February 2019 - 42

technically speaking
BY ERIK RUNKLE
Selecting an LED Fixture
Horticultural lighting fixtures containing arrays of lightemitting
diodes (LEDs) abound. Established plant lighting
companies, companies with general illumination but not
horticultural lighting expertise, and small start-up companies are
increasingly developing and marketing them.
With so many LED products now on the market, and more
on the way, choosing the right LED fixture is a crucial decision.
Below are factors to consider when choosing an LED fixture for
plant applications.
Application. There are three major lighting
applications: low-intensity greenhouse lighting to create
long days and regulate flowering, moderate-intensity
supplemental greenhouse lighting to increase growth,
and high-intensity sole-source lighting for indoor use.
Each LED fixture model usually only has one application,
so be sure they match. For example, sole-source lighting
fixtures are usually not designed for greenhouse use.
Durability. Fixtures should be constructed to tolerate
horticultural (greenhouse) conditions such as high
humidity and high temperature. Recently, the American
Society of Agricultural and Biological Engineers released
a second standards document on methods for measuring
and testing of LED products for plant applications. This
document formalizes the need for horticultural lighting
fixtures to withstand damp and moderately hot conditions.
Longevity. The standards document also requires
Erik Runkle is
professor and
floriculture
Extension specialist
in the department
of horticulture at
Michigan State
University.
reporting of long-term performance changes to LEDs (i.e., the
expected lifetime). For a product to be listed by the DesignLights
Consortium (DLC), LED fixtures should operate at least 36,000
hours before light output decreases to 90 percent of its initial
value. Therefore, look for product guarantees or warrantees that
meet or exceed this value.
Efficacy. The photosynthetic photon efficacy (PPE) refers to the
total quantity of photosynthetic photons emitted from a fixture
per second (µmol·s-1
) per Watt of electricity. The unit for efficacy
is µmol·J-1, with higher numbers indicating a more efficient fixture.
While this value is an important factor when choosing a fixture,
it can be overemphasized. As a guideline though, look for LED
products with a minimum PPE of 2.0 µmol·J-1
least 2.5 µmol·J-1
and preferably at
. Do not purchase products unless the PPE is
explicitly reported.
Light uniformity. The intensity and variability of light emitted
from fixtures to a growing surface are equally important. Growers
should have a target light intensity at installation, as well as a
tolerance in uniformity. Generally, the installed light intensity
should not deviate more than 20 percent across the growing area.
Cost. The greatest barrier to commercial implementation
of LEDs is economics. In some cases, such as seasonal use of
supplemental greenhouse lighting, conventional fixtures (such as
Figure 1. There are a number of factors to consider when choosing
an LED (or conventional) lighting fixture for plant applications.
high-pressure sodiums) are still more economical. Work with a
reputable lighting company to generate a lighting plan, which will
indicate the number and position of fixtures needed to deliver the
desired light intensity. LEDs are usually more expensive, but can
lower operating costs offset the higher investment cost within a
reasonable (three or four year) period?
Ease and cost of installation. Some LED fixtures are easier and
less expensive to install than others. Be sure the installation cost,
including additional electrical supply (if needed), is included when
you compare costs for different lighting fixtures.
Light spectrum. LED fixtures have a wide range of emission
spectra, but most emit a whitish, pinkish, or purplish light. Usually
the purple light-emitting fixtures have the highest PPE values,
but they can be rather unpleasant to work under, especially at
night. Thus, while fixtures are primarily for plant applications, also
consider the visual effects.
Minimal shading. Some LED (as well as conventional) fixtures
are quite large, which when installed in a greenhouse, shade
plants below. Consider the surface area (length × width) of the
fixture and the number of fixtures needed to deliver the desired
intensity, with the objective of minimizing the shading factor.
Customer service. There is a lot of demand for horticultural
lighting now, and some companies are better than others at
returning phone calls, answering questions in a timely manner,
etc. Ask them for customer referrals or testimonials, as well as
fixture warrantees and/or guarantees. If some fixtures fail, how
will they respond? Will the company still be in business?
Eligible for utility rebates. Work with your utility company to
determine whether you can obtain a rebate if your fixture meets
certain criteria. Some rebates can be large enough to change the
investment decision of one lighting fixture over another. Thus,
include any rebates in your economics calculations.
42 FEBRUARY 2019 GPNMAG.COM
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February 2019

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