PPFD is the metric that tells you how much usable light is actually reaching your plants. Understanding it removes the guesswork from light placement, helps you choose the right fixture for your space, and explains why two lights with identical wattage can produce very different results.
This guide covers what PPFD measures, the target ranges for different crops and growth stages, and how to apply that information in a real Australian grow room.
What PPFD Measures
PPFD stands for Photosynthetic Photon Flux Density. It measures the number of photons in the photosynthetically active range (400 to 700 nanometres) that land on a given surface area per second. The unit is micromoles per square metre per second, written as µmol/m²/s.
In practical terms, PPFD tells you the light intensity at a specific point in your canopy. A reading of 600 µmol/m²/s at canopy level means 600 micromoles of photosynthetically active photons are hitting each square metre of that surface every second.
PPFD is measured at a specific point and height. The same fixture will produce different PPFD readings at different distances from the canopy, at different positions across the coverage area, and at different heights within the tent. This is why PPFD maps showing the distribution of light across an entire coverage area are more useful than a single peak number.
PPF, PPFD and Efficacy: Why the Difference Matters
PPF (Photosynthetic Photon Flux) measures the total photon output of a fixture in all directions. PPFD measures what actually reaches the plant canopy in a specific footprint. Efficacy measures how efficiently the fixture converts electricity into those photons, expressed in µmol/J.
When comparing grow lights, efficacy and PPFD distribution are the numbers that matter. A fixture with high PPF but poor optics delivers a lot of photons toward the tent walls rather than the canopy. A fixture with high efficacy and well-designed optics delivers more usable light per watt to the plants.
The SANlight EVO Series is built around secondary optic technology specifically to address this. Asymmetric beam angles concentrate photon delivery across the target footprint rather than toward the tent walls, which is why the EVO produces high canopy PPFD at lower wattage than many competing fixtures.
PPFD Targets by Growth Stage
These targets represent the ranges at which most crops reach their light saturation point. Above the saturation point, additional light does not increase photosynthesis and can cause photoinhibition in some crops.
**Propagation and seedlings:** 100 to 300 µmol/m²/s
Young plants have not developed the leaf structure and pigment density to handle high light intensity. Starting low and increasing gradually as plants develop reduces stress and supports healthy early root development.
**Vegetative growth:** 400 to 600 µmol/m²/s
Most crops in active vegetative growth respond well in this range. Leafy greens and herbs are often at their light saturation point at the lower end of this range. Fast-growing fruiting crops benefit from the upper end.
**Flowering crops with CO2 supplementation:** 600 to 900 µmol/m²/s
With CO2 raised above ambient levels (typically 1000 to 1500 ppm), the light saturation point of many crops increases and they can utilise higher PPFD productively. Without CO2 supplementation, pushing above 800 µmol/m²/s provides diminishing returns for most hobby growers.
**High-demand flowering crops without CO2:** 600 to 800 µmol/m²/s
Cannabis, tomatoes, cucumbers, and similar high-demand crops in flower can utilise light effectively up to around 800 µmol/m²/s under standard atmospheric CO2 levels. Beyond this, gains become marginal and heat management becomes more demanding.
**Leafy greens and herbs:** 200 to 400 µmol/m²/s
Lettuce, basil, spinach, and most leafy greens are light-saturated at relatively low PPFD. Running these crops under high-intensity flowering lights wastes electricity and can cause tip burn and bleaching. Purpose-built propagation and leafy green fixtures, or a SANlight FLEX II, deliver appropriate intensity without overshooting.
DLI: The Number That Ties It All Together
DLI stands for Daily Light Integral. It measures the total number of photons a plant receives over a full day and is calculated by multiplying average PPFD by the number of light hours.
A plant receiving 600 µmol/m²/s for 18 hours per day receives the same DLI as a plant receiving 900 µmol/m²/s for 12 hours per day. Both approaches can produce the same total light dose, though the intensity and photoperiod interact with plant physiology in ways that differ by species.
DLI targets for common crops:
Leafy greens: 12 to 17 mol/m²/day
Vegetative cannabis and tomato: 20 to 30 mol/m²/day
Flowering cannabis: 35 to 50 mol/m²/day
Fruiting tomatoes and cucumbers: 25 to 35 mol/m²/day
Understanding DLI helps you decide whether to increase light intensity or extend photoperiod when plants are underlit, and whether to shorten the light period rather than raise the fixture when intensity is too high.
How to Measure PPFD in Your Grow Room
The accurate way to measure PPFD is with a dedicated quantum flux meter, also called a PAR meter. These measure photons in the photosynthetically active range and give you actual canopy readings rather than manufacturer claims.
For most hobby growers, manufacturer PPFD maps produced by reputable brands are a reliable starting point. SANlight publishes independently verified photometric data for the EVO Series across multiple hanging heights and footprint sizes, which gives you accurate expectations before you purchase.
What matters most is not the peak PPFD at the centre of the footprint but the average PPFD across the full growing area, and the uniformity ratio showing how much the intensity drops from the centre to the edges. A fixture with 900 µmol/m²/s at the centre and 300 µmol/m²/s at the edges produces very uneven plant development. A fixture with 700 µmol/m²/s at the centre and 550 µmol/m²/s at the edges produces consistent results across the full canopy.
Matching Fixtures to Tent Size
The most common mistake in light selection is choosing a fixture by wattage rather than by target PPFD across the specific footprint.
For an 80x80cm tent targeting 600 to 800 µmol/m²/s in flower, the SANlight EVO 3-80 is purpose-engineered for that exact footprint. Running a larger fixture in a smaller tent does not increase yield proportionally and creates heat and intensity management problems.
For a 120x120cm tent in flower, the SANlight EVO 4-120 or EVO 5-120 delivers the coverage footprint and photon distribution that the space requires. Running a single smaller fixture in a larger tent leaves the edges of the canopy underlit regardless of how much you raise or lower it.
Match the fixture to the footprint first. Then consider wattage and intensity as secondary factors.
PPFD and Australian Summer
One practical consideration for Australian growers is that high PPFD targets during summer heatwaves compound heat management challenges. Some growers reduce light intensity by raising fixtures or dimming during the hottest weeks rather than fighting the resulting heat with additional cooling.
This is a valid approach. A plant receiving slightly lower PPFD in a stable 24 degree environment will generally outperform a plant receiving peak PPFD in a 32 degree environment. Temperature stress reduces the plant's ability to utilise high light intensity. When you cannot fully control temperature, reducing the light demand on your climate management system is the pragmatic choice.
For the full range of grow lights available for Australian indoor growers, browse the Grow Lights collection.