| Try to use dielectric_material_photon (i think this). connect this node to your (for example) blinn1SG to photonshader in mental ray (Blinn) tip. If you create all right you have a good caustic in low time. I've forgoten cearly. If you need, i'll write your tomorow. This way is the most easy to create a caustic effect.
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Sorry for the mistakes :)
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