The most common mistake people make with planted tanks is to not understand photosynthesis and the visible spectrum of lighting that affects plant growth. Most people choose lighting solely based on the Kelvin temperature of a bulb. This tells you very little about what type of light within the spectrum is being emitted and at what strength. Visible light is on a scale in nanometers (radiated wavelength) from 400nm (violet) to 700nm (red). Simple matter of photosynthesis: plants can only utilize light that is absorbed. Bright light is essential yet only a portion of this white light is used for photosynthesis. The blue and red zones of the visible spectrum are the most beneficial to plants. Green plants appear green because it is reflected light. How "bright" a light appears has more to do with how much light is output in a given area visible to the human eye, with "brightness" being at a maximum in the green spectrum (middle of visible spectrum, or around 550nm).
Lighting for a planted tank should not be chosen on color temp alone. It is true that 'full spectrum' bulbs are referred to as bulbs between 5000 Kelvin (K) and 6500 K and are considered to be best for planted tanks. Yet this does not indicate what wavelength in nanometers the bulb is actually emitting. If you want to optimize plant leaf development (blue light) and stem elongation and color (red light) you need light in both the blue and red spectra for photosynthesis. You need a mix of blue and red for your plants, and green for you (brightness as perceived by humans). If your lighting looks extremely bright and your plants seem ultra-green, it means that you have lighting that outputs strongly in the green spectrum. Do not equate this with good lighting for your plants, because plants don't use light in the green spectrum for photosynthesis. Sunlight peaks in the blue spectrum at 475 nanometers (nm). This is a shorter wavelength than red light and is used by both plants and algae. As light passes through water the intensity decreases. The shorter wavelength blue light penetrates water better and more quickly than red, which is slower and absorbed more quickly. Chlorophyll, the photosynthetic pigment used by plants traps blue and red light but is more efficient with red light at 650 – 675nm. Blue is used at the same rate as red because it is more available for reasons mentioned above.
For green plants the lighting peaks that are most important:
Chlorophyll-a: 430nm/662nm
Chlorophyll-b: 453nm/642nm
Carotenoids: 449nm/475nm
Red pigmented plants use more light in the blue area of the spectrum.
Beyond choosing lighting that is optimal for photosynthesis, as above, you should choose lighting with the color temperature that best suits the aesthetic goals of your tank. So, don't obsess about color temperature beyond how you want your tank to look. From a color temperature standpoint, blue-colored light will enhance blues in your fish. Green-colored light will make the tank look bright to humans and enhance the green color of your plants. Red-colored light will enhance the reds in your fish, and any red plants.
Lux is lumens/square meter, so they are similar. They are both defined in terms that are meaningful to human perception of light – not plants. They stress the amount of energy in the green band to which humans are most sensitive – not plants.
Artificial light sources are usually evaluated based on their lumen output. Lumen is a measure of flux, or how much light energy a light source emits (per unit time). The lumen measure does not include all the energy the source emits, but just the energy with wavelengths capable of affecting the human eye. Thus the lumen measure is defined in such a way as to be weighted by the (bright-adapted) human eye spectral sensitivity.
Lumen ratings are usually available, but when you use them you have to keep in mind what they mean. Lamp A can have a higher lumen rating than lamp B and appear brighter to you, while lamp B provides more useful light for plants. Compare the lumen ratings for cool white and GroLux bulbs of the same wattage and you will see what I mean. A 40-watt cool white bulb is rated at 3050 lumen; a 40-watt GroLux bulb (not the wide spectrum) is way lower at 1200 lumens. The big difference is because GroLux lamps provide very little green light and cool whites provide a lot of green light. I have found it best to provide a mix of lighting to a planted tank. The GroLux bulb is perhaps the best plant bulb available but it has very little green light so the visual effects of your tank will look dim and purplish. Yet if you add some other lighting such as a Philips 6500K the effect is more pleasing to the eye and still beneficial to the plants. I find that the GroLux along with a GroLux wide spectrum (89 Color Rendering Index) has a great effect for use as dawn/dusk lighting. (A Sylvania rep. told me it was best to use both together.)
Kelvin rating and lumens does not equate for plants. The Kelvin scale is more of how your tank will look to you/us and is totally subjective. It is true that the lower Kelvin ratings like 3000K will have more red light and a 10,000K will have more blue light. Lumens are meaningless for plants, as green plants do not utilize green light for photosynthesis. A higher lumen rating at the same wattage often means greener light. Lumen is a rating weighted entirely towards human perception. It has little to do with the value of a light for either growing or viewing plants.
The Kelvin rating is an indication of color temperature. The higher the temperature, the more blue the light. Here's a rough scale:
- Reddish/Yellowish Endpoint -
Incandescent Light: 2700K
Daylight: 5500K
Blue Sky: 10,000K
- Blue Endpoint –
Don't be fooled by color temperature as an indication of what wavelength of light may or may not be present. The emitted wavelengths of light for two bulbs with the same color temperature could be wildly different. Therefore, color temperature is not what you should use to determine useful light for growing plants. It will, however, give you an idea of how things in your tank will look. For example, the sky has a color temperature of 10,000K and looks blue. Lighting that has a higher color temperature, indicating that it is bluish, does point to the fact that blue wavelengths are dominant. This, in turn, just means that it will activate green plants in the blue range, which is a good thing, and enhance blue fish. Red photosynthetic pigment is less efficient at utilizing light and requires stronger light as a result. The less efficient red carotenoid pigment must rely on blue and some green light as well as more intense lighting. There are some plants that that are able to change the pigment they use for photosynthesis depending on available lighting. We see this in red-leaved plants that turn green if the lighting is too low, not enough blue and/or green light. Alternatively, some green leafed plants produce red foliage when closer to the light source or with overly bright lighting.
The Kelvin color designation of a particular bulb is not always true to the black body locus line on a CIE Chromaticity map. This is why some 5000K bulbs look yellow and others white, especially when trying to compare a linear fluorescent with a CF or MH. This is where Kelvin ratings of bulbs can fall prey to marketing schemes/hype.
The standard measure that quantifies the energy available for photosynthesis is "Photosynthetic Active Radiation" (aka "Photosynthetic Available Radiation") or PAR. It accounts with equal weight for all the output a light source emits in the wavelength range between 400 and 700 nm. PAR also differs from the lumen in the fact that it is not a direct measure of energy. It is expressed in "number of photons per second". The reason for expressing PAR in number of photons instead of energy units is that the photosynthesis reaction takes place when a photon is absorbed by the plant; no matter what the photon's wavelength is (provided it lies in the range between 400 and 700 nm). In other words if a given number of blue photons is absorbed by a plant, the amount of photosynthesis that takes place is exactly the same as when the same number of red photons is absorbed. This is why it is so important to get the spectral output of a bulb before deciding if is a 'good plant light'. You may need to add/mix bulbs to get a lighting that has good visual effects for the human eye and proper light for plants because 'plant bulbs' tend to be purplish. There is an additional term called "Photosynthetic Usable Radiation" or PUR which takes in to account blue and red light only.
I don't understand why people insist on distinguishing between lamps on the basis of their color temperature. No lamp renders color correctly or looks natural unless its Color Rendering Index (CRI) rating is very high. When CRI is over 90 the color temperature shouldn't make much difference; colors rendered accurately will always look about the same regardless of the Kelvin rating. Many bulbs render red and orange colors poorly and give you a look with very flat color contrasts. Other bulbs produce a lot of green light and don't render either blue or red very well at all.
CRI or Color Rendering Index is an indication of how close the light is to daylight (full spectrum) on a scale from 0 to 100 with respect to how it makes objects appear. In the case of the Philips PL-L 950, the CRI is 92, so it has pretty good color rendering properties. Two bulbs with the same Kelvin temperature but different CRI ratings can produce very different appearances. Compare a 5000K that has an 80-something CRI with a 5000K that has a 90-something CRI. The 80 CRI bulb is very bright, but it renders greens with a distinct yellow cast. The 90 CRI bulb is dim, but it renders rich colors across the whole spectrum.
Whether or not a bulb looks "natural" to you is totally subjective. It depends in part on what you're used to. If you only see the world under cool white fluorescents then that is probably what looks natural to you. If you live somewhere with frequently hazy or overcast skies then you may be accustomed to "natural" light having a color temperature near 7000K. If you live somewhere with clear skies and infrequent cloudy days then your natural light might have a color temperature closer to 5000K. If you are used to north skylight then maybe a color temperature close to 10,000K seems more natural. In any case of actual natural light the light will render colors pretty well. That is usually not the case for fluorescent lamps with a high Kelvin temperature rating. If you want a high K lamp that does render colors accurately then you might try finding the Philips C75. It has a 7500K color temp and a 90+ CRI. It could be hard to find and a bit pricey.
Plants will grow with ordinary bulbs as they tend to have both some blue and red emissions. The problem is that they also have wavelengths between 500 and 600nm, which algae likes. Green algae and green plants use the same pigments for photosynthesis (chlorophyll a/b & carotenoids). So, light that helps one helps the other. The algae that are different are the blue-green algae (cyanobacteria), which contain Phycocyanin and absorb light heavily in the low 600nm (orange-red), which is unfortunately present in most standard fluorescents. In the planted aquarium artificial light should ideally peak (or be stronger) in the red area of the spectrum. The tanks’ appearance can be compensated (balanced) with blue light and some green light for brightness to the human eye. Strong blue light will cause plant growth to be more compact and bushy and will also tend to promote algae growth. So remember to balance 2/3 red to 1/3 blue light emissions.
Bulbs sold as generic plant/aquarium bulbs usually have OK energy in blue and not much in red. A bulb sold as a generic "sunshine" bulb may or may not have some useful red, depending on the bulb. You can put any fluorescent lighting on your tank and do OK, but if you want to maximize plant growth, it's best to compare lighting options and, if possible, try to find the graphs/data for spectra output, rated life and output decay over time. Unfortunately, CF bulbs haven’t caught up with linear bulbs in the ability to offer light (tri-phosphor type) in the proper areas of the spectrum.
Fluorescents lose efficiency over time. Some lose more than others - some bulbs may only suffer 10% drop in output, while others may drop 30% or more in the same time frame. The less the drop over time, the less you have to replace them, depending on your application. Linear fluorescent tubes should be changed out every six months and compact fluorescents every year.
Fluorescent bulbs marketed for aquaria are often more expensive and not necessarily better than generic versions. They are also not necessarily marketed correctly. Many bulbs offer spectral output graphs. However, many of these graphs are measured in relative power on the Y-axis rather than a known reference like watts per nanometer per 1000 lumens. All that 'relative power' lets you know is that 100% is the highest peak at a given nanometer and all other peaks are relative to this. So, don't be fooled by nomenclature and packaging (marketing hype).
Aquatic plants quickly respond to changes in light conditions and are more highly evolved than algae and are able to regulate photosynthesis more quickly than algae, which are biologically less advanced. Therefore, creating a ‘siesta’ period in the middle of the lighting period is effective at curbing algae. Plants are able to start photosynthesis once there is sufficient light. Algae need a long and uninterrupted lighting period to function properly. Intensity and duration will also be detrimental to algae growth. Create an hour dawn/dusk lighting period at the start and end of the lighting period to simulate natural lighting with the ‘siesta’ period in the middle of the intense lighting period. Duration depends on many variables such as type of lighting, size tank, intensity of the lights, etc. The point of this is to say that algae prevention is not a black art that involves estimation of color temperature. There are a few specific things that cause algae, mostly including excess nutrients (phosphate, nitrate) combined with light that is useful for photosynthesis. Fix the water chemistry and you should be able to get rid of the algae without impairing the total light available to your plants in areas of maximum activation for photosynthesis.
Re: Lighting Spectrum and Photosythesis 
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