contribuations/sec = samples/sec * efficiency /100.

 

So 100kS/s @ 200% eff = 200kC/s.

 

It's just a stat we added awhile back to make it a bit easier to see the net effect of some change you made to the scene that could affect both sample rate and effiency (like changing light sampling strategy or adding a portal).

 

Efficiency is just % of rays that managed to catch a light, but with path tracing algorithms you can hit more than once per ray, which is why the thing is almost always above 100%. (tbh, I wish we reported it it as a decimal rather than a precent, ex 3.52 instead of 352%, but i can't get find enough traction for the idea). Earlier someone posted " What we might be doing looking at lux's efficiency and s/s speed is like measuring a car's fuel efficiency by how cold the air conditioning is". Not quite, it's more like comparing a cars speed and handling....but testing each car on a completely different road.

You cannot compare stats between scenes, there's FAR too many things can affect low long it takes to trace a ray and how many light hits each one is getting. That's true of any render engine, you can have two completely legitimate render jobs with times that vary by an order of magnitude or two. If you want to dig into the stats bar, give this a read: http://www.luxrender.net/wiki/LuxRender_1.0_Interface#Status_Bar