3D printing revolutionizes rapid prototyping of high-precision optical lenses

(Nanowerk Spotlight) Optical lenses play a crucial role in a vast array of applications, from microscopy and astronomy to photography, medical devices, and machine vision systems. Traditionally, the fabrication of these lenses has relied on a complex, multi-step process involving grinding, polishing, molding, and coating. Each step is essential in achieving the desired optical properties and quality, but the process is time-consuming, often requires specialized equipment, and can be cost-prohibitive for low-volume, customized lenses.
In recent years, the emergence of additive manufacturing, or 3D printing, has shown promise as a faster and more cost-effective alternative for producing complex parts, including optical components. Among the various 3D printing techniques, material jetting and vat photopolymerization (VPP) have been the most widely used for optical applications due to their high precision, fast printing speed, and the availability of transparent materials. However, achieving the optically smooth surfaces required for high-quality lenses has remained a challenge due to the inherent limitations of the layer-by-layer printing process.
The primary obstacles have been the presence of stair-stepping defects, which manifest as pixelated steps within each printed layer (lateral defects) and as layered steps along the building direction (vertical defects). These microscale surface imperfections can significantly degrade optical performance. Previous efforts to mitigate these defects have included using grayscale exposure to blur pixel edges, reducing pixel size, and employing post-processing techniques such as meniscus coating, grinding, and polishing. However, these methods have either been limited in their effectiveness or have introduced additional time-consuming steps, undermining the key benefits of rapid 3D printing.
Now, a team of researchers from Purdue University has developed a customized VPP-based lens printing process that addresses both lateral and vertical stair-stepping defects by integrating two key innovations: unfocused image projection and precision spin coating. By slightly defocusing the curing image during printing, the researchers were able to largely eliminate lateral pixelation without sacrificing build size. They then applied a carefully controlled spin coating process to smoothen the layered steps along the building direction.
The team published their findings in Advanced Functional Materials ("3D Printing of Optical Lenses Assisted by Precision Spin Coating").
vat photopolymerization based 3S printing
a) VPP-based 3D printing setup. b) Projection image for a single printing layer. Light intensity of focused image c) and unfocused image d). SEM images of printed samples using focused image e) and unfocused image f), and after the spin coating process g). h) 3D printed lens by utilizing the unfocused image and the precision spin coating. (Image: Reproduced from DOI:10.1002/adfm.202407165, CC BY)
The precision spin coating process is a critical step in this new technique. While spin coating on curved surfaces was previously considered unpredictable and unrepeatable, the researchers conducted extensive experimental, numerical, and mathematical modeling to precisely control and predict the coating profile. They discovered that the printed stairs do not affect the coating profile if the amount of liquid is sufficient to cover the staircases, allowing them to treat the 3D-printed surface as a smooth substrate. Furthermore, they found that the coating thickness is insensitive to initial thicknesses and can be analytically predicted as a function of time and lens profile. This enables the accuracy of the coating thickness to be controlled within an impressive 1 μm.
Using this innovative approach, the researchers successfully demonstrated the precision fabrication of multi-scale spherical, aspherical, and axicon lenses with diameters ranging from 3 to 70 mm using high-clarity photocuring resins. The 3D-printed lenses exhibited exceptional surface quality, with less than 1 nm surface roughness and 1 μm profile accuracy. Optical characterization revealed that the lenses achieved a maximum modulation transfer function (MTF) resolution of 347.7 lp/mm and demonstrated superior imaging quality across the visible spectrum with minimal distortion.
To further showcase the versatility of their technique, the researchers printed various functional optical components, including microlens arrays with diameters as small as 0.8 mm, compound parabolic concentrators with a large acceptance angle of 45°, and even an assembled Keplerian laser beam expander with a magnification ratio of 1:2. They also extended their method to fabricate lenses from other optical materials, such as polydimethylsiloxane (PDMS), by 3D printing negative molds and using the same spin coating and post-processing steps.
The significance of this breakthrough lies in its potential to revolutionize lens manufacturing by enabling rapid, low-cost, and highly customizable fabrication of precision optical components. This technology could accelerate innovation and prototyping in fields such as custom eyewear, vision correction devices, scientific instrumentation, and medical devices. By eliminating the need for time-consuming and expensive traditional lens manufacturing processes, this 3D printing technique could democratize access to high-quality optical components and open up new possibilities for freeform optics and advanced imaging systems.
The researchers also discussed several areas for future investigation, including further optimization of printing parameters for non-axially symmetric lenses, exploration of new photocurable materials with tailored optical properties, and the potential for spin coating different resins with varying refractive indices and Abbe numbers to create advanced multi-material lenses with enhanced functionality, such as anti-reflection, achromatic correction, or reduced optical loss.
As the field of additive manufacturing continues to evolve, the ability to rapidly produce smooth, precise, and customized lenses could become a powerful tool for researchers, engineers, and designers across a wide range of industries. With ongoing refinements and the development of new materials, 3D printing has the potential to become the fourth generation of lens making, ushering in a new era of optical innovation and customization. This groundbreaking research represents a significant step forward in realizing that vision, paving the way for faster, more accessible, and more adaptable optics that could transform a multitude of applications.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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