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Over the last 25 years, the field of optical design has transformed dramatically. We've witnessed advancements not only in design tools but also in the complexity and variety of systems being developed. This evolution raises an interesting question: are the design tools driving the complexity of optical systems, or are the increasingly demanding design problems pushing the development of better tools? In this blog post, we¡¯ll explore these intertwined changes, focusing on key aspects such as color correction, direct optimization of as-built performance, modal analysis, and optimization over pressure and temperature.
Dr. John Rogers
Color Correction: Increased Challenges and Better Tools
In the era of film photography, lens design targets were relatively modest, typically focusing on the Modulation Transfer Function (MTF) at 30 cycles per millimeter. Modern optical systems, however, often aim for 4K resolution, significantly increasing the demands on color correction.
The traditional rule of thumb for secondary color correction, using ordinary glass types, has become inadequate for these higher resolutions. Achieving exceptional color correction now often requires special glass types and advanced optimization techniques. For instance, CODE V¡¯s Global Synthesis and Glass Expert tools have become invaluable for optimizing glass choices and correcting chromatic aberrations effectively.
One of the most significant advancements is the ability to accurately simulate images. This allows designers to demonstrate the expected performance to customers directly, leading to more precise specifications and better communication. For example, Figure 1 illustrates how image simulation can reveal chromatic separations that might not meet customer expectations, prompting tighter specifications and improved designs.
Figure 1. Simulated pixel images. (a), With three wavelengths, with the primary and secondary lateral color indicated. (b) The same pixel image simulated with a quasi-continuous sampling of 21 wavelengths. (c) The simulated pixel image for the system after reselection of glass types and reoptimization.
Direct Optimization of As-Built Performance
Traditional optimization focused on the nominal optical system, often leading to the as-built performance being significantly worse than the as-designed performance. Recent advancements, however, have introduced Simulated As-Built (SAB) optimization, which incorporates tolerances into the optimization loop. This approach ensures that the optimizer targets the as-built performance directly, leading to more robust and reliable designs.
For example, a study using Global Synthesis revealed a significant lack of correlation between optimization error functions and as-built RMS wavefront error. By employing SAB optimization, we are now able to achieve designs that perform exceptionally well in the as-built state, as shown in Figure 2.
Figure 2. Results of SAB optimization
Modal Analysis
We have recently introduced the concept of tolerance eigenmodes, providing deeper insights into the as-built performance of optical systems. This technique, akin to modal analysis in mechanical engineering, identifies the dominant aberration patterns and their field distributions.
For instance, Figure 3 shows the second eigenmode of a wide-angle objective, revealing a combination of field tilt and field-linear, field-asymmetric astigmatism. Understanding these modes allows designers to implement more effective compensators and make informed adjustments during the design process.
Figure 3: Mode #2 of a wide-angle objective: field-linear, field-asymmetric astigmatism plus image tilt
Optimization Over Pressure and Temperature
A notable advancement in optical design is the ability to optimize systems simultaneously over different temperatures and pressures. This approach, which considers thermal expansion and thermo-optical coefficients, ensures that the system performs reliably across a range of environmental conditions.
For example, Figure 4 demonstrates the performance of a de-cemented Double Gauss lens optimized for a nominal temperature but evaluated at extreme temperatures.
Figure 4. Performance of the de-cemented Double-Gauss Lens, optimized at 22? C only, but evaluated (a) at -100? C and (b) evaluated at +100? C. The vertical scale on the aberration plots is ¡À0.060 mm
Using the Multi-Environment Coupling (MECo) feature of CODE V, we reoptimized the system for three environmental configurations simultaneously. We also used Glass Expert to select glasses that produce good imagery for all three environments simultaneously. With these techniques we achieved a design with stable aberration correction over the entire temperature range, as shown in Figure 5.
Figure 5. Performance of the de-cemented Double-Gauss lens, optimized simultaneously at +22? C, -100? C, and +100? C: (a) evaluated at -100? C and (b) evaluated at +100? C. The vertical scale on the aberration plots is ¡À0.060 mm
Conclusion
The field of optical design has seen remarkable advancements over the last 25 years, driven in part by the increasing complexity of design problems and in part by the development of sophisticated tools. As we look to the future, it will be fascinating to see how these tools and techniques continue to evolve, meeting the ever-growing demands of optical systems design.