The UV wavelenghts commonly used for photolithography are known
as "long-wave UV" and are typically in the range of about 360 to 375
nm. Most glasses and many plastics will pass this range rather
easily. As wavelenght shortens to mid-wave and short-wave, UV
absorption by most glasses and plastics increases substantianlly. At
254 nm (short-wave UV) for example, perhaps only a few percent of the
UV will pass thru common glass. Special glasses are then required,
quartz glass is one example.
> Doesn't glass (or at least most kinds of glass) tend to absorb UV
rays? I
> was under the impression that this was the case. As a result I
first tried
> using acrylic, and after finding that it scratches too easily,
ended up
> using the thinnest glass I could find -
>
http://www.krupin.net/serendipity/uploads/pcb_making_exposure_frame.jpg
>
> It sounds to me that using thick glass would seriously slow down the
> exposure time, especially for people who are using a less powerful
light
> than the one you have.