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Credit: by Sumit Kumar, Zhen Tong and Xiangqian Jiang

Freeform Optics brings precision optical systems into a new ear, delivering superior imaging in compact packages, or otherwise impossible functions. Surfaces that are axially unbalanced or have no axis of rotational invariance are called free-form surfaces. Advanced free-form optical designs, complementary to ultra-precise manufacturing and metrology techniques, have enhanced the living, thinking, and observing power of existing humans. Imaginations related to space explorations, portability and accessibility have also come alive in the present day with freeform optics. The optical, space, automotive, defense and many other industries are directly dependent on the time and cost involved in the complete production of pre-designed free-form optics.

In a new article published in the International Journal of Extreme Manufacturing, a team of researchers led by Dr. Zhen Tong from the Center for Precision Technology at the University of Huddersfield, UK, have comprehensively summarized the current state of progress in free-form optics, its methods of design, manufacture, metrology and their applications. The main objective of this review is to answer certain questions such as; What is our new understanding regarding free-form optics? Where are we in terms of developments and applications of free-form surfaces in optical systems? How many effective tools have we developed in the design, manufacturing and production aspects? What are the main challenges in producing free-form optics?

Various free-form optics and systems are designed with different methods such as partial differential equations, customization methods, point-to-point mapping, simultaneous multiple surface method, and aberration-based performance optimizations. All final designs obtained using these methods are directly or indirectly associated with ultra-precision machining for fabrication at the nano or sub-nanometer level. To meet the demands of today’s market, the process chains depend on the types of manufacturing, namely, make-to-order, make-to-assemble, and make-to-stock. Ultra-precise machining along with figure correction techniques are the most reliable technologies for developing free-form optics that meet the desired requirements of topographical errors such as low, mid, and high spatial frequencies. With multi-axis ultra-precise diamond cutting, the complex shape can be manufactured with high precision and optically smooth surface can be obtained.

Surface shape metrology will continue to be in high demand as a key enabler to conforming to manufacturing line criteria. The complexity of metrology increases with the increase in the degree of freedom of the free-form optics to be tested. Measurement problems for free-form optics increase with differences in sag, slopes, depths, surface roughness, measurement speed, environmental factors, temperature control, and aperture size . Due to these factors, the cost of the entire freeform optic increases, therefore, an appropriate balance must be maintained between the controllable parameters to keep the cost of the final product within a limited range. Metrology can be performed in several ways, such as in-situ monitoring and offline testing during and after component manufacturing.

Prof. Dame Xiangqian Jiang (Director of EPSRC Future Metrology Hub, CPT), Dr. Zhen Tong (Head of Ultra-Precision Machining Group, CPT) and Mr. Sumit Kumar (PhD Scholar) identified some critical challenges in the field of design, manufacture, and metrology of free-form optics as follows:

“There is no standard definition or tolerances for freeform surfaces that can be classified by their performance. Is it possible to determine the relationship between surface roughness and the specified tolerance at the design stage? »

“The design can be done using commercial optical design software such as Code V, Zemax, etc., however, complex optical system optimizations take a long time to reach their optimal overall solutions.”

“Is it possible to develop a repeatable system to determine when and where the intended free-form optical surface should be positioned for optimal optical performance?”

“For new free-form optics, manual manufacturing stress analyzes in the design phase are unreliable, impractical and impossible. Undoubtedly there are custom software packages for freeform optics and they will continue to become more mainstream. However, quick and reliable solutions are needed.

“As long as we want light, the demand for free-form optics will exist. How far have we come in adopting complete sustainable manufacturing solutions for free-form optics? »

“Development towards a special measurement system for modern optics of all sizes and characteristics that can perform all types of measurements. This can have advantages such as reduction of costs, measurement time, complexity, wear, data processing and increased planning and production time.

The researchers suggested that free-form optics should follow the principle of 3F, that is, form, fit and function. In achieving the 3Fs of freeform optical components and freeform optical systems, time plays a major role. Therefore, especially for free-form optics, advanced cost-effective processes must be developed that do not trade designer effort, reduce assembly and test time, and also reduce energy consumption and waste of materials.

About IJEM:

International Journal of Extreme Manufacturing (IF: 10.036) is a new multidisciplinary, anonymous double-reviewed, fully open-access journal covering only areas related to extreme manufacturing. The journal is dedicated to publishing original articles and reviews of the highest quality and impact in areas related to extreme manufacturing, ranging from fundamentals to processes, measurements and systems, as well as materials, structures and devices. with extreme functionality.

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