Disregarding coma, trefoil, and other higher-order effects are not recommended for lens assemblies with a large number of elements or for optically sensitive systems since simplified SI models often do not sufficiently reproduce the wavefront error or irregularity in the system. Two commonly used simplified SI models include fitting a surface to either a 50/50 combination of spherical aberration and astigmatism or 100% astigmatism. Surface irregularity (SI) modeling can be as simple or complex as needed. A sensitivity analysis can help reveal which factors have a greater impact on the design and can help designers prioritize their focus. diamond turning vs batch process), and a particular technician’s style. The truth is that a number of factors actually influence the shape of the distribution, including the number of lenses in the batch, the fabrication process (e.g. For this reason, the distribution for the center thickness of a batch of lenses will likely be skewed to the higher side of the tolerance ( Figure 3).įigure 3: A lens will often be oversized to allow for downstream corrections, skewing the distribution of the value of dimensions like center thickness or diameter within a given batchĭuring a Monte Carlo analysis, it is often falsely assumed that the distribution for center thickness follows a symmetric normal distribution, without skew or kurtosis. This provides the manufacturer with a larger tolerance window within which to work when refining other dimensions or specifications, such as surface accuracy or surface quality, which naturally reduce the center thickness dimension. Similar to the diameter of a lens, the center thickness is usually kept intentionally thick during early stages of fabrication. Therefore, it is important to understand the statistical models and probability distributions that may be used to control specification tolerances. Monte Carlo simulation is the most common analytical method for modeling the performance probability of an optical system. Statistical modeling has become a more widely used tool in the optics industry over the last decade. Lenses with hemispherical (radii of curvature less than or equal to 0.7 times the diameter) or near flat (a sag equal to or less than 100μm) surfaces should also be avoided if possible, as this is also difficult to manufacture. Ensure that |∆r| is greater than 2mm so that the lens can be centered. The Karow or Z-factor, which measures the ability for a lens to center itself automatically between bell chucks, also known as bell clamps, is given by:įigure 2: This meniscus lens has radii that are nearly concentric. It is suggested to keep the edge thickness above ~0.7mm, at a diameter 1mm larger than the intended final diameter of the lens. Anticipating the need to oversize the diameter of the lens, designers need to ensure that the edge thickness is not too small and not too sharp, such that the edge would be prone to damage. One common technique optical manufacturers use when machining glass or other materials to the final specification is to oversize the lens diameter early in the fabrication process. Remember, optical software is just a tool and the user must pay careful attention to review the outputs. Geometry Considerationsĭesign software digitizes the optical planning process and offers tools to ease the detailed and time-consuming procedure however, one must be aware that most optical software does not always warn of–or–prevent the user from–creating physically impossible or difficult to manufacture solutions. Designers must consider the individual lens element geometry, the assembly setup, and the tolerancing models when creating an optic from scratch. For this reason, it is imperative to recognize the nuances of optical manufacturing, paying careful attention to the statistical assumption of models and manufacturing practicality. Occasionally, a lens may appear to succeed in conception but fail in one of the subsequent phases of manufacturing, assembly, or testing. A successful lens design succeeds not only in the creation of a working model but also in manufacturing, assembly, testing, and implementation.
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