The Science

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

Light quantity is the most directly correlated factor to biomass yields. Photosynthesis is a quantum process, and can be quantified on a photon basis. 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 potential of the lamps capabilities. This does not take into account any loss factors such as distance from light source to the canopy or reflections of walls and such. 

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 PLC640, with a PPF of 1700µmols+, and a coverage area of 4’x4′(1.48645m^2) we get 1700÷.1.48645=~1144µmols/m^2/sec PPFD. This is a good way to estimate, but does not take into account real world losses getting the photons from fixture to the canopy. 

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 in a single measurement. 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 the whole grow area and at what relative intensity. 

Light quality

A better known term for quality is spectrum. Spectrum is the essentially the colors of light that are emitted from a source. Different colors are made by photons at different energy levels. They 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”

2-6RelativeQuantumEfficHEIDI

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.

 The green and yellow region are many times misunderstood, but are the major penetrating force 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.