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.
Summary
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.
APPENDIX:
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.
Conversion factors for typical metal halide sources
Typical lighting level (can vary widely based on application) PAR Watts/sq. meter
watts-m-2
Micro-einsteins or
µ-mol-m-2.s-1
Lux
lumens- m-2
Foot-candles
lumens- ft-2
Dark Variable Variable Variable Variable
Low 22 100 6,000 550
Medium 45 200 12,000 1100
High 75 350 21,000 1900
Very High 135 600 36,000 3300
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.
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.