Light and Plant Growth
There are many different ways of assessing light. But there are certain methods and terms that pertain specifically to plant growth and its symbiotic relationship with light. Quantity(or intensity) and quality(or spectrum) are 2 very basic but sometime misunderstood factors. They are also the 2 most important, so let’s look at them closer.
Light quantity is the most directly correlated factor to biomass yields. Photosynthesis is a quantum process, and can be quantified on a photon bases. There are three ways of measuring and testing light quantity.
PPF(photosynthetic photon flux)- is the unit of measurement that shows the total output of a lamp or light source. It is a measurement in µmols/sec of photons emitted by the source. It is a very useful figure because it shows the ability and/or possibility of the lamps capability.
Average PPFD(photosynthetic photon flux density)– As we have said before, it is the relationship between light and plants that makes up a grow light, and grow space is a one of the most essential factors. 1 sq meter is the area unit which studies and scientist have standardized for the measurement unit PPFD. The “D” signify density, meaning how much PPF per area(1m^2). Studies of many varieties of plants have determined that 700-1000µmols/m^2 average is the optimal range for higher light crops. The PPF(total output) of the lamp ÷ by the area in m^2 will give you the average PPFD of the area based on the PPF available. In the case of the CX300, with a PPF of 720µmols, and a coverage area of 3’x3′(.836m^2) we get 720÷.836127=~861µmols/m^2/sec.
Instantaneous PPFD(PAR meters)- Many people have seen instantaneous PPFD readings from tools like PAR meters. These are very useful to see how light is distributed and to check direct intensity levels relative to plant growth. How ever they do not tell the PPF of the lamp, or a true average PPFD of the entire canopy. They are only an extrapolated square meter figure based on that single point in space. As one single measurement these meters can be very misleading, especially with LEDs and their target beams. But when used as a network of instantaneous data points, we can see how well a lamp or source is covering an area and at what relative intensity.
A better known form of light quality is spectrum. Spectrum is the essentially the colors of light that are emitted from a source. Different colors are made by photons at certain energy levels and are broken down into nanometers(nm’s). When it comes to photosynthesis, the differences in the nm of a photon, or it’s color, changes it’s potential effectiveness to drive photosynthesis. This was shown by Dr. McCree in 1972, when he studied and averaged 22 different species of both indoor and field grown plants by their carbon fixation in response to different wavelengths of light. When graphed for every nm in the spectrums range(320nm -780nm), we get what is known the McCree Relative Quantum Efficiency curve, or RQE. To this day, is the only scientifically accepted “action spectrum”
Notice that there is no extreme differences across the range as some are lead to believe. Though red and blue nm’s are driving forces in photosynthesis and physiological development, they are no way the only useful or necessary nm’s for maximum physiological expression and biomass yields. A full and targeted spectrum is needed so that photosynthesis is maximized and that biological processes are triggered correctly.
Now look at our 3500K spectrum, with the McCree RQE curve overlayed
It fits perfectly with the McCree action spectra in addition to its high photon output. Maximizing the use of every photon it can produce. As you can see the white is a targeted balance of the whole spectrum from 380nm-780nm. The blue is elevated over standard spectrums to increase essential oil and terpene production as well as induce shorter nodal spacing. The green and yellow region are many times misunderstood, but is the major penetrating area of the spectrum and aid in full plant development. The upper yellow through red is the driving force in growth and development of flowering and fruiting plants. In the 3500k white, the bulk of the photons are in the prime photosynthetic areas of the spectrum , 400-700, but there are also ideal ratios of far red that initiate biological processes and photomorphological effects.