When LEDs arrived on the market, their tremendous efficiency and competitive price attract more and more people work on it. They changed the playing field. Lumens, lux and candela finally became obsolete metrics for determining light requirements for plants.
Therefore,PAR, PPF and PPFD become more exactly measu rement in photosynthetics lighting applications esp. LED grow light. The purpose of this article is to explain what these terms mean and share scientific methods in lighting selection.
1.What is PAR?
Usually, people use PAR to mean a type of LED fixtures. But now the PAR we talk about is Photosynthetically Active Radiation. It defines the type of light (scientists call light “electromagnetic radiation”) needed to support photosynthesis in plant life. Plants absorb lighting energy into chemical energy by photosynthesis, while the chemical energy support the plants growth.
As we all know, some light (like the light from a candle) is visible 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” which includes invisible X-rays, radio waves and infrared visible light such as sunlight, and light from a red or blue LED.
Interestingly, most spectrum plants use are same and visible. However, the wavelength we see brightest is not the one which influence best to plant growth.
So, PPF and PPFD is the key to know how PAR influences plant growth. When selecting a lighting system or fixture to promote photosynthesis, there are three important measurements. How much light the fixture produces? How much lighting the plant absorb? 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 plant growth, development, yield and quality 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. Tadiation called physiological radiation. Besides promoting photosynthesis, it also affect other physiological activities.
Photosynthetic Photon Flux (PPF).
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”, called μm.
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