Growers often ask for the single best PAR number for cannabis. There is no one number that runs every tent, because the right intensity depends on growth stage, the strain, the temperature, the CO2 level and how evenly the light spreads. What you can do is understand the three terms that matter, PAR, PPFD and DLI, measure them properly at the canopy, and read the research honestly. This guide does that, with help from Purdue University Extension and Utah State University.

PAR, PPFD and DLI in plain words

PAR stands for photosynthetically active radiation. It is the band of light, roughly 400 to 700 nanometers (a standard unit for light wavelength), that plants use for photosynthesis. When a lamp is marketed as “PAR light,” that is a description of the waveband, not a measurement.

PPFD is what you actually meter. Purdue Extension explains that “most horticultural researchers measure instantaneous light in micromoles (μmol) per square meter (m-2) per second (s-1)” of PAR. It notes this quantum unit counts the photons used in photosynthesis “that fall on a square meter (10.8 square feet) every second.”

DLI is the daily total. Purdue Extension and Utah State both use the daily light integral, which, in the words of the Utah State study, “totals all the light a plant receives over a day.” DLI is reported in moles per square meter per day.

Why foot-candles and lux mislead

Many cheap meters read foot-candles or lux. Purdue Extension warns that these are “photometric” units “based on the amount of visible light detected by the human eye (primarily green light),” and concludes that “foot-candles are focused on people and not appropriate for indicating plant photosynthesis.” A phone app that reads lux can tell you whether a spot is brighter or darker than another spot, but it cannot measure PPFD.

What the research found about high light

A 2023 Utah State University graduate thesis grew medical cannabis in a controlled environment under light levels above summer sunlight. They note that at Logan, Utah, “during the peak of summer, the sun’s light intensity can reach about 2000 μmol·m-2·s-1 at solar noon” and that “the peak DLI during summer is around 60 mol·m-2·d-1.”

Their finding: as light intensity increased, biomass, flower yield and cannabinoid content per unit area increased. They also saw tradeoffs. With more light, they “observed a reduction in height, higher flower compactness, and density in the cultivar ‘Fun-dip’, a decrease in CBD concentration in the cultivar ‘Jack’, and increased resource allocation to stems in the cultivar ‘Jack’.”

Two things follow. First, in these trials, plants under light above summer sunlight produced more biomass and flower per area. Home results will vary. Second, more light did not help every measure in every cultivar. A home grower also pays for every photon, so the useful question is how much light your plants, your room and your power bill can support, not what the highest possible number is.

How DLI ties PPFD to your light schedule

DLI connects intensity to hours. The math is fairly simple:

DLI = PPFD × hours of light × 3,600 ÷ 1,000,000

For example, a canopy reading of 600 μmol/m²/s for 12 hours gives 600 × 12 × 3,600 ÷ 1,000,000, or about 25.9 mol/m²/day. The same 600 for 18 hours gives about 38.9. That is why a photoperiod plant in flower on 12 hours needs a higher PPFD to reach the same daily total as an autoflower running 18 hours. Our guide on DLI for autoflowers covers that in more depth.

How to measure PPFD: step by step

1. Use a quantum (PAR) sensor. Purdue Extension describes “a light quantum sensor” that “measures instantaneous light intensity (preferably in µmol.m-2·s-1).” That is the right tool for PPFD.
2. Measure at the top of the canopy. Hold the sensor where the top leaves are, not at the pot rim or on the floor.
3. Keep the sensor level and clean. Purdue says “it is important to keep all light sensors level and clean to assure accurate readings.”
4. Take a grid of readings. Measure the center, the corners and the edges. A single center reading often hides weak corners.
5. Average the grid readings. Write down every reading, then take the average and note the lowest value.
6. Work out DLI. Multiply your average PPFD by your light hours, then by 3,600, and divide by 1,000,000.
7. Change one thing at a time. Raise or lower the light, or dim it, then measure again. Give the plants several days before you judge the change.
8. Log your readings. Purdue notes a sensor on a data logger can record “at some defined interval (such as once every 15 to 60 seconds).” Even a notebook of weekly readings helps you compare runs.

Matching light to the rest of the room

Light is one input among several. If you raise intensity, the plants often use more water and nutrients, and the canopy can run warmer. In the Utah State thesis, yield rose as light increased. Our own advice, not the study’s: plants under stronger light also need enough water, nutrients and airflow, and very high light without a matched room can stress plants. Raise light gradually, watch leaf temperature and watering, and keep airflow steady.

Troubleshooting light intensity

Leaves near the top are bleaching or turning pale yellow. That pattern at the very top, close to the lamp, often points to too much light or heat at that spot. Raise or dim the light and re-measure the top of the canopy.

Plants are stretching with long gaps between nodes. Stretch can come from genetics, but low light makes it worse. Check PPFD at the canopy before blaming the strain.

Corners lag behind the center. Your grid of readings will show it. Move plants, add reflective walls or adjust fixture spacing, then measure again.

Readings jump around. Check that the sensor is level and the cable or app is steady. Purdue stresses level, clean sensors for accurate readings.

Your meter only reads lux. Lux cannot be turned into PPFD reliably, because the conversion depends on the light’s spectrum. Use it only for side by side comparisons under the same lamp.

Seeds and light planning

Plant height and flowering type change how you use light. A short autoflower under an 18 hour schedule and a tall photoperiod strain under 12 hours often need different setups, depending on PPFD, DLI, genetics and the room. Plan your canopy before you buy seeds, and see our guide on fixing bent and broken stems if heavy colas start to fold under strong light.

FAQ

What is the best PAR for cannabis?

There is no single best number. The right PPFD depends on growth stage, strain, temperature and CO2. Measure PPFD at the canopy and track DLI rather than chasing one figure.

What is the difference between PAR and PPFD?

PAR is the 400 to 700 nanometer light band plants use. PPFD is the measurement, in micromoles per square meter per second, of PAR photons reaching a surface.

Can I measure PPFD with a lux meter or phone app?

Not accurately. Purdue Extension says foot-candles and lux are based on what the human eye sees and are not appropriate for indicating plant photosynthesis. Use a quantum sensor.

Does more light always mean more yield?

Not always. A Utah State graduate thesis found yield and cannabinoid content per area rose with light in a controlled trial, but it also saw tradeoffs such as lower CBD concentration in one cultivar.

How do I calculate DLI?

Multiply PPFD by the hours of light, then by 3,600, and divide by 1,000,000. The result is DLI in moles per square meter per day.

Sources

• Purdue University Extension, HO-238 Measuring Daily Light Integral in a Greenhouse (https://ag.purdue.edu/department/hla/extension/extension-publications-library/ext-pubs/ho-238-w.html)
• Utah State University graduate thesis (2023), Responses of Medical Cannabis to Daily Light Integrals Higher Than Summer Sunlight, USU DigitalCommons (https://digitalcommons.usu.edu/etd2023/379/)