An overview of a typical illumination system design cycle

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This lesson provides an overview of a typical design cycle for illumination systems. You will learn the various steps within an illumination design, to save you time and to avoid critical mistakes. 

Authored By Katsumoto Ikeda

Introduction: Why understanding the design cycle is important as an optical designer

The design cycle is a guide of sorts that illustrates the various steps in optical design. By identifying the different steps within a design cycle, it is possible to understand the objective of each step, so we know the exact task at the appropriate time, and we can anticipate the future tasks to expect as the design goes forward. With a firm guide to follow in place, there are no surprises in the design process, and we can even take preemptive action for a future step that we can foresee will cause potential problems.

Let's take a look at a typical illumination design cycle and see the various steps for an illumination system.

The design cycle is our guide to lens design

In an optical system, but more specifically in an illumination system, there are many steps from design to manufacture. Below, we have presented a shared roadmap that illustrates the different steps needed in the process of illumination design. The various steps can be grouped into two main groups, the design stage, and the manufacturing stage.

(Reference: Illumination Engineering, R. Koshel, IEEE Press)

Observation of the above design cycle shows that for the design phase, there are iterations, but the iterations are contained within the design phase. Notice that the fabrication state has iteration paths that go back to steps in the design phase. This reverting in the steps means that during fabrication and manufacturing, we may encounter a problem that brings to light that the base design was not suitable since we did not take into account a crucial tolerance parameter. It is easy to see that by covering our tracks early in the design cycle, we can potentially avoid frequent iterations, if any. The crucial iterations that bring us closer to the beginning of the design cycle can cause a loss in time that can fatally delay the project.

Let's take a look at each step from the design cycle above and break it down.

The Design Phase

• Concept: The concept for the systems is determined in this step. This means that the possible optical systems, the specification requirements (or goals of the system), and the description of the system are determined. There is the possibility that this step has no involvement with the optical designer if an optical part is governed by a larger goal in the overall system. For example, the optics in a flashlight can only be so big. Another example, the optical requirements of an automotive headlamp are required by law.
• Baseline: In the baseline, the efficiency of the system, the light distribution, the chromatic properties, the cost of the system, and the volume or size of the system is determined. Also, the étendue analysis is performed to evaluate what kind of light source is needed to reach the specification goals. It is good practice to have a preemptive analysis of tolerance at this time, to reveal the most common sensitivities in the system. Such a preemptive tolerance analysis can significantly reduce the manufacturing problems that can arise later in the tolerancing phase, potentially reducing the time for a successful product launch. Perhaps counter-intuitively, spending time on the tolerance in this stage will save us time down the road.
• Literature: The optical designer researches possible literature taking into account the eventual goal of the project. For example, critical optical systems can be determined at this stage, whether or not a reflective or refractive optical system is a suitable choice or not. More often than not, this step is skipped by a more experienced optical designer that has experience in a similar system.
• Initial study: This step can be used to figure out the most effective optical system to achieve the specification requirements. It is common practice to test multiple optical configurations of numerous optical systems to compare if the optical system has not yet been determined. This step can be the very barebones or initial design of the system, with just the light source and the lens. When this step is completed, the optical configuration of the design is decided, and the chosen path is much more concrete.
• Design: This is the meat of the design process, where the most critical optical parameters are optimized. Also, the tolerance parameters are determined at this stage. At the end of this step, the optical design goals should be met, albeit for an untoleranced optical system.
• Optimization: The optimization step is to introduce further optimizations to account for perturbations to the optical system to improve the performance of the optical design. This step is an improvement upon the previous step, and the parameters of the optimization do not differ in this step. At the end of this step, the untoleranced optical system exceeds the design specifications. The untoleranced performance must exceed the design specifications since in the prototyping and manufacturing stage will introduce changes to the system parameters that decrease (and do not increase) the optical performance.

The Fabrication Phase

Tolerance

The tolerancing step overlaps the design phase and the manufacturing phase since it is the final step of the lens design, while the first step of conveying the optical tolerance toward the manufacturing phase. From a design point of view, the crucial tolerance parameters are determined in the design steps. The two main aspects of focus are the sensitivity tolerance analysis and the Monte-Carlo tolerance analysis. In the sensitivity analysis, the optical performance of each of the tolerance parameters is perturbed, and the system is analyzed for the performance of the specifications. In Monte-Carlo analysis, all of the parameters are randomly perturbed to analyze a statistical result. The setting of the Monte-Carlo analysis parameters is essential to ensure that the results reflect the real manufacturing error. Unfortunately, this step is often ignored either due to time constraints, or lack of experience, or the abstract nature of illumination systems compared to hard numbers like MTF for imaging systems. However, ignoring this step often leads to the primary reason that manufactured illumination systems do not meet the required optical specifications.

Fabrication

The optical components and the non-optical components of the illumination system are drawn in a 3D or 2D CAD layout to complete the optical system. Each component is manufactured and assembled.

Testing

Testing has two types of test for two subsets of tests. One of the two types of tests is the mechanical measurement, while the other type is functional measurement. One subset of the test is the testing of components, and the other is the testing of the entire system. Therefore, four overall testing steps may be needed in an illumination system.

An example of a mechanical component measurement can be the shape of an aspherical lens, while an example of a functional component measurement can be the focal length of one lens within the system. An example of a mechanical system measurement can be the alignment of the lenses as a whole, while an example of a functional system measurement is the optical performance of the completed optical system. Most often, the most critical measurement is the optical functional measurement of the entire system, and if this meets the requirement goals, nothing needs to be fixed. Quite often, however, the completed system will not perform as desired, and individual component measurements are required in order to find out the culprit. Note that a thorough tolerance analysis will significantly help to find the error.

Although the above design cycle shows a typical design cycle for illumination design, depending on the complexity of the illumination design, there may be fewer steps, or there may be more steps involved. The complexity of the illumination system can increase the number of steps, since there may be a more involved tolerance step and a complicated testing step. The experience of the optical designer can reduce the number of steps, since the concept, baseline, literature, and initial study may be speedy. The difficulty of the optical requirements may increase the number of steps, as there may be several iterations in the optimization step and the tolerancing step.

Reference: Illumination Engineering, R. Koshel, IEEE Press