Homebrew PCBs

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.jp
g
> 
> 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.