Shafts of light - not shafting

I’m trying to get the effect of shafts of light coming down through the surface of the water toward the bottom of a lake. Now I though that I’d test this out, so I got a plane, textured it, with the texture itself on ALPHA so that only some parts of the material was transparent whilst others weren’t.

Easy enough, until I put a spotlight with Buf.shadow on and cranked up the HALO STEP to about 7. No matter what I did (Z-trans or Ray trace) I couldn’t get the shafts of light to go THROUGH the material, instead of round it, can someone help me?

cut out the parts of the mesh you want the halo to pass through. keep it out of sight of the camera and you should get some nice light shafts. If you need the surface of the water to be in the frame it will block the halo spots to, you will need to separate your lights to different layers or compost the image of your light shafts and under water scene together.