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1) What is light?

Light is essential to our vision and plant growth but the way our eyes and plants react to this light are entirely different processes. While the overall physics and science of lighting can be complex we’re going to reduce it to it’s bare elements here and primarily concentrate on the important plant/light interactions and how we measure that light.

WAVES: All light consists of electromagnetic waves in a spectrum that range from the low end ultraviolet (UV) to the high end infrared (IR) of the spectrum. The wavelength is measured in nanometers (nm) and consists of both visible and invisible light.

PARTICLES: Light is measured in PHOTONS which are a quantum or individual unit. Since individual photons possess tiny amounts of energy, photons are measured in units of moles (mol), which are 6.02 x 1023 photons. Micromoles (µmol) are one-millionth of a mole.

QUALITY: Photons have different amounts of energy, determined by their wavelengths. Light quality is the relative number of light particles at each wavelength. Light quality refers to the spectral distribution of light, or the relative number of photons of each portion of the spectrum, both visible and invisible that our light emits.

PAR LIGHT: During the photosynthetic process where the plant turns light into its energy it requires certain wavelength spectrums which we refer to as Photsynthetic Active Radiation or PAR for short. While PAR light spectrum ranges between 380-720 nm the region brightest to human vision (555 nm – Green, Yellow and Orange) has the least effect on plants. Consequently lightmeters that measure human vision levels (lumens,lux,footcandles) are not as effective as quantum type meters in determining if plant lighting levels (YPF, PPF) are actually being met.

MEASURING PAR: Quantum light meters differ from visible light meters in that they will tell you how much many photons are striking a square meter per second. This can be taken as a moment in time ‘incident reading’ at the plant and the unit of measurement will be measure in micromoles and expressed as µmol/m2. To give you some reference, using a quantum meter, sunlight on a cloudless day would measure 2,000 µmoles at the leaves.

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