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 nonsequential 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.
The lifetime of the ray can look something like the following 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
For nonsequential 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 "MonteCarlo" 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. Multicore 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, nearfield distribution.
 The 3dimensional 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.
Useful articles on simulation methods

How to add coatings and scattering functions to NonSequential objects In NonSequential mode, it is often the case that we need to apply scattering profiles or coatings to specific surfaces of an object. These properties can be defined on any face of a 3D nonsequential object. This article explains the concept of a Face Number in OpticStudio, discusses how these properties can be set in OpticStudio, and reviews some issues that can arise in the process. 

What scattering models are available in OpticStudio? This article provides a summary of the surface and bulk scattering models available in OpticStudio. It describes the bidirectional scattering distribution function (BSDF) used by different builtin scattering models as well as the DLL scattering models. It also provides a general guideline on when to use each scattering model. 

How to use importance sampling to model scattering efficiently Modeling surface scattering is essential in many optical systems and is critical for stray light analysis. Accurate modeling of surface scattering can require a large number of rays, which may drastically increase computation time. OpticStudio has two features that improve scattering efficiency: Scatter To and Importance Sampling. In this article, we discuss the details of each and perform a stray light analysis using importance sampling. 

Overview of photoluminescence simulation in OpticStudio Photoluminescence is the phenomenon where photons are absorbed in a medium and part of the absorbed energy is reemitted as photons. There are two categories of photoluminescent emission, fluorescence, and phosphorescence. Each of these can be modeled in OpticStudio using the photoluminescence bulk scattering model available in NonSequential mode. 

How to simulate atmospheric scattering using a Mie model This article describes the implementation of the DLL Mie scattering model in OpticStudio. An example of using this model to simulate scattering in the atmosphere is provided. The sample system consists of two configurations. Configuration 1 models the Rayleigh limit, where scattering is mainly due to water droplets in the atmosphere. Configuration 2 models the scattering from much larger particles which shifts scattering from the Rayleigh limit to the Mie domain. 

Polarizationsensitive scattering in OpticStudio This article describes how to simulate polarizationsensitive bulk scattering and fluorescence using a custom DLL in OpticStudio. The bulk scattering model defined in MSP.dll (available for download with this article) considers the polarization of incident nonsequential rays and simulates how the polarization and direction of propagation change with each scattering event. This DLL can also be used to simulate fluorescence in combination with Mie scattering. Both fluorescence and polarizationsensitive scattering are essential for modeling biological imaging. This article summarizes seven experiments that use the MSP DLL bulk scatter model. 

How to model a partially reflective and partially scattering surface This article describes how to model a partially reflective surface which diffusely scatters a fraction of incident energy into a specific distribution. Demonstrated here are cases of scattering combined with partial absorption, as well as partial specular reflection. 

How to use tabular BSDF data to define the surface scattering distribution In many cases, scattering data is delivered as a text file of experimental values that may not be easily modeled. This article discusses how to use tabular BiDirectional Scatter Distribution Function (BSDF) data for defining surface scattering properties that may not be able to be described by Lambertian, Gaussian, or ABg models that are built into OpticStudio. Surface scattering is a significant effect to consider when analyzing both the illumination and stray light characteristics of a system. While OpticStudio provides Lambertian, Gaussian, and ABg models, in many cases, the desired distribution is provided in terms of a table of measured data for the BiDirectional Scatter Distribution Function (BSDF). 

How to perform stray light analysis In this article, we will demonstrate how to use Filter Strings to analyze and characterize rays with specific optical properties by evaluating the amount of moonlight that contaminates the detector of a Cassegraintype telescope when viewing a distant star. 
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