November/December 2022 - 42

technically speaking
BY ERIK RUNKLE
Getting Started With Supplemental
Greenhouse LED Lighting
The use of electric lighting inside greenhouses is nothing new,
but the technology and sophistication of lighting have advanced
dramatically in the last several years. Unlike conventional
fixtures (i.e., high-pressure sodium), light-emitting diodes (LEDs)
literally come in all shapes and sizes, emit different colors and
intensities, and have a range of energy
efficacies. This can make the process
of choosing a lighting product
daunting. This article outlines
how to get started with
supplemental greenhouse
lighting with LEDs.
1. Determine the desired
light intensity.
Supplementing sunlight
is most useful during lightlimiting
conditions; for
example, when the daily light
integral (DLI) from sunlight is
less than 10 moles per square
meter per day (mol∙m-2
∙d-1
).
The first step is to determine
Erik Runkle is
professor and
floriculture
Extension specialist
in the department
of horticulture at
Michigan State
University. He can be
reached at runkleer@
msu.edu. Reference
to companies,
commercial products
or trade names
does not imply
endorsement or bias
against those not
mentioned.
the light intensity you want to deliver,
which is based on how much light
you receive from the sun and the
minimum target DLI for your crop(s).
For example, if you receive an average
of 5.5 mol∙m-2
∙d-1
during your darkest
month from the sun, and your target DLI is 10 mol∙m-2
an additional 4.5 mol∙m-2∙d-1 needs to come from
4.5 mol∙m-2
∙d-1
∙d-1
supplemental lighting.
The instantaneous intensity needed to deliver this
is calculated by converting mol to µmol,
dividing that value by the hours (h) you plan to light each day,
then dividing that value by 3,600 (the number of seconds per
hour). For example, if you plan to light 18 h∙d-1
∙d-1 = 4,500,000 µmol∙m-2
:
* 4.5 mol∙m-2
* 4,500,000 µmol∙m-2∙d-1 / 18 h∙d-1
* 250,000 µmol∙m-2∙h-1 / 3,600 s∙h-1
∙d-1
= 250,000 µmol∙m-2
= 69.4 µmol∙m-2
∙s-1
2. Quantify the growing area and constraints.
Several characteristics of the growing space need to be
measured, including the dimensions of the greenhouse growing
area. The maximum hanging height of light fixtures also needs
to be determined. Identify placement options and constraints for
fixtures considering overhead obstructions, especially energy/
shade curtains, and electric service capacity.
3. Contact multiple companies for quotations.
Make initial contacts with several lighting companies
or lighting distributors, considering their knowledge and
42 NOVEMBER/DECEMBER 2022 GPNMAG.COM
∙h-1
,
experience with plant lighting. A lighting representative
should engage with you to understand your lighting needs and
propose different options. Ask them for referrals of greenhouse
customers and contact them to assess their experiences with
the company and product. Expect timely communication with
the lighting company/distributor
and move on if you don't receive
sufficient attention.
4. Understand efficacy.
One of the more important metrics
of a lighting fixture is its efficacy,
which refers to how efficient it
converts electricity into light useful for
photosynthesis. The efficacy metric is
in micromoles of photons within the
photosynthetically active waveband
per watt of energy consumed. Efficacy
has a unit of micromoles per joule
(µmol∙J-1
or µmol/J), and higher
values mean the fixture is more
efficient. Target a minimum fixture
efficacy of 2.5 µmol∙J-1
.
Choosing an LED product for supplemental
lighting can be a daunting task.
5. Only consider LEDs that are a
qualified horticultural product by the
DesignLights Consortium (DLC).
This helps ensure the lighting fixture
is at least moderately efficient, can
withstand greenhouse conditions,
has at least a five-year warranty and meets other performance
minimums. For more, visit www.designlights.org.
6. Consider the light spectrum for people.
The most efficient LEDs emit mostly red light, but, in most
cases, at least some blue light also needs to be delivered
for acceptable plant growth. A combination of blue and red
light creates a purple light that is energy efficient but visually
unpleasant. The inclusion of white LEDs in a fixture slightly
decreases the fixture efficacy but creates a softer pink or
whiter light that improves color rendering and creates a more
comfortable work environment.
7. Compare lighting solutions.
Obtain multiple lighting plans from different companies and
compare costs to purchase and install, including eligibility of
rebates and incentives from utility providers. Also consider the
fixture efficacy, light spectrum, uniformity of the lighting plan,
product warranty, fixture longevity, projected shipping date and
nontangibles like customer service. A lot of the LED product
information is on the horticulture qualified product list (Hort
QPL) of the DLC website. When possible, discuss options with an
unbiased plant lighting expert.
http://www.designlights.org http://www.GPNMAG.COM
November/December 2022

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