Lighting14 min read

What Do PPFD, DLI, PAR, and Lux Mean for Grow Lights?

Understand PPFD, DLI, PAR, and lux, calculate daily light, and measure grow-light intensity at canopy height.

Truleaf Editorial

August 12, 2026

Lettuce plants growing beneath an LED array in a controlled-environment chamber

Key point: PAR describes a range of light wavelengths. PPFD measures how many photons in that range reach a square meter each second. DLI adds those photons across the day. Lux measures brightness for human vision, so it cannot be converted to PPFD with one universal formula.

These terms answer different questions about the same lighting setup. Once you keep those questions separate, grow-light labels and meter readings become much easier to use.

The four terms at a glance

Term	What it tells you	Unit	Plain-language use
PAR	The traditional 400 to 700 nanometer waveband used for plant-light measurement	No unit by itself	Defines the wavelength range being discussed
PPFD	The photon flux arriving at a surface each second within the stated PAR range	µmol m⁻² s⁻¹	Shows light intensity at the plant right now
DLI	The photon total received by a surface over one day	mol m⁻² d⁻¹	Shows the daily light dose
Lux	Illuminance weighted for human vision	lx	Shows how bright light looks to people

PAR is often printed beside a number on a fixture chart, but PAR itself is not a reading. A chart labeled “PAR map” is usually showing PPFD values measured across an area. Check the unit before interpreting the numbers.

What is PAR?

Photosynthetically active radiation (PAR) is the traditional 400 to 700 nanometer waveband used in plant-light measurement. It sets boundaries around the wavelengths included in a conventional PPFD reading.

Because PAR names a spectral range, it does not tell you how much light reaches a plant. Saying that a lamp “has PAR” leaves out the quantity, the measurement plane, and the distribution across the grow area. PPFD supplies the quantity for a particular place and moment.

The 400 to 700 nm boundary is a measurement convention, not a claim that every photon outside it has no effect on plants. Far-red light needs a separate caveat, which we cover below.

What is PPFD?

Photosynthetic photon flux density (PPFD) measures the incident photon flux per unit area per second within the stated PAR waveband. Its unit is micromoles per square meter per second, written µmol m⁻² s⁻¹.

For a grower, PPFD answers: how much measured photosynthetic light is reaching this part of the canopy right now?

The location matters. A reading taken in the bright center beneath a fixture does not describe the dimmer edges. A reading taken above the leaves does not describe the light that reaches the canopy. Researchers and extension specialists therefore specify the measurement plane and use repeated readings or logging when they need to describe more than one point in space or time.

PPFD also says nothing about how long the light remains on. That is the job of DLI.

What is DLI?

Daily light integral (DLI) is the accumulated photosynthetic photon total received per square meter over a day. It combines intensity with time and is written in mol m⁻² d⁻¹.

When PPFD stays constant during a fixed light period, calculate DLI with this equation:

DLI = PPFD × light hours × 0.0036

The conversion factor comes from 3,600 seconds per hour and 1,000,000 micromoles per mole.

For example, a constant PPFD of 200 µmol m⁻² s⁻¹ produces:

200 × 12 × 0.0036 = 8.64 mol m⁻² d⁻¹ over 12 hours.
200 × 16 × 0.0036 = 11.52 mol m⁻² d⁻¹ over 16 hours.

You can rearrange the equation when you know the DLI and photoperiod:

PPFD = target DLI ÷ (light hours × 0.0036)

A target of 12 mol m⁻² d⁻¹ spread across 16 hours requires an average PPFD of about 208 µmol m⁻² s⁻¹. This is an arithmetic example, not a recommendation for a particular crop.

The constant-PPFD equation works well for an indoor fixture with a stable output and schedule. If sunlight enters the space, the intensity changes through the day. A logger that integrates readings over the full photoperiod gives a more defensible DLI than one spot reading multiplied by the number of hours.

DLI Planning and Logging Workbook

Use a measured PPFD value and a documented light schedule when you estimate DLI. A fixture setting or a center reading is not a canopy average, and sunlight or programmed dimming makes the one-line constant-PPFD calculation less representative.

For a constant indoor schedule

Record four inputs:

Field	What to enter
Measurement plane	The canopy height where the sensor was placed
Average PPFD	The mean of readings taken across the mapped grow area
Light hours	The number of hours at that stable output
Fixture state	Hanging height, position, and dimmer setting

Then calculate:

DLI = average PPFD × light hours × 0.0036

For an example canopy average of 240 µmol m⁻² s⁻¹ over 14 hours:

240 × 14 × 0.0036 = 12.096 mol m⁻² d⁻¹

Rounded to two decimal places, the estimated DLI is 12.10 mol m⁻² d⁻¹. This is an arithmetic example, not a crop target.

For a schedule with several stable periods

Calculate each period separately, then add the results:

Period	PPFD (µmol m⁻² s⁻¹)	Hours	DLI contribution (mol m⁻² d⁻¹)
Low output	120	2	0.864
Full output	260	10	9.360
Low output	150	4	2.160
Daily total		16	12.384 mol m⁻² d⁻¹

The method works because each row applies PPFD × hours × 0.0036 to a period with a reasonably stable output. If intensity changes continuously, use a light logger that integrates readings rather than treating a few spot checks as the full day.

Keep the biology attached to the number

Save the PPFD pattern and photoperiod beside the DLI. Two schedules can reach the same daily total without producing the same response, so do not treat intensity and duration as freely interchangeable.

A reusable record can look like this:

Use a crop- and stage-specific reference for the target, and copy its spectrum, photoperiod, and environmental context when those details are available. The workbook checks delivery and arithmetic; it does not create a universal target.

Why lux cannot give you a universal PPFD value

Lux is designed around the spectral sensitivity of human vision. PPFD counts photons across a stated plant-light waveband without applying the same visual weighting. Two light sources can therefore show the same lux while delivering different PPFD values, or the same PPFD while producing different lux readings.

The relationship changes with the spectrum. Sunlight, older lamp types, and different light-emitting diode fixtures do not share one fixed lux-to-PPFD factor. Both historical conversion research and modern measurements found source-dependent differences.

A source-specific conversion can be valid when you know the spectral distribution or have a tested factor for that exact source. A generic online formula cannot promise the same accuracy across different fixtures and spectra.

A phone lux app can still help you compare relative brightness across the same setup. It may show that one corner is darker than another. It does not turn the phone into a calibrated quantum sensor, and its lux reading should not be presented as a measured PPFD value.

How to measure PPFD at canopy level

Use a calibrated quantum sensor, sometimes sold as a PAR meter, when you need a PPFD measurement. Use a spectroradiometer when the spectral distribution itself is the question.

Follow this repeatable process:

Set the measurement plane. Place the sensor at the top of the intended canopy, where the upper leaves will receive light.
Keep the sensor level. A planar sensor needs a consistent horizontal orientation. Tilting it changes the light it receives.
Measure more than the center. Take readings at representative positions across the grow area. Keep the same positions when comparing fixture height or dimmer settings.
Keep the sensor clean. Dust or residue on the detector can affect the reading. Follow its cleaning instructions.
Check correction guidance. Some sensors need a source-specific correction because their spectral response is not perfectly uniform. Use the manufacturer’s correction for the light source when required.

Record the fixture, dimmer setting, hanging height, sensor height, measurement positions, PPFD readings, and light hours together. Those details make a later comparison meaningful.

One PPFD reading does not describe the whole grow area

Fixture output is rarely identical at every point under the lamp. A center reading can look strong while the edges receive much less light. Measuring a small grid at canopy height shows both the average and the spread across the area.

The reviewed evidence does not support one universal grid size or number of measurement points. Match the sampling pattern to the area you need to describe, and repeat the same pattern after changing fixture height, position, or output.

A grow-light chart can provide a useful starting point if it names the fixture settings, hanging height, area, and measurement plane. Your own canopy-level readings are still the best check for your setup.

Canopy PPFD Mapping Protocol

A repeatable light map lets you compare the average and the variation across the grow area. Keep the measurement plane, grid positions, fixture settings, and sensor orientation unchanged when you compare two setups.

Build the map

Mark the grow area's boundaries and the intended canopy height.
Choose representative measurement positions across the area, including the center and edges.
Draw the positions as a simple grid and label them so you can return to the same points.
Warm up and operate the fixture according to its instructions, then record its hanging height, position, and dimmer setting.
Hold the sensor level at the canopy plane and record one PPFD value at each position.
Repeat the same grid after any change you want to evaluate.

There is no universal grid size in the reviewed evidence. Use enough positions to describe the area you intend to grow in, and state the grid when you share or compare the result.

Summarize the readings without hiding the edges

Suppose a nine-point map produced these example readings in µmol m⁻² s⁻¹:

	Left	Center	Right
Back	180	220	190
Middle	230	260	225
Front	175	215	185

The summary is:

Average: (180 + 220 + 190 + 230 + 260 + 225 + 175 + 215 + 185) ÷ 9 = 208.9 µmol m⁻² s⁻¹
Minimum: 175 µmol m⁻² s⁻¹
Maximum: 260 µmol m⁻² s⁻¹
Spread: 260 - 175 = 85 µmol m⁻² s⁻¹

At a constant 14-hour photoperiod, the average would give an estimated DLI of:

208.9 × 14 × 0.0036 = 10.53 mol m⁻² d⁻¹

The values are invented to show the calculation. They are not a fixture specification, an acceptable-uniformity threshold, or a crop recommendation. The minimum and maximum remain visible because an average alone can conceal dim or bright areas.

Compare one change at a time

Use the same table for a before-and-after check. Change one item, such as fixture height, fixture position, or dimmer setting, then repeat every grid point. Compare the full maps as well as their averages. If you change the sensor plane, grid, and fixture output together, you cannot tell which change produced the difference.

Save the raw readings with the summary. A compact map record should include the date, fixture, dimmer setting, hanging height, sensor model, correction applied if required, canopy plane, grid dimensions, every PPFD reading, average, minimum, maximum, photoperiod, and estimated or logged DLI.

DLI does not replace photoperiod biology

The same DLI can come from brighter light over fewer hours or dimmer light over more hours. The arithmetic total may match, but plant responses do not always match.

In a controlled cucumber seedling experiment, researchers held DLI constant while changing PPFD and photoperiod. The treatments produced differences in plant form, biomass allocation, pigments, carbohydrates, and other measured responses. An independent experiment with lettuce and mizuna also found growth and physiological differences when the same DLI was delivered through different PPFD and photoperiod combinations. These results apply to the tested crops and conditions, but they demonstrate why DLI cannot erase the separate effects of intensity and duration.

A cross-crop review also found that responses vary with species, production stage, temperature, development, and other environmental conditions. A separate indoor iceberg lettuce experiment showed a nonlinear response across the tested DLI treatments in one cultivar and vertical hydroponic system.

Use a crop- and stage-specific source to choose a target. Then use PPFD and photoperiod to calculate and measure how your setup delivers it. A universal “low, medium, high” table would hide the crop, cultivar, stage, spectrum, environment, and schedule that give a target its meaning.

The far-red caveat

Traditional PPFD uses the 400 to 700 nm PAR range, so it does not count far-red photons from 700 to 750 nm. That does not make far-red inactive.

ASABE’s public record confirms that its S640 standard covers quantities and units for electromagnetic radiation used with plants. The complete controlled standard was not available for this review, so the wavelength definitions here rely on the full peer-reviewed sources instead.

In controlled lettuce studies, Zhen and Bugbee found that far-red photons contributed to canopy photosynthesis when combined with photons from 400 to 700 nm. Far-red also affected canopy form and light capture under the tested spectra.

Note: Far-red performance depends on the spectrum, canopy, and comparison being made. In a later lettuce experiment, far-red photons contributed to photosynthesis when combined with shorter wavelengths, but their photosynthetic activity was lower than that of red photons under the tested substitution treatments, and final dry weight did not change. This does not contradict the evidence that far-red can contribute; it limits any one-to-one efficiency claim.

This evidence supports a careful limit on conventional PPFD: it may not describe every plant response to a fixture’s spectrum. It does not show that far-red alone is equivalent to shorter wavelengths, nor does it mean an extended range has replaced the traditional convention in every meter, standard, or growing guide.

A practical way to use all four terms

Start with a crop- and stage-specific light reference. Use its unit exactly as written and keep its photoperiod and environmental context attached.

Measure PPFD at several canopy positions with a suitable sensor. Adjust fixture height or placement, or change output based on the pattern across the grow area, then measure again.

Calculate DLI from the measured average PPFD only when the light output stays reasonably constant. If intensity changes during the day, log and integrate the readings instead.

Treat lux as a human-vision measurement. Convert it to PPFD only when you have a validated factor for the exact light source.