February 2021 - 31

RESEARCH
QUANTUM
Radiometric
J∙cm-2
Yearly Minimum
Yearly Maximum
Yearly Average
∙d-1
49
2,951
1,230
Instantaneous average
(per second)
µmol∙m-2
11
669
279
∙s-1
Cumulative
(daily light integral)
mol∙m-2
1.0
57.8
24.1
∙d-1
Table 1. Example showing the yearly
minimum, maximum, and average
solar radiation conversion from
joules per square centimeter per day
(J∙cm-2∙d-1) to micromoles per square
meter per second (μmol∙m-2∙s -1) to
the daily light integral in moles per
square meter per day (mol∙m-2∙d-1) for
a greenhouse in a northern latitude.
TERM
Daily light integral
(DLI)
(mol∙m-2
∙d-1
)
Electromagnetic
radiation
Foot-candle (fc)
Joule (J)
Mole (mol) or
micromole (µmol)
Nanometer (nm)
Photon or quantum
Photosynthetically
active radiation
(PAR)
Photosynthetic
photon flux density
(PPFD)
Photometric unit
(fc or lux)
Quantum unit
(μmol∙m-2
∙s -1
∙d-1
).
DEFINITION
Cumulative amount of PAR that is delivered each
day. It is expressed as moles of light (mol) per
square meter (m-2
(mol∙m-2
; 10.8 square feet) per day (d-1
) or
Light is a form of energy called electromagnetic
radiation, which varies in duration (energy over
time), quality (wavelength or color), and intensity
(the amount of light at each wavelength or color).
Illuminous photometric unit based on the amount
of visible light detected by the human eye (primarily
green light) and therefore, not appropriate for plants.
Scientific unit for energy that measures the capacity
to do work or generate heat.
Very large constant number used in chemistry
(6.02 × 1023
of a mole.
A unit of wavelength [billionths of a meter (10-9
m)]
Individual packets or particles of light energy. The
amount of energy in each photon depends on its
wavelength (color).
Photons with wavelengths between 400 to 700 nm,
which are capable of stimulating photosynthesis.
The amount of PAR reaching a unit area (usually m2
per unit of time (usually second; s).
Measurement of light based on its perceived
brightness and thus, is biased toward people.
1 fc = 10.8 lux.
Measurement of the number of micromoles (μmol) in
the PAR range that reaches one square meter (m2
every second (s), or μmol∙m-2
)
∙s -1
)
Radiometric unit
(W∙m-2
)
. Quantum meters
are the most appropriate instrument to measure
light intensity for photosynthesis and plant growth
because it is not biased toward the human eye.
Measurement of radiant energy.
COMMUNITY INVOLVEMENT
Short-wave radiation Wavelengths from 100 to 2,800 nm
Ultraviolet (UV)
radiation
Watt (W)
Wavelength
(nm)
Wavelengths from 100 to 400 nm, although most
radiation with wavelengths less than 300 nm is
blocked by the ozone layer.
A unit for the rate of energy use or irradiance.
1 Watt = 1 J∙s-1
HERITAGE PRESERVATION
The distance between two crests of a wave.
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Sidebar. Terms, units, and definitions related to plant lighting applications.
GREENHOUSE PRODUCT NEWS 29
PLANT BENEFITS
ENVIRONMENTAL ACTION
)
LEADS THE WAY
The America in Bloom national awards program brings out the best in hometowns
and empowers communities to excel. Lead the way and experience the benefits by
registering today.
) and a micromole is one millionth
computer displays is only for PAR radiation (from 400 to 700 nm).
However, almost all pyranometers measure total short-wave radiation
and not just the energy in PAR. Therefore, we need to use a different
conversion factor.
Approximately 43% of the energy from the sun is within the PAR
range, so we need to use the conversion factor of ≈1.96 (rather than
4.57) μmol∙m-2
∙s-1 per watt of energy. Check with your environmental
control computer company to find out what units their system reports.
Some provide instantaneous units in J∙cm-2
∙s-1 or J∙m-2
provide cumulative units (the radiation sum per day) of J∙cm-2
J∙m-2∙d-1. Table 1 provides an example of light data measured outside a
greenhouse located in the Midwest.
∙s-1 while others
∙d-1
or
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February 2021

Table of Contents for the Digital Edition of February 2021
