Useful optical simulation methods in illumination design

Zemax OpticStudio

This article is part of the [Tutorial Series] Illumination Systems Fundamentals free tutorial.

This lesson provides an introduction to different optical simulation methods in illumination design. You will learn how to set up your system in terms of how the ray trace operates, for best results. The lesson also provides useful links to articles that teach specific simulation methods.

Authored By Katsumoto Ikeda

Understanding ray tracing in illumination systems

The shape of the lens is rarely determined by analytical calculations alone. In actuality, many optical phenomena affect the rays, such as Fresnel loss (polarization), dispersion, material absorption, diffusion. We need to take these optical phenomena into account when designing and simulating the illumination system. For non-sequential ray tracing of illumination systems, few cases require diffraction and interference, but a careful evaluation of the optical system is required.

Ray tracing methods:

Treat the light as a bundle of rays
- rays have position, direction, power, wavelength
- diffraction is not considered
Ray tracing
- rays start at the source
- rays hit a surface, change properties (direction, power) due to reflection or refraction
- when rays do not have prospective surfaces, or when power is below the threshold, ray tracing finishes

Schematically, it can look like the following image.

Ray tracing schematic

The lifetime of the ray can look something like the following flowchart.

Ray tracing flowchart

There are several parameters of distribution we need to consider in a simulation:

angular distribution of the source
spatial distribution of the source
spectral distribution of the source
diffusion distribution of diffusing surfaces

Spatial and angular distribution

For non-sequential analysis, rays are used for illumination. Within one ray, there is one position, one direction, one value of power, and one wavelength or color information. For the simulation of the optical system, we use a statistical representation of multiple rays to get our result. The randomness of the rays is determined by using each distribution and setting the direction, power, and wavelength of the rays. This type of simulation method of the rays is called a "Monte-Carlo" simulation, which means a random distribution of rays. Since one ray only has limited information of the entire light source, if we don't use enough rays the results can be inaccurate. To increase the signal to noise, we need to increase the number of rays. Multi-core CPUs and GPU calculations can increase the speed of calculations.

The optimal resolution of the detector with respect to the number of rays is explored in Lesson 2, Detectors, but too few rays gives a lot of noise, and too many rays is a waste of computational resources and more importantly, time.

Possible simulation methods used in illumination design

The more accurate the model of the illumination system, the more accurate the simulation will be:

The light source: angular distribution, spatial distribution, spectral distribution, near-field distribution.
The 3-dimensional objects in the system: the shape will determine the reflection and refraction of the rays.
Materials: the index of refraction, transmission, and to a lesser extent, the diffusion parameters in the material.
The optical properties of surfaces: the transmission, reflection, diffusion of the optical surface will determine how the rays interact with the surface.

Just as computer code only does what the program tells it to do, the results of ray tracing behave only as our settings. The accuracy of our results depends on our modeling, and how close we are to the system in real life in the components that matter. On the other hand, modeling too many minute details can be a waste of time, and it is up to the designer to discern what parameters are essential in the simulation and what is not. Although complicated, the diffusion properties and the modeling of light sources can be critical in the design.