August 2022 - 42

technically speaking
BY ERIK RUNKLE
Futuristic Light(ing) in Horticulture
The science and engineering of crop production continues
to advance with new or more effective technologies. This is
especially true with the production of specialty crops inside
greenhouses and indoor (vertical) farms. There have been
significant developments in the most important - and often
the most limiting - factor for plant growth: light. While lightemitting
diodes (LEDs) are leading the way (and arguably
advancing the most rapidly) there are other technologies that
supplement or modify the solar spectrum that could also have
applications in horticulture.
LEDs
LEDs have become the primary fixture type for new
greenhouse lighting installations and the dominant
lighting technology for indoor farms. Their efficacy of
converting electricity into photons for plant growth has
increased dramatically, as has their light output, stability
and reliability.
Cost (on a light output basis) has also decreased
over time. However, there are still opportunities for
Erik Runkle is
a professor and
floriculture
Extension specialist
in the department
of horticulture at
Michigan State
University. He can be
reached at runkleer@
msu.edu.
improvements in LED lighting in horticulture, including:
* New phosphors and combinations. Most white
LEDs are blue LEDs covered by a material that
down-converts much of the blue light into longer
wavelengths (green and red light). These materials,
called phosphors, continue to be refined. Specific
objectives include developing a semi-tunable
emission spectrum and increasing the colorrendering
index (CRI). For example, white LEDs
can be developed to have a specific tint (such as a
greenish-white color) that when combined with red
LEDs can improve the efficacy and/or CRI of an
LED fixture.
* More efficient and thermally stable. LED fixtures
are much more effective than conventional lighting
fixtures, but the magnitude varies dramatically from
one product to another. LED efficiency continues
to improve, although most of the possible gains
have already been achieved in some of the newest
products. Still, efficiency improvements of at least
10 to 15% are realistic in the next several years.
* Ultraviolet (UV) light. Potential applications for
UV LEDs in horticulture include suppression of
pathogens and sanitation/sterilization. LEDs on
the market today that emit UV light are generally
expensive, inefficient and have a short lifetime.
However, meaningful developments are being made
primarily because of demand to kill viruses related to
the recent pandemic.
ORGANIC LEDs
Organic LEDs (OLEDs) contain thin layers of organic
compounds sandwiched between two thin-film electrodes, which
42 AUGUST 2022 GPNMAG.COM
Figure 1. In addition to LEDs, there are other technologies that
supplement or modify the solar spectrum that could also have
applications in greenhouses and indoor farms.
emit light when energized by electricity. In contrast, " regular "
LEDs have positive and negative regions and light is emitted
when an electric current flows from one end to the other.
OLEDs are commonly used in digital displays such as television
screens and smartphones, but their application in horticulture is
limited because of their relatively low light output.
There are transparent OLEDs that (as the name implies)
allow most light to pass through them, and conceivably could
be developed into a greenhouse glazing or shading material.
While conceptually interesting, cost and low output of light
are significant barriers to commercialization of this
specific application.
QUANTUM DOTS
Quantum dots are microscopic particles or crystals (millionths
of a millimeter in diameter) that semi-conduct electricity and
emit specific wavelengths (colors) of light when excited by moreenergetic
light or electricity. Quantum dots have a variety of
applications and, in horticulture, can be incorporated into plastic
materials (films or sheets) to modify the solar spectrum.
In particular, quantum dots can be used to convert a portion
of the high-energy photons (UV and blue light) from sunlight
into less-energetic, longer-wavelength photons (usually red and
far-red light).
UV and blue light typically suppress extension growth of plants,
and thus plants may be slightly taller and/or have larger leaves
under a solar spectrum modified by quantum dots. This can
increase the light capture of plants and increase overall plant
growth, but at the expense of compactness, which is often a
desired trait for ornamentals.
A challenge with quantum dot films is that they decrease overall
light transmission, and thus are likely more suitable for regions
with relatively high light levels. The incorporation of quantum
dots into greenhouse glazing materials (potentially combined
with photovoltaics) is conceptually very interesting.
http://www.GPNMAG.COM
August 2022

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