Light is the all-important source of energy used by plants in photosynthesis. But light is not just one simple, well-defined entity. The plant response is influenced by light quality, intensity and duration.
The energy contained in light is absorbed in the chlorophyll of plants. Not all wavelengths of light are utilized with equal efficiency. Looking at a chlorophyll/light absorption curve, one can deduce that red and blue light are more effective than green. This is logical. Plants do not use all of the green light. They reflect it. This is why plants appear green.
When electric light sources produce light, the wavelength (color) is determined by many things. For instance, a tungsten filament in an incandescent bulb emits more light with long wavelengths (the reds) when it is relatively cool and more short wavelengths (the blues) when it is very hot.
Other sources emit light in special ways. Metallic vapor lamps are quite efficient for high intensity lighting on larger areas. Fluorescent tubes are the most economical and convenient in limited areas. In the fluorescent tube, special phosphors become excited as electricity is supplied to them and give off energy in certain wavelengths. It is possible to design these tubes to emit primarily red, green, yellow or the "color" of your choice. Not all are equally efficient, but this would appear to have a practical application, since plants do not use all wavelengths with equal efficiency.
To achieve better efficiency in the response of plants to fluorescent light, special tubes have been designed which emit proportionately more blue and red wavelengths of light. In doing so, the green-yellow-orange fraction has been reduced. The loss in this portion of the spectrum, even though it is slightly efficient has not been compensated by enough increase in the blue-red portion of the spectrum to result in increased plant growth.
Critical experiments show that maximum growth of most plants under cool white fluorescent lights will be equivalent to or better than that obtained under the blue-red phosphors. Work by V. A. Helson, Canada Department of Agriculture, Ottawa; and J. W. Bartok and R. A. Ashley, University of Connecticut, indicate that there is no advantage to the use of blue-red fluorescent tubes except for aesthetic purposes. Some of the names of these special tubes are Grolux, Plant-Gro, Plant Light, Vita Light and Optima. The higher cost of these fluorescent tubes may be justified on experimental or aesthetic grounds but is hardly warranted on the basis of plant growth.
Did you know that the recommended amount of light for a planted tank is 7000 - 15000 LUX. Oh LUX is Lumens per square meter. Do you know how many Lumens your bulbs put out? Well most of us do not but if you are here is a more detailed link, and one more. But really most of us do not think about it in LUX as much as the old rule 1.5 to 4 watts per gal. And boy, that too is a large range for determining the right amount of light. Which one do you choose? Well it depends on a few things, like what type of plants you have, and this site can help you there(towards the end). Another consideration is this the depth of the tank and as you may have noticed LUX is sq meters and doesn't consider depth. A third thing is the spectrum of the bulb you use, plants want blue and red light the rest is almost extra and algae really do not care. Also how far above the tank are your lights. For standard florescent bulbs you want to get them as close as possible. So the bottom line is there is no one answer to the lighting question as with many of the variables in the hobby. The right amount of light depends on a lot of factors one of which can be how fast do you want your plants to grow.
by Wayne Vandre, Horticulture Specialist