This is the final image we will try to achieve.

Please note the following points:
- I wrote this tutorial to explain a friend the basics of caustics.
-This tutorial does not go deeply into physical-lightning considerations.
- Further experimenting and testing are required for photo-realistic results.
- Stay in the RENDER module (keyboard shortcut 3)

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Because I'm a cool guy I've already prepared a scene that you should be able to drag-drop from the xsi netview :
caustic_start.scn(169k)

did it work? I sincerely hope so :-))
If if did not, you can download it here: caustic_start.zip(48k)
if you render, preview or render region you'll get something like this:
(don't forget to select the good camera!)

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

Note: if your picturelooks a bit different it's possibly due to the fact that XSI merges scenes when importing
so you'd still have default light and ambiance values (delete the def. lights and set the 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 similar to configuring a global illumination calculation.

- First we will configure the light to be a photon emitter.
- Secondly, we have to define 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 click 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 (called cube and cube1 !)

Now modify the common property called visibility (using the selection button on the right panel)
and check both checkboxes 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 later!!

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if you render you should get results 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 picture like this one.
Some basic things for improving quality are: - increasing the number of photons and increasing the number of raytracing depth or photon depth

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