Aquarium Lightning
A few notes about Kelvin:* Plant chlorophyll absorbs light at wavelengths of 300 to 700 nm (a Kelvin rating of about 6400 strikes a good balance here, which is why this is the best Kelvin temperature for freshwater plants and symbiotic zooxanthellae in corals).
* The lower the "K", the more yellow, then red the light appears, such as a 4500 K bulb.
* The higher the "K", the bluer the light appears, such as a 20, 000 K bulb.
*Higher Kelvin Color Temperature lights penetrate water more deeply, even more so in saltwater, however there is less of the infrared "PAR spike" as well. * The human eye sees mostly sees light around 5500K.
* Candle flame = 1850 – 1900 K
* Sunlight (1 hour after dawn) = 3500 K
* Typical summer light (sun + sky) = 6500 K
* Cool white fluorescent = 3400 K
In regards to the original question, I don't think there is a definitive Kelvin rating that is best for plant growth. The measurement itself is only meaningful to humans, as it generally indicates what kind of hue the light gives off as perceived through the human eye. Lower values are more red-ish in appearance, while higher values appear more and more blue.
That said, shorter wavelength blue light penetrates much farther into the water than the longer wavelength red. This may be why 6700k+ Kelvin bulbs seem to be better for plant growth, since the blue light is more readily available for the plant to use compared to the lower Kelvin lights.
However, plants actually use a broad range of the light spectrum for photosynthesis... not just the blues. This is known as PAR, or Photosynthetically Active Radiation. Basically, it is the output of light needed by plants for the process of photosynthesis. Photosynthesis actually uses the very much of the 650 to 670 nm range of the light spectrum which appears to humans as deep orange to red color. It also uses much of the 430 to 475 nm range of the spectrum (varying shades of blue), but not quite as much as the red range. The range of colors which human eyes are most sensitive to are the yellow/green range (550-620nm) is almost completely unused in photosynthesis, which is why most plants appear green in color (reflected light). It also explains why many warm/cool white bulbs are so terrible for growing plants--they mostly have the yellow/green tint that humans are so sensitive to but plants don't use at all.
So what should you look for in a bulb if the Kelvin rating isn't the best measurement to use for growing plants? I would recommend trying to find the spectral power distribution graphs for the light bulbs you plan to use. Most manufacturers of aquarium-specific light bulbs will put this graph right on the cardboard case of the light bulb for you to read. You want the graph to look like the letter "M"... good bulbs for growing plants will have a higher concentration of light being output in the 430-475nm range, almost no power in the 500-600nm range, and another large curve or spike around 650-675nm.
If you decide to buy a cheaper bulb from places like Home Depot or Lowes (which I don't necessarily think is a bad thing at all), you may have to dig a little deeper for your information. Try looking around the manufacturer's website. Many of them have the spectral power distribution data on their sites if you poke around enough.
Here are some of the graphs for the bulbs mentioned earlier in the thread:
The four most important criteria in determining the light you need are:
• Lumens per watt
• Lumen focus (as well a restrike, although less of an issue as per my research and tests than focus)
• PAR (this is important to understand as well as related useful light energy)
• Watts (or watts per gallon, however this term is way over used/simplistic)
• Lumens per watt
• Lumen focus (as well a restrike, although less of an issue as per my research and tests than focus)
• PAR (this is important to understand as well as related useful light energy)
• Watts (or watts per gallon, however this term is way over used/simplistic)
Here are a few Kelvin numbers:
*Absolute zero = 0 K (-273.15C)
*Waters freezing point = 273.15 K (0 C)
*Waters boiling point = 373.1339K (100C)
Kelvin is used in the lighting industry to define the Color temperature of a bulb. Higher color temperature lamps above 5500 K are "cool" (green–blue) colors, and lower color temperature lamps below 3000 K are "warm" (yellow–red) colors.
*Absolute zero = 0 K (-273.15C)
*Waters freezing point = 273.15 K (0 C)
*Waters boiling point = 373.1339K (100C)
Kelvin is used in the lighting industry to define the Color temperature of a bulb. Higher color temperature lamps above 5500 K are "cool" (green–blue) colors, and lower color temperature lamps below 3000 K are "warm" (yellow–red) colors.
A few notes about Kelvin:* Plant chlorophyll absorbs light at wavelengths of 300 to 700 nm (a Kelvin rating of about 6400 strikes a good balance here, which is why this is the best Kelvin temperature for freshwater plants and symbiotic zooxanthellae in corals).
* The lower the "K", the more yellow, then red the light appears, such as a 4500 K bulb.
* The higher the "K", the bluer the light appears, such as a 20, 000 K bulb.
*Higher Kelvin Color Temperature lights penetrate water more deeply, even more so in saltwater, however there is less of the infrared "PAR spike" as well. * The human eye sees mostly sees light around 5500K.
* Candle flame = 1850 – 1900 K
* Sunlight (1 hour after dawn) = 3500 K
* Typical summer light (sun + sky) = 6500 K
* Cool white fluorescent = 3400 K
*The 6500 Kevin bulbs have produced the best freshwater plant growth (as this Kelvin lamp generally has more of the infrared nm spike need by "higher" plants, but still has some of the 425-500 nm blue).
In regards to the original question, I don't think there is a definitive Kelvin rating that is best for plant growth. The measurement itself is only meaningful to humans, as it generally indicates what kind of hue the light gives off as perceived through the human eye. Lower values are more red-ish in appearance, while higher values appear more and more blue.
That said, shorter wavelength blue light penetrates much farther into the water than the longer wavelength red. This may be why 6700k+ Kelvin bulbs seem to be better for plant growth, since the blue light is more readily available for the plant to use compared to the lower Kelvin lights.
However, plants actually use a broad range of the light spectrum for photosynthesis... not just the blues. This is known as PAR, or Photosynthetically Active Radiation. Basically, it is the output of light needed by plants for the process of photosynthesis. Photosynthesis actually uses the very much of the 650 to 670 nm range of the light spectrum which appears to humans as deep orange to red color. It also uses much of the 430 to 475 nm range of the spectrum (varying shades of blue), but not quite as much as the red range. The range of colors which human eyes are most sensitive to are the yellow/green range (550-620nm) is almost completely unused in photosynthesis, which is why most plants appear green in color (reflected light). It also explains why many warm/cool white bulbs are so terrible for growing plants--they mostly have the yellow/green tint that humans are so sensitive to but plants don't use at all.
So what should you look for in a bulb if the Kelvin rating isn't the best measurement to use for growing plants? I would recommend trying to find the spectral power distribution graphs for the light bulbs you plan to use. Most manufacturers of aquarium-specific light bulbs will put this graph right on the cardboard case of the light bulb for you to read. You want the graph to look like the letter "M"... good bulbs for growing plants will have a higher concentration of light being output in the 430-475nm range, almost no power in the 500-600nm range, and another large curve or spike around 650-675nm.
If you decide to buy a cheaper bulb from places like Home Depot or Lowes (which I don't necessarily think is a bad thing at all), you may have to dig a little deeper for your information. Try looking around the manufacturer's website. Many of them have the spectral power distribution data on their sites if you poke around enough.
Here are some of the graphs for the bulbs mentioned earlier in the thread:
