Optics come in many shapes and sizes. From lenses with diameters of several meters for use in large telescopes to nanofabricated microlenses only a few microns in size, there are optics being produced across a wide variety of length scales. The same is true for optical shapes. Particularly for lenses where different surface constructions can help reduce image aberrations, optics can be fabricated with several different geometric designs.
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A perfect, aberration-free optical system will focus light from a point object to a single image point. Any spreading in space of the focused image is known as an aberration and can have a significant impact on the performance of an optical task. However, the fabrication of optics capable of achieving truly aberration-free performance is an impossible task.
Controlling and minimizing aberrations is one of the main challenges in optical engineering and optic design. Finding ways to create optical coatings and surfaces that do not introduce additional aberrations or are not affected in an inhomogeneous manner by strains introduced by mounting is incredibly important in the design of new components.
One recent development in the type of optical surfaces being produced is the creation of more freeform surfaces. Historically, limitations with computational power made it impossible to solve the high order polynomials that were required to model and understand the imaging behavior of such surfaces.1 There were also limitations in the availability of fundamental theories to describe the aberrations generated by freeform surfaces making the optical design of systems even more challenging.
Now, with the creation of afocal three-mirror anastigmats, greater computational processing availability, and the need for miniature digital viewfinders, freeform optics have undergone a period of rapid development. Freeform optics have become widely used in several industries, including remote sensing, manufacturing, lighting, and transportation. With further improvements in ultra-precision machining and greater reliability and reproducibility making smaller tolerances achievable, the complex-shaped construction of freeform optics seems likely to become even more commonplace in optical devices.
What are Freeform Optics?
Freeform optics are optics where at least one surface has no translational or rotational symmetry about the axis normal to the mean plane. This is a highly unusual design type when compared with the typical spherical and aspherical geometries that have long been favored for optical components. Heralded as the ‘next-generation of modern optics’,2 while freeform optics are not trivial to fabricate, their unique surface structures have opened up completely new possibilities with the flexibility of their design.
For example, with freeform optics, it is possible to combine several optical elements in a single component, including aberration correction, increase in depth of field, and expansion of the field of view. Normally, this would require the use of several components and so freeform optics are ideal tools for device miniaturization. They can also be manufactured to be particularly lightweight and compact.
Freeform optics can be individually customized for each application and the surface structure optimized for the desired optical properties, such as greater light throughput or higher resolution. The surfaces of the freeform optics can also be functionalized with particular structures or kept smooth.
Evolution of Freeform Optics
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Applications of Freeform Optics
Although freeform optics are challenging to design and manufacture, the flexibility of these optics are particularly beneficial when working with extended light sources such as light-emitting diodes (LEDs). With LEDs available in a dizzying range of colors and with very high brightness, they have become one of the light sources of choice for optical applications.
However, LEDs often suffer from poor divergence and it can be challenging to control the behavior of the light exactly. Freeform lenses can be used to compensate for this dispersion to provide equal and consistent luminance distribution and dramatically increase the throughput of the device.3 They have become widely used in the display and projection industry for this reason.
The decrease in weight and device miniaturization has made freeform optics also incredibly popular in head-mounted devices.4 It is now possible to create very lightweight displays that can display 6-focal-plane depth-fused 3D scenes that demonstrate a significant improvement on the multi-focal-plane displays from previous generations of optical devices.
Head-mounted displays have become popular for virtual reality gaming and training simulations. Generating 3D images that are sharp, clear and do not have too much bulk has been a significant challenge that advances in freeform optics have helped to overcome.
While there are still manufacturing challenges with verification of the surface shape and quality, advances in computer-aided manufacture and design is making it even easier to create complex optical shapes. With their ability to work across the electromagnetic spectrum and for advanced imaging applications, freeform optics are likely to become more commonplace in many applications.
References and Further Reading
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Plummer, W. (2021) Freeform optics for imaging. Optica, 8(2), 161.
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Fang, F. Z., Zhang, X. D., Weckenmann, A., Zhang, G. X., & Evans, C. (2013) Manufacturing Technology Manufacturing and measurement of freeform optics. CIRP Annals - Manufacturing Technology, 62(2), 823–846. https://doi.org/10.1016/j.cirp.2013.05.003
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Fournier, F. and Rolland, J. (2008) Optimization of freeform lightpipes for light-emitting-diode projectors Appl. Opt. 47, 957-966
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Hu, X., & Hua, H. (2014) Plane head-mounted display using freeform optics. Optics Express, 22(11), 13896–13903. https://doi.org/10.1364/OE.22.013896
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