The diffraction limited spot size is denoted by the circle in the drawing. The points show intersections of rays traced to focus for different wavelengths (colors) of light, in this case red, green and blue. This type of intercept plot, (commonly called a spot diagrams), is a common diagnostic tool used in optical design.
The behavior of traced rays can also be optimized to achieve a desired light distribution or spot size. A benefit of ray tracing within software is that the process can be heavily parallelized (and sped up via other methods), allowing much faster simulations than would otherwise be possible.
In imaging software, a relatively small number of rays are needed for an accurate simulation (10 – 1000 rays). The goal of designing imaging systems is to get the best image possible. Typical performance metrics are modulation transfer function (MTF), point spread function, and spot size.
For illumination software, you are trying to control the distribution of light and are typically not concerned with forming an image. In this case, you’ll need many more rays, which are typically traced (1000 to millions) with a process called a Monte Carlo simulation. You define a light source, trace millions of rays, and optimize the system to make a desired illumination pattern.