User Manual

www.we-online.com ANO003a // 2018-08-01 // RiB 1
Advantages of LED L
ighting in Horticultural
Applications
Application Note
ANO003 // DR. RICHARD BLAKEY
1 Introduction
High intensity discharge (HID) lamps are the current industry standard
used by the artificial lighting greenhouse industry because of their
economic viability and providing a consistent, adequate spectrum for plant
growth
[1]
. Light emitting diodes (LED) provide a multitude of advantages
as horticultural lighting sources but early difficulties, primarily cost and
intensity, limited their implementation in horticultural applications.
However, rapid advances in LED design and manufacture have closed the
gap to traditional discharge based lighting technologies and are now
becoming an economically viable alternative to HID sources, especially for
high-value crops
[2]
, which some have called “a monumental shift”
[3]
. The
following AppNote compares the advantages of LEDs with traditional HID
light sources for horticultural applications. Although the properties have
been addressed in different sections, they are highly interrelated. Gains in
one performance characteristic will compromise others. For an
introduction to the use of LEDs in horticultural applications, please refer
to ANO002 LEDs - The Future of Horticultural Lighting.
2 Output Intensity
Initially, the intensity of LEDs was too low to be of practical use in
horticulture, being more suited to indicator lights and control panel
backlighting. The intensity of light that can now be generated by LEDs
means photosynthetic photon flux (PPF) output is comparable to that of
HID sources when used in clusters. The output intensity of lighting is
usually expressed in lumens, as humans perceive light, which is biased
towards the sensitivity of the eye. However, photosynthesis and plant
growth is driven by photons and so is quantified as PPF. This is especially
important when comparing LEDs that can generate specific wavelengths
of light. As radiant energy is inversely proportional to wavelength, “red
photons” have a lower radiant energy content resulting in more photons
being generated per unit of input energy. This means that although blue
LEDs have higher radiant flux than red LEDs, the difference in PPF is much
closer (Figure 1). It is difficult to compare the output intensity of LED and
HID sources in a useful way due to a number of factors including, the
number of LEDs, the inherent radiation pattern of the devices (LEDs are
unidirectional while HID lamps have an omnidirectional broad emission
pattern), and the use of reflectors and lenses. The aim is to maximize the
transfer of the emitted light from the light source to the plant leaves. It
may be therefore, more interesting to consider how light is delivered to
the plants. There is no perfect emission distribution pattern but there are
some that are more suitable for certain greenhouse configurations.
Precision overhead luminaires and lenses can be used to control the
emission pattern of HID devices and focus light to the plant growth areas.
This is necessary in small greenhouses with widely separated cultivation
areas. Canopy photon capture efficiency of above 90% can be achieved
in this manner, regardless of the light source. But capture rates near to
100% can be achieved using LED intracanopy lighting
[4]
. The heat
generated by HID fixtures makes intracanopy lighting infeasible.
Figure 1: Comparison of PPF and radiant flux of the WL-SMDC Deep Blue (150 353 DS7 4500) and Hyper Red (150 353 HS7 4500)
0.E+00
2.E-08
4.E-08
6.E-08
8.E-08
1.E-07
1.E-07
1.E-07
2.E-07
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm
Photon Flux (µmol ⋅ s
-1
)
Radiant Flux (J ⋅ s
-1
)
Wavelength
Deep Blue (Radiant Flux) Hyper Red (Radiant Flux) Deep Blue (Photon Flux) Hyper Red (Photon Flux)