PPF divided by lit area = PPFD. A 100W lamp at 2.7 µmol/J over 0.4 m² delivers ~675 PPFD before losses. Same wattage at 1.2 µmol/J delivers 300 PPFD.
Q04How do I increase DLI without buying a bigger lamp?: Lower the lamp (PPFD scales inversely with distance²) or extend the photoperiod. Both come with limits — too close burns plants; too long triggers bolting in lettuce and other long-day-sensitive crops.
Q05Is more light always better?: No. Plants saturate. Lettuce above DLI 20 doesn't grow faster — it tipburns. Tomato above DLI 40 sees diminishing returns and increased water demand. Match DLI to crop, not lamp capacity.