PAR, PPF and PPFD: the three numbers that matter

A fixture's box says 1,700 µmol/s. A meter held at canopy height under that same fixture reads 640 µmol·m⁻²·s⁻¹. Both numbers are correct, and only one of them tells you whether your plants are getting enough light. This page follows that number from the diode to the leaf, so you can read any spec sheet and know exactly what to check with a meter.

PAR waveband: 400–700nm; the convention this page uses
Example fixture PPF: 1,700µmol/s; what the fixture emits, factory-measured
Flower-stage canopy PPFD: 600–1,000µmol·m⁻²·s⁻¹; what the canopy should receive

The band your plant actually uses

Photosynthetically active radiation (PAR) is the slice of the spectrum, roughly 400–700 nanometres, violet through red, that drives photosynthesis. The 400–700 nm convention comes from classic quantum-yield measurements across 22 crop species, which found two broad peaks in the plants' photosynthetic response, a shorter one in the blue around 440 nm and a taller one in the red around 620 nm ^[1]. PAR on its own doesn't mention a fixture, a room, or a plant. It just names the waveband everything else in this page is measured across.

What a fixture claims: PPF

Photosynthetic photon flux (PPF) is the total number of PAR photons a fixture emits every second, in micromoles per second (µmol/s). It's measured with the fixture running inside an integrating sphere, a chamber that catches every photon leaving the fixture in every direction and counts it, whatever the beam angle or reflector shape. That test happens once, at the factory, with no room, no mounting height and no plant anywhere near it. Independent programmes such as the DesignLights Consortium's Horticultural Qualified Products List retest submitted fixtures against their own published requirements, which is the closest thing going to an outside check on a manufacturer's own PPF number ^[2].

PPF is a property of the fixture, full stop. Two growers running the identical 1,700 µmol/s fixture, one in a 0.6 × 0.6 m (2 × 2 ft) cabinet and one in a 1.5 × 1.5 m (5 × 5 ft) tent, are working with the same PPF and very different amounts of light on their plants.

What your canopy actually gets: PPFD

Photosynthetic photon flux density (PPFD) is photons landing on one square metre per second, in micromoles per square metre per second (µmol·m⁻²·s⁻¹), at one specific point and height. It's literally what a quantum sensor placed at that point reads. Move the sensor 15 cm (6 in) sideways or 10 cm (4 in) closer to the fixture and the number changes, because PPFD describes a location, not a fixture.

That's the whole relationship in one line: PPF is a single number stamped on the box; PPFD is a different number at every point on your canopy, and it's the one that decides how your plants grow.

Fig. 1PPF is everything the fixture emits. PPFD is what actually lands at one point, which is the number the sensor reads.Horus

Same PPF, two very different PPFDs

Because PPFD depends on distance and spread as well as PPF, identical fixtures can deliver very different canopy PPFD. Mount a fixture low and let its beam cover a small footprint and the same total photon output concentrates into a high PPFD over a small area. Raise it, or run a wider beam angle, and that photon output spreads over more canopy at a lower PPFD per point. Neither setup is wrong; they're different trade-offs between intensity and coverage, and a spec sheet's PPF number alone cannot tell you which one you've built.

You can estimate the ballpark before you buy a meter, then confirm it with one.

Read the fixture's PPF and its rated footprint2 min
Take a fixture rated 1,700 µmol/s PPF, a plausible spec for a bar-style LED sized for a 1.2 × 1.2 m (4 × 4 ft) tent, mounted at 45 cm (18 in) above the canopy. The manufacturer's beam chart says that at that height, its light spreads fairly evenly across roughly a 1.3 × 1.3 m (4.3 × 4.3 ft) footprint.
Divide PPF by the footprint area1 min
Footprint area is 1.3 × 1.3 = 1.69 m². Average PPFD is 1,700 ÷ 1.69, which is about 1,000 µmol·m⁻²·s⁻¹.
Treat that as an average, not a reading
This arithmetic tells you the ballpark you're working with, at the flower-stage ceiling in this example, not what any individual plant is actually receiving. Two identical fixtures with different beam angles can produce the same 1,000 µmol·m⁻²·s⁻¹ average and still leave corner plants at very different real readings. See light uniformity and edge effect for how uneven that spread usually is.
Warning No single point on a real canopy sits exactly at the average. The centre typically reads well above it, and the corners well below.
Confirm it with a quantum sensor at several points
Measure at the centre and at two or three points toward the edges, at actual canopy height, in the middle of the light period. That set of readings, not the box number and not the arithmetic above, is what you act on. See measuring PPFD with a quantum sensor for how to take a reading that isn't skewed by your own shadow or a nearby wall.

Why a lux meter or your phone will mislead you

A quantum sensor is built to weight photons across the PAR band roughly evenly, because a photon at 450 nm and a photon at 650 nm both drive photosynthesis about as effectively, photon for photon. Lux meters, and the light sensors in phone cameras, are calibrated to match human vision instead: a curve that peaks sharply around 555 nm, in the green, and drops off steeply toward the deep red and blue wavelengths plants use heavily. A fixture with a red- and blue-heavy spectrum can read as dim on a lux meter or a phone app while delivering plenty of usable PAR, and a green-heavy white fixture can read the opposite way.

Watts tell you the electricity bill, not the light

Wattage is electrical input, drawn from the socket, and it says nothing on its own about how much of that power leaves the fixture as usable PAR photons rather than heat. Take two illustrative 240 W fixtures, not real products: one rated 2.2 µmol/J emits about 528 µmol/s, another rated 2.8 µmol/J emits about 672 µmol/s, roughly 27% more usable light for the identical electricity bill. "X00W equivalent" marketing claims compare informally to an old HPS wattage class and carry no standard behind them at all. The numbers that actually describe light output are PPF, PPFD and efficacy; see watts are not light: reading fixture specs for how to read a full spec sheet, and photon efficacy and what it costs you for the running-cost arithmetic.

What PPFD to aim for, stage by stage

Fig. 2Canopy PPFD roughly quadruples from a fresh clone to a CO2-enriched flower room.Horus

The same bands as a table, for a quick lookup:

Stage	Canopy PPFD (µmol·m⁻²·s⁻¹)
Propagation / seedling	100–250
Vegetative	300–600
Flower	600–1,000
Flower with CO2 enrichment	900–1,500

These are industry-common canopy targets, not a hard biological ceiling. In one controlled indoor trial across eight PPFD levels, dry inflorescence yield rose roughly linearly from 116 to 519 g/m² as PPFD rose from 120 all the way to 1,800 µmol·m⁻²·s⁻¹, with no clear saturation point across that range, while cannabinoid potency did not differ between light levels ^[3]. That result is from one trial, specific cultivars, indoor LED, ambient CO2 — read as "the biological ceiling is higher than almost anyone runs," not as "everyone should run 1,800." Cost, heat load and canopy uniformity set the practical ceiling well below that ambient-CO2 result for nearly every home and commercial room; see when does CO2 enrichment let you push more light for where that ceiling actually moves.

PPFD tells you the instant rate of light hitting the canopy. How long you run that rate each day is a separate decision with its own number, DLI; see DLI: why daily light is the number to manage and the DLI calculator for the arithmetic that turns today's PPFD reading into a daily dose. None of this changes what your jurisdiction allows you to grow or how, which is covered in the law section.