A single ray is generally not adequate to to model a wavefront. Instead, a collection of rays is used. The most common collection of rays is a rectangular grid.

Many ZEMAX features, such as PSF or MTF analysis, use grids of rays to sample the entire pupil. Usually, these grids are specified by N x N, where N is an integer power of 2. For example, the grid sizes are typically 32 x 32, 64 x 64, 128 x 128, etc.

The higher the pupil sampling, the greater the accuracy of the computation and the longer the data will take to compute.

The amount of energy associated with each ray is proportional to the pupil area associated with each ray. If more rays are traced in the pupil, each ray will have less energy.