@Miker

...i thought the 20h and 21h referred to the render time, 21 hours to render an image!!!

I wouldn't have the patience to wait as long especially that complex renderings monopolize my computer that has only 2 cores.
Previous tests on caustics take less than 2 minutes.


@mazoola

According to the Sunflow wiki

It's also important to note that at this time IBL and Sunsky do not emit photons, therefore, no caustics are viewable when using these lights.

I had seen this info but I thought that caustics would still be visible with the use of the desk lamp.

...you might want to try rendering your scene using a textured sky (or without a textured sky, if one was present for your first render)...

Previous tests on caustics use a sky without texture.
I did other tests with a blue sky texture but I get the same result: no caustics when it's daylight (sun icon).

With enhance external brightness enabled.
sky without texture (sun icon)
(moon icon)
sky with texture (sun icon)
(moon icon)