[Radiance-general] Perfectly Diffuse Surface?

[schwaj3 at rpi.edu]

Hello, I'm an undergraduate student who is new to Radiance. I would like
to know how to set up a perfectly diffuse surface material. Plastic
seemed like it would be the right choice, but I learned that
" If the surface roughness (a5 ) is zero or the specularity (rs ) is
greater than the threshold (t s ) then a ray is traced in or near the
mirror direction. "

Could anyone show me an example material which is perfectly diffuse?