[Radiance-general] [Radiance-material definition] glass

[pillo]

i m a radiance beginner as you can see reading what follows..;-)
i m wondering how  to describe glasses (the same refraction index) that 
are different in thickness... (5mm, 10mm...)
(i m modeling rooms with different windows, single and double glassed 
ones)

void glass glass15mm
0
0
tr tg tb

where "ri" is 1.51

but where it is thickness?

should i use the interface primitive and two surfaces at different 
distances?

thanks in advice for helping me

pillo.pillo@tin.it