If it's actually in the rules that materialized spirits can't see through transparent glass or Plexiglas, then that's overruled at our table on the grounds that it's really, really stupid. Page reference, please (just out of curiosity, it won’t negate a ruling for our table on the grounds that we haven’t abdicated all common sense). So that’s not an issue.
QUOTE (Street Magic P. 114 @ "Astral Visibility")
Determining cover works the same way on the astral
plane as it does in the physical world (see pp. 140–141, SR4).
Shadows of physical objects in the astral plane may be drab and
insubstantial, but they are still opaque and can prevent targeting.
Items that are transparent or mirrored in the real world
(like a car window) simply impair visibility as astral shadows.
Since there are no ranged weapons on the astral plane and spell
targeting depends on seeing your target, hiding behind physical
shadows works as well as hiding behind a vibrant aura.
plane as it does in the physical world (see pp. 140–141, SR4).
Shadows of physical objects in the astral plane may be drab and
insubstantial, but they are still opaque and can prevent targeting.
Items that are transparent or mirrored in the real world
(like a car window) simply impair visibility as astral shadows.
Since there are no ranged weapons on the astral plane and spell
targeting depends on seeing your target, hiding behind physical
shadows works as well as hiding behind a vibrant aura.
it's not in the rules for materialized spirits. it's in the rules for the entire astral plane, which is what a spirit sees even while manifested, because they're still dual-natured and are therefore using astral perception. and why would it be stupid? the astral plane is different. it has many completely different laws from the physical plane in other respects, and indeed the rules for visibility and 'light' are completely different. you don't use the standard spectrum of light in the astral, why would it matter if a given material allows the standard spectrum of light to pass through or not? the question is "does this object allow astral 'light' to pass through unimpeded", and the answer is "no, it doesn't". and there's nothing unreasonable about that, any more than the fact that you can float right through that same window on the astral plane is unreasonable. different plane, different natural laws.