LightTools Enewsletter

February 2017

Quick Tip: Getting the Most Out of a LightTools Hybrid Simulation

LightTools features three types of Monte Carlo simulation methods: forward simulation, backward simulation, and hybrid simulation.

Each method has advantages and disadvantages, and benefits are subject to the geometry present in the model and type of analysis required. Depending on the characteristics of your model, you may observe dramatic ray trace efficiency differences between each model. In some cases, one type of simulation may achieve similar accuracy that is dozens of times faster than another method. This tip discusses hybrid simulation, LightTools’ newest simulation method, which was added to the software in version 8.4.

Forward and Backward Simulation Review

The most common and intuitive method is forward simulation, which traces rays in the direction light will propagate. In forward simulation, rays start from the source, interact with geometry, and are collected on receivers.

Forward simulation

Forward simulation: backlight system

Another method is backward simulation, where rays start from receivers and trace backward. Backward rays that eventually intersect a source are included in the illumination pattern.

Backward simulation

Backward simulation: illuminance test point analysis with a reflector

Hybrid Simulation Overview

The third method, and the newest method in LightTools, is the hybrid simulation. A hybrid simulation traces multiple “passes” that start with a forward ray trace, where illumination data is gathered on scattering surfaces. The second part of each pass is to trace backward rays, which will use the scatter data on surfaces from the forward pass to determine their contribution to the illumination pattern on the receiver.

Hybrid simulation

Hybrid simulation

Systems Where Hybrid Simulation Can Be Efficient

The characteristics of models where a hybrid simulation will be of value are:

  • Systems with multiple surfaces with optical properties that are broad scatterers (i.e., Lambertian, Gaussian, etc.)
  • Models with smaller sources relative to their geometry
  • Models with point sources or collimated sources that do not have direct line of sight with the receiver

Note that hybrid simulations do not currently support all LightTools capabilities, such as systems with ray data sources, polarization, or volume scattering. See the Simulation Quick Reference table in Chapter 4 of the Illumination Module User’s Guide for a larger list of supported capabilities for forward, backward, and hybrid simulations.

Controls for Hybrid Simulation

The controls for setting up a hybrid simulation are available in the Hybrid Mesh Manager. You’ll see controls for the number of rays in the forward and backward parts of each pass. These controls are analogous to the standalone Forward and Backward Simulation controls.

Hybrid Mesh Manager

The right balance of forward rays, backward rays, and refinement passes will depend on simulation accuracy needs and your PC. As with the other Monte Carlo simulation types, increasing the number of rays will improve accuracy, but will also increase simulation time and memory usage. Another consideration is the ratio of the number of forward rays traced and the number of backward rays traced. As a rule of thumb, the number of forward pass rays should be larger than the number of backward pass rays traced.

A unique control of the hybrid simulation method is a refinement pass. A refinement pass is the number of forward/backward iterations to perform, and can be used to manage the amount of memory used during simulation. Each refinement pass will allow you to effectively increase the number of forward pass rays traced without increasing the memory requirements. If you need additional accuracy but your PC is limited on memory, try increasing the number of refinement passes, while leaving the forward pass rays and backward pass ray settings the same.

True Color Spatial Luminance

Refinement  Pass 1

Refinement passes = 1

Refinement  Pass 2

Refinement passes = 2

Refinement  Pass 10

Refinement passes = 10

Another unique control in the Advanced tab of the Hybrid Mesh Manager is the Kernel Radius. During a forward pass, illumination information is collected on each surface using the forward rays. On the backward pass, the backward rays will utilize the illumination information for any forward rays that intersect within a defined collection area. The size of the collection area is controlled by the Kernel Radius control.

Kernel Radius

While LightTools can automatically set this control, you may also manually set it. In general, the Kernel Radius should be several times smaller than the smallest dimension on your scattering surface. Kernels that are too large will blur details and slow the simulation, while kernels that are too small will introduce noise. Achieving an average of 200 samples inside the collection area (kernel) are considered ideal.

Spatial Luminance

Large kernel radius

Large kernel radius, ~1,000 samples in kernel 
Distribution is smooth, but details are blurred

Moderate kernel radius

Moderate kernel radius, ~200 samples in kernel 
Balance of detail and smoothness

Small kernel radius

Small kernel radius, ~30 samples in kernel
Higher detail, but noisier results

The resources in the next section have additional guidelines for setting these controls.

Learning More

You can find out more about hybrid simulation on the Customer Support Portal. You’ll find various presentations and videos to help you get going with this type of simulation:

There is also a hybrid simulation example in the LightTools Example Model Library under Tools > Example Model Library > Program Features > RayTrace/Hybrid.

If you have further questions about hybrid simulation, you can contact LightTools Technical Support at

IODC Illumination Design Competition

The International Optical Design Conference will occur on July 9-13 in Denver, Colorado this year. The illumination design problem has been posted on the website, and entries are due by April 1, 2017.

International Optical Design Conference

Submit your entries today! For more information, please visit their website: