metal halide height

[Tim Marvin]

1/8 inch will warp for sure.

[Instar]

Acrylic will block (filter might be a better word) from 9% to 11% of the light, mostly in the ultraviolet range, but, only these percentages.
Glass will block from 30 something (possibly a bit less with an optically clear perfect glass - expensive) to nearly 50 something depending on tint and thickness.