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Parametric measurement improves efficiency aspheric machining


An aspheric surface is a general term for surfaces that deviate from a spherical surface. Aspheric surfaces include gyrosymmetric aspheric surfaces, off-axis aspheric surfaces, and freeform surfaces.

Compared with the spherical surface, which has only two degrees of freedom of curvature radius and aperture, the aspheric surface has higher degrees of freedom, thus realizing more functions. On the one hand, aspheric surfaces can achieve high-order aberration correction, thereby obtaining higher imaging quality; On the other hand, one aspheric surface can achieve the effect that can only be achieved by multi-piece spherical mirrors, which is conducive to reducing the volume and weight of the optical system. This is especially important in modern optical systems that need to balance system load with image quality.

High-precision measurement of aspheric parameters is the basis for aspheric manufacturing. Recently, the research group of Professor Hao Qun and Professor Hu Huang from the School of Optoelectronics of Beijing Institute of Technology published a review article on Light: Advanced Manufacturing entitled “Measurement techniques for aspheric surface parameters”.

This paper analyzes the difference between aspheric parameter measurement and traditional surface shape error measurement, reveals the intrinsic nature of parameter measurement, puts forward new views on the classification of existing aspheric parameter measurement techniques, and outlines possible future trends and challenges.

Figure 1 Modulation of the beam by optical components. Source: Vee

Why measure parameters?

There are two main types of aspheric measurement techniques, surface shape measurement and parameter measurement. Polygon shape refers to the three-dimensional distribution of the surface to be measured in the airspace. The measurement of a polygon shape is usually expressed in terms of surface height, which is a function of (x,y) coordinates in length. Aspheric parameters are well-defined physical quantities that can be derived from polygons. Both surface shape and parameter measurements can be used to evaluate the quality of aspheric surfaces.

Surface shape measurement can obtain the deviation between the measured surface and the ideal model in the airspace, so as to effectively guide optical processing, which is widely used in optical manufacturing. Theoretically, as long as the surface shape error of each surface in the system is small enough, the optical system can meet the requirements. However, in a real system, it is rare to assign a very small face shape error to each surface, as this would significantly increase the cost. How to accurately distribute the processing index of each surface in the optical system is an important topic in optical processing.

Parameter measurement can obtain the deviation between the measured surface and the ideal model in the beam modulation function, so that it can effectively evaluate whether the measured surface can perform the expected role in the optical system. After measuring the parameters of each aspheric surface in the system, the measurement results are substituted into the optical design software, and whether the surface to be measured can be determined whether the surface to be measured meets the system requirements through ray tracing.

In short, surface shape measurement provides direction for machining, and parametric measurement provides a suitable end point for machining. A mature, economical and efficient measurement scheme requires the rational use and combination of two measurement methods.

Classification of parameter measurement methods

According to the processing method of the measurement data, the author divides the parameter measurement methods into two categories: generalized fitting law and curvature center method.

1. Legality in a broad sense

Use the data acquired in the instrument directly to fit the parameters. The authors define this approach as generalized pseudo-legal. Generalized fitting can achieve high-precision measurement of all aspheric parameters, and has a wide range of applications in aspheric parameter measurement. Depending on the type of measurement data, generalized fitting can be divided into three types: direct fitting (absolute surface shape), interference (wavefront aberration), and geometry (surface gradient).

1) Direct legal drafting

After accurately obtaining the absolute surface shape of the aspheric surface, the aspheric parameters can be obtained by fitting it directly. Due to the gradual maturity of aspheric surface measurement technology, this method has been widely used. However, accurate measurement of absolute surface shapes of aspheric surfaces is generally more expensive and less efficient.

2) Interference Law

Interferometry is a high-precision optical measurement method with good traceability of test results. The author’s research group has conducted research in the field of aspheric interferometry for many years, which can achieve high-precision measurement of surface shape and parameters at the same time. However, the test setup is complex, the cost is high, and the measurement range is relatively small.

Figure 2 Interferometric measuring device. Source: Light: Advanced Manufacturing

3) Geometry

Compared with the interferometric method, the test results of the geometric method lack traceability, but its test device has a simple structure, low cost and a large test range.

2. Center of curvature method

The data acquired in the instrument is used to locate the center of curvature of the aspheric surface. The authors define this method as the center of curvature method. The center of curvature method is mostly used to measure the radius of curvature of quadratic aspheric surfaces, and its versatility is poor, and the higher-order coefficients of aspheric surfaces cannot be measured.

Prospects

Thanks to the gradual maturity of aspheric surface shape measurement methods, direct fitting has the advantages of high precision and high versatility, and has been widely used in aspheric parameter measurement. The interferometric, geometric and curvature-centric methods have different advantages in test accuracy, versatility and cost control.

Figure 3 Characteristics of various measurement methods. Source: Light: Advanced Manufacturing

In recent years, methods and techniques for aspheric parameter measurement have flourished. In different test scenarios, there are different applicable methods for different surfaces and parameters to be measured. However, with the advancement of society and technology, the development of various advanced optical systems always brings new challenges. Considering accuracy, cost and versatility will be the only way to develop parameter measurement technology. (Source: Advanced Manufacturing WeChat public account)

Related paper information:https://doi.org/10.37188/lam.2023.019

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