Ray Depth / Ray Bounce / Ray Recursion Limit
It’s a common parameter in most if not all software that is capable of raytracing, and a common consideration in minimising the rendering time of scenes. When you raytrace a scene, the software fires a set number of rays for calculating the light per pixel of your rendered scene. When a ray reaches the surface of an object, the natural behavour is for that light to bounce on to the next surface, and the next, and so on. In the natural world this happens all around us, and of course nothing is needing to be calculated to achieve it. It just happens. When 3D software simulates this, it is desirable to do this the least number of times possible while achieving the result you are aiming for. This is your Ray Depth or Ray recursion Limit. We can tell the software how many consecutive bounces to calculate for each ray before stopping.
For scenes containing little or no reflective and/or refractive materials, the limit can be kept quite low, 4 would suffice, maybe even lower. If you have glass or gem stones, metals such as chrome or gold, then the Ray Depth becomes much more critical.
Looking at each image in turn, not surprisingly, calculating zero bounces means the objects in the scene effectively receive no light, and therefore render as black silhouettes. The first seven images have an extra ray bounce each time, so more detail is revealed in the reflections and refractions as each calculated ray can travel further through the scene. Images eight and nine have eight and sixteen bounces respectively, which even in this scene yields some gains, but not very much. Compared to six bounces eight does give a good benefit, but beyond eight the gains are minimal.
The final render just highlights the worth of Indirect Ground Illumination (also termed commonly as Caustics) when using reflective or refractive materials.
Although these examples are rendered with Keyshot the same principles apply with any software.