In this short how-to you will learn how to easily setup Corona to render caustics!
Start with a blank scene just to be sure that everything is at default state. You can use this technique on your current scene after you achieved the correct result on a blank scene first.
Create a basic Plane and a Circle:
Now we create the main light which is a disc light in this example.
We start with a Radius of 1.0 which could be changed later if necessary. Be sure to change the Directionality to 0.6 to get a punchy light source.
Short render test without making any changes yet.
Now we head over to the material editor to setup the material for the Circle.
Make a simple Red Chrome material.
Why Red? You will see later.
We already got some slightly visible Caustics with default render settings but that's yet not enough.
So we now head over to the render settings to set it up for our needs.
First of all change the whitepoint to 0.01. Because we want the full power of whites.
We counteract by decreasing the EV to -6.0 to reduce the burned out whites.
Make sure your debug mode is activated.
Now we make the last changes to the engine.
Change the render engine mode to PPM................
Ok now we have the caustics rendering properly we need a technique for the white highlighting on the ground plane. It won't work if you just exlude the ground plane from your light. So we need something different.
My current solution is a very simple one. Just change the white balance to 3000. While our chrome produces red caustics and the ground plane is tinted to blue you can isolate the ground highlights in photoshop with ease.
If you select the red channel in photoshop you will see the caustics more isolated from the ground highlights. Unfortunately you won't eliminate them fully but i think that's already enough!
If you wan't to increase the sharpness of the caustics itself, just decrese the radius of the light and increase the light intensity. By that you will get even sharper results.