Lighting is described in terms of its QUALITY (Spectrum) and its INTENSITY (P.A.R.).
Light is made up of photons that travel in waves. We talk about them in terms of wavelengths. The photons that affect plants are found in the range between ultraviolet and infrared, near the visible end of the spectrum, at wavelengths between 350 and 750 nm (Fig. 2).
• Near UV (350-380 nm) : These wavelengths help fight certain microorganisms and improve plants’ immune systems.
• Visible (380-700 nm) : This is the range of light that is used in photosynthesis. Plants absorb mainly blue and red light. In general, blue light (430-500) promotes growth and red light (630-680) promotes blooming and fruiting.
• Near IR (700-750 nm) : Also known as “Deep Reds”, they act as radiant heaters and help sun-loving plants to grow tall.
Only photons with wavelengths between 400 and 700 nm (close to the visible range) contribute to photosynthesis. These photons are called « Photosynthetically Active Radiation ». The PAR of a lighting system can be measured when it is used alone (PPF-PPE) or within a complete growing configuration (PPFD).
LIGHTING SYSTEM ONLY
The lamp is placed in an Ulbricht sphere (Fig. 3), and the number photons it emits every second is measured. This gives the « Photosynthetic Photon Flux »,(PPF, measured in μmol/s), which assesses the potential photosynthetic power of the lighting system. Dividing the PPF by the electricity used gives the « Photosynthetic Photon Efficacy » (PPE, measured in μmol/s/w or μmol/d), which is used to assess the potential photosynthetic efficacy of a lighting system per watt consumed. Here are some example PPE values : Compact fluorescent lights (CFL, TCL, T5, etc.) = Less than 1 μmol/s/w, discharge lights (HPS, MH, CMH, etc.) = 1 to 2 μmol/s/w, light-emitting diodes (LED): 2 to 3 μmol/s/w at present.
COMPLETE GROWING SYSTEM
The number of photons received by the grow space per square metre are measured every second. This is called the « Photosynthetic Photon Flux Density », (P.P.F.D. measured in μmol/s/m2). This makes it possible to evaluate the real photosynthetic power, per m2 of a grow space. The PPFD is represented by a top view of the grow space, with measurements taken every 15 x 15 cm (6″ x 6″) (Fig. 4). To simplify, we have specified three colours for three PPFD levels.
Level I : Less than 300 μmol/s/m2 = Propagation, seedling, light growth, vegetative.
Level II : 300 to 600 μmol/s/m2 = Growing of Sun-loving plants, intensive growing of shade-loving plants.
Level III : Over 600 μmol/s/m2 = Intensive growing of sun-loving plants.
There are two main types of LEDs. Monochrome LEDs emit photons that vibrate at a single frequency. Depending on their wavelength, they are called: 450 nm blue, 660 nm red, etc. White LEDs have a broader spectrum. White LEDs are in fact a monochrome blue LED with a phosphor coating that shifts and flattens the spectrum towards yellow by decreasing the energy of certain photons (Fig. 5). Depending on their colour temperature, in Kelvin, they are called: 6500 °K Cool White, 5000/4000 °K Warm White, 3000/2700/2100 °K Yellow White. This method produces LEDs that can be used for everyday lighting as well as for growing, due to their broader spectrum.