Untangling the Differences Between PAR, PPF, PPFD, and PFD

PAR, PPF, PPFD, and PFD Introduction

Greenhouse and horticultural professionals will  encounter various terms related to light measurement. This article discusses the four terms PAR, PPF, PPFD, and PFD. These terms have been used interchangeably in literature and can cause confusion. We want to clear this up once and for all.

1. What is PAR? Background of PAR metric
2. What is PPF?
3. What is PPFD?
4. What is PFD?
5. Further subdivision of PFD
6. Measuring PFD – Spectral PAR Meters
7. Deep Dive: What is a mole (mol), micromole (μmol)?

What is PAR? Background of PAR metric

Photosynthetically Active Radiation (PAR) has been the cornerstone of light’s impact on plant growth and photosynthesis. In the 1960s, PAR measured light between 400-700 nm, the wavelengths crucial for photosynthesis. It used either watts per second (W/s), or watts per square meter per second (W/m²/s).  Effectively, it measured the “energy” from light needed to trigger and sustain plant growth.

What is PPF?

Photosynthetic Photon Flux (PPF) refers to the number of photons emitted within the PAR range (400–700 nm) per second. It is expressed in micromoles per second (μmol/s). Unlike traditional measurements based on energy (watts), PPF focuses on photon count, which more accurately represents the light available for photosynthesis. This shift in focus allows growers and researchers to better assess the quality and effectiveness of their lighting setups for plant growth.

PAR to PPF

In the 1970s, scientists began to question whether measuring light in watts truly reflected the needs of plants. They recognized that photosynthesis responds to the number of photons received, not just the total energy. As a result, plant science transitioned from using generic radiation metrics like PAR intensity to adopting PPF as a more precise and biologically relevant measurement. This shift marked a fundamental improvement in horticultural lighting evaluation.

What is PPFD?

Photosynthetic Photon Flux Density (PPFD) measures the number of photons within the PAR range that land on a specific surface area per second. It is expressed in micromoles per square meter per second (μmol/m²/s). While PPF tells us how many photons are emitted from a light source, PPFD tells us how many of those photons actually reach the plant canopy. This makes PPFD one of the most practical metrics for growers to evaluate whether their crops are receiving sufficient light for photosynthesis.

PPF to PPFD

As horticultural lighting research advanced, scientists refined PPF by adding the concept of surface area. This led to PPFD, which translates the total photon output (PPF) into the actual photon delivery over a defined growing area. Growers use PPFD to strategically position lighting fixtures and ensure uniform photon distribution across all plants. This ensures that no section of the crop is underlit or overexposed, directly impacting plant health and yield consistency.

Note: mol & μmol explained (here)

What is PFD?

Photon Flux Density (PFD) is a broader version of PPFD, measuring the number of photons across an extended wavelength range—typically from 350 nm to 800 nm—that reach a given surface area per second (μmol/m²/s). Unlike PPFD, which is restricted to the traditional PAR range (400–700 nm), PFD includes additional wavelengths such as near-UV (below 400 nm) and far-red (above 700 nm), which have been shown to influence plant morphology, flowering, and secondary metabolite production.

PPFD to PFD

While PPFD served as a key benchmark for photosynthetic light, it became clear that plant responses extend beyond the 400–700 nm range. New research highlights how blue light below 400 nm and far-red light above 700 nm play essential roles in plant physiology—stimulating stem elongation, improving flowering, or triggering photomorphogenic responses. In response to these findings, PFD emerged as a more inclusive metric, allowing growers and researchers to evaluate total photon exposure across the full spectrum relevant to plant development.

Further subdivision of PFD

Another further refinement is the subdivision of PFD into even more specific color wavelengths such as PFD-B (Blue), PFD-R (Red), PFD-FR (Far Red), PFD-UV (Ultra Violet).

Chlorophylls, the primary pigment responsible for photosynthesis, absorb light most efficiently in the spectrum’s red (PFD-R) and blue (PFD-B) regions. PFD-B, with its shorter, more energetic wavelengths, is vital during early growth stages as it influences leaf expansion, branching, and overall plant structure. PFD-R extends the range of light absorption for the plant and contributes to the overall pool of available photosynthetic energy.

PFD-FR, along with PFD-R data is critical in understanding and affecting seed germination, stem growth, flowering, and fruiting.  See article on Photomorphogenesis (here).

PFD-UV data is important for plant health.  Mild UV exposure can trigger a host of responses such as defense against pests and pathogens. It can increase antioxidant content to protect against sunlight exposure, boost the immune system and nutritional content.

Summary

In conclusion, the evolution from PAR to PPF, PPFD, and PFD has had a staged development, where ambiguities have crept into the literature and conversation.  However, understanding these terms in a clear context will help growers and scientists bridge the gap between traditional farming and new technologies.

• PAR – is now an umbrella term covering the entire topic of measuring light quantity for plant growth and includes PPF, PPFD, and PFD.
• PPF (Photosynthetic Photon Flux) measures light quantity (photons) from the 400-700 nanometer range and is measured as μmol/s.
• PPFD (Photosynthetic Photon Flux Density) measures light quantity between 400 to 700 nanometers and is measured in μmol/m²/s.
• PFD (Photon Flux Density) measures light quantity between 350 to 800 nanometers and is measured in μmol/m²/s.  PFD can be further subdivided into PFD-B, PFD-R, PFD-FR, and PFD-UV by Spectral PAR meters.

We hope we have untangled these sometimes loosely used terms, and brought some clarity to your knowledge base.  Please comment if you have any questions or suggestions.

References:

UPRtek – Photomorphogenesis, sibling to Photosynthesis

UPRtek – Photosynthesis: What, Where, How, and Why?

UPRtek – The Mighty Chlorophyll Molecule.