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Updated daily light integral (DLI) maps now available

New DLI maps have been created from an updated database that includes data from 1998 to 2009.

Daily light integral (DLI) is the amount of photosynthetically active radiation (PAR) received each day as a function of light intensity and duration. DLI maps display the ambient light delivered daily during each month across the entire United States. The original maps released in 2002 were researched and developed by Jim Faust at Clemson University and Joanne Logan at the University of Tennessee.

Continue reading Updated daily light integral (DLI) maps now available

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LEDs have the potential to change how crops are grown

The use of LED grow lights to provide specific light wavelengths could allow growers to increase nutritional values of edible crops, enhance the intensity of foliage and flower color and improve the postharvest longevity of ornamental and edible crops.

Improvement in the light intensity delivered by light emitting diodes (LEDs) is helping to expand their use for the production of both edible and ornamental crops. Research with LEDs has been going on for about 30 years. Only within the last 10 years have increases in the light
intensities of LEDs allowed researchers to study the direct effects of narrow wave bands of light on plant physiology.


“LEDs are now available to deliver all blue, all red, all green, all yellow light or mixtures,” said University of Tennessee plant sciences professor Dean Kopsell. “White LEDs are almost a broad spectrum light source. White LEDs are actually mostly blue light with a little bit of red, yellow and green light with a white phosphor over them.”

Kopsell and his colleagues at the University of Tennessee are studying the impact individual types of light can have on the nutritional qualities of edible crops. Their work is focusing on crops that can be produced relatively quickly in 25-35 days, including microgreens and baby greens. They have also begun looking at some herbal crops including basil, tarragon and chives.

Researchers at the University of Tennessee are finding that exposing plants like brassicas to blue light is having a significant effect on their nutritional values. Photos courtesy of Dean Kopsell, Univ. of Tenn.


“Some of the unique things we are finding are when we change the light quality environment, going away from broad band light sources like fluorescent, incandescent and HIDs, and exposing plants to narrow band wavelengths of red and blue light, many things are changing in the plants. These narrow bands of light are having an effect on several plant quality parameters from a metabolic standpoint.”


Potential of specific light wavelengths


University of Tennessee researchers have found that exposing plants to narrow wavelengths of the light spectrum has resulted in the increased production of antioxidants and anti-carcinogenic compounds within the plants.

“What is even more interesting is some of the primary metabolites like the mineral nutrients are also increasing,” Kopsell said. “We are shifting the light ratios and putting more blue light into the mix. Blue light is close to the ultraviolet (UV) range and has higher energy values than red light. Because of the higher energy level associated with blue light, the more blue light we are exposing the plants to, it seems the more significant the results are on nutritional values.

“We haven’t got hard data yet, but everything that we can see, smell and taste, these blue lights not only affect nutrient uptake, and anti-oxidant metabolism, but they also affect aromatic compounds and flavor compounds. They make them more intense.”

Although researchers have only recently begun to study the impact of narrow light wavelengths on plant physiology, Kopsell said this will be the major use of LEDs in future applications.

“Not only is a grower going to be able to select the type of light and intensity from the LED manufacturer, but eventually the grower will know when is the critical time to apply a specific amount of light to a crop. One of the things that we have seen with these short term crops is using the light as a finishing-off treatment. The crops are grown under regular light conditions like any grower would have the ability to do and then just before harvest the plants would receive a specific type of light for a certain period of time. This light treatment would stimulate the plant physiology uptake and metabolism right before the plants go to the retail market.”

Kopsell said research exposing leafy brassicas to blue light prior to harvest has intensified pigments and green leaf color.

“We increased the green pigments in the leaves so that they looked more vibrant,” he said. “Other research has shown that UV light increases the anthocyanin compounds in leaf lettuce. Providing a little UV light, which is blocked out in most greenhouse environments, at the right time, a grower can get a crop to color up quickly before the plants are shipped out. What we have done with leafy greens to intensify the color of the leaves can also be done with petal tissue. By changing the light quality a grower could get more vibrant flower colors.”

Need for fine tune management

Kopsell said whether plants are grown outdoors, in a greenhouse or in a closed controlled environment with artificial light, the plants are using specific wavelengths from the available light source.

“Horticulture, floriculture and agronomic researchers know how much light is needed in order to produce crops with broad spectrum light,” he said. “The million dollar question that hasn’t been answered is how much light is needed from LEDs to achieve that same level of production? It is going to be less than the daily light integral (DLI) from a broad spectrum light source. But, right now we can’t tell you how much less it’s going to be.

“Applying specific light wavelengths when the plants need them, whether it’s for juvenile growth, flowering or fruiting, we don’t have a good grasp on the amount of light that the plants actually need. If a grower is only going to supply his plants with red and blue light, how much less light can a grower use in that production system?”
One of the reasons that plants will not require as much light from LEDs is because of the reduction in light stresses.
University of Tennessee studies have shown LED grow lights provide  a less stressful light environment for plants.

“Providing specific types of red and blue light, the amount of stress on plants is reduced because the plants don’t have to tolerate the light not being used for metabolism and physiology,” he said. “We have data that shows LEDs provide a less stressful light environment for plants. So we have to determine how much less light is needed. It is going to require an extra level of management to know what kind of light, how much light and when to apply it. Growers are going to be able to use LEDs to fine tune the light environment. It’s going to depend on the crop, how it’s being grown, where it’s being grown and how the crop will be used. Is it an ornamental, edible or medicinal crop? It’s not going to be as easy as sticking a seed or cutting into a substrate and letting Mother Nature take control. It’s really going to take some fine tune management. But the future looks bright so far.”


For more: Dean Kopsell, University of Tennessee, Plant Sciences Department, Institute of Agriculture, Knoxville, TN 37996-4561; (865) 974-1145;


David Kuack is a freelance technical writer in Fort
Worth, Texas; our corporate website at
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Meeting the fertilization needs of greenhouse lettuce

Greenhouse lettuce can be a successful container or
hydroponic crop for ornamental plant growers looking to give edibles a try.

By David Kuack
Ornamental plant growers considering producing an edible
greenhouse crop may want to try lettuce. Neil Mattson, associate horticulture
professor at Cornell University, said lettuce is a plant with moderate
fertility needs.
“Grown hydroponically, lettuce has somewhat lower
fertility needs than a greenhouse tomato crop,”

Ornamental plant growers interested in growing edible
crops may want to try lettuce. It can be produced in
containers with a growing medium or hydroponically
in troughs or a float system (pictured).
Photos courtesy of Cornell University

Mattson said. “Grown as a
container crop, lettuce is relatively similar to petunia. However, lettuce has somewhat
greater calcium needs. Growers can produce a relatively good crop of lettuce in
containers, if they use a complete fertilizer at a moderate strength of 150
parts per million nitrogen.”

Mattson said head lettuce can be produced in containers similar
to a bedding plant crop. The seed would be planted into a plug tray for three
to four weeks. Transplanting the plugs into larger containers, the crop could
be finished in four to six weeks depending on light and temperature levels.
He said baby leaf lettuce can be grown in flats. The seed
is directly sown into the growing medium and grown for three to four weeks
until plants reach suitable size.
Calcium deficiency

Leaf tipburn is a physiological disorder that can occur
when growing greenhouse lettuce. It can greatly impact the salability of a

“The main reason that tipburn occurs is the lettuce is
growing too fast under high light,” Mattson said. “For lettuce, the target
daily light integral is 17 moles per square meter per day. The light level should
be lower if there is poor air flow. If the light level goes higher than 17
moles, the rapid growth of young leaves is affected. There may be an inadequate
calcium supply, especially as the lettuce heads begin to mature and close. If
there is not enough air flow and not enough transpiration by the young leaves,
then not enough calcium can reach the leaves through the xylem sap. This can
cause tipburn to occur. It’s a case of pushing the plants too fast.”
tipburn in lettuce is not a result of a lack of calcium
supplied to the plants,
but an inability of the plants to
transport enough calcium to the young leaves.
Mattson said in many cases, tipburn is not a result of a
lack of calcium supplied to the plants, but an inability of the plants to
transport enough calcium to the young leaves.
“For container-grown lettuce, there is typically enough
calcium if the growing medium has a lime charge and if the fertilizer water
solution contains more than 50 ppm calcium,” he said. “Many common bedding
plant fertilizers, including 20-20-20, 20-20-20 and 21-5-20, do not contain
calcium. These fertilizers are typically used with tap water sources that
contain moderate alkalinity. In many cases, these tap water sources also
contain sufficient calcium.”
Mattson said it is important for growers to test their
water sources to make sure adequate calcium is being supplied, either from the
water source or added into the fertility program. If calcium needs to be added,
calcium nitrate is most commonly used. However, calcium nitrate is not
compatible with most complete fertilizers.
“Usually if a grower has to add calcium, it can be done
using a separate stock tank or a separate injector,” Mattson said. “One
strategy is to use a separate injector for the calcium nitrate in a series with
a 20-10-20 fertilizer that is being added with a second injector. Adding 50 ppm
calcium from calcium nitrate should be sufficient.
“An alternative method of calcium application, if a
grower has only one injector is to rotate between two separate stock tanks, one
for calcium nitrate and one for the bedding plant fertilizer. A grower would then
rotate between the two fertilizers. For example, for two days he would use the
20-10-20 fertilizer and on the third day he would use the calcium nitrate
applied at 150 ppm.”
Production with
organic fertilizers

Mattson has been able to grow a relatively good crop of
container-grown lettuce using granular organic fertilizers incorporated into
the growing medium.

“We incorporated poultry-based organic fertilizer (Sustane
8-4-4) into the growing medium at a rate of 8 pounds per cubic yard for both
the seed germination and transplant growing mixes,” he said. “That provided
good fertility, but for optimum yields I would also suggest making some liquid
organic fertilizer applications, maybe two to three times a week as the plants
get older.”
Mattson said the organic granular fertilizer he used is
temperature-dependent and is broken down by soil microbes. Sustane 8-4-4 has a
45-day release period, but under very warm greenhouse temperatures Mattson has
noticed quicker release rates. He said there are other slow release organic
fertilizers with different release periods. For example, Verdanta EcoVita lists
a 75-100 day release period.
electrical conductivity and pH

One strategy that Mattson recommends growers do periodically
is to monitor the electrical conductivity (EC) and pH levels.

“Monitoring EC will help growers determine if the plants
are receiving sufficient fertility,” he said. “If a grower is incorporating a
slow release fertilizer, this is a good indicator of when additional fertilizer
needs to be added. An under-fertilized plant will show yellow lower leaves from
nitrogen deficiency.”

electrical conductivity (EC) can help avoid
under fertilizing lettuce plants,
which show yellow
lower leaves caused by nitrogen deficiency.

Mattson said monitoring pH is important as it impacts
nutrient availability. He said lettuce isn’t commonly susceptible to iron
deficiency, but it will start to show up when the pH starts to increase above
“Monitoring EC and pH is especially important in
hydroponics,” he said. “A good grower who is producing his crop in a growing
medium in containers will monitor the pH every week or two. The pH may change
over the course of a week by maybe one unit.
“Growing hydroponically, a grower should be monitoring
the pH every day and make adjustments. Depending on the type of fertilizer and
the quality of the water, the pH in a hydroponic set up could change two units
in a day.”
Optimizing lettuce

Mattson said light and temperature are going to be the
drivers for how long it takes to finish a lettuce crop. Whether a grower is
producing the crop in containers with growing medium or hydroponically
shouldn’t have any effect on the length of production.

He said plant spacing can also impact the size of the
lettuce head. If plants are grown in small containers and spaced pot-to-pot,
the lettuce heads may not reach full size.
For greenhouse lettuce, Cornell University researchers
developed a hydroponic production model that enables growers to produce a
lettuce crop from seeding to harvest in 35 days if temperature and light
intensity are at optimum levels.
“When the light level isn’t optimized, a lettuce crop can
take more than 100 days from seeding to harvest,” Mattson said. “High pressure
sodium lamps would be the best lamps to use if a grower is looking to provide
supplemental light in a greenhouse to increase the daily light integral. For
the Cornell model we adjust the amount of light in the greenhouse based on the
amount of outdoor light. Seventeen moles per square meter per day is the daily
light integral we are aiming for with the model. The optimum temperature for
plant development is about 75ºF
during the day and 65ºF
at night.”

For more: Neil
Mattson, Cornell University, School of Integrative Plant Science; (607)

David Kuack is a freelance technical writer in Fort
Worth, Texas;

Visit our corporate website at

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LEDs found to have multiple uses on multiple crops

Researchers at Purdue University are finding LEDs can
have positive effects on both ornamentals and leafy vegetables.

By David Kuack

As more research is done with light emitting diodes
(LEDs), scientists are discovering new ways to use the lights on ornamental and edible
plants. Researchers at Purdue University have done extensive studies on annual
bedding plants, comparing the growth of seedling plugs and vegetative cutting liners
under LEDs.
“My goal is to continue to do research with LEDs because
we are finding new and exciting results, especially with the indoor production
of young plants and microgreens,” said associate horticulture professor Roberto
Lopez. “Some of the work that we have been doing has shown the benefits of
“If you would have asked me two years ago if I would ever
try to produce plugs indoors and not in a greenhouse, I would have said no. If
you would have asked me five years ago if I would be working on greens or
vegetables, I would have said no. Now I am doing both of those things with

Purdue University graduate student Joshua Craven
and associate horticulture professor Roberto Lopez are
studying the effects LED
lights have on ornamental
plants and leafy vegetables.
Photo by Tom Campbell, Purdue University

No need for

Lopez and former graduate student Wesley Randall found
that greenhouse-grown seedling plugs of impatiens, marigold, petunia, vinca and
zonal geranium did as well or better when supplemented with LEDs compared to
plugs supplemented with light from high pressure sodium lamps. What Lopez found
surprising was the quality of the plugs produced in a growth room with LEDs as
the only light source.

“LEDs produce better plugs when they’re grown indoors
than when they are grown in a greenhouse with sunlight supplemented with light
from LEDs or high pressure sodium lamps,” Lopez said. “It is amazing how good
the plugs look grown in an indoor multilayer production system with LEDs. The
plugs are compact, sturdier and greener with a similar root and shoot dry mass
to greenhouse-grown plants supplemented with light from LEDs or high pressure
sodium lamps.”
One crop that Lopez said they are still “tweaking” with
LEDs is petunias.
“Petunias, which are long day plants, when moved from an
indoor grow room equipped with red and blue LEDs, encountered a slight delay in
flowering in the greenhouse,” he said. “We are going to see if exposing the
plants to far-red LED light prior to moving them into the greenhouse will
induce them to flower.”
Using LEDs to
intensify leaf, flower color

Lopez said many of the annual spring bedding plants grown
in greenhouses in northern climates are produced under low light levels. The
result is that some plants don’t produce the same intense foliage colors that
they would if they were grown outdoors.

“Plants grown in glass greenhouses are not exposed to the
sun’s ultraviolet light because it is blocked by the glass,” he said. “The
result is that crops like zonal geraniums and purple fountain grass (Pennisetum setaceum ‘Rubrum’) don’t
“color up” like they would outdoors. One of the things we noticed with zonal
geraniums was the dark patterns on the leaves stood out much more when the
amount of blue light was increased. We hypothesized and found it was a result
of an increase in anthocyanin production. We have also looked at geraniums that
have very dark foliage and found not only does leaf color darken, but flower
color can be made darker by exposing market-ready plants to red:blue LEDs.”
Lopez said the change in leaf color due to anthocyanin
production was also dramatic for purple fountain grass.
“Purple fountain grass is a very popular ornamental
species produced by many growers,” Lopez said. “Grown in the greenhouse, the
leaves appear to be dull green and not very purple. We found that putting the
plants under a combination of red and blue LEDs for one to two weeks of what we
are calling “end-of-production lighting” resulted in an attractive purple
color. UV light is what stimulates anthocyanin synthesis.”
He said in the case of purple fountain grass, only the
leaves exposed to the LED lights change color. Those leaves not exposed to the
LED light remain green.
Expanding studies
to vegetable crops

Seeing the positive results that occurred with LEDs and
purple fountain grass, Lopez and PhD student W. Garrett Owen expanded the
research to red leaf lettuce to see if they could produce a similar response.

“Trying to produce red leaf lettuce can be difficult for
greenhouse growers if they are producing crops under low daily light integrals
(DLIs),” Lopez said. “Growers producing red leaf lettuce under low DLIs are
essentially producing green lettuce.
“We placed red leaf lettuce under the same LED treatments
used for purple fountain grass and the plants colored up in three to five days.
Based on our research, red leaf lettuce and purple fountain grass can be placed
under a 50-50 red and blue LED combination prior to harvesting or shipping
triggering anthocyanin formation.”
on Purdue University research,
red leaf lettuce can be placed under a
50-50 red
and blue LED combination prior
to harvesting triggering anthocyanin formation
the lettuce‘s red color.
Photo courtesy of Roberto Lopez,
Purdue University
Based on the results related to LEDs and anthocyanin
formation, Lopez said the studies may be expanded to look at the impact of LED
light on ornamental cabbage and kale. “Growers, especially those in the South,
have a hard time coloring up ornamental cabbage and kale,” he said. “It is
primarily a temperature response, as the night temperatures get cooler the
plants start to color up.”
Lopez and Owen did a small study placing ornamental cabbage
and kale under LEDs that resulted in a minimal color change. When
greenhouse-grown plants were grown under cool night temperatures and exposed to
LEDs, they exhibited the most intense color.
“What we are proposing is for growers in warmer climates
who have access to coolers, is to use a cool temperature/LED treatment,” he
said. “We will be conducting this study next fall. Smaller container sizes like
4-inch pots, could be rolled on carts into a cooler and exposed to cool
temperatures and LED lights for three to four days prior to shipping enabling
the plants to color up.”

Another study conducted by graduate students Joshua
Gerovac and Joshua Craver looked at the effect of LEDs on the growth of three
different microgreen species (kohlrabi, mustard and mizuna) in an indoor
multilayer production system. The study included three different light
qualities and three different DLIs (light quantity).

“Overall what we have seen is as the DLI increases, this
is for three microgreen species we trialed, the length of the hypocotyl,
basically the height of the microgreen, decreases,” Lopez said. “The more light
the plants are provided, the more compact they are. If the plants received 6
moles of light, they were much taller than if they received 18 moles of light.
Depending on the growers’ market, some customers might want microgreens that
are a little leggier or they might want plants that are more compact. That will
depend on market preference.”
The ideal LEDs

Lopez said the ideal vertical LED light module would
contain all of the wavelength colors.

“The vertical LED light with all the different colors
would enable growers to turn them on when they need them and off when they
don’t, depending on the stage of plant growth,” he said. “Once flowering begins
a grower doesn’t want stem elongation. Far-red light works for flowering so the
far-red would be turned on for the minimum amount of time required for
flowering. If the grower wants to increase the amount of anthocyanin in the
leaves or flowers, he can turn on the red and blue light near the end of the
crop. To be able to turn on specific colors when a growers needs them, that is
something I envision happening with LEDs.”

For more:
Roberto Lopez, Purdue University, Department of Horticulture and Landscape
Architecture; (765) 496-3425;;

David Kuack is a freelance technical writer in Fort
Worth, Texas;

Visit our corporate website at

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Daily light integral: a better way to measure greenhouse light

Measuring daily light integral (DLI) provides a more
accurate reading of the amount of photosynthetic light being received by

By David Kuack

Do you measure the light in your greenhouse to ensure
your plants are receiving an adequate amount of light? If you are using a
footcandle meter to measure the light intensity you are not getting a true
measurement of the amount of light received by the plants. Measuring the light
in footcandles or micromoles per square meter per second (µmol/m2/s) is
a measurement of instantaneous light, that is, the amount of light at the time
the measurement is made. An instantaneous measurement made under sunny or cloudy
conditions may not provide an accurate evaluation of the total amount of light
perceived by the plants over the course of a day.

“When you think about light over the course of a day,
light is extremely dynamic,” said assistant horticulture professor Chris Currey
at Iowa State University. “From sunrise to sundown, there are variations in
light. If growers take a single light measurement early in the day, they may be
underestimating the amount of light. Alternatively, if the light measurement is
made later in the day, growers may be overestimating the light level.
Instantaneous light levels change over the course of a day.”
Visible light vs.
photosynthetic light

Currey said another issue with measuring light with a
footcandle meter is that it measures the light that is visible to the human

“The sun produces a broad spectrum of light,” he said.
“Photosynthetic light, which is the light plants can use for photosynthesis and
is defined as photosynthetically active radiation (PAR), is the light that
occurs between 400 and 700 nanometers. So if a footcandle meter measures light
that is visible to the human eye, this includes wavelengths outside of PAR.
Consequently, footcandle measurements tend to overestimate the amount of light
for plant growth.”
More accurate

Currey said it is the total amount of photosynthetic
light that is going to impact how a greenhouse crop is going to grow.

“Daily light integral (DLI) is the sum of photosynthetic
light over the course of a day,” he said. “In production situations where there
is a static light source it is relatively easy to determine the DLI. Examples
of a grower producing a crop under a static source of light include growing
plants under high pressure sodium lamps in a warehouse or tissue culture plantlets
grown under LED lights in a laboratory. In these situations the light levels
are not going to change throughout the day.
“In most greenhouse environments light levels change
throughout the day. So to take one instantaneous measurement to indicate the light
level isn’t the best way to describe the total amount of PAR light available.
It’s better to look at the total amount, which is the DLI.”
Currey said a grower could estimate the DLI by using
hourly measurements.
“A grower could go out into the greenhouse every hour and
measure the light intensity,” he said. The grower would then use these
measurements in some calculations that would give the DLI. Realistically, very
few growers are going to determine DLI this way.”
There are a number of instruments available for measuring
DLI. Currey said one of the best ways to determine DLI is to use a quantum
sensor that measures PAR. The unit of measurement for DLI is moles per square
meter per day (mol/m2/day).
“In many cases the quantum sensor is connected to a data
logger that records the light measurements,” he said. “The quantum sensor can
be attached to a data logger that can record frequent instantaneous light
measurements which can then be integrated into a cumulative total for the day.
The quantum sensor can also be hooked up to a greenhouse environmental control
computer, such as an Argus or Priva, which can calculate the DLI.”
Expanding use of

Researchers at Clemson University used light measurements
collected by the National Oceanic Atmospheric Administration to developed
monthly DLI maps for the United States.

Monthly daily light integral maps for the United States
were developed using light measurements collected by
the National Oceanic Atmospheric Administration. The
 maps are based on historical averages.
Photo courtesy of Michigan State University
Currey said the maps are based on historical averages.
“The DLI maps give you a good average,” he said.
“Poinsettia growers will tell you each growing season is different. Sometimes
they will have bright, sunny Novembers and other years, it’s dark and cloudy.
These maps provide a good indication of light levels, but there are going to be
variations between years. That’s why it is important for growers to measure DLI
so they know what is happening in their greenhouses and be able to react,
including using supplemental lighting to increase DLI.”
Currey said that research has been done to quantify the
DLI necessary for specific crops.
“The optimum DLI is going to vary for different crops,”
he said. “For African violets, a DLI of 6 mol/m2/day is enough to
produce a good crop. For poinsettias, 10-12 mol/m2/day are needed to
grow an acceptable quality crop. For cut roses, the DLI needs to be above 20
mol/m2/day to produce a good crop.”
Growers are using daily light integral with their environmental
controls and supplemental lighting to provide their plants
with optimum photosynthetically active radiation levels.

Currey said some growers are using DLI, their
environmental controls and supplemental lighting to provide their plants with optimum
PAR levels.

“It is kind of like predictive lighting,” he said.
“Growers have their lights turn on when the light intensity is below a certain
level and turn off when the light goes above a certain level. This way a grower
is not adding light during the brightest time of the day when the light level
is at or above the light saturation point for photosynthesis. Growers can use
lighting set points so that if they are not achieving the target DLI for a
crop, they can have the lights turn on sooner or turn off later to ensure
plants receive enough PAR. As a crop goes later into the spring, a grower is
likely going to lower that trigger light intensity because there is going to be
more natural sunlight so less supplement light is needed.”

For more: Christopher
Currey, Iowa State University, Department of Horticulture; (515) 294-1917;

David Kuack is a freelance technical writer in Fort
Worth, Texas; dkuack@gmail.

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Horticulture Lighting Basics

Introduction:  Of all the environmental variables in controlled environment agriculture (CEA), light is quite possibly the least controlled and most confusing.  Yet, light is such an important factor.  Aside from temperature, light is arguably the most important environmental factor which affects plant growth.  Light is what drives photosynthesis (the conversion of light energy to chemical energy), photomorphogenesis (the light-driven plant development processes), and photoperiodism (plant recognition of daylength).  So what is light?  What we so commonly refer to as light is really a small segment of radiation within the overall electromagnetic spectrum (Fig. 1).  Electromagnetic radiation is measured in nanometers (one billionth of a meter, 1 x 10-9 meters, or 0.000000001 meters).  One human hair is approximately 50,000 nm in diameter.  There are three basic types of “light”: radiometric, photometric, and quantum. 
Radiometric is based on radiant energy or Joules (J) and the passage of that energy is a Joule per second (J·s-1).  One J·s-1 is equal to one Watt (W).  The rate at which surfaces intercept energy is expressed at W·m­2.  Radiometric measurements are inappropriate for plant growth as they often measure light below and above that which plants use.  However, that is not to state that they are useless.  These units are very important when dealing with the energy balance of a plant or greenhouse system. 
Photometric measurement is based on how the human eye perceives light.  In the U.S., the footcandle (fc) is the preferred photometric unit.  Elsewhere in the world the most common photometric measurement unit is the lux (lumens·m-2).  However, growers need to understand the limitations of photometric units.  Again, photometric units are based on how the human eye perceives light, which at a maximum is approximately 560 nm (or green light) and a waveband that is not correlated with plant photosynthesis.  Photometric measurements can be used to determine maximum light in a greenhouse and to determine light transmission (%) outside versus inside.  Furthermore, photometric units are instantaneous and do not accurately represent the amount of light that a plant has received over a 24-hr period. 
Quantum measurements are based on the number of photons (light particles) within a defined waveband (400 nm to 700 nm). This defined waveband is known as photosynthetically active radiation or PAR.  Photosynthetic photon flux density (PPFD) quantifies (in micromoles, µmol) the number of photons of light used for photosynthesis falling on a square meter per second (µmol·m-2·s-1, see Fig. 2).  So, PAR is the range, PPFD is the measurement. However, most people simply refer to the measurement as PAR.  A µmol is 1×10-6 (or one millionth) mole.  One mole is Avogadro’s number, 6.02×1023 particles.  If marbles were placed end to end over the surface of the Earth, one mole of marbles would represent a layer of marbles 50 miles deep!  Unlike radiometric and photometric measurements, quantum measurements are directly correlated to photosynthesis and plant production.  Furthermore, quantum measurements can be both instantaneous and accumulated (daily).  This is great because plants are really light counters.  You can think of a plant’s leaves as nature’s solar panels.  Therefore, it is better to know how much light is received by a plant over a 24-hr period versus taking an instantaneous measurement.  However, if you must take an instantaneous measurement to get an understanding of your maximum available light it is recommended to do so between 1100 to 1300 hr.  If you wanted to quickly measure rainfall over a 24-hr period you could set a rain gauge outside for 5 or 10 minutes and calculate the estimated rainfall.  Of course you are presuming that the volume of water falling in the gauge is uniform.  Is it?  How about light?  Does the sun simply turn on and off and project a uniform amount of light over the course of a 24-hr period?  Of course not, so why not measure light constantly over a 24-hr period?  The cost of light measurement equipment may be one reason not to do so.  However, there are companies which offer affordable light measurement technology.  Light measurements can provide valuable information.  The accumulated light a plant receives is correlated to plant quality and yield.  Are you familiar with Daily Light Integral (DLI)?  DLI is the light intensity and duration of PAR received each day and is expressed as mol·m-2·d-1 or mol·d-1. 
DLI is directly correlated to plant growth, development, yield, and overall quality.  Attributes of ornamental crops grown under optimal DLI are smaller and thicker leaves, shorter internodes, higher root mass, higher branching and flowers.  This should not be surprising.  Higher DLI is a common reason why plants finished in late spring generally are of higher quality than those produced earlier in the production season.  This is not to diminish the impact of temperature, which is another discussion.  Low DLI can be an indication that the greenhouse glazing needs to be cleaned or replaced; shade cloth needs to be removed or stowed; hanging basket density is too high; and/or supplemental lighting is needed.  The average target DLI for propagation of cuttings is 4 to 5 mol·d-1.  For bedding plants, containerized crops, and perennials the optimal DLI should be between 10 to 12 mol·d-1.  For food crops, a good rule of thumb is >12 mol·d-1.  For greenhouse tomatoes, it has been shown that for every 1% increase in light there was a subsequent 1% increase in yield.  For lettuce, 17 mol·d-1 is optimal.  So what is your DLI?  Your first step in DLI measurement can be to familiarize yourself with estimated outside DLI.  Dr. Jim Faust at Clemson University has developed DLI maps by month across the United States (Fig. 3).  However, keep in mind that average transmission loss in any given greenhouse can range from 40% to 70%.  Therefore, the next step is to begin measuring DLI.  While several companies offer light measurement tools for instantaneous light measurement (more photometric than quantum), only a few offer tools that will measure and calculate DLI.  Nevertheless, these tools are quite affordable.  A grower can begin measuring DLI for as little as $200.  More sophisticated sensors and data loggers range from $500 to $1,000 and up.
Summary:  Visible radiation, i.e. light, is one of the least controlled and often most confusing environmental growth factors.  Radiometric measurement refers to radiant energy and is practical when measuring the energy balance of a system.  Photometric measurement is based on how the human eye perceives light.  Photometric measurement is useful when determining maximum light levels and light transmission loss within a greenhouse.  Quantum measurement is well-suited for plant growth because it relates to light used for several plant-driven processes including photosynthesis.  Quantum measurements can be both instantaneous and accumulated.  DLI provides and accurate measurement of PAR over the course of a 24-hr period.  Plants grown under optimal DLI are generally of higher quality than plants grown under low DLI.  Increasing DLI can be achieved by adding supplemental lighting.  Please contact me at with additional comments or questions.  Keep growing and make it a great day!

Figure 1.  The electromagnetic spectrum.  The visible spectrum encompasses approximately 380 nm to 780 nm.  Photosynthetically active radiation or PAR is 400 nm to 700 nm.
Figure 2. Image depicting photons of light falling on one square meter.

Figure 3.  Map of outdoor DLI by month throughout the United States.
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