February 2021 - 30

RESEARCH
MAKING SENSE OF
Light Sensors
Learn to convert radiation data from your environmental computer into daily light integral.
BY ROBERTO G. LOPEZ AND ERIK S. RUNKLE
D
uring the winter, greenhouse growers in Northern regions often
become concerned that their plants are not receiving enough
light to produce good quality crops. However, many do not
know what light levels their crops are receiving because they do not
understand the radiometric units their environmental computers display
each day. In this article, we discuss light sensors, light units, and how
to convert the units your environmental computer displays into more
meaningful values.
Plants use and respond to wavelengths of solar radiation ranging
from UV-B (as low as 280 nm) to far-red (FR) light (as high as 750 nm
or so). However, photosynthesis is mainly stimulated by wavelengths of
light between 400 and 700 nm, which is known as photosynthetically
active radiation (PAR). Outside of the PAR range, energy can contribute
less or not at all to photosynthesis but may influence the concentration
of pigments (UV radiation), regulate photoperiodic and elongation
responses (FR radiation), and increase plant temperature (infra-red; IR).
These wavebands as well as other light terms and units are defined in
the sidebar.
There are several units used by scientists and growers to measure light
both inside and outside the greenhouse. For plant applications, we should
use light units that represent the number of photons within PAR since
they are what drive photosynthesis and thus plant growth. Therefore, we
need to use quantum units. Since the number of photons is so large, we
measure them in moles (mol) of photons, and 1 mol = 6.022 × 1023
micromole (μmol) equals 1 × 10-6
. One
mol so therefore, 1 μmol = 6.022 × 1017
Most environmental control computers are connected to a pyranometer
mounted to a weather station outside the greenhouse (Figure 1). This
sensor measures total short-wave energy from 300 to 2,800 nm from the
sun and sky. This includes UV radiation, photosynthetic (and visible) light,
near infra-red (NIR), and IR. These measurements can be used to assist
in an array of climatic and irrigation decisions and environmental control
settings such as heating, cooling, shading, irrigation, and supplemental
lighting. Figures 2 and 3 show screenshots as examples of radiation sums
reported by environmental control computers.
Pyranometers measure total short-wave energy, but we are interested in
photons within PAR. A pyranometer provides radiometric units, which are
typically expressed in watts per square meter (W∙m-2
meter and second (J∙m-2
∙s-1). (Note that 1 W = 1 J∙s-1
∙s-1 equals 1 W∙m-2
interested in PAR, we need quantum units in μmol∙m-2∙s-1. Therefore, we
need to do some math to convert from radiometric to quantum units.
As noted above, 1 J∙m-2
or J∙m-2∙s-1 ≈ 4.57 μmol∙m-2∙s-1 for sunlight (conversely, 1 μmol∙m-2
0.22 W∙m-2) can be used IF the W∙m-2
or J∙m-2
28 FEBRUARY 2021 GPNMAG.COM
. The approximation that 1 W∙m-2
∙s-1
=
∙s-1 that your environmental
Figure 1. A pyranometer mounted on a weather station atop a greenhouse.
(Photo: Jonah Brown)
) or joules per square
.) Since we are most
.
Chuck Jagger, director of production at Henry Mast Greenhouse in Byron
Center, Michigan is downloading accumulated radiation data from their
environmental computer to calculate the daily light integral (DLI) for the
previous day.

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February 2021

Table of Contents for the Digital Edition of February 2021