Quote -
In the RW you often need more light behind an object than is shining on it.  I have found it is often the same in 3D. Do you have a stronger light source behind the curtain?

 

On the matt-transparancy I have found a little goes a long way - 5-15% will probably do you.'

 

Another (more difficult) option would be to make a transparancy map like you do for lace or fishnets with a tight weave

No idea how to make transparency map, so I'll plump for the soft option and turn the matt-transparency setting to something lower. I do have a stronger lightsource behind the curtain. The backdrop is a mesh light, though it's only marginally stronger than the curved mesh I have shining on the figure - the plane mesh is at 8.0000000 while the curved mesh is at 6.5000000.

The exhiliration of obscenity, the obscenity of obviousness, the obviousness of power, the power of simulation... ravishing Hyperrealism... mind blowing!