January 2022 - 42

technically speaking
BY ERIK RUNKLE
A Closer Look at LED Efficacy
Arrays of light-emitting diodes (LEDs) have become the
predominant type of fixture for new plant lighting installations.
While LEDs typically have many advantages to conventional
fixtures, they are primarily chosen because of their effectiveness
at converting electricity into light that is useful for plants. For
horticulture applications, fixture efficacy is quantified by the
number of photons with wavelengths from 400 to 700 nm (the
photosynthetically active radiation waveband) emitted by a
lighting device divided by the input power to emit that light. This
term is referred to as the photosynthetic photon efficacy
(PPE) and is in the unit of micromoles of photons per
joule (µmol∙J-1
).
Until 2014, the most effective horticultural LED
fixtures were similarly effective as the most effective
conventional fixture, the double-ended high-pressure
sodium with an electronic ballast. Both had efficacies
of around 1.7 µmol∙J-1
. The technology of LEDs
1.9 µmol∙J-1
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.
has significantly advanced since then, as has the
transparency and rigor of LED performance metrics
specific to horticultural lighting.
In October of 2018, the nonprofit DesignLights
Consortium (DLC) established technical requirements
for LED-based horticultural lighting products. A
product that meets or exceeds these metrics ensures
it is relatively energy efficient, long lasting, safe to
use, and tolerant of growing conditions (e.g., high
humidity). Among the criteria is a minimum PPE of
, although because there is a 5% tolerance to
this value, technically the minimum PPE is 1.81 µmol∙J-1
. This
threshold value will likely increase in the next few years.
In the beginning of 2020, there were just over 50 LED
products on the DLC's horticultural lighting qualified product list
(QPL) and the highest fixture efficacy was around 2.8 µmol∙J-1
.
As we enter 2022, there are over 600 products on the DLC's
horticultural QPL and the PPE value ranges from 1.81 to 3.69
µmol∙J-1
. Therefore, although a product on this list can be
considered energy efficient compared with conventional lighting
fixtures, there is wide variation in product efficacy. Of these
600+ LED products, over half have a PPE of at least 2.5 µmol∙J-1
but only about 6% have a PPE of at least 3.0 µmol∙J-1
. The PPE
values of qualified LED products can be found online at
https://qpl.designlights.org/horticulture.
While the efficacy of an LED fixture depends on several factors,
the primary determinant is the composure of the individual
diodes. Red LEDs are roughly 30% more effective than blue
LEDs, and blue LEDs are about 20 to 25% more effective than
white LEDs. Therefore, the most efficient fixtures contain 90 to
95% red LEDs and 5 to 10% blue LEDs (Figure 1). LEDs that
contain mostly red with some white LEDs have slightly lower PPE
values. While a fixture that includes white LEDs will invariably
Figure 1. Because red photons have less energy than those with
shorter wavelengths, the most effective LED fixtures usually emit
at least 90% red light.
have a lower PPE value, the emitted light can improve the
color rendering of plants and create a more pleasant working
environment.
Additional factors that influence LED fixture efficacy include
the drive current and operating temperature. A low drive current
can increase fixture efficacy, but at the consequence of reduced
light output. LED efficacy decreases as temperature increases,
so effective passive (e.g., by heat sinks) or active (e.g., by fans
or flowing water) heat dissipation is critical to achieve high
efficacy as well as LED longevity. Finally, power supplies vary
in efficacy and are typically less efficient when the spectrum,
intensity, or both are tunable.
The efficacy of individual diodes and thus LED fixtures
has increased dramatically the last several years, but future
gains will likely occur at a slower pace. The reason is that the
efficacies of the best-in-class LED fixtures are approaching
their theoretical maxima. Compared to the most effective LED
products on the market today, realistic estimates are that
horticultural LEDs can increase in efficacy by around 25%, but
that could take a decade.
42 JANUARY 2022 GPNMAG.COM
https://qpl.designlights.org/horticulture http://www.GPNMAG.COM
January 2022

Table of Contents for the Digital Edition of January 2022
