Flatness in Precision Optics: Aspect Ratio and the “Potato Chip” Effect

Posted by Steve Rowe on

When it comes to precision optics, surface quality is only half the battle. The other half is flatness—how well an optic maintains a uniform surface without warping or distortion. For applications in imaging, metrology, laser systems, and aerospace, even nanometer-level deviations in flatness can cause performance issues. We have covered the topic of flatness in previous posts, but two key factors often overlooked in discussions of flatness are aspect ratio and the so-called “potato chip effect”. Understanding these factors help engineers design more reliable components and possibly avoid costly surprises during fabrication and system integration. 

The aspect ratio of an optic is the proportion of its longest diagonal measurement across the surface in relation to its thickness, and is a critical factor in achieving and maintaining surface precision. For example, a square optic with sides of 50 mm has a diagonal of about 72 mm; if the thickness is 10 mm, the aspect ratio is 72:10, or 7.2:1. In general, once the aspect ratio exceeds about 5:1, fabrication becomes increasingly difficult because the reduced thickness offers less structural stability, making it harder to consistently maintain flatness and surface form.  

One common deformation in thin optics is the potato chip effect. As the name suggests, this refers to an optic warping into a slightly twisted, saddle-shaped surface resembling a potato chip. This deformation may not be noticeable to the observer, but it can be easily detected with an interferometer. While the potato chip effect may introduce only nanometer-scale deviations, in high-precision optical systems, this can shift focal planes, distort wavefronts, and reduce overall system resolution. Aspect ratio is critical to the overall structure as uneven pressure or removal rates across the surface can introduce residual stress, especially in thin substrates during polishing. The material homogeneity itself can be a factor if there are variations in thermal expansion, refractive index, or residual stress in the glass, which itself can promote warping. 

There are options for optical designers and engineers to minimize aspect ratio and potato chip risks 

  1. Optimize aspect ratio early – Match thickness to aperture size for the intended flatness tolerance. 

  1. Use low-expansion materials – Ultra-low expansion glass or fused silica can mitigate thermal stresses. 

  1. Design compliant mounts – Use kinematic or stress-relieved mounts that don’t over-constrain the optic. 

  1. Involve your manufacturer – It’s important for your engineers to know manufacturing constraints. Working with your optical manufacturer during the design process enables discussions on sub-aperture polishing and material selection. 

Flatness in precision optical glass isn’t just a matter of polishing skill—it’s tightly tied to geometry and stress management. Aspect ratio determines whether an optic is mechanically stable or prone to distortion, while the potato chip effect illustrates how thin optics can warp under subtle forces. By accounting for both, designers can ensure that optics meet stringent performance requirements in both the lab and the field. The team at Esco Optics is always willing to share insights on fabrication methods to provide solutions for our clients. Please reach out to us at Sales@EscoOptics.com 

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