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. However, 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 of surface. Clearly, both lumens and lux (or foot-candles) refer specifically to human vision and not to the way plants see light. How then should the rating for plant lighting be accomplished? There are two basic approaches to develop this rating: measuring energy or counting photons.
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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 indicate 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 this rating.
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 "µmol." A µmol 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 µmol.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.
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Photosynthesis and Photomorphogenesis
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 UV light.
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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 Layout
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
400 µmol.m-2.s-1 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 halide lamp.
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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 rating.
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.