Well i'm sure someone will prove me wrong but i'm gonna take a shot at this
I would consider all three "diffusion" however they do have different purposes they all diffuse he beam of light
in one way or another. "
Frost" is going to soften the
edge of the beam of light it is typically not directional and depending on the density of the particular
frost it will give you more or less of a "smooshed"
edge much like running the
lens tube forward or backward on an
ellipsoidal unit. I tend to find using
frost will give you a more consistent result when using to soften the
edge of light. For example in side light I will use
Rosco 119 to soften the
edge instead of running the
barrel, I've found it gives me a smoother transition between lanes and positions, I do the same for front washes if I do indeed not want to have hard edges on the beam. I've also used
frost in some situations where three lights are not quite wide enough to
cover an area and I don't have any other fixtures to fill in the holes sometimes it gives you just enough glow to make it work (I try not to do this but sometimes you have to cheat).
Silk is different but kind of does the same thing. Typically silk is directional, it will have what appear to be lines running through it. If you you were to put silk in an
ellipsoidal pointing straight down at the
stage with the lines running up and
down stage it would diffuse the beam left to right and if you run the lines in the silk left to right it would diffuse the light upstage to
downstage. If that makes sense, the beam of light would appear to oval shaped. I use silk in
cyc lights all the time to help with the scallop
pattern you get when strip light are placed close to the object you are lighting, I run the silk upstage to
downstage thus diffusing the beam left to right helping to eliminate the ugliness that is
scalloping.
Just remember when using
frost it does change the fixtures transmission of light. i.e. The heavier the diffusion the less light you get out the front of the
fixture.
I know this is fairly detailed in respect to my feelings on diffusion but I hope it helps.