PPFD from Distance

Estimate canopy PPFD from fixture spec and hang height. Use to plan a grow space without buying a quantum sensor first.

Estimate canopy PPFD

Pick a fixture (optional — fills the spec fields below from the OAT catalog) — enter specs manually —

Fixture PPF (μmol/s) Total photosynthetic photon flux from spec sheet

Coverage area (ft²) e.g., 4×4 = 16 ft²; from fixture spec

Hang distance (in) From fixture face to canopy

Fixture type LED quantum board / bar (flat) LED bulb / single-source HPS / MH (point source + reflector) CMH (point source)

Wall reflectivity None / unreflective Mylar / Panda film (~10% lift) Tent reflective interior (~15%) White paint (~5%)

Estimated avg canopy PPFD —μmol/m²/s Across coverage area at canopy distance

DLI at 12h photoperiod —mol/m²/d Cannabis flower

DLI at 18h photoperiod —mol/m²/d Veg / lettuce

Center-vs-edge variance — Approximate uniformity

This is an estimate using a simplified model. For precision, use a quantum sensor (Apogee SQ-500 series, Bluelab MultiMedia, etc.) and measure your actual canopy PPFD at multiple points.

How this estimate works

The calculator uses a hybrid model:

1. Average PPFD over coverage area: total PPF (μmol/s) divided by coverage area (m²) at the fixture's design distance gives the rated average PPFD. This is the headline number on most fixture spec sheets.
2. Distance adjustment: hanging closer raises PPFD non-linearly (concentration effect for flat fixtures, more inverse-square for point sources). Hanging farther lowers it.
3. Reflective wall lift: enclosed spaces with reflective walls bounce additional photons back to the canopy. Mylar adds 8-12%; tent fabric 12-18%; flat white paint 4-6%.
4. Variance estimate: flat fixtures (LED quantum boards, bars) have moderate uniformity; point sources have a hot center and dim edges.

Actual delivered PPFD always varies across the canopy. Real fixtures aren't uniform — center is brighter, edges dimmer. Manufacturers publish PPFD-at-distance grids that show this; a reasonable middle-of-canopy reading from a quantum sensor is the only authoritative value.

Distance vs PPFD trade-offs

Hang distance	Effect
12-18" (close)	Highest PPFD; risk of light burn / leaf bleaching at edges; smaller effective coverage; significant heat above canopy
18-24"	Common cannabis flower hang range with high-PPE LEDs
24-36"	Common veg + finishing hang; lower PPFD, broader coverage, gentler heat
36-48"	Lettuce / herb / propagation hang; low-DLI crops
48"+	Mother plants, propagation chambers, supplemental greenhouse hang

For cannabis flower, the rule is "as close as the canopy can tolerate without bleaching." Look at the top-most leaves; if they're cupping, twisting, or looking like sunburn, lift the lights 3-4". Tunable LEDs let you dim at close hang as a less-energy-efficient alternative.

Coverage area math

Manufacturer-published PPF averages are rated for the design coverage area. At the same hang distance:

• Smaller area than rated: PPFD is higher (you're concentrating photons), but uniformity drops
• Larger area than rated: PPFD drops below the average; edges receive less than rated

For a 1000W LED rated for 5×5 ft (25 ft² ≈ 2.32 m²) at 800 PPFD, hanging it over 4×4 ft (16 ft²) at the same distance raises center PPFD significantly but creates an uncovered border. Two fixtures overlapping a 4×8 ft canopy delivers more uniform PPFD than one fixture covering the same area.

Free under CC BY 4.0. Cite as "OAT PPFD from Distance Calculator (openagriculturetechnology.com)".