Question about translucent material

[psycho...@gmail.com]

I'm trying to make a perfectly transparent sphere to place in my
scene. It should be absolutely invisible (I have a counter in the
geometry that counts the number of photons that transmit through it).

I've set the reflectance to 0.0 and the transmissivity to 1.0 but I'm
not sure where to set kd, ks, and roughness in order to get them to
exist but not appear.