Test two: In test two,
a “Background Shader” was applied to the same dummy surface (Background Shader is a Maya default shader,
it allows catching reflection,
specularity,
and shadow without rendering the object itself).
It was assumed that this method would provide an acceptable result,
which only renders the caustics area and keeps rest of the parts with a value of 0,
so it could be used for compositing purposes.
However,
the result was not successful and could see some small values in the background where supposes to be completely black.