My R. macrandra japan red is very read under 10k and blue LEDs...However you can't really see the redness unless you put them under a normal 6500k bulb. I think this is the issue. Most LEDs render reds very poorly.
LEDs and fluorescent lights don't produce spectrums that match certain color temperatures. Instead, LEDs tend to all have a big, wide peak in the blue area, and a much smaller peak in the red. Fluorescents have narrow peaks that are spread around, depending on exactly what phosphors are used. Because our eyes are relatively insensitive to red light, in order to see intense reds we need our lights to supply a big peak in the red area, making most of the LEDs we use somewhat deficient in that regards. But, if our goal is growing healthy plants, that look great in our aquascape, why do we need intense reds?
Yes, my goal is to grow healthy plants. But also to bring out all the beauty the plant has to offer as well. There's no denying that some red plants just have a certain 'WOW' factor. If you can grow healthy vibrant red plants, then why not have your cake and eat it too?
I have Ecoxotic 8000 Panorama's plus 2 magenta strips and my R macranda Japan looks nice and red. Even with the magenta strips off they look red.
Pics or it didn't happen!
I think it's CRI - color rendering index. Natural light (sun) has lots of light in the various colors. CRI of natural light is 100. Surprisingly incandescent bulbs are also ~100 (probably since they are blackbody radiators, same as the sun). CRI isn't the same as light "color".
Artificial light isn't as well spread out. CRIs can drop into the 80s and lower. Really low for the old "fish tank bulbs".
You can still grow plants (good PAR), might still look good to the eye (color temp), but might be missing some of the spectrum.
A mix of bulb colors (LED or otherwise) is what lots of people do to help spread the spectrum out.
I've got a 50/50 of warm white and cool white 10W LEDs (8 of each). I'm toying with swapping some out for some blues and/or reds as it looks a little warm for my tastes.
The RGB LEDs look interesting too - red, green and blue in one unit. Needs it's own ballast however.
Interesting..... I'm really looking for a commercially available premade fixture though. The thing stopping me from taking the plunge with LED's is that I haven't seen a tank running them growing nice red plants.