To increase calculation speed for the spotlight shadow rays, the render plug-in divides the spotlight cone into rectangular regions, and again builds a sorted list of the objects within each region. Usually the lions share of the render time is spent tracing the shadow rays. The shadow ray is the ray shot from the scene towards each light when the intersection between the eye ray and the scene is found. The smaller the grid cell size, the more memory it takes and the more memory it takes to build, but the faster the final render.
The width and height of each grid cell in pixels. Advanced Rhino Render Settings Render acceleration grids Screen grid, cell size
