Let’s consider what happens when plants are grown under generated light. As a graduate student, and then as a young Assistant Professor beginning his academic career, I conducted plant nutrition studies in growth chambers under generated light. The growth chamber was used so that the environment surrounding the plants could be controlled, and therefore the treatment effects obtained would not be the result of interacting influences that exist in an uncontrollable environment, such as that which occurs in either a greenhouse or outdoors. Day length, air temperature, relative humidity, etc. can be controlled in a growth chamber. But the question that was always a concern – “was that observed in my plants an artifact of the light conditions since the generated light did not duplicate sunlight in terms of ether intensity or wavelength composition?” I never was able to satisfactorily answer that question.
Interestingly, many plants will grow very well under a wide range of generated light that varies in both intensity and wavelength composition. Significant advancements have been and continue to be made in light-generating device design, so that today growers have a variety of light- generating sources to choose from that will provide adequate light conditions for good plant growth. As with so many things today, it is not the lack of sources and lamps, it is the overload of information on light-generating devices and sources that can make selection challenging. On the internet, there are over 60 million websites dealing with this subject!
There are two primary applications for the use of artificially generated light: the supplementation of normal sunlight or as the only source of light. For light supplementation, its primary use is to extend the sunlight period, rather than adding additional light during the normal sunlight period. The possible exception would be when during sunlight hours, there is a significant reduction in sunlight intensity due to dense cloud conditions.
Initially, most of the artificial light sources were lamps that generated light within specific wavelength ranges. In the growth chambers I used mentioned above, two types of lights were installed, fluorescent lamps generating light in the shorter wavelengths of the spectrum, and interspaced were incandescent light bulbs that generated light in the longer wavelengths. Today, most of the lamps available have a fairly wide wavelength spectrum of generated light or have a specific wavelength range in order to affect a particular plant characteristic, such a foliage color, physical shape or flowering habit.
When selecting a particular light generating device, light intensity and wavelength range characteristics are the primary factors for consideration. In addition, initial and operating costs, life span, as well as heat generation are additional factors that can influence a selection.
Light measurement can be complicated since it involves two factors, light intensity at a point in time and light accumulation over a period of time. Using a light measuring device, such as a PAR meter, light intensity is measured at a moment in time. Plants respond to both factors, light intensity at a moment of time, as well as light accumulated over a period of time. If light accumulation is a factor, why not grow plants in continuous light when light intensity is low? Good question. Unfortunately, plants do not grow well in continuous light. One researcher thought he could get around this factor by exposing plants to continuous flashing light. He obtained some interesting plant responses, but these same plants grew better when there was at least an equal light and dark period.
Foot candles is one of the units for expressing light intensity, although researchers prefer micomoles per square meter per second of Photosynthetically Active Radiation, known by its acronym PAR, that portion of the spectrum that activates photosynthesis in green plants. To convert foot candles of sunlight to micomoles of PAR, multiply by 0.20. Total sunlight accumulated during the course of a day is called the “daily light integral,” expressed as moles/day. To convert from an average 1,000 foot-candles of sunlight to moles/day over 12 hours, multiply 1,000 x 0.00071 x 12 hours = 8.5 moles/day.
There are 4 types of light-generating lamps that one can choose: metal halide (MH) [referred to also as high intensity discharge lamps (HID) whose color index can be changed depending on the halide used], high pressure sodium (NPS), fluorescent, and Light Emitting Diode (LED). Each has its unique characteristics in terms of spectrum coverage, energy efficiency, and suitability for a particular plant-influencing factor. In addition, distance above the plant canopy, plant canopy coverage under the lamp, and the need for reflectors are equally important considerations. Some lamps also require a ballast system for their operation.
The energy emitted by these light-generating devices is measured in terms of its color temperature, expressed in degrees Kelvin (oK), with those generating light mostly at the lower end of the temperature spectrum at 1500oK are considered “cool” and those at the upper end of the spectrum at 6000oK as being “hot.” The Color Rendering Index (CRI) is a rating scale (0-100) that indicates the accuracy of the colors that can be perceived under a light source. Most light generating devices have a CRI that ranges between 60 and 80, with natural sunlight having a CRI of 100.
In order to provide a balance among these various characteristics, it is not uncommon to place a mix of lamps over a crop, usually placing fluorescent and/or incandescent lamps in with either HID or HPS lamps.
For comparison with sunlight energy, total light generated in moles/day by HID lamps, the multiplier is 0.00054 and for HPS lamps, 0.00047, in the equation given above instead of 0.0071 which demonstrates that the total foot-candles of energy generated by these lamps are considerably less than that for sunlight. So, there is a way to go to make a lighting-generating device that duplicates sunlight!
J. Benton Jones, Jr. has a PhD in Agronomy and is the author of several books. Dr. Jones has written extensively on hydroponic growing and outdoor vegetable gardening employing sub-irrigation hydroponic growing systems.