In general, there seems to be a lot of confusion about what exactly color temperature
is and whether or not it's of significance to a planted tank, so I decided to create a thread to share information and debate.
I've seen a lot of posts across various forums where people extol the virtues of a bulb's color temperature rating with regards to photosynthetic suitability. I've seen people claim that “kelvins” are equally as important as “watts” (watts don't tell the entire story either, though that's an entirely separate topic). I've seen people ask what color temperature the bulbs being used are when someone else reports a plant problem, as if color temperature were a factor. I've also seen people say that others might see better results if they purchased a 6700K bulb, but I've never seen any of these people say exactly
why they feel color temperature is so important.
Keep in mind that I am not a physicist or lighting science expert, I've done no experimentation on my own, and I don't guarantee the accuracy of any information in this thread. This information is presented
as I understand it, not as gospel.
So with that in mind, I'll attempt to answer a few basic questions and share my opinion on the importance of color temperature ratings on bulb.
What do plants require?
Plants require (among other factors that are beyond the scope of this topic) light energy for photosynthesis. The light is absorbed by chlorophyll molecules, with chlorophyll a absorbing most strongly around 430nm and 660nm, a chlorophyll b absorbing most strongly around 450nm and 640nm. This likely varies by species, but as a general range that's about right.
Those wavelengths indicate (among other things) what color humans perceive the light to be:
Red: 625 - 740
Orange: 590 - 625
Yellow: 565 - 590
Green: 520 - 565
Cyan: 500 - 520
Blue: 435 - 500
Violet: 380 - 435
So in general, most plants require more intense lighting at the 430-450nm (low blue) and 640-660nm (low/middle red) ranges than elsewhere on the spectrum. Keep this in mind, we'll come back to this topic.
What do we mean by “color temperature”?
In order to answer this question, we need to have a common understanding of a few key terms.
Kelvin – A kelvin (K) is a unit used to measure temperature. The Kelvin scale is a thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero (0 K). Note that kelvins are not technically referred to as degrees, so the correct written representation would be 5000K instead of 5000°K. (I make this mistake often)
Black-body radiator - A black-body radiator is a theoretical object that absorbs all electromagnetic radiation that falls on it, but radiates varying wavelengths of light dependent upon the thermal temperature of the radiator. As the temperature of the black-body radiator increases, the color of light emitted follows the Planckian locus illustrated here.
Given those definitions,
color temperature indicates what temperature, measured in kelvins, a black-body radiator would need to be heated to in order to radiate a given color of light.
What does that mean for aquarium hobbyists? Rather than attempt to paraphrase an already excellent article, I'll quote Dr. Sanjay Joshi:
Quote:
Does this mean that a lamp being sold as a color temperature of 20000K is a black-body radiator and has an actual physical temperature of 20000K? No, since the lamps are not black-body radiators! To be able to assign a color temperature to a light source there must be a color match as well as a spectral match to a black-body radiator. The spectral output of fluorescent lamps and metal halide lamps does not match with the black-body spectral irradiance. Hence, the term color temperature, in fact, does not apply directly to these light sources. What it really means is that if we were to compare the lamp's color to a black-body at 20000K, it would appear the same to a human observer. The technically correct term for this is Correlated Color Temperature (CCT) which is defined as the value of the temperature of the black-body radiator when the radiator color matches that of the light source. CCT implies a color match to a black-body at the specified temperature, but there is no spectral match.
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One of the key phrases in this quote is “if we were to compare the lamp's color to a black-body at 20000K,
it would appear the same to a human observer.” The correlated color temperature ratings on bulbs are strongly dependent upon human perception, not true spectral equality.
Dr. Joshi goes on to say:
Quote:
This now brings up the issues of matching lamp color to color temperatures of the black body. Once we start talking about color, we have to remember that color is not a physical property but a physiological response created in the brain by the visible light seen by the eye. As someone adequately surmised, "Color is only a pigment of your imagination."
To be able to work with color mathematically, scientists have developed a mathematical means to specify color - where color is specified by numerical values called color coordinates or chromaticity. Correlated Color Temperature (CCT) can be determined by mathematical formula to find the chromaticity coordinates of the black body's color temperatures that are closest to the light source's chromaticity.
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One thing many bulb manufacturers provide is something called a spectral power distribution graph, such as this example from GE's website:
SPX35.jpg
http://www.gelighting.com/na/busines...ion_curves.htm
The SPD graph gives you a good idea of what wavelengths the bulb emits and in what relative intensities. This is helpful if you know the wavelengths needed for photosynthesis, as discussed earlier.
While I don't pretend to understand the math behind calculating correlated color temperatures, I do know that mathematics often allows more than one set of variables to produce the same result (2 + 3 and 4 + 1 both produce the same result, despite being unique sets of inputs), meaning it's possible to have two bulbs with the same color temperature rating that have vastly different SPD graphs (one example can be seen
here).
This phenomenon of two bulbs looking the same to humans but actually emitting different wavelengths of light is called metamerism. The fundamental reason for metamerism, as Dr. Joshi discussed in his article, is that color is a sensation rather than a property of an object. Human eyes can register the same sensation from an almost infinite variety of combinations of different light wavelengths.
Metamerism is the number one reason why I personally place virtually no importance on color temperature ratings as an indicator of photosynthetic suitability. If two bulbs can have the same correlated color temperature value but emit different spectral ouputs, how can you tell from the CCT value alone which is better for plants? In short, you can't. The only way I know of to be sure which bulb produces more photosynthetically active radiation (PAR) is to use a PAR meter.
In fact, a poster at plantedtank.net by the name of i4x4nMore did a limited test with a PAR meter, and actually saw slightly better results with a 3200K bulb than a 5500K bulb, despite the common “get a full-spectrum bulb for plants” mantra. Admittedly the test was limited and the differences small, but if the conventional wisdom were correct, he should have seen results in
favor of the 5500K bulb, not against it. His tests (and a plethora of other interesting information) can be found
here.
In summary, I find color temperature ratings helpful to determine how the tank will look to human observers, but I prefer to rely on other factors (SPD graph, power, reflector, distance from water, depth of tank, etc.) for judging the benefit to my plants.
For those interested in further reading, here's where I obtained the vast majority of information in this post:
http://en.wikipedia.org/wiki/Visible_spectrum http://www.americanaquariumproducts...._Lighting.html http://www.aquatic-eden.com/2007/12/...-aquarium.html http://www.biologie.uni-hamburg.de/b-online/e24/3.htm http://www.gelighting.com/na/busines...ion_curves.htm http://www.lrc.rpi.edu/programs/nlpi...es/measure.asp http://www.personal.psu.edu/sbj4/aqu...lideLamps1.htm http://www.reefkeeping.com/issues/20...sj/index.php#0 http://www.sylvania.com/LearnLightin...rDistribution/
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