Hydrolife Magazine April/May 2017 (Canada Edition)

grow
Whether you are using horticulture
lighting systems to provide supplemental
lighting in a greenhouse or solesource
lighting indoors, it is important
that you know the quality and quantity
of light that your fixtures are providing to
your plants. There are numerous metrics
used to measure light; however, certain
ones were developed for applications
related to human vision, while others
were developed specifically for measuring
light that influences plant growth
and development.
HUMANS USE LUMENS
It goes without saying that plants
perceive light very differently than
humans. Where plants use light to
drive photosynthesis and other lightmediated
physiological responses,
humans and many other animals use
something called photopic vision in
well-lit conditions to perceive colour
and brightness of light. Lumen is a
unit of measurement based on a model
of human eye sensitivity in well-lit
conditions, which is why the model
is called the photopic response curve
(Figure 1). As it shows, the photopic
response curve is bell-shaped and shows
how humans are much more sensitive
to green light than blue or red light.
LUX and foot candle meters measure
the intensity of light (using lumens) for
commercial and residential lighting
applications, with the only difference
between the two being the unit of area they
measure (LUX uses lumen per square metre
and foot candle uses lumen per square
foot).
The fundamental problem with using
LUX or foot candle meters when measuring
the light intensity of horticulture lighting
systems is the underrepresentation of blue
(400–500 nanometres) and red (600–700 nm)
light in the measurement. Humans may
not be efficient at perceiving light in
these regions, but plants are highly
efficient at using blue and red light
to drive photosynthesis.
PLANTS USE PAR
Plants primarily use wavelengths of light
between 400-700 nm to drive photosynthesis
(Figure 1), which is why this range is
called photosynthetically active radiation
(PAR). PAR is a much-used (and often
misused) term related to horticulture lighting.
PAR is not a measurement or metric
like feet, inches, or kilos. Rather, it defines
the type of light needed to support photosynthesis.
The amount and spectral light
quality of PAR are the important metrics
to focus on. Quantum sensors are the
“HUMANS MAY NOT be efficient at perceiving light in these regions, but plants
are highly efficient at using blue and red light to drive photosynthesis.”
PAR
Relative Photosynthetic Efficiency (%)
100
80
60
40
20
0
Lumens
300 400 500 600 700 800
Wavelength (nm)
Figure 1
Graph depicting the average photosynthetic efficiency of plants and the spectral
range that photosynthetically active radiation (PAR) and lumens are measured.
20 grow. heal. live. enjoy.
myhydrolife.ca