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Lighten Up!

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Plants have evolved over many eons through natural selection (except maybe in Kansas) to use the energy available to them in Sunlight as efficiently as possible. This part of the spectrum which plants use is defined as Photosynthetically Active Radiation(PAR). PAR light output is usually expressed in units of micro-moles(umol) or micro-Einsteins(uE)of quanta per Watt(W) and measured in umol or uE per square metre per second ((umol*m-2)*s-1 ((umol/m2)/s) or (uE*m-2)*s-1 ((uE/m2)/s)) of Photosynthetic Photon Flux Density(PPFD).

Human eyes have developed to utilize different wavelengths for vision as seen in the (rough) graph which shows the relationship between the daylight spectrum that reaches the Earth and the wavelengths used by plants and humans. As you can see, plants only use certain wavelengths of the electromagnetic spectrum, peaking in the blue and red regions, while the human eye can best see light in the yellow area, with the "Lumen" being used to relate the amount of light put out by bulbs that the human eye can see.

Click here for the graph: http://www3.sympatico.ca/elaine.best/lig...

Supplemental lighting is required in greenhouses and grow-rooms when insufficient daylight is available. The artificial light source of choice must have an output that will match, as closely as possible, the spectrum plants required for photosynthesis. While the amount of Lumens put out by bulbs can give a rough estimate of which bulb to use, PAR ratings are more useful when choosing lights for plants and conversion factors are available to convert from Lumens to PAR ratings for most light sources. Metal Halide (MH) and High Pressure Sodium (HPS) lamps put out about 14 umol PAR per klux, White fluorescent give ~12 and Incandescent give ~20, but HPS can give up to 120-140 klumens per 1000 Watts while MH give only 80-100, Fluorescent give ~70 and Incandescent give only ~20. "Gro" bulbs put out slightly more. Compact fluorescent bulbs can put out close to ~3000 lumens @ 40 watts. Halogen bulbs can also be useful bacause of ease of implementation. Greenhouses and many university Phytotrons use various types and combinations of bulbs.

When no daylight is available at all (i.e. indoors), some supplemental MH and fluorescent lighting is usually used because the HPS lamps put out a reddish spectrum which can lead to plants getting "leggy" (spindly) if some additional blue light isn't added.

The accepted value for optimal growth of lettuce & spinach is 200-300 umol*m-2*s-1 PAR PPFD, so a 1000W HPS system putting out 140 klumens @ 14 umol*m-2*s-1/klux will give

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