Plants & 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 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 colour 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

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that as technology has improved and efficiency of light sources has advanced, the numbers given there are somewhat outdated.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 photo mapping 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 re-lamp 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.


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Typical lighting level (can vary widely based on application) PAR Watts/sq. meter
Micro-einsteins or
lumens- m-2
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



This is an image of Sunmaster Grow Lamps in a green house