7
Understanding PAR Light
Daily Light Integral (DLI)
If photons were raindrops, light meters would show the intensity of a
rainstorm. A five minute rainstorm may look impressive, but often
provides less water than an all
-
day drizzle. As cumulative rainfall is
measured with a rain gauge, the cumulative quantity of light is
measured by recording and summing the light intensity over time,
by convention, a 24 hour period.
The LightScout
®
Quantum PAR Light Meter can also determine the
total number of photons incident on one square meter over a 24
-
hour period. This is defined as the Daily Light Integral (DLI) and is
displayed in moles per square meter per day. The typical range for
DLI is 3 to 26 moles m
-
2
day
-
1
. This is useful for understanding the
average amount of light your crop sees in a day. One mole is equal
to one million micromoles.
Each type of plant has a different DLI range for optimal growth. DLI
is directly correlated with plant quality, and a minimum amount of
light is required for marketable plants. Measuring DLI can guide in
decisions regarding shade cloths and supplemental lighting.
Cosine Response (Sun Angle Effect)
This meter is designed to accurately display light intensity readings
±
80° from vertical. As illustrated below, less light is hitting your crop
as the light source (sun) is tilted from vertical. This is called cosine
effect and is accurately measured by the specially shaped diffuser
on the top of the light sensor. Ideally a horizontally projected light
source should show no available light for your crop regardless of its
intensity. Conversely, a directly overhead light source will have
100% of its light available for your crop.
