LightTools Enewsletter

May 2019

Quick Tip: Simulation Diagnostics with Ray Paths in LightTools

LightTools ray paths provide an excellent way to visualize rays and their impact on an analysis of receiver data. A ray path is a list of surface zones that a collection of rays encounter during a simulation. All rays that share the same sequence of surface zones are collected into a single unique ray path. Analysis tools are available for you to filter these ray paths on specific criteria. This filtering is helpful to identify undesirable rays (for example, stray light) and diagnose potential issues in the model, such as geometry or modeling errors. 

The following figure shows an example of ray paths collected in a LCOS projector system in which the LCOS device is divided into a 3x3 array of specific color bands. Ray paths allow you to analyze the paths for each color band individually because the rays in each band share a unique ray path.

It is also possible to analyze stray light in this model using ray paths. For example, we can analyze the impact of light where rays bounce multiple times in the final projection lens element by filtering the paths to show only the rays that have a specific sequence of surfaces that rays hit. 

This tip focuses on how to use ray paths to diagnose modeling errors such as positioning errors and geometry errors, which can arise from poor quality imported geometry.

While the Ray Report for a forward simulation that you can turn on in the Simulation Input dialog box (Ray Trace > Simulation Input) can help you assess the overall impact of terminated and leaked rays, ray path tools can help you diagnose the actual causes of these issues.

Ray paths are typically collected on a given receiver. In this example, we will focus on collecting ray paths on all possible surfaces, regardless of receivers in the model.

During a simulation, rays can encounter error conditions for various reasons. For example:

–      Rays may terminate due to dropping below a power threshold

–      Rays may leak due to modeling errors

–      Other miscellaneous reasons, such as incorrect model geometry


When errors are found in the simulation, they are reported on the Console window as error/warning messages.

These are brief messages, and they do not provide a direct way to visualize the rays that generate the error conditions. Ray paths can help diagnose these types of issues. We will use a simple model to illustrate basic concepts in the next section. More practical examples are described in the full presentation (link provided at the end).

Model Positioning Error

Consider the simple model shown below. Two blocks are placed next to each other with a small air gap. Because of a modeling error, the second block is tilted slightly backward. This small error in the model produces errors during the simulation. Note that the message indicates that surfaces Cube_2.TopSurface and Cube_2.RightSurface are possibly overlapping with other geometry.

However, preview rays do not indicate any obvious issue. With the default settings, preview rays show the first 100 rays in a simulation, and those rays may not necessarily encounter overlapped area of the model.

By enabling ray paths for all rays and re-running the simulation, we see the following output in the model. Each color corresponds to rays in different ray paths.

In the Ray Paths dialog box (Analysis > Ray Path), we can see all the ray paths collected during the simulation. Sorting the Final Surface column enables you to visualize rays that cause the error condition reported in the simulation. We see two paths where the final surface is the same surface mentioned in the error message issued during the simulation: Cube_2.TopSurface and Cube_2.RightSurface. In this model, we do not expect any rays to terminate before passing two blocks on to the far-field receiver.

The corresponding ray paths can be seen in the model by enabling these ray paths and disabling all other ray paths.

In this model, the error condition can be corrected by simply removing the tilt for the second block.

For more detailed examples and a ray path primer, see the training presentation on the Customer Support Portal.

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Introduction to Illumination Design Using LightTools

July 15-17, 2019
Pasadena, CA

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