Does anyone know what the first or most obvious symptoms are of not-enough-light in indoor ferns?
It would be helpful to say "Watch out for XYZ, and try giving them more light if you see that".
If the extra light was focused on only some of the plants, and those plants improved while others didn't, then low-light was probably the problem.
I was surprised to read ANYONE advising putting a incandescent bulb over any kind of plant. I saw that in some article I scanned for ferns. Unless they were trying to give them heat as well as reddish light.
Well, maybe the Koch brothers would suggest it, to increase consumption of electricity and coal.
If you need more light, I would add a CFL bulb - a spotlight bulb with built-in reflector, or a regular "pretzel" CFL bulb screwed into a down-aiming reflector on a light pole. Cheap and you can get any brightness you want, up to maybe 1500 lumens. But I bet two 800-lumen bulbs would be cheaper and more efficient than one 1500-lumen bulb.
That article made it sound right that a redder color is better for ferns, hence another "warm white" bulb would be good.
But I also bet that the warm LED bulb you already have gives enough excess red. If you just like a cooler color in your home, get a cooler color. Doubling the brightness has to be better for the plant than fine-tuning the color.
P.S. You were concerned about the light cover? Since the light that supports photosynthesis is in the visible range, and the cover lets visible light through, I doubt it filters out anything that would have helped the plant. If it absorbed infrared, it would get hot and then re-radiate infrared, so even there I don't think a cover hurts much. Just keep it clean.
