Watts, Lumens, Photons and Lux
As the importance of artificial
light in the plant growing industry has increased, lamp manufacturers
have begun to rate lamps specifically for plant needs.
This article discusses and compares the different measures
of " light level" that are currently used for plant growth
and hydroponic applications. Light level is one of the important
variables for optimizing plant growth, others being light quality,
water, carbon dioxide, nutrients and environmental factors. The
appendix describes a step-by-step approach to developing a simple
lighting layout using the PAR watt ratings of light sources.
In recent years, it has become increasingly
cost-effective to use artificial lights for assisting plant growth.
Lighting costs and lamps have become less expensive, and very efficient
light sources are now available in high wattages. These developments
along with the ability to preserve and transport plants and produce
as well as special new products in demand today have resulted in
a lucrative market for hydroponic products, that is, products grown
Artificial light can be used for plant
growth in three different ways:
- To provide
all the light a plant needs to grow
- To supplement
sunlight, especially in winter months when daylight hours are
- To increase
the length of the "day" in order to trigger specific
growth and flowering.
PAR and Plant Response Curve
Just as humans need a balanced diet,
plants need balanced, full-spectrum light for good health and optimum
growth. The quality of light is as important as quantity. Plants
are sensitive to a similar portion of the spectrum as is the human
eye. This portion of the light spectrum is referred to as photosynthetically
active radiation or PAR, namely about 400 to 700 nanometers in wavelength.
Nevertheless, plant response within this region is very different
from that of humans.
The human eye has a peak sensitivity
in the yellow-green region, around 550 nanometers. This is the "optic
yellow" color used for highly visible signs and objects. Plants,
on the other hand, respond more effectively to red light and to
blue light, the peak being in the red region at around 630 nanometers.
The graphs below show the human eye response curve and the plant
response curve. Note the vast difference in the contours.
In the same way fat provides the most
efficient calories for humans, red light provides the most efficient
food for plants. However, a plant illuminated only with red or orange
light will fail to develop sufficient bulk. Leafy growth (vegetative
growth) and bulk also require blue light. Many other complex processes
are triggered by light required from different regions of the spectrum.
The correct portion of the spectrum varies from species to species.
the quantity of light needed for plant growth and health can be
measured, assuming that all portions of the spectrum are adequately
covered. Light for plants cannot, however, be measured with the
same standards used to measure light for humans. Some basic definitions
and distinctions follow that are useful in determining appropriate
ways to measure the quantity of light for hydroponic plant growth.
Measuring Light for Humans:
Lumens and Lux
First, how do we measure light quantity
for humans? The obvious way is based on how bright the source appears
and how "well" the eye sees under the light. Since the
human eye is particularly sensitive to yellow light, more weight
is given to the yellow region of the spectrum and the contributions
from blue and red light are largely discounted. This is the basis
for rating the total amount of light emitted by a source in lumens.
The light emitted from the source is
then distributed over the area to be illuminated. The illumination
is measured in "lux", a measurement of how many lumens
falls on each square meter of surface. An illumination of 1000 lux
implies that 1000 lumens are falling on each square meter
of surface. Similarly, "foot-candles" is the term for
the measure of how many lumens are falling on each square foot
Clearly, both lumens and lux (or foot-candles)
refer specifically to human vision and not to the way plants see
How then should the rating for plant
lighting be accomplished? There are two basic approaches to develop
this rating: measuring energy or counting photons.
PAR Watts for Plants
Watts is an objective measure of energy
being used or emitted by a lamp each second. Energy itself is measured
in joules, and 1 joule per second is called a watt. A 100 watt incandescent
bulb uses up 100 joules of electrical energy every second. How much
light energy is it generating? About 6 joules per second or 6 watts,
but the efficiency of the lamp is only 6%, a rather dismal number.
The rest of the energy is dissipated mainly as heat. Modern discharge
lamps like high pressure sodium (HPS) and metal halide convert (typically)
30% to 40% of the electrical energy into light. They are significantly
more efficient than incandescent bulbs.
Since plants use energy between 400
and 700 nanometers and light in this region is called Photosynthetically
Active Radiation or PAR, we could measure the total amount of energy
emitted per second in this region and call it PAR watts. This is
an objective measure in contrast to lumens which is a subjective
measure since it is based on the response of the subjects (humans).
PAR watts directly indicates how much light energy is available
for plants to use in photosynthesis.
The output of a 400 watt incandescent
bulb is about 25 watts of light, a 400 watt metal halide bulb emits
about 140 watts of light. If PAR is considered to correspond more
or less to the visible region, then a 400 watt metal halide lamp
provides about 140 watts of PAR. A 400 watt HPS lamps
has less PAR, typically 120 to 128 watts, but because the light
is yellow it is rated at higher lumens (for the human eye).
"Illumination" for plants
is measured in PAR watts per square meter. There is no specific
name for this unit but it is referred to as "irradiance"
and written, for example, as 25 watts/square meter or 25 w/m2.
Another means of measuring light quantity
for plant growth involves the understanding that light is always
emitted or absorbed in discrete packets called "photons."
These packets or photons are the minimum units of energy transactions
involving light. For example, if a certain photosynthetic reaction
occurs through absorption of one photon of light, then it is sensible
to determine how many photons are falling on the plant each second.
Also, since only photons in the PAR region of the spectrum are active
in creating photosynthesis, it makes sense to limit the count to
PAR photons. A lamp could be rated on how many actual tiny photons
it is emitting each second. At present no lamp manufacturer does
Instead, plant biologists and researchers
prefer to talk of the flux of photons falling each second on a surface.
This is the basis of PPF PAR with PPF standing for Photosynthetic
Photon Flux, a process which actually counts the number of photons
falling per second on one square meter of surface. Since photons
are very small, the count represents a great number of photons per
second, but the number does provide a meaningful comparison.
Another measure appropriate for plant
growth, called YPF PAR or Yield Photon Flux, takes into account
not only the photons but also how effectively they are used by the
plant. Since red light (or red photons) are used more effectively
to induce a photosynthesis reaction, YPF PAR gives more weight to
red photons based on the plant sensitivity curve.
Since photons are very small packets
of energy, rather than referring to 1,000,000,000,000,000,000 photons,
scientists conventionally use the figure "1.7 micromoles of
photons" designated by the symbol "Ámol." A Ámol
stands for 6 x 1017 photons; 1 mole stands for 6 x 1023
photons. Irradiance (or illumination) is therefore measured in watts
per square meter or in micromoles (of photons) per square
meter per second, abbreviated as Ámol.m-2.s-1
The unit "einstein" is sometimes
used to refer to one mole per square meter per second. It means
that each second a 1 square meter of surface has 6 x 1023
photons falling on it. Irradiance levels for plant growth can therefore
be measured in micro-einsteins or in PAR watts/sq. meter.
These three measures of photosynthetically
active radiation, PAR watts per square meter, PPF PAR and YPF PAR
are all legitimate, although different, ways of measuring the light
output of lamps for plant growth. They do not involve the human
eye response curve which is irrelevant for plants. Since plant response
does "spill out" beyond the 400 nanometer and 700 nanometer
boundaries, some researchers refer to the 350 750 nanometer
region as the PAR region. Using this expanded region will lead to
mildly inflated PAR ratings compared to the more conservative approach
in this discussion. However, the difference is small.
Plants receiving insufficient light
levels produce smaller, longer (as compared to wide) leaves and
have lower overall weight. Plants receiving excessive amounts of
light can dry up, develop extra growing points, become bleached
through the destruction of chlorophyll, and display other symptoms
of excessive stress. Plants are also damaged by excessive heat (infrared)
radiation or extreme ultraviolet (UV) radiation.
Within the acceptable range, however,
plants respond very well to light with their growth rate being proportional
to irradiance levels. The relative quantum efficiency is a measure
of how likely each photon is to stimulate a photosynthetic chemical
reaction. The curve of relative quantum efficiency versus wavelength
is called the plant photosynthetic response curve as shown earlier
in this section.
It is also possible to plot a curve
showing the effectiveness of energy in different regions of the
spectrum in producing photosynthesis. The fact that blue photons
contain more energy than red photons would need to be taken into
account, and the resulting curve could be programmed into photometry
spheres to directly measure "plant lumens" of light sources
instead of "human lumens." This is likely to happen at
some point in the future. In fact, manufacturers like Venture Lighting
International provide PAR watt ratings for their Sunmaster
line of lamps designed for the plant growth market.
The main ingredient in plants that
is responsible for photosynthesis is chlorophyll. Some researchers
extracted chlorophyll from plants and studied its response to different
wavelengths of light, believing that this response would be identical
to the photosynthetic response of plants. However, it is now known
that other compounds (carotenoids and phycobilins) also result in
photosynthesis. The plant response curve, therefore, is a complex
summation of the responses of several pigments and is somewhat different
for different plants. An average is generally used which represents
most plants, although individual plants may vary by as much as 25%
from this curve. While HPS and incandescent lamps are fixed in their
spectral output, metal halide lamps are available in a broad range
of color temperatures and spectral outputs. With this in mind, the
discriminating grower can choose a lamp that provides the best spectral
output for his specific needs.
In addition to photosynthesis which
creates material growth, several other plant actions (such as germination,
flowering, etc.) are triggered by the presence or absence of light.
These functions, broadly classified as photomorphogenesis, do not
depend much on intensity but on the presence of certain types of
light beyond threshold levels. Photomorphogenesis is controlled
by receptors known as phytochrome, cryptochrome, etc., and different
plant functions are triggered in response to infra red, blue or
Plants "see" light differently
than human beings do. As a result, lumens, lux or footcandles should
not be used to measure light for plant growth since they are measures
used for human visibility. More correct measures for plants are
PAR watts, PPF PAR and YPF PAR, although each in itself does not
tell the whole story. In addition to quantity of light, considerations
of quality are important, since plants use energy in different parts
of the spectrum for critical processes.
Designing a Simple Lighting
Step 1. Determine required
irradiance levels in PAR watts/square meter
What is a "good" level of
lighting for plant growth? This level depends on a number of factors,
including plant type, stage of growing cycle, response to increased
light levels, among others. Recommendations offered in technical
brochures or articles should be treated as rough guidelines. Within
a broad range, plants grow faster with more light; therefore the
cost of electrical power versus the benefit of faster or higher
growth plays a role.
Since lamp to lamp variations, light
depreciation over life, fixture degradation from dirt and line voltage
fluctuations all contribute to variability, calculating to three
decimal places is unnecessary!
As an example, if a specific technical
brochure recommends a PPF PAR irradiance of
for your plants, the table below shows that you need approximately
85 PAR watts/square meter. The conversion factors between PPF PAR,
PAR Watts and lux depend on the light source. For example, a 400
watt HPS lamp has more lumens than a 400 watt metal halide lamp
but fewer PAR Watts. Depending on the color temperature of the metal
halide lamp, there can be small variations in the conversion factors.
The table below provides a general
guideline for metal halide light sources. Conversion factors for
HPS sources are similar except that about 10% higher lux or foot-candle
levels are required to achieve the same PAR watts/square meter.
factors for typical metal halide sources
lighting level (can vary widely based on application)
For a more technical discussion of
the conversion factors among various types of light sources, refer
to Langhans and Tibbits, "Plant Growth Chamber Handbook",
North Central Regional Research Publication No. 340, Iowa State
University (1997). Be aware, that as technology has improved and
efficiency of light sources has advanced, the numbers given there
are somewhat outdated. Additionally, the article refers to metal
halide as one standard light source with a specific spectral output.
In reality, metal halide is a generic name, and almost any kind
of spectral output can be provided from a custom designed metal
Step 2. Next calculate (or
measure) the area you wish to illuminate in square meters.
Example: For a 12 meter x 6 meter area,
this = 72 sq. meters.
Step 3. Area x required PAR
watts per square meter = total PAR watts required
Total PAR watts required = 85 PAR watts/sq.
meter x 72 sq. meters = 6120 PAR watts
Step 4. Estimate PAR watts
required at source (typically 50% higher than in step 3)
If half the light is lost in the fixture,
walls, etc. twice as many PAR watts are needed from the source.
If 1/3rd of the light is lost (a reasonable estimate
for most cases), then 50% more PAR watts are needed from the sources
(lamps) than the figure calculated in step (3).
Therefore (1.5) x 6120 =9180 PAR watts.
Step 5. Select a lamp of appropriate
wattage (e.g. 400 watt, 1000 watt, etc) and calculate its PAR watt
A 400 watt lamp may have 140 PAR watts,
a 1000 watt lamp may have 380 (or 420) PAR watts. Higher wattages
mean fewer fixtures and are therefore more economical; however they
lead to greater variations in light level. Be alert for the phenomenon
of photomapping where plants in areas of higher illumination grow
taller than those in darker areas, essentially mapping out the irradiance
contour for the area! For purposes of this example, we will select
a 1000 watt lamp with 400 PAR watts.
Remember that these lamp ratings refer
to initial light values, and all light sources depreciate over the
life of the lamp. If you are designing to average or maintained
light levels, start at 20% to 30% higher. Be sure to relamp before
the depreciation reaches an unacceptable light level.
Step 6. Calculate the total
number of lamps (or fixtures) needed
To determine the total number of lamps
required, divide the total source PAR watts needed by the PAR watts
per lamp 9180/400 =22.95. For this sample calculation, the number
is approximately 23 or 24 fixtures.
Step 7. Use a Grid to Design
Your Fixture Layout
A square grid or a "staggered"
grid may be used to minimize light level variations across the growing
area. For example, 24 fixtures can be shown on a 6 x 4 grid or on
an 8 x 3 grid. Remember, the higher the ceiling height, the more
space is possible between the fixtures. If you find that there will
be too many "dark" areas in the regions between fixtures,
you may choose a lower wattage lamp and increase the number of fixtures.