micromole per square meter second
Photon flux densityexact by definition
The micromole per square metre per second is the working unit of horticultural lighting — PPFD, photosynthetic photon flux density. It counts photons between 400 and 700 nm landing on a surface, expressed as micromoles of them, and is exactly 10⁻⁶ mol/(m²·s) by prefix.
Why it is written : Older literature writes the same quantity as μE/(m²·s), for "microeinstein" — one einstein is a mole of photons. The einstein was never accepted into SI and the two units are numerically identical, so a 1980s paper quoting μE is directly comparable to a modern meter reading μmol.
Watch out: Lux and foot-candles cannot be converted to PPFD without knowing the spectrum. Lux weights light by human eye sensitivity, which peaks in the green that plants reflect, so the same 10,000 lux is roughly 130 μmol/(m²·s) under sunlight, 120 under a white LED and 145 under high-pressure sodium. Any single conversion factor is a spectrum-specific approximation.
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The unit follows from the discovery that photosynthesis is a photon-counting process: within the photosynthetically active band, one photon drives the same photochemistry regardless of its energy, so the useful measure is a count and not a power.
About the micromole per square meter second
Plants are not paid in watts. They are paid in photons, and only those between about 400 and 700 nm — the photosynthetically active band. That is why horticultural lighting is measured as a photon count rather than as power or as illuminance.
The numbers worth carrying: full summer sun at midday is about 2,000 μmol/(m²·s). An overcast winter day outdoors might be 100. A well-lit indoor grow runs 400 to 900 depending on the crop and whether CO₂ is enriched; leafy greens are content at 200 to 400, while a fruiting crop at full canopy will use 600 to 1,000. Above roughly 1,500 most crops saturate, and the extra photons become heat rather than sugar.
The measurement trap is that PPFD is a reading at a point, and light from an overhead fixture falls off sharply toward the edges of the space. A single reading directly under the lamp is the most flattering number available and tells you almost nothing about the canopy. Take a grid — nine points is the usual minimum — and work with the average and the spread, because the corners are where the disappointing plants grow.
One micromole per square meter second in every photon flux density unit
| 1 | micromole per square meter second (μmol/(m²·s))this unit | |
| 0.001 | millimole per square meter second (mmol/(m²·s)) | |
| 0.000001 | mole per square meter second (mol/(m²·s)) |