November 26, 2019

How to work around the limitations of low-cost, compact imaging

How to work around the limitations of low-cost, compact imaging

As there is increasing pressure for optical engineers to create compact imaging to fit within thinner and lighter housing for applications such as AR/VR headsets, heads up displays, lidar systems, medical imaging, and more, there is also the demand to make those systems manufacturable at a lower cost. But there are certain limitations associated with this low-cost, compact imaging. Following is a look at the limitations—as well as the enabling technologies that can help engineers work around them. 

Limitations
At last year’s Envision, LightPath Optical Engineering Manager Jeremy Huddleston discussed limitations of compact, low-cost imaging systems. Zemax customer LightPath is a global leader in optical technology—offering optics and photonics solutions for the industrial, defense, telecommunications, testing and measurement, and medical industries—and has significant experience on the topic. Following are three points from Huddleston’s presentation: 

  1. Material choice impacts the cost of the system. Being budget restrained means choosing less expensive materials. But for low-cost materials that scale to high-volume, it’s important to keep in mind that the infrared region is restricted by the opacity of plastics and oxide glasses. So, it’s a balance between desired cost and opacity. 

  2. The desire for a smaller size dictates the design form. Having a low effective focal length (EFL) induces a wide field of view (FOV). Space limitations may not allow the lens to be situated far enough away from the focal plane, so the number of elements/surfaces are limited, and the aperture stop is impacted. Unintentional vignetting and lack of brightness can occur. 

  3. The F-number will be low.The push for smaller sized imagers has driven down the detector pixel sizes as well. This requires low F-number optics to compensate for small pixel detectors. 

Enabling technologies
There are several enabling technologies that help engineers get around the limitations of designing low-cost, compact imaging. These include: 

  • Precision glass molding, which enables low-cost aspheres with fewer elements. Offered by LightPath, extra time and attention is applied to dial-in mold tooling, which results in a highly repeatable lens form at low cost. 

  • Diamond-turning, also offered by LightPath, uses ultra-precision mold tooling for high-volume production and enables diffractives for achieving chromatic correction.

  • Chalcogenide Materials for IR Applications, another technology offered by LightPath, is an amorphous (moldable) material for IR and broadband applications. This is low-cost compared to most IR materials. 

  • Wafer-level optics are large arrays of lenses fabricated simultaneously on “wafers.” This can be achieved through molding and/or etching processes.

Analyzing your system to evaluate performance
OpticStudio includes a suite of tools to analyze the performance of any optical system. In addition to the classic analysis functions, OpticStudio offers Full-field Aberration analysis to improve freeform designs; contrast analysis for MTF optimization; and Image Simulation to produce photorealistic images of object scenes.

Engineers can also customize OpticStudio to suit their needs with the ZOS-API. They can create standalone applications, build their own analyses, and control OpticStudio externally using C#, C++, MATLAB and Python. Zemax Programming Language (ZPL) enables them to write their own macros to automate repetitive processes.

Huddleston says, “Compact, low-cost imaging systems have a high risk of violating standard assumptions and approximations commonly used in design. Even certain enabling technologies, such as aspheres and diffractives, increase the risk that these assumptions will be violated. Optical designers need to be aware of these risks and utilize the thorough documentation and analysis capabilities within OpticStudio to properly evaluate designs and report accurate metrics. Optical designers are best positioned to help validate models and apply best practices using the tools that OpticStudio provides.”

For more on how to properly analyze designs that violate standard approximations, scrutinizing common imaging lens metrics including the effective focal length, the F-number and relative illumination, horizontal filed-of-view and distortion, and diffractives, access Huddleston’s Envision presentation here.

Optimizing for as-built performance
A new feature called High-Yield Optimization in OpticStudio makes it easier for engineers to design low-cost, compact optical and imaging systems. The conventional optical design approach results in designs that are very sensitive to manufacturing and alignment errors, which means the optical product is difficult to repeatedly manufacture successfully. High-Yield Optimization, on the other hand, produces designs that meet tight performance specifications, provide a higher manufacturing yield, and lower manufacturing costs through less waste.

Engineers can use OpticStudio to:

  • Optimize for as-built performance, rather than nominal performance.

  • Account for common manufacturing defects in the design process.

  • Find optical design solutions that have both good image quality and rays with low angles of incidence, which reduces the tolerance sensitivity of the resulting design when fabricated.

 

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