Freeform optics bring precision optical systems into a new ear, delivering superior imaging in compact packages, or functions otherwise impossible. Surfaces that are axially unbalanced or have no axis of rotational invariance are known as freeform surfaces. Advanced freeform optical designs supplementary to ultra-precision manufacturing and metrology techniques have upgraded the lifestyle, thinking, and observing power of existing humans. Imaginations related to space explorations, portability, accessibility have also witnessed sensible in today's time with freeform optics. The optics, space, automotive, defense industries and many more are directly dependent on the time and cost involved in the complete production of advance designed freeform optics.

In a new paper published in the International Journal of Extreme Manufacturing, a team of researchers, led by Dr Zhen Tong from the Centre for Precision Technology, University of Huddersfield, United Kingdom, have summarized comprehensively the present state of the art of advances in freeform optics, its design methods, manufacturing, metrology, and their applications. The main aim in this review is to address certain questions like; What is our new understanding regarding freeform optics? At what stage have we reached in terms of developments and the applications of freeform surfaces in optical systems? How many efficient tools we have been developed in aspects of design, fabrication, and production? What are the main challenges in freeform optics production?

Various freeform optics and systems are designed with different methods such as partial differential equations, tailoring methods, point-to-point mapping, simultaneous multiple surface method, and aberration-based performance optimizations. All the final designs obtained using these methods are directly or indirectly associated with the Ultra-precision machining for fabrication at the nano or sub-nanometric level. To meet the requirements of the current market, process chains are dependent on the manufacturing types i.e. make-to-order, make-to-assemble and make-to-stock. Ultra-precision machining along with the figure correction techniques are the most trusted technologies for the development of freeform optics that fulfil the desired requirements of topographical errors such as low-, mid-, and high-spatial frequencies. With the multiple axes ultra-precision diamond cutting, one can fabricate the complexed shape at high accuracy and obtain a smooth optical surface.

Surface shape metrology will continue to be in high demand as a vital enabler for conforming to manufacturing chain criteria. Metrology's complexity increases with the increase in degree-of-freedom of freeform optics to be tested. Measurement problems for freeform optics rise with the sag differences, slopes, depths, surface roughness, measurement speed, environmental factors, temperature control, and aperture size. Because of these factors the cost of the whole freeform optics increases, therefore a proper balance must be maintained between controllable parameters to keep the final product cost within a limited range. Metrology can be done in several ways such as in-situ monitoring and off-line testing during and after fabricating the components.

Professor 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) have identified a few critical challenges in the field of design, manufacturing, and metrology of freeform optics as follows:

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

"Design can be accomplished with the help of commercial optical design software such as Code V, Zemax, etc., however, the optimizations of complicated optical systems take a long duration of time to reach their optimum global solutions."

"Is it possible to develop a repeatable system for determining when and where the intended freeform optical surface should be positioned for optimal optical performance?"

"For novel freeform optics, manual analyses of manufacturing constraints in the design phase stand unreliable, impractical, and impossible. No doubt, there are custom software for freeform optics and will continue to become mainstream. However, fast and reliable solutions are required."

"As long as we want light, the demand for freeform optics will exists. How far we are to adopt complete solutions of sustainable manufacturing for the freeform optics?"

"Development towards special measurement system for modern optics of any size and features that can perform all types of measurements. This can have benefits such as reduced cost, measuring time, complexity, wear, data-processing and increase in planning time and production."

Researchers have suggested that the freeform optics must follow 3F's principle i.e. form, fit, and function. In achieving the 3F's of the freeform optical components and freeform optical systems, time plays a major role. Therefore, particularly for freeform optics advanced economical processes should be developed that does not negotiate the designer's efforts, reduces the time of assembly and testing and also reduces the energy consumption, and wastage of materials.

Research Report:Advances in the design and manufacturing of novel freeform optics

Related Links
University of Huddersfield
Space Technology News - Applications and Research

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