Indoor horticultural lighting guide: PAR, PPFD, DLI & fixture sizing
Designing an effective indoor lighting system requires understanding photobiology rather than human-eye lumens or basic electrical wattage. Plants do not see light as brightness; their photosynthetic pigments absorb individual photons within the Photosynthetically Active Radiation (PAR) waveband (400 to 700 nm, extending to 750 nm with far-red). Sizing fixtures accurately prevents stunted vegetative growth from photon starvation and leaf chlorosis from light bleaching.
Understanding Core Horticultural Metrics
- PPF (Photosynthetic Photon Flux — μmol/s)
- The total number of PAR photons emitted by a grow light fixture every second. This is the true optical horsepower of the light, independent of distance or canopy area.
- PPFD (Photosynthetic Photon Flux Density — μmol/m²/s)
- The instantaneous density of photons landing on a 1-square-meter area of plant canopy per second. PPFD varies with hanging height and reflector beam angle.
- DLI (Daily Light Integral — mol/m²/day)
- The total cumulative "dose" of light energy received by the plant canopy over an entire 24-hour photoperiod.
The Daily Light Integral (DLI) Equation
DLI (mol/m²/day) = PPFD (μmol/m²/s) × Photoperiod (Hours/Day) × 3,600 sec ÷ 1,000,000
Simplified: DLI = PPFD × Photoperiod (Hours) × 0.0036
Example: Lettuce under 250 μmol/m²/s for 18 hours/day receives: 250 × 18 × 0.0036 = 16.2 mol/m²/day DLI.
Crop Lighting & DLI Target Requirements
| Crop / Growth Phase | Target PPFD (μmol/m²/s) | Daily Photoperiod | Target DLI (mol/m²/day) |
|---|---|---|---|
| Microgreens & Seedlings | 100 – 200 | 16 – 18 hours | 6 – 12 mol/m²/day |
| Butterhead & Romaine Lettuce | 200 – 300 | 16 – 18 hours | 14 – 18 mol/m²/day |
| Vegetative Herbs & Brassicas | 350 – 500 | 18 hours | 22 – 32 mol/m²/day |
| Fruiting Tomatoes & Peppers (Ambient CO₂) | 600 – 900 | 12 – 14 hours | 30 – 45 mol/m²/day |
| High-Intensity Flowering (Enriched 1200ppm CO₂) | 1,000 – 1,500 | 12 hours | 45 – 65 mol/m²/day |
Optical Beam Footprint & Inverse-Square Geometry
Hanging height drastically impacts both coverage footprint diameter and center-to-edge light uniformity. As fixture height doubles, photon intensity drops according to the inverse-square law, while illuminated surface area quadruples:
- 120° Wide-Angle LED Bars: Maintain a 12 to 18-inch hanging height above the canopy for superior multi-fixture overlap and uniform PPFD distribution without localized hot spots.
- 90° Concentrated Optical Lenses / COBs: Require a 24 to 36-inch hanging height to prevent photon bleaching directly below the center diode array.
Three Worked Lighting Calculations
1. Commercial 4x8 ft (32 sq ft / 2.97 m²) Indoor Lettuce Bench
Target: 250 μmol/m²/s PPFD. Total required photons = 250 × 2.97 = 742.5 μmol/s PPF. Using high-efficiency LED bars (2.6 μmol/Joule): 742.5 ÷ 2.6 = 285 Watts total power required. Two 150W 4-foot LED strip fixtures hung 14" high provide perfectly uniform canopy illumination.
2. 5x5 ft (25 sq ft / 2.32 m²) High-Intensity Fruiting Tent
Target: 800 μmol/m²/s across a 12-hour flowering cycle (34.5 mol/day DLI). Total photon requirement = 800 × 2.32 = 1,856 μmol/s PPF. Sized for a commercial 650W to 720W full-spectrum multi-bar fixture.
3. Supplemental Greenhouse Lighting in Winter
Natural winter solar DLI inside a glass greenhouse is 8 mol/day. To reach the tomato target of 26 mol/day, artificial lights must supply a 18 mol/day deficit. Running lights for 14 hours/day: required supplemental PPFD = 18 ÷ (14 × 0.0036) = 357 μmol/m²/s.
Common Grow Lighting Mistakes
- Measuring grow lights in Lumens or Lux: Lux meters measure human eye sensitivity (peaking at green 555nm light), ignoring critical red and blue photosynthetic photons. Always use a calibrated Quantum PAR sensor measuring PPFD.
- Over-driving PPFD without supplemental CO₂: At ambient CO₂ (400 ppm), photosynthesis saturates around 800 to 900 μmol/m²/s. Pushing light to 1,200+ μmol/m²/s without elevating CO₂ to 1,200 ppm causes photo-inhibition and bleached leaf tips.
- Hanging lights too close to save power: Placing a fixture 6 inches above plants creates a scorching center hotspot while leaving outer corners dark. Maintain recommended manufacturer standoff distances.
Calculate environmental balance and power expenses with our VPD Calculator, Hydroponic Cost Calculator, and Sun Exposure Calculator.