Quote - > Quote - so the mathematical pure sphere is in the code?

and the code tells Lux if the sphere is bigger or smaller? 

Yes in the LuxRender engine. Hit-testing a ray against a sphere (a point and a radius) is mathematically the 2nd simplest thing to execute in ray tracing. The simplest is hit-testing against an infinite plane.

Even one triangle is more complicated than a sphere. A low-poly spherical mesh is about 100 times more expensive than a sphere.

thanks for the info. it makes sense now.