When LEDs arrived on the market, their tremendous efficiency and money saving potential changed the playing field and lumens, lux and candela finally became obsolete metrics for determining light requirements for plants.
Recently, people began referring to PAR, PPF and PPFD as ways to measure light in photosynthetics lighting applications. The purpose of this article is to explain what these terms mean, correct some common misunderstandings and help growers understand how the science behind these terms can be used to determine proper light levels to grow happy, thriving plants.
What the is PAR?
Our LED fixtures emit extremely high PAR levels, PAR (Photosynthetically Active Radiation) is a much used (and often misused) term. It defines the type of light (scientists call light “electromagnetic radiation”) needed to support photosynthesis in plant life. Through photosynthesis, plants convert light energy into chemical energy, which is the food they use to grow and thrive.
As we all know, some light (like the light from a candle) is visible to the human eye and some (such as infrared) is not. Scientists define different types of light by their “wavelengths”, these different wavelengths make up the electromagnetic radiation “spectrum”. This spectrum includes X-rays, radio waves and infrared light (none of which are visible to the human eye) and light that we can see such as sunlight, and light from a red or blue LED.
Interestingly, plants use roughly the same part of the spectrum that’s visible to the human eye, but the wavelengths we perceive to be the brightest (i.e. green light) are not the most efficient wavelengths for photosynthesis.
So, the first thing to understand about PAR that it is the part of the electromagnetic radiation spectrum (light) which is useful for plants and algae to activate photosynthesis, the point is PPF and PPFD. When selecting a lighting system or fixture to promote photosynthesis, there have three measurements parameters are important: how much light the fixture produces, how much of that light is available to the plants and how much light the plant receives during the photoperiod.
Photosynthetically Active Radiation (PAR). It’s the basic energy to produce biomass which affects the growth, development, yield and quality of plants directly. PAR defines what’s kind of light source can support plant photosynthesis. The wavelength range of light source to promoting green growth is more wider than photosynthetically active radiation wavelength range, it is roughly at the 300nm to 800nm range, this part of radiation called physiological radiation. Besides it can promote photosynthesis, it also affect other physiological activities.
Photosynthetic Photon Flux (PPF). A measurement of the total light (photons) emitted by a light source each second. PPF tells us how much PAR is emitted by a light source. Measured in “micromoles per second” and expressed as μmoles/second.
Photosynthetic Photon Flux Density (PPFD). The measurement of the light (photons) that reach the target each second. PPFD is measured over a one meter square area in “micromoles per square meter per second” and expressed as μmol/m2/s.