March 2023 - 14

PRODUCTION
since they can be turned on and
off rapidly without any negative
consequences. That said, it
makes sense to leave the LED
fixtures on or off for at least five
minutes at a time, just so
the lights don't start to flicker
when the sunlight level is right at
the threshold.
A refinement to threshold
A quantum sensor inside a
greenhouse. Note that the
light sensor is mounted
above the supplemental
lighting system, so it only
measures sunlight, not
supplemental light.
energy savings, some utility
companies are starting to
provide rebates for " smart "
lighting controllers, in addition
to rebates for efficient
LED fixtures.
THINK ABOUT LIGHT
SENSORS
Both threshold and DLI
control that can be achieved only
with dimmable LEDs is to provide
only enough supplemental light
to reach a specific threshold. For
example, if the threshold is set at
a PAR value of 400 µmol/m2/s,
and the current sunlight level is
350 µmol/m2/s, it is possible to
control how much supplemental
light is provided. In this case,
adding another 50 µmol/m2/s
of supplemental light would
allow you to reach the threshold
value. Few lighting control systems currently have this
functionality, but we will likely see more of this in the future.
DLI control is the most advanced lighting control
system that aims to make sure that the crop gets a similar
amount of light each day. Rather than controlling based
on instantaneous sunlight levels, the goal is to achieve
a consistent daily light integral (DLI, the total amount of
PAR in the greenhouse in a 24-hour period). The control
system continuously monitors the amount of sunlight,
adds it up over the course of the day and determines
whether supplemental light is needed.
Good DLI control systems can provide very consistent
lighting conditions from day to day by providing more
supplemental light on cloudy days and less on sunny
days. DLI control can be achieved using both HPS fixtures
(see Cornell's LASSI system) or LED fixtures. Although
the dimming capabilities of many LED fixtures in principle
provide better control, a good lighting control system
can be very effective with either HPS or LED fixtures. That
said, not all lighting control systems are created equal
and some are more accurate than others. When looking
into these systems, ask about how well the system really
does control DLI and whether this has been verified by an
independent third party.
Based on my experience, a good DLI control system
not only ensures consistent growing conditions, but
also reduces energy use by 20 to 30% compared to
less advanced controllers. And on an annual basis, that
can be a significant amount of money. Because of these
14
MARCH 2023 GPNMAG.COM
control rely on accurate light
readings from one or more
light sensors. Think about
where the light sensor you use
to control lights is mounted.
Is it inside or outside of the
greenhouse? If it is outside,
do you know how much of
the outdoor sunlight actually
gets into your greenhouse?
Probably less than you think,
but you should measure it. And
in many cases, light sensors
mounted outside of the greenhouse are pyranometers,
which measure the total energy of the sunlight, and
not PAR sensors. In principle that is not a major issue,
since the energy of sunlight can be converted to PAR by
multiplying by 2.02.
The other issue related to light sensors is maintenance.
Ideally, light sensors should be cleaned regularly, since
dust buildup on the sensor will affect the readings. In
addition, light sensors should be recalibrated every
one to two years. This is especially important for older
models of light sensors, which often are not sealed very
well and allow water vapor to get into the sensor. That
water vapor can affect the calibration and eventually
make the sensor inaccurate. This seems to be less of
an issue with newer sensors, since reputable sensor
companies now do a much better job sealing the
sensors. Remember that not cleaning or calibrating
your light sensors may result in bad readings, which will
inevitably affect the lighting control system. To check the
calibration of your light sensors, there is a free online
tool available at www.clearskycalculator.com. Make sure
to read the instructions carefully.
Marc van Iersel is a professor of horticulture at the University
of Georgia and co-founder of Candidus Inc., a lighting control
company. This work was supported by USDA-NIFA-SCRI
(project #2018-51181-28365, Lighting Approaches to Maximize
Profits). For more information about this lighting research, visit
www.hortlamp.org.
http://www.clearskycalculator.com http://www.hortlamp.org http://www.GPNMAG.COM
March 2023

Table of Contents for the Digital Edition of March 2023
