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Join us for this rare, behind-the-scenes look at the history of the LightTools illumination design optimizer as we sit down with Dr. Bill Cassarly, one of the visionaries behind LightTools and an award-winning illumination design consultant. Bill shares insights from his decades of experience designing illumination systems and creating tools for designing illumination systems.
Dr. Bill Cassarly
What was the state of illumination design before LightTools optimizer was released?
After years of development, the LightTools optimizer was released in the early 2000s. Back then, software that could predict the output of an illumination system using Monte Carlo simulations was still a new technology. While it was common to use optimization with imaging systems, the use of illumination optimization was typically done by manually “tweaking” models. LightTools changed that forever when it provided the first commercial illumination optimizer.
How has the optimizer evolved since that first version?
The initial commercial release of illumination optimization provided a user interface for easily defining the variables and merit function, which made the optimizer something that could be used by an illumination engineer instead of just software modeling experts.
In a subsequent version, the optimization algorithm was adjusted to consider Monte-Carlo simulation noise. Back in the early days of the LightTools optimizer, I spent a week trying to optimize a model before realizing that it needed more rays, so the optimizer was enhanced to provide feedback about how much noise was in the merit function and to make sure that variable adjustments were significant. Additional improvements have been made in ray trace speed. While knowing if a change made a system much worse was relatively easy, knowing that a change made a system better requires time. As computers became faster, it became much more practical to quantify illumination performance in a reasonable amount of time.
Early days of the Backlight Pattern Optimizer
In later versions, LightTools also added design algorithms for special use cases--Backlight Pattern Optimization for displays and the Freeform Designer for tailoring freeform surfaces to achieve a desired beam pattern. These design tools incorporate special optimization algorithms tailored for specific applications and provide a simple user interface for defining the setup. These algorithms can be used in combination with the LightTools optimizer by running them during conventional optimization and tolerancing.
LightTools optimization of a backlit display
Freeform model in LightTools
What are some good strategies for maximizing the utility of the LightTools optimizer?
Typically, starting simple and adding complexity as the design matures is the key to effective design. If you add too much complexity at the beginning of the process, the optimizer can spend a lot of time searching unimportant portions of the design space. This often means using simple setups that trace quickly and then switching to more detailed setups once the basic design space is understood. Fundamentally, if you do not understand what variables are meaningful, you waste time optimizing details that can end up not mattering. The Parameter Analyzer included with LightTools helps to automate analyzing how variables impact output performance and the optimization merit function.
Parameter Sensitivity Analyzer in LightTools
What was the motivation for the tolerancing feature?
The LightTools Tolerance Manager was added so that designers could understand if their illumination design was manufacturable. While it is often a design step that is skipped, it is a critical aspect of doing a robust design. It is often used in conjunction with the LightTools Parameter Analyzer.
Single parameter tolerancing in LightTools
Can you share a design example of the game changing capability of the optimizer/tolerancer?
The game changing aspect of the LightTools optimizer is that it is integrated into a tool with a large collection of features. As LightTools has matured, additional features to support parameterizing a model have been added. In the early days of LightTools optimization, doing this often required writing and compiling code for user-defined variables and user-defined merit functions. Those features still have their place, but routine design cases have mostly been replaced by the use of Parametric Controls to link model parameters. For example, a handful of parameters can be used to automatically adjust the layout of an illumination system as well as the shape of optical surfaces. And these user-defined parameterizations of a model can be used along with the LightTools Configuration Manager to allow you to compare different setups (e.g., before and after optimization) or even automatically reoptimize the setup as additional details are added to the model.
Merit Function convergence when optimizing an optical system
How did the LightTools optimizer/tolerancer make your illumination design workflow more productive?
The optimizer is a tool to help the designer be more productive. It does not replace the designer, but it removes many of the tedious aspects of improving a design. Before the optimizer was available, I often spent hours and days manually tweaking variables to make the system better. With the optimizer, the designer can focus on what makes their illumination system better and let the software take care of doing the tedious work.
Dr. Bill Cassarly, Senior Scientist, Illumination Engineering, is a driving force in the movement to create the field of computer-aided illumination engineering. Before joining Synopsys/ORA, Bill worked at GE and received his Ph.D from the University of Pennsylvania. He holds 50 US patents and has worked extensively in illumination system design, sources, photometry, light pipes, and non-imaging optics. Bill was awarded the GE Corporate “D. R. Mack Advanced Course Supervisor Award” for his efforts in the training of GE Engineers, submitted the winning solution to both the 2006 and 2010 IODC Illumination Design Contests, and is an SPIE Fellow.
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