After reading TFA, it appears that the author thinks that 'transparent' means clear to the eye, as opposed to tinted. It's a measure of how much of the desired spectrum passes through, i.e. the transmissivity.
"Completely transparent" means 100% transmissivity. Sorry mate, not available.
Besides, TFA describes the action as deliberate internal reflection of some incident light to the sides, so the transparency has gotta be worse than the intrinsic material, since some light is forced to not pass through.
A useful discussion would have been the tradeoff between window transmissivity and power per unit area...