How to use Ray filters??

Hi

My problem is that I've rays coming out from a subject. I want the rays to intersect only one other object and ignore the rest of the bodies.

I guess I've to use Ray.userPreFilterCallback() . But how do I go about using it. Where should I use it?

I'm creating the ray inside of a collision callback and want it to intersect only the participating bodies of the collision and ignore the rest.

Plz advice. Tell me if my problem is not clear. I need it to be solved for sure .. ..

regards

Nikhil