Cheri's post is loaded with different subjects, that it brought out my inner Bill Nye.
1) Plants can adjust to different light levels. They may skip a year of blooming if moved to lower light, but will compensate and do fine as long as the reduction is not too drastic.
2) In nature, most phals are understory plants that get no direct light whatsoever. Tropical forests can be dense! The best-grown phals I've ever seen were in a greenhouse so algae-covered that it was just plain dark.
3) Low-E windows reduce the light transmission to about 75-80% - but that's light that is perpendicular to the glass. All windows reflect a good bit of the incident light that comes in at an angle, which is precisely what sunshine does.
4) Roberta hit on a good point about the glazing being the "light source" to indoor plants. Sunlight does not really go through glass (or other glazing materials), it is absorbed on one side, travels through it and is reradiated with reduced energy on the inside. The intensity really does drop off at an inverse square of the distance, but because the area of radiation is so large, the level doesn't actually drop off that rapidly.
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