LIGHTING FUNDAMENTALS
PAR, PPF, PPFD and DLI Explained
Learn how the principal horticultural lighting measurements differ—and how commercial growers use them to evaluate fixtures, light distribution and the total light delivered to a crop.
IN THIS GUIDE
Four Measurements, Four Different Questions
PAR, PPF, PPFD and DLI are related, but they do not describe the same aspect of horticultural lighting. Understanding the difference is essential when comparing LED fixtures or planning a commercial lighting system.
PAR
Which wavelengths are being discussed?
PPF
How much PAR light does a fixture produce?
PPFD
How much PAR light reaches a square metre each second?
DLI
How much PAR light accumulates during the day?
AT A GLANCE
PAR, PPF, PPFD and DLI Comparison
| Measurement | What It Describes | Typical Unit | Practical Use |
|---|---|---|---|
| PAR | The wavelength range of light commonly associated with photosynthesis. | 400–700 nm | Defines the traditional photosynthetic waveband. |
| PPF | The total quantity of PAR photons emitted by a light source every second. | µmol/s | Compares the total photon output of fixtures. |
| PPFD | The quantity of PAR photons arriving at a specific surface every second. | µmol/m²/s | Evaluates intensity and distribution at crop level. |
| DLI | The cumulative quantity of PAR photons received over the daily photoperiod. | mol/m²/day | Connects light intensity and duration to daily crop exposure. |
WAVELENGTH RANGE
What Is PAR?
Photosynthetically Active Radiation (PAR) traditionally refers to light within the wavelength range of 400 to 700 nanometres. This is the range most commonly used when discussing the photons that drive photosynthesis.
PAR is not a measurement of how much light a fixture produces. Instead, it defines the portion of the light spectrum being considered. A fixture cannot meaningfully be described as having “more PAR” without also specifying a quantity such as PPF or PPFD.
Important distinction
PAR identifies a wavelength range. PPF and PPFD quantify photons within that range.
What About ePAR?
Extended photosynthetically active radiation, commonly called ePAR, expands the measured range to 400 to 750 nanometres. It includes far-red photons from 700 to 750 nanometres in addition to the traditional PAR range.
When comparing specifications, confirm whether a value represents traditional PPF or extended photon flux. Measurements based on different wavelength ranges should not be treated as directly equivalent.
FIXTURE OUTPUT
What Is PPF?
Photosynthetic Photon Flux (PPF) measures the total number of PAR photons emitted by a light source every second. It is expressed in micromoles per second (µmol/s).
PPF is useful for comparing the total photon output of grow lights. However, it does not explain where those photons travel, how evenly they are distributed, or how much light ultimately reaches the crop canopy.
PPF and Fixture Efficacy
System efficacy = PPF ÷ input power
System efficacy describes how efficiently a fixture converts electrical power into photosynthetic photons. It is normally expressed in µmol/J.
For example, a 600-watt fixture producing 1,680 µmol/s has a system efficacy of 2.8 µmol/J. Efficacy is important when evaluating operating efficiency, but it should be considered together with light distribution, reliability, spectrum and project requirements.
CANOPY INTENSITY
What Is PPFD?
Photosynthetic Photon Flux Density (PPFD) measures the quantity of PAR photons arriving at one square metre every second. It is expressed in micromoles per square metre per second (µmol/m²/s).
Unlike PPF, which describes total fixture output, PPFD describes the light intensity at a particular location. PPFD changes with mounting height, fixture spacing, beam distribution, reflective surfaces and the position of the measurement point.
Why a PPFD Map Matters
A single centre-point reading cannot describe the lighting conditions across an entire growing area. A PPFD map uses multiple measurement points to show how light is distributed across the crop canopy.
- Average PPFD indicates the mean intensity across the measured area.
- Minimum and maximum PPFD reveal the range between darker and brighter areas.
- Uniformity helps evaluate how consistently the crop receives light.
Higher is not always better
Effective lighting design aims for the appropriate crop-level PPFD and distribution—not simply the highest possible reading at the centre of a test area.
For example, a 600-watt fixture producing 1,680 µmol/s has a system efficacy of 2.8 µmol/J. Efficacy is important when evaluating operating efficiency, but it should be considered together with light distribution, reliability, spectrum and project requirements.
DAILY LIGHT
What Is DLI?
Daily Light Integral (DLI) represents the total quantity of PAR photons received by one square metre during a day. It is expressed in moles per square metre per day (mol/m²/day).
DLI combines light intensity and duration. Two crops can receive the same DLI through different combinations of average PPFD and photoperiod, although the biological response may not always be identical.
DLI = PPFD × light hours × 3,600 ÷ 1,000,000
In a greenhouse, total crop DLI may include natural sunlight and supplemental electric lighting. Because sunlight varies throughout the day and season, accurate greenhouse planning normally relies on measured or estimated solar DLI rather than a single instantaneous sunlight reading.
PRACTICAL TOOL
Calculate DLI From PPFD and Photoperiod
Enter an average PPFD and the number of light hours to calculate the resulting daily light integral.
PUTTING IT TOGETHER
A Practical Lighting Example
Consider a grow light with a PPF of 1,680 µmol/s installed above a crop. The fixture’s total output is 1,680 µmol/s, but this does not mean the crop receives a PPFD of 1,680 µmol/m²/s.
After the mounting height, fixture spacing and growing area are considered, a canopy measurement might show an average PPFD of 500 µmol/m²/s.
Fixture output
1,680 µmol/s PPF
Average canopy intensity
500 µmol/m²/s PPFD
Photoperiod
16 hours
Daily light integral
28.8 mol/m²/day
This example demonstrates the relationship between the measurements: PPF describes what the fixture emits, PPFD describes what reaches the canopy, and DLI describes the accumulated light received during the photoperiod.
COMMERCIAL APPLICATION
Using These Measurements in a Lighting Project
PAR, PPF, PPFD and DLI each provide useful information, but none should be used alone to select a horticultural lighting system. A commercial project should also consider:
- Crop and cultivar
- Growth stage
- Target photoperiod
- Available natural sunlight
- Facility dimensions
- Mounting height and spacing
- Required uniformity
- Lighting spectrum
- Electrical infrastructure
- Environmental controls
Photometric layouts and project-specific calculations help translate fixture specifications into a lighting plan designed for the actual crop area and production objectives.
KEY TAKEAWAY
Remember the Difference
PAR
Defines the traditional photosynthetic waveband.
PPF
Measures the fixture’s total photon output.
PPFD
Measures photon intensity at the crop surface.
DLI
Measures the total light accumulated each day.
PROJECT-SPECIFIC LIGHTING SUPPORT
Need Help Applying These Lighting Metrics?
Fixture specifications are only one part of a successful lighting project. Share your crop, facility and production requirements with CultiLight, and our team will help identify a lighting approach suited to your objectives.