For better or worse, we do “judge a book by its cover.” Appearance can override our better judgment. It’s tempting to judge the quality of an optical element based on its appearance, but that overlooks the element’s role within the overall optical system. In this article we’ll consider three types of surface irregularities and explore how each type can affect optical performance.
Surface Curvature refers to the difference between the overall shape of the fabricated part and the curvature of the optical design. In most cases, curvature errors are impossible to see from visual inspection. Because they affect an optical element’s power and wavefront, these “invisible” errors can actually have an enormous effect on performance, depending on the placement of that optical element. Fortunately, curvature irregularities can be quantified through interferometric measurements of wavefront errors, which simplifies setting numeric tolerances.
Surface Roughness is the point-to-point irregularity across the surface of the lens that causes light to scatter. In severe cases surface roughness creates a “cloudy” appearance. Surface roughness has two major effects: reducing transmission efficiency and creating “stray” light. It is notoriously difficult to set meaningful manufacturing specifications on surface roughness because 1) it is difficult to optically model the effects of scattering and 2) it is difficult to set a numeric value that accurately describes the level of roughness.
Surface Defects are any of a number of localized, visible flaws such as scratches, digs, and chips. Quality inspectors visually classify defects by their apparent size using a comparison chart, and such classification is ambiguous. Often a surface defect is strictly cosmetic with little impact on an optical system’s performance, but that is not always true. For example, in high-energy systems, a surface defect can create potentially destructive hot spots.
The irony, when comparing these three types of surface irregularity, is that “invisible” surface curvature usually has the strongest effect on performance, whereas a visible scratch may have no effect at all. Setting correct surface specifications requires an understanding of the system’s application and the role of each optical element within the system. By optimally tolerancing each surface, the overall cost of an optical system can be greatly reduced. Getting those tolerances right requires skill and experience – and Ross has that. Let us help you design a system with performance that is not just surface deep.
Want more tips for improving optical design and specification? Download our Tolerancing Guide.