Now right mouse click on the MR. Area Omni and disable Affect Specular and Affect Diffuse as the image below shows.
After doing this, the MR.Area Omni will not anymore affect the illumination of the room. We will use that MR.Area Omni only to create the caustic effects. Do it this way, we have much more control of the caustic position, color and the brightness. Disable the shadows of the MR.Area Omni. Now select the MR.Area Omni and all the other objects, except for the lights that you have created for illumination(Free area lights).
Right mouse click on all of the selections and select properties like in the image below.
Go to the Mental Ray panel and check Generate Caustics.This is necessary in order to have a caustic effect on the objects. Now we must go to the Indirect Illimination panel (panel for rendering), and enable Caustics.
To improve the quality of the caustics (better/clearer shapes) you must go to Light Properties and put a very high number in average caustic photons per light. I put a bit less than one million. For test renderings, I normal put about 10.000 as I find that is enough. The higher the number, the better the effect will be. Of course it will take much more longer to render all of the scene.
You can use very high numbers in the multiplier (MR.Area Omni) to create a very bright caustic effect. If you change the light color of the MR.Area Omni you will change the color of the caustics (blue omni color = blue caustics).
To have a blue color of the illumination under water the water material should have a blue color in diffuse and transparency like in the image below.
Well I hope you liked this tutorial.