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Kelvin quandries

I always thought 5,000- 6,500 Kelvin was an optimum colour temperature
for plant growth. Even 4,500K OK but too yellow for aesthetic

At least two manufacturers are selling 8,000K bulbs

http://www.adaeurope.com/boti.html  Click Na goods.
Says: "The perfect light for plant photosynthesis. Na lamp rovides the
spectrum of light at 8,000K as close to the natural environment as
possible. The famous Green Glow 604 which provides the ideal amount of
light for the plant according to their condition"

In the Spring 2000 copy of Planted Aquaria Magazine, Zoomed have
advertised FloraSun, an 8,500K high intensity fluoro tube which
"maximizes the photo-biological process in plants" "Emission spectra
closely matches the absorption curve of Chlorophyll-A, promoting
maxiumum photosynthesis" They suggest that it can be used with their
6,500K UltraSun - full spectrum "Trichromatic Super Daylight". Has UVA
emissions to "stimulate feeding and mating behaviour" and a "CRI rating
which maximizes color rendition and contrast perception". And then if
you were not confused enough, a 5,500K TropicSun tube on the same page
which " simulates natural sunlight".

I assume the combination of the 8,500K and 6,500K is to try to optimise
viewing potential of the fishes and maximize photosynthesis.

So what's natural - what's optimal ? Can we improve on nature by
increasing the Kelvin to 8,500 ? Is this too blue ? Do we have Kelvin
creep here?

Any comments/experiences ?

{I'm at 5,200 K myself}

Ken Brook