[newbie question] trouble creating coloured window glass


First, an apology if this is not the right place to post this kind of
question, but it was the only suitable-looking mailing list I could find.

I'm new to Radiance, although I think I've got the hang of the basics now.
I'm working on an archaeological model---an Anglo-Saxon church building. The
problem is creating some coloured glass for a window. I have created a
successful sky file, and the sunlight pours through the window (when there's
no glass) to leave a window-shaped highlight on the floor and illuminating
the rest of the room via ambient bouncing. When I tried adding a
window-shaped polygon and assigned it the "glass" material using 0.96 0.96
0.96 as my rgb transmissivities (as advised in the online manual) everything
was still fine. However, when I specified the following:

void glass blue_glass
3 0.2 0.2 0.96

the light entered the window, but instead of appearing blue inside the room,
it was a very low-intensity white. Experimentation seemed to indicate that
the light would always be white, and that it seems to adopt an intensity
related to the *lowest* of the values 0.2 0.2 0.96, ie 0.2. (Indeed, 0 0 1
gives no light at all, not blue light.) I've since tried all sorts of
things, including the following "illum" approach recommended in the online
intro chapter to the "Rendering with Radiance" book:

skyfunc brightfunc window_dist
2 winxmit winxmit.cal
void glass window_glass
3 0.96 0.96 0.96

window_dist illum RAL5013_Cobalt_blue
1 window_glass
3 0.2 0.2 0.96

but the light was still white and not blue. Then I tried using the "trans"
material with some success, in that the light coming in was blue! But I lose
the nice window-shaped sun highlight on the floor---the blue light is just
diffusely spread around. Indeed, when I use no ambient bounces, I just get a
completely black image (instead of a white, window-shaped highlight inside
an otherwise completely black image, which is what I was getting when I used
"glass"). Changing the values for the "trans" material doesn't seem to
correct this.

What's more, I'm totally guessing the values to use, which kind of defeats
the point seeing as I'm trying to make visually accurate images. I'm sure
the correct values are tied up with the "glass" material---if only I could
get it to be coloured!

I'd be really grateful for any help with this.

Many thanks,
Anton Prowse