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.

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.

PAR to PPF

A pivotal shift in  understanding plants emerged in the 1970s. Scientists argued that quantifying radiation energy (wattage) alone wasn’t sufficient. Instead, they proposed measuring the number of photons emitted instead of wattage, because the impact on photosynthesis is more directly and precisely measurable.

This led to Photosynthetic Photon Flux (PPF), quantifying the number of photons within this critical range over a determined time period (number of photons per second or μmol/s).

PPF to PPFD

Another refinement led to Photosynthetic Photon Flux Density (PPFD), taking PPF and adding a dimension, surface area. Thus PPF (μmol/s) led to PPFD, which is the number of photons striking a certain surface area in one second (μmol/m²/s). The “surface area” is important because growers can use this data to situate grow-lights so that enough photons are distributed equitably over a farming area.

Note: mol & μmol explained (here)

PPFD to PFD

As we mentioned, PPFD includes only the photons present between 400-700nm. Nevertheless, recent scientific revelations unveiled the significance of certain wavelengths beyond this range that still affect plant physiology.  This includes blue light below 400nm and far-red infrared, above 700nm.

Consequently, Photon Flux Density (PFD) is another refinement, a wider spectrum spanning 350nm to 800nm, encapsulating the non-traditionally defined photosynthetically active range.

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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.