Originally Posted by aquabillpers
Thanks for your responses!
Please pardon my obtuseness.
Are we saying that all Kelvins in the 400 - 700 nm range produce about the same PAR values, all other things being equal?
Would the answer be the same for both LED and tube-generated light? Why?
Since par diminishes with the distance from the light source, what would be the difference between a a fixture designed for a deep tank and a shallower one, given the same footprint? More LED's? More voltage?
"Kelvin" is the absolute temperature in the Celcius temperature scale - what used to be Centigrade degrees. It is the name of the absolute temperature scale. A Celcius degree is the same as a Kelvin degree and is a measure of temperature. The wave length of light is often measured in nanometers or "nm". 400 to 700 nm wave length is the range of wave lengths, or colors, of light that plants can use for photosynthesis. When light sources are given a Kelvin temperature it means that a black body at that temperature would radiate light with about the same intensity vs wave length as the light source produces.
Light fixtures are not designed for any specific shape of aquarium, other than that their length is always intended to be equal or less than the aquarium length. If a light is suspended high above the aquarium the light intensity will vary much less over the full height of the aquarium than for light fixtures sitting right on the top of the tank. But, with most LED lights, the large angle cones of light from each individual LED will cause a lot of spill over light with the light high above the tank.