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Plastic (and glass) aspheric molded lenses with precision tolerances continue to enable ever more ambitious (mobile camera) compact aspheric design. So let’s discuss a few tips for compact aspheric lens design.

Spherical surfaces are often used in classical lens designs because they are easy to manufacture. Spherical surfaces have limitations, however, and one of those limitations is spherical aberration, which is an optical effect that causes incident light rays to focus at different points when forming an image. This effect can result in a less-than-perfect blurred image in optical systems. Aspheric surfaces, on the other hand, focus light to a small point, creating comparatively no blur and improving image quality.

While surfaces with spherical shapes are easier to make, many more spherical surfaces are generally needed to reduce image blurring compared to aspheric surfaces. This means that an aspheric lens can be used to replace multiple spherical lenses, thereby creating a device that is smaller, lighter, and potentially less expensive to produce.

As lens systems become more compact and complex, and image resolution becomes ever more critical, aspheric surfaces in optical systems are increasingly important. Unfortunately, traditional methods of aspheric design present inherent complications when it comes to manufacturing and testing these components. To step up your aspheric lens designs and work more efficiently, try these CODE V design and analysis tips.

Combine Glass and Plastic Elements to Correct for Change in Lens Focus with Temperature

Many optical systems need to operate over a wide temperature range. For example, the same cell phone camera module needs to produce superior imagery both in the harsh winters of Chicago and in the searing summers of the Death Valley, which can have a temperature difference of about 90 degrees Fahrenheit. Optical systems for military and aerospace applications are expected to perform well in an even wider range of temperature environments. Temperature change affects many parameters of an optical system, including element radii, element thickness, refractive index, and airspace thickness. Designing optical systems for a large temperature swing is a complex task. One trick for designing such optical systems is to combine glass and plastic elements.

Glass and plastic elements have diverse thermal properties that allow designs to maintain focus over a temperature range.? Use the CODE V macro function @MTF_PEAKFOC to evaluate the change in lens focus with temperature (thermal focal shift).?

Specifications for the sample lens are as follows:?

• Best focus at 20C: -1.4 um?
• Best focus at 45C: 0.0 um?
• Best focus at 70C: 1.1 um?
• Thermal Focal Shift?

= [(+1.1um) – (-1.4um)]/[70C-20C]?

= 0.05um/degree

Distill Data of Interest with Macro-PLUS?

Due to the complexity of compact aspheric designs, making sense of the analysis results can be a daunting task. For example, a through-focus MTF plot produced by CODE V includes a total of 30 curves, as shown below in the figure on the left. It is difficult to observe the relationship between the peak focus and the field from the large number of curves presented. In this case, we can use a macro to plot the Peak MTF focus versus field based on the through-focus MTF data, as shown in the figure on the right. Using Macro-PLUS to produce a distilled representation provides much clearer data visualization.

Align Peak Foci to Improve Depth of Focus?

For classical lens design forms, the best foci typically lay on a curved image surface defined by Petzval Curvature.? In contrast, for compact aspheric designs, the best foci may lay on a highly aspheric surface.? Including the function @MTF_PEAKFOC in your optimization file to constrain the peak focus to a plane will improve the design depth of focus.? In general, designing for a larger depth of focus improves as-built performance and may improve thermal performance as well.

Summary

CODE V provides design and analysis capabilities that enable optical engineers to take advantage of the unique image quality and cost benefits offered by designing with aspheres. Contact our Optical Engineering Services team for help with your most challenging design projects at optics@synopsys.com.