February 2021 - 32

RESEARCH
Figure 2. A screen shot of a Priva environmental computer weather station. Note how
the radiation sum the previous day was 272 joules per square centimeter per day.
Figure 3. A screen
shot of an Argus
environmental
computer weather
station. Note how
accumulated
radiation sum
( " Accum Light " )
was 114 joules per
square centimeter
per day.
CONVERSIONS IN ACTION
Let's say you want to calculate the daily light integral (DLI) that is
reaching your greenhouse (outside) each day, and your environmental
computer provides a radiation sum in J∙cm-2
1,230 J∙cm-2
1,230 J∙cm-2
∙d-1. First, let's convert cm2
∙d-1 × 10,000 cm2
to m2
per m2
= 12,300,000 J∙m-2
Next, let's convert days into hours and then into seconds:
12,300,000 J∙m-2
∙d-1
∙d-1 / 24 h per d / 3,600 s per h = 142.4 J∙m-2
∙s-1
Since 1 W = 1 J∙s-1, we can convert this to W∙m-2 so it has a value of
142.4 W∙m-2. As discussed earlier, there is 1.96 μmol∙m-2∙s-1 of PAR for
every 1 W∙m-2 of short-wave radiation, so we can then convert this into
quantum units:
142.4 W∙m-2
× 1.96 μmol∙W-1 = 279.0 μmol∙m-2
∙s-1
This is the average value received during a 24-hour period. We can
convert this from an instantaneous quantum unit of μmol∙m-2
daily light integral (DLI) cumulative unit in mol∙m-2
279 μmol∙m-2
∙s-1
∙d-1
∙s-1
∙d-1
:
× 3,600 s per h × 24 h per day =
24,105,600 μmol∙m-2
into mol∙m-2
Since there are 1,000,000 μmol per mol, we can then convert this
∙d-1
:
24,105,600 μmol∙m-2
∙d-1
24.1 mol∙m-2
/ 1,000,000 μmol per mol =
∙d-1
This is the DLI delivered outdoors, since your sensor is located
outside. For plant growth, we need to know what the DLI is inside your
greenhouse; this depends on light transmission through the greenhouse
glazing, the greenhouse structure, and overhead obstructions such as
hanging baskets and lighting fixtures. The light transmission inside a
greenhouse and to your crop is usually between 60 and 80% when there
is no shading (whitewash or shade screen).
30 FEBRUARY 2021 GPNMAG.COM
to a
∙d-1. Let's use an example of
:
The transmission percentage can be determined by taking a light
reading outside the greenhouse on a clear day around solar noon, then
going inside and measuring light intensity in your greenhouse at plant
height. You can then calculate what percentage of light outdoors is
transmitted inside. In this case, any kind of light sensor can be used to
determine the light transmission percentage.
For example, if you measured 1,400 μmol∙m-2
925 μmol∙m-2
approximately:
∙s-1
outside and
∙s-1 inside, then your light transmission percentage is
925 μmol∙m-2∙s-1 / 1,400 μmol∙m-2
∙d-1
∙s-1
= 66%
Using the example above, the indoor DLI would be 24.1 mol∙m-2
0.66 = 15.9 mol∙m-2
.
The light transmission percentage of a greenhouse changes over time,
especially when whitewash is applied or removed, or when new glazing
is installed. Therefore, it should be measured periodically to improve
accuracy of your readings.
To conclude, if your environmental control computer measures shortwave
radiation and reports it in the unit of J∙cm-2
∙d-1
∙d-1 or conversely, 1 J∙cm-2
convert that value into DLI using this conversion: 1 mol∙m-2
J∙cm-2
∙d-1 = 0.0196 mol∙m-2
∙d-1
.
For growers who do not have an environmental control computer that
measures radiation sum, then the daily light integral outside can be
estimated at your location by using DLI maps that are on the American
Floral Endowment website at endowment.org/dlimaps (for U.S. locations)
or SunTracker Technologies' website at dli.suntrackertech.com (for
worldwide locations). Next, calculate the greenhouse light transmission
percentage to estimate the DLI delivered to your crops inside.
Roberto G. Lopez (rglopez@msu.edu) is an associate professor and Controlled
Environment and Floriculture Extension specialist and Erik S. Runkle
(runkleer@msu.edu) is a professor and Floriculture Extension specialist
at Michigan State University.
, then you can
∙d-1
= 51.0
∙d-1
×
http://www.endowment.org/dlimaps http://dli.suntrackertech.com http://www.GPNMAG.COM
February 2021

Table of Contents for the Digital Edition of February 2021
