What does “rectangle light options” directional parameter do?
It focuses the distribution of light rays from (nearly?) 180 degrees down to 90 degrees to the surface of the light. You can think of it as adjusting a broad diffuse light into more of a spotlight. A killer feature (especially when combined with a texture map) and especially when RT supports it so we can tune it better 
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Directional parameter changes the angle of light emitted from the plane.
When Directional = 0 - the light is emitted from the place in 180 degree angle - the effect is very smooth light , and very soft shadows.
If Directional is set to 1 - the light is 100% directional-parallel - the shadows are very sharp - and the illumination is very strong.
Everything in between 0 and 1 - is blending between these two effects.
A killer feature if it worked right. I recently submitted a bug to Dmitri that caused massive noise and strange patterns when using this feature.
It mostly works right. In the scene that Dmitry sent, without any of the extra plugins, the material renders perfectly fine. I didn’t get a chance to debug with all the plugins installed yet (colorcorrect, bercon maps and vraybump2normal).
There is nothing wrong with the light in that case; just the probability for a secondary reflection ray to hit the light at a suitable angle is very small. You can remove the noise by turning off the “affect reflections” option for the light.
Best regards,
Vlado
Vlado, thanks for the info. Is there a way to clean up the noise while keeping affect reflection checked for the light?
Regards and thanks
Oliver
I installed all the plugins, but the noise effect was unrelated to them I think. I reproduced it using a single material.
No, it’s not the plugins; like I said I could reproduce the issue eventually. Nothing I can do about it for the time being though, it’s just part of the whole physically-based rendering concept There are some work-arounds like I suggested above.
Best regards,
Vlado