What distinguishes PPFD from PPF?

If you seek to comprehend the comparison of grow lights, you have probably encountered the metrics PPF and PPFD.
Nonetheless, you may be perplexed regarding the significance of these indicators and how to interpret them to facilitate an informed purchase decision. This article examines the fundamental distinctions between PPF and PPFD.

The distinction between PPF and PPFD as defined.

PPF and PPFD are acronyms pertaining to the quantification of light emitted by a light source or present in a specific region.
PPF - Photosynthetic Photon Flux
PPFD - Photosynthetic Photon Flux Density
What is a photosynthetic photon? A photon is an individual particle of light that may assume several wavelengths. Entities capable of facilitating photosynthesis are regarded as photosynthetic photons.
This specifically encompasses photons with wavelengths ranging from 400 nm to 700 nm.
PPF and PPFD quantify the number of photons. The essential distinction is that PPFD quantifies the density of photons incident on a certain surface, whereas PPF assesses the overall quantity of photons emitted by a light source.
In the figure below, each dot signifies each individual photosynthetic photon. PPFD refers to the quantity of photons that strike a specific surface, whereas PPF denotes the aggregate number of photons emitted by the light source.
The illustration should illustrate two further characteristics of PPFD.
The proximity to the light source correlates positively with the PPFD value. This phenomenon occurs because of the dispersion of light as one distances from the light source.
The centre of the beam often exhibits the greatest PPFD measurement. As one distances from the centre, the PPFD will diminish.

 

Units of measurement for PPF and PPFD

PPF - micromoles per second
Photosynthetic Photon Flux Density - micromoles per second per square metre
Both PPF and PPFD quantify the total photon count. This is evidently a substantial quantity, hence the unit micromoles (μmol) per second is employed. A micromole is approximately comparable to 6 x 10^17. Additionally, given we are quantifying the rate of these photons, this measurement is expressed per second.
PPF denotes micromoles per second, whereas PPFD represents micromoles per second per square metre. This is to ascertain the number of photosynthetic photons that strike a square metre every second.
 

When to utilise PPFD versus when to employ PPF

PPFD must always be accompanied with a specified distance and location. Most manufacturers will provide PPFD statistics; nevertheless, ensure that you consider:
Proximity to the luminous source
Geographical location and if this is averaged throughout a certain area
Without this information, a meaningful comparison of PPFD is unattainable.
PPFD may develop from numerous lights illuminating a single region.
PPF, conversely, quantifies the quantity of Photosynthetically Active Radiation emitted by each individual grow light. Meaningful comparisons between lights may be made by evaluating their PPF measurements. Be advised that the beam angle may influence the subsequent PPFD measurements based on the goods used.
 

Differences in measurement methods between PPFD and PPF

PPFD quantifies the intensity of light incident on a surface, so even a compact, portable light meter or spectrometer can assess the quantity of PAR that impacts a surface. These are often more economical and may be linked to cellphones for field usage.
PPF, conversely, quantifies the quantity of Photosynthetically Active Radiation released by an individual light source in all directions. Consequently, more advanced instruments are necessary. A goniosphere or integrating sphere is generally required. These gadgets capture light produced from all angles and subsequently measure the total light output.