User Manual

www.we-online.com ANO003a // 2018-08-01 // RiB 3
Advantages of LED L
ighting in Horticultural
Applications
Application Note
4 Light Quality
The key advantage of LEDs here is the ability to adjust and optimize the
total light spectrum. This can be used to enhance and improve
photosynthetic efficiency and control developmental phases
[8]
but also to
reduce the amount of wasted light and therefore energy. Because of their
monochromatic output, a number of LEDs with different wavelengths can
be used to configure light “recipes” specific to species, cultivars and
growth phases
[9]
. This is opposed to HID sources that have a fixed output
spectrum, which supply sufficient quantities of light in some wavelengths
while providing excessive or deficient quantities at others (Figure 5).
Additionally, the light recipe cannot be modified to suit a plants
development (Figure 6). There are currently a number of projects that use
feedback control to optimize the light recipe (and other parameters) to the
growth stage of plant. These systems use cameras, usually in the visible
or infrared spectrum.
The ultraviolet region (UVA and UVB, 280 to 400 nm) is currently a very
interesting topic in horticulture. Sunlight consists of 9 % UV (percent of
PPF) while HID sources emit a fixed level of 0.3 to 8 % UV radiation
(percent of PPF)
[10]
. With LEDs, it is very easy to control the level of
exposure. Deficient levels of UV can interrupt development in some plant
species
[11]
. HID sources have minimal far-red radiation (710 to 740 nm),
which LEDs are capable of efficiently generating.
The importance of far-red radiation can be found in ANO004. Green
LEDs (530 to 580 nm) are not usually directly utilized in LED fixtures as
these frequencies were thought to be less important for photosynthesis.
However, these wavelengths have better penetration through the canopy
and can be important for development and response mechanisms
[12]
.
Light in this wavelength range are usually delivered using white (phosphor)
LEDs that also augment blue wavelengths.
Figure 5: Typical Emission Spectra of Light Sources used in Horticulture.
The green shaded area represents the action spectrum of photosynthesis meaning any peaks outside of this is wasted energy
Figure 6: Possible Light Recipes used in Different Developmental Phases of Plants
0.0
0.2
0.4
0.6
0.8
1.0
400 nm 450 nm 500 nm
550 nm 600 nm
650 nm
700 nm
750 nm
Relative PPF (arb.)
Wavelength
High Pressure Sodium Metal Halide Fluorescent Typical LED Recipe
0
0.2
0.4
0.6
0.8
1
400 nm 600 nm 800 nm
Flowering
0
0.2
0.4
0.6
0.8
1
400 nm 600 nm 800 nm
Wavelength
Growth
0.0
0.2
0.4
0.6
0.8
1.0
400 nm 600 nm 800 nm
Relative Intensity (Arb.)
Propagation