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There are various methods to evaluate light. Some of them focus on the light that can be used by plants and these are the ones we will take a closer look at: “Lumen is for humans!” When dealing with light there are two important factors that are often misunderstood, namely light quantity and quality. Below we will cover the two terms and explain what they mean.
Light quantity or the amount of light correlates directly with the production of biomass – the more light, the more biomass (up to a certain point, of course). As photosynthesis is a quantum process, it can be quantized on basis of photons. In the following, we will present three ways of measuring light quantity.
PPF (photosynthetic photon flux) – is the measuring unit which references the overall performance of the light spectrum relevant for plant growth. PPF measures how many photons are emitted by a given light source. The data is denoted in μmol/s. PPF is a very important factor because it shows how much output a light source/lamp will produce.
Average PPFD (photosynthetic photon flux density) – the “D” stands for density and yields information on how much PPF is hitting an area of 1m². PPFD values are denoted in μmols/m². Studies on various plant types have shown that 700-1000μmols/m² represent an optimum average PPFD for light-hungry plants. Without the additional help of CO2, values that are higher than1000μmols/m² are not recommended.
Direct PPFD value (PAR meter) – many know PAR meters and the PPFD values measured by these devices. They are highly useful to see how the light spreads over an area and to measure how much light is hitting a certain point. These values, however, say nothing about the overall light output in relation to the whole area. This why PAR meters are used to measure light at several points of the surface area in order to calculate an average PPFD value.
A widely known form of assessing light quality is to measure the spectrum of available light. An analysis of the light spectrum emitted by a source essentially tells you which different wavelengths are available and how the light is distributed among these wavelengths. The different colours of the light spectrum emerge due to the different energy levels of the photons. The more energy a photon has, the more its colour tends towards blue. On the other hand, the less energy a photon has, the more it tends towards the reds.
Concerning photosynthesis, these colours are used to determine the potential effect a given light has on plant growth. This was shown by Dr McCree in 1972. He examined 22 different types of plants in regard to how their carbon fixation reacted to different wavelengths of light. The results led to what today is known as the McCree Relative Quantum Yield curve or RQE-curve. Up until today, this is the only scientifically recognized study on how potent different wavelengths are in relation to plant growth.
It is important to note, however, that the important wavelengths are not restricted to the red and blue spectrum. While the red and blue wavelengths are driving forces when it comes to photosynthesis and the physiological development of plants, they are not the only beneficial or indeed necessary wavelengths to ensure maximum physiological development and high mass yields. The full white spectrum is necessary to maximize photosynthesis and to correctly trigger biological processes.
If we look at the 3500K spectrum of the Sunflow being overlayed with the McCree RQE curve we can observe the following:
The spectrum provided by the LED-chips we use perfectly fits the McCree curve. The LED’s white spectrum delivers a deliberate balance of the whole spectrum between 380nm-780nm. The blue wavelengths increase the production of essential oils and terpenes. In addition to that the Sunflow’s light spectrum effects shorter distances between the nodes (nodes = junction points between leaves and branches).
The often misunderstood region between green and yellow wavelengths is responsible for balanced and healthy plant growth. The orange to red wavelengths is the driving force behind the plant’s fruiting phase and an important factor when it comes to flowering and crop growth.