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Light measurement values and their importance for plant growth

1. November 2017
Pro-emit Team
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Light measurement values and their importance for plant growth

Why LUX and lumen are no longer up to date

Plants don't grow faster if you pull on their stems. They will grow faster, however, if you provide light conditions that are geared towards healthy and sustainable growth. The question is which measurement values you should consider when planning your lighting conditions?

Opinions are divided concerning this subject. While LUX and lumen values are still widely used among plant breeders, science is offering more and more arguments against this simplified view of light and its properties in relation to plant growth. New findings imply that the average photosynthetic photon flux density (PPFD value) is of greater importance towards plant growth than LUX and lumen values. To understand why this is the case, we need to cover some of the basics concerning photosynthesis and light measurement values.

Photosynthesis and how it is impacted by light

For our purposes the term "photosynthesis" designates the process of converting water and carbon-dioxide (CO2) into glucose and oxygen used by plants. The basic requirement for this energy conversion is radiation energy and chlorophyll. While water is being gathered by the roots of the plant, radiation energy is provided by the sun in a natural environment.

While humans need oxygen to breathe and produce carbon-dioxide when breathing out, plants work the other way around in that they need CO2 in order to produce oxygen. For this process to work, however, the plant is not only dependent on CO2 but also on radiation energy, that is, light. This is provided by the sun in the form of photons. As energy carriers photons are of significant importance concerning the growth of plants and need to be available in sufficient quality and quantity according to the needs of the individual type of plant. While nature takes care of this by providing the ideal light spectrum of the sun, managing artificial grow light setups to ensure optimal sustenance for plants is a complex process.

To understand which measurement value is best for evaluating the photosynthetic process we need to cover some more basics.

Lumen and LUX, PPFD and PAR

The lumen value describes the luminous flux based on light intensity and solid angle. In a nutshell the lumen value of a light source tells us at which rate light is produced and made available. It does not, however, offer information on the light intensity that actually reaches a certain area because it is not concerned with the dispersion angle of the light source. Further, the lumen value does not take into account the number of photons hitting the illuminated area. As such while the lumen value is useful when ascertaining a setup for straight forward illumination purposes, it does not convey much useful information when it comes to grow light setups.

The LUX value is derived from the lumen value and designates the light intensity that hits a certain area. Since it is not concerned with the number of photons hitting that area, however, the LUX value - just like the lumen value - really is not ideal when it comes to grow light setups and assessing the quality of light emitted by a source.

A solution to this problem is to introduce the measurement value PPF (photosynthetic photon flux). The PPF value is denoted in μmol/s and measures the number of photons being emitted by a given source. If put in relation to an illuminated area, the average number of photons hitting that area is the basis for calculating the average photosynthetically active photon flux density (PPFD value). As such the PPFD value is highly suited to ascertain the quality of light emitted by a given source. The direct PPF value of a given illuminated point is termed its PAR value. While PAR values yield information on certain points of the illuminated area, they should always be considered in relation to the overall PPFD value.

To assess the effect of a grow light on a plant the photosynthetic photon flux density PPFD is what really matters because unlike LUX or lumen values, the PPFD will yield information on the average number of photons actually reaching the plant. To get optimal results when plant breeding, however, you will also have to take into account the available wavelength spectrum of the emitted light.

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