This is the final image we will try to achieve. Please note the following points:
- I'm not at all a lightning guru.
- I wrote this tutorial to explain a friend tha basic of caustics.
-This tutorial does not go deeply into physico-lightning considerations.
- Further experimenting and testing are required for photo-realistic results.
- Stay in RENDER (key 3)

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Because i'm a cool guy i already prepared a scene that u should be able to drag-drop from the xsi netview :
caustic_start.scn

did it work? I sincerly hope so :-))
if you render, preview or render region you get something like this:
don't forget to select the good camera!

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so we are going to change this cube to a glass looking material.
get-material->phong
under transparency/reflection tab -> transparency to 0.6 ( use CTRL to move all 3 sliders)

Note: if your picture seems a bit different it's possibly due to the fact that XSI merges scenes when importing
so u still have default light and ambiance values (erase def. lights and set ambiance to 0)

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The Caustics.

Ok. Now we are going to add some "realism" by working with caustics lights.
There are 3 things to configure before a caustic render can begin.
Please note that the procedure is the same as configuring a global illumination calculation.

- First we will configure the light to be a photon emitter.
- Secondly, we have to say which surface will transmit, receive (or both) the photons.
- and finally tell mental ray to calculate the caustic in the scene.

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Configure the light:

select the spot and modify -> shader: (should be a white spot with intensity of 2)
under the Photon tab click the caustic checkbox and set the following values:
Energy : RGB 0.7
Energy Intensity : 12000
Number of emitted photons Caustic : 2400
(more photons = longer the render time but accurate results)

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Configure the objects:

select the cube and the ground (poeticely called cube and cube1 !)

Now modify there common property called visibility (using the selection button on the right panel)
and put a check in both checkbox under the caustic tab.
You can of course optimize performances by choosing which object transmits or receives or not (!)

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Configure the render options:

here are the values and i'm too lazy to copy them so please have a look at the following image:

again note that this is just an introduction... i'll add the comments on optimizing and adjusting the values as soon as I understand how to !!

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if you make a render you should have a result like this no?

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Ok so that's it for now... it's late and i'm going to bed soon.
Just add a refraction index of 1.4 and an area spot and get a pitcture like this one.
Some basic things for improving quality are: - increase the number of photons, increase the number of raytracing depth or photon depth

You can also change the values for different results...
Just experience it!