Photosynthetically Active Radiation
Photosynthetically Active Radiation (PAR) is a crucial concept for indoor growers to understand as it refers to the portion of the electromagnetic spectrum that is essential for photosynthesis in plants.
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose and oxygen, using carbon dioxide and water.
PAR encompasses the wavelengths of light between 400 to 700 nanometers (nm), which corresponds to the visible light spectrum. These wavelengths are particularly effective at driving the photosynthetic process in plants. The measurement of PAR measures each wavelength equally i..e Blue, green and red photons contribute to photosynthesis equally.
However not all wavelengths of light have the same effect on photosynthesis. The McCree curve, also known as the McCree action spectrum, is a graphical representation that illustrates the relative efficiency of different wavelengths of light in driving photosynthesis in plants. It was developed by the American botanist Warren L. McCree in the 1970s. Although a little outdated and not accurate for all plant types the McCree curve proposes that different wavelengths of light have varying levels of effectiveness in photosynthesis.
You can see from the McCree curve that in general red photons (600nm to 700nm) are the most photosynthetically efficient, green (500nm to 600nm) a little less efficient and blue (400 to 700nm) the least efficient. UVA and Far red photons also contribute to plant growth but with reducing efficiency for wavelengths the further outside the PAR range you look.
To further understand PAR, two related terms are often used: Photosynthetic Photon Flux (PPF) and Photosynthetic Photon Flux Density (PPFD).
Photosynthetic Photon Flux (PPF)
PPF is a measure of the total number of photons (individual particles of light) within the PAR range that strike a given area over a specific time period, typically expressed as micromoles per second (μmol/s). It quantifies the total amount of light energy available for photosynthesis. PPF provides a valuable metric for evaluating the overall light output of a light source, such as a lamp or a natural light source, in terms of its effectiveness for supporting photosynthesis.
With respect to grow lights this is measured in an integrated sphere which measures the total PAR output of the fixture, not the amount of PAR delivered to the plant canopy.
Photosynthetic Photon Flux Density (PPFD)
PPFD takes PPF a step further by accounting for the area over which the photons are distributed. It measures the number of PAR photons that reach a specific surface area (usually expressed in square meters) per unit of time. PPFD is a crucial parameter for understanding how much light plants are actually receiving in their immediate environment, as it accounts for factors such as light source distance, light angle, and shading. PPFD is measured in micromoles per square meter per second (μmol/m²/s) and provides a more practical measure for assessing the light available to plants at a specific location.
In summary, Photosynthetically Active Radiation (PAR) is the range of wavelengths in the visible light spectrum that is essential for photosynthesis in plants. Photosynthetic Photon Flux (PPF) quantifies the total amount of PAR photons emitted by a light source, while Photosynthetic Photon Flux Density (PPFD) accounts for the amount of PAR photons that reach a specific surface area. These metrics are crucial for optimizing lighting conditions in indoor cultivation, greenhouses, and other environments where plants are grown to ensure they receive the appropriate amount of light for healthy growth and maximum photosynthetic efficiency.