For tens of millions of years, plants have evolved to make the most of a single source of light: the sun.
Although artificial light will never be a perfect substitute for the sun, we have nevertheless succeeded in designing lighting under which plants thrive, both in their vegetative and flowering phases.

Below, we look at a few concepts that are essential to understanding LED lighting for horticulture.

- Colors and the light spectrum
Visible light is an electromagnetic wave between 400 and 700nm in length.
As shown in the diagram below, each wavelength in the visible field corresponds to a color. This chromatic palette is called the "spectrum".

Unlike other types of lighting, LEDs (Light-Emitting Diodes) have the particularity of emitting monochromatic light within a very limited wavelength range (approx. 10nm). (We'll see later that this property can be altered by the addition of a phosphor).
It is also possible to combine several LEDs with different wavelengths within a single light. This lighting will then emit several wavelengths, and the light perceived by the eye will be the sum of the colors of each LED.

- Light and Photosynthesis
The process of photosynthesis enables plants to synthesize the organic matter necessary for their development from light energy.
Photosynthesis is triggered by certain wavelengths of the light spectrum, mainly in the blue and red tones.
The diagram below shows the wavelengths involved in photosynthesis:

- Intensity
To assess the intensity of light, we'll look at three measurements: "lumen", "lux", PAR and PPFD.
The "lumen" is a measure of the total amount of visible light emitted by a light source. The higher the number of lumens, the more "intense" the light source.
The "lux" is a unit for measuring the luminous intensity received per unit area.
Whereas the lumen is concerned with the light source, the lux is concerned with the illuminated surface. Distance from the light source is accompanied by an exponential drop in lux, i.e. light received.
PAR (Photosynthetically Active Radiation) is the radiation that plants can use in the photosynthesis process. So to speak, the basis of lumen is human perception, while the basis of PAR is plant perception.
PPFD is a unit of measurement for PAR received by a given surface. It's a kind of plant lux.

Here are some reference measurements, for an ordinary plant:

- Power consumption
Like all electrical appliances, the Watt is used to measure the amount of electricity consumed by horticultural lighting.
With its excellent light output / power consumption ratio and long life, LED is the most efficient of all lighting.

- White light
There is a unit of measurement called "Kelvin" for determining the various shades of white.
The scale ranges from 1,000 K (warm yellow) through all shades of white to 10,000 K (cool blue).
Below is an illustration of the Kelvin scale.

- COB (Chip On Board)
COB technology concentrates a large number of LED units on an aluminum plate. The result is a more reliable, more powerful and more homogeneous light source.
By selecting the wavelength of each individual LED in a COB, we can create a precise, "made-to-measure" spectrum.
What's more, color mixing on a COB takes place "at source", unlike using 3W single LEDs scattered across a panel.

- White LEDs
As we saw earlier, an LED emits a very narrow wavelength of around 10nm, i.e. monochromatic light.
However, white light does not have a wavelength of its own: it is in fact a vast spectrum encompassing all colors.
Therefore, in order to produce white light, "white LEDs" combine a blue LED with a phosphor. The light produced by the blue LED is absorbed in part by the phosphor and transformed by it into colors ranging from green to red. It is the superposition of this emission and the unabsorbed blue that recreates the spectrum of white light.
As illustrated in the graph below: