Way too much emphasis is placed on Kelvin or actually Kelvin approximations. This is an almost laughable measurement for choosing fluorescent lamps.
K,
K rating, or Kelvin is often referred to as the color temperature of light produced from the bulb. The color temperature of light is derived from the temperature to which you would have to heat a blackbody radiator to make it glow the same color. However, the output from a blackbody radiator is continuous across the visible spectrum but, fluorescent tubes emit an interrupted spectrum, so this means you can't accurately define a fluorescent light as having a Kelvin color, it only approximates it. Bulb manufacturers have stamped bulbs with
K ratings to give us an idea of how the color of the light compares to sunlight. Sunlight should be about 5500K. Lower values should look more yellow/orange/red and higher values should look more white/pink/blue. It doesn’t always work out that way, some manufactures use "secret" formulas for determining the
K rating they give to their bulbs. (Hagen's PowerGlo is 18000K ?) Make no mistake. Two 6500K lamps from two different manufacturers may look very different to the eye and may be comprised of very different spectral patterns. Meaning, the manufacturers combine differing amounts of each wavelength to create their lamps. It's like saying you have 25 cents. You could have a quarter, or you could have two dimes and a nickel, or you could have one dime and three nickels, or you could have two dimes and five pennys, etc. all of which equal 25 cents, but the combination of parts can vary greatly. All this would be important, except that plants aren't that picky. Plants are highly adaptable...they have to be. They adapt to cloud cover, water turbidity, bright sun, etc. Different dissolved organics in the water will absorb differing wavelengths of light and all this can change frequently due to runoffs from rain, leaf litter, etc. The main point is this, some lamps may be more efficient, but if you provide enough intensity you need not be overly concerned about Kelvin...except in how your color rendering preferences align with Kelvin ratings. For example, I really like how actinic lamps intensify colors on my fish and plants, so when space permits I always use one actinic lamp...and no, actinic lamps do not promote algae. As stated by several others, any lamp with a
K rating from 5000K to 10000K should be pleasing to your eyes in how they render colors and the plants will adapt to whatever you place over them.
