Is anybody out there that can explain UV light with regard to transmission
through glass and plastic, plant requirements of the stuff and how they use
it and if those UV light blocking windows are good or bad.

I can shed some light, no pun intended, on the subject of UV light transmission
and insulated glass units.

Most insulated glass units will block some of the UV radiation in the 300-380
nm range. This includes the red spectrum which some plants need to flower.

Standard insulated glass will block 43% of the UV and allow 82% visible light
transmittance while insulated units that have a special coating on the inside
of the glass, Low E, and are argon gas filled will block up to 87% of the UV
while allowing 71 % of the visible light transmittance.

Some of these specialty coatings will block almost 100% of the red spectrum and
could affect flowering of some plants. The company I work for has been
producing garden windows, among other types, for the past ten years that has
insulated glass incorporating a special Low E coating and has never had any
complaints from consumers related to the growing of flowering plants.

There are some greenhouse manufacturers, Four Seasons is one, that uses
insulated glass units that have a Low E coating and are argon gas filled.
Perhaps someone on this forum has one and could weigh in on the subject.

As for plastics most fiberglass greenhouse coverings and lexan insulated sheet
material contain coatings to prevent UV degredation and do not affect the
flowering of orchid or other plants. If they did not they would turn yellow
very quickly and begin to decay.