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Fabrication of Microlens Array and Its Application: A Review

Authors:
  • Shenzhen Technology Unversity

Abstract

Microlens arrays are the key component in the next generation of 3D imaging system, for it exhibits some good optical properties such as extremely large field of view angles, low aberration and distortion, high temporal resolution and infinite depth of field. Although many fabrication methods or processes are proposed for manufacturing such precision component, however, those methods still need to be improved. In this review, those fabrication methods are categorized into direct and indirect method and compared in detail. Two main challenges in manufacturing microlens array are identified: how to obtain a microlens array with good uniformity in a large area and how to produce the microlens array on a curved surface? In order to effectively achieve control of the geometry of a microlens, indirect methods involving the use of 3D molds and replication technologies are suggested. Further development of ultraprecision machining technology is needed to reduce the surface fluctuation by considering the dynamics of machine tool in tool path planning. Finally, the challenges and opportunities of manufacturing microlens array in industry and academic research are discussed and several principle conclusions are drawn.
Yuan et al. Chin. J. Mech. Eng. (2018) 31:16
https://doi.org/10.1186/s10033-018-0204-y
REVIEW
Fabrication of Microlens Array and Its
Application: A Review
Wei Yuan1,2, Li‑Hua Li1,2* , Wing‑Bun Lee1,2 and Chang‑Yuen Chan1,2
Abstract
Microlens arrays are the key component in the next generation of 3D imaging system, for it exhibits some good opti‑
cal properties such as extremely large field of view angles, low aberration and distortion, high temporal resolution
and infinite depth of field. Although many fabrication methods or processes are proposed for manufacturing such
precision component, however, those methods still need to be improved. In this review, those fabrication methods
are categorized into direct and indirect method and compared in detail. Two main challenges in manufacturing
microlens array are identified: how to obtain a microlens array with good uniformity in a large area and how to pro‑
duce the microlens array on a curved surface? In order to effectively achieve control of the geometry of a microlens,
indirect methods involving the use of 3D molds and replication technologies are suggested. Further development
of ultraprecision machining technology is needed to reduce the surface fluctuation by considering the dynamics
of machine tool in tool path planning. Finally, the challenges and opportunities of manufacturing microlens array in
industry and academic research are discussed and several principle conclusions are drawn.
Keywords: Microlens array, Ultraprecision machining, 3D image system, MEMS
© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License
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and indicate if changes were made.
1 Introduction
Natural compound eyes are extensively prominent in
the biological optical systems of many diurnal insects
or deep-water crustaceans, and such eyes consist of a
mosaic of hexagonal ommatidia that work as tiny opti-
cal units [14]. Unlike single aperture eyes, natural com-
pound eyes are characterized as having extremely large
field of view angles, low aberration and distortion, high
temporal resolution and infinite depth of field [3, 4].
However, the compound eye image system has intrinsic
low resolution and sensitivity [4]. e image resolution is
subject to both the number and size of the ommatidia. If
the image resolution of compound eyes increases to the
same level as the human aperture eye, the radius of the
overall lens would be at least 1 meter [5].
Although the image resolution and sensitivity of com-
pound eyes are relatively low, microlens arrays, the arti-
ficial counterpart of natural compound eyes, still have
crucial potential in a variety of applications in image
systems, under the condition that high-resolution is not
always required. For example, microlens array are more
suitable in the extremely miniaturized imaging systems
and 3D light field cameras [6]. In addition, the high
quality microlens arrays were applied in color imaging
systems, 3D image acquisition systems and fingerprint
identification systems [79].
Since the 1980s, the fabrication of microlens array is
realized by different methods, such as Micro-electrome-
chanical Systems (MEMS) based technologies [1017]
and ultraprecision machining technologies [1820].
However, little work has been focused on the comparison
of these method in terms of the surface finish, form error
and the efficiency of production. One of the major chal-
lenge in the fabrication of the microlens array is the fabri-
cation and assembly accuracy in a large area [1921]. As
the image resolution of a compound eye optical system is
increased with the number of microlens and the radius
of each microlens unit, enlarging the overall size of a
microlens array can make up the deficiency. However, to
achieve the required uniformity in a large area is very dif-
ficult [19]. Another challenge for microlens fabrication is
Open Access
Chinese Journal of Mechanical
Engineering
*Correspondence: lihua.li@polyu.edu.hk
2 Shenzhen Branch of State Key Laboratory of Ultra‑precision Machining
Technology, PolyU Shenzhen Research Institute, Shenzhen 518000, China
Full list of author information is available at the end of the article
Page 2 of 9
Yuan et al. Chin. J. Mech. Eng. (2018) 31:16
producing microlens array on a flexible layer or a curved
surface. e curved artificial compound eye is similar to
the eye of the fruit fly Drosophila, which is more com-
patible and has a larger Field of View (FOV) [22]. Such
curved compound eye imaging systems may have great
potential in terrestrial aerial vehicles, visual reality sys-
tems, surveillance etc. Image detectors, such as con-
ventional complementary metal-oxide-semiconductor
(COMS) and charge-coupled device (CCD), are planar
and not suitable for curved image systems. Recent devel-
opments [23] in flexible technologies enable the forma-
tion of microlens arrays on flexible substrates which are
bent to a spherical surface. Similar to the problem in the
fabrication of planar compound eye, the requirement of
precise alignment of the photodetector and microlens is
hard to achieve.
In the light of the above, this paper aims to review the
latest research on the progress of microlens array fabrica-
tion technologies. In Section2, the operation principle of
the compound eye is briefly introduced to provide back-
ground for the design of microlens array. In Section3, the
state-of-art technologies, including the direct and indi-
rect methods for fabricating microlens array are reviewed
and compared. Section 4 describes the applications of
microlens array. Finally, the challenges and opportunities
of manufacturing microlens arrays in industry and aca-
demic research are discussed and several principle con-
clusions are drawn in Section5.
2 Principle of Compound Eyes
In nature, compound eyes can be categorized into 2
types, e.g., apposition compound eyes and superposition
compound eyes, as shown in Figure1. In natural apposi-
tion compound eyes, the light through each ommatidia
is received by only one photo receptor [1, 2]. In contrast,
every photo receptor in the superposition compound eye
is able to acquire light from several ommatidia. erefore,
superposition eyes are much more light-sensitive, and
more suitable for deep-water crustaceans living in dim
light. However, the main drawbacks of superposition eyes
are the aberrations as the consequence of the combination
of light from different ommatidia. erefore, the artificial
compound eyes are mainly of the apposition form.
As shown in Figure1(a), in nature, the ommatidia of
apposition compound eyes are arranged on a curved
surface of radius Re and the receptors of diameter d are
distributed on the focal points of the ommatidia. e
geometric size of each ommatidia is denoted by the pitch
D and focal length f. e acceptance angle
�ϕ
and inter-
ommatidial angle
�Φ
can be expressed as follows[24]:
(1)
�ϕ
=
d
f
2
+
D
2
,
where
D
is the full width maximum of the Gaussian
approximation of the Airy function.
In most of artificial compound eye optical systems,
microlens array which is the counterpart of ommatidia,
is arranged on a plane (Figure 2) to fit with CCD and
(2)
�Φ
=
D
Re
,
Figure 1 Natural compound eyes: (a) apposition compound eyes,
(b) superposition compound eyes
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Yuan et al. Chin. J. Mech. Eng. (2018) 31:16
CMOS. Moreover, the fabrication process of microlens
array on a plane is much simpler.
For each unit of the microlens array, the geometric size
is determined by the pitch (D), the height (h), the radius
of curvature (Ru) and contact angle (θ), shown in Fig-
ure3. ese parameters can be measured through optical
microscopy, scanning electron microscopy as well as con-
tact profilometry. e quality of the microlens often is
denoted by the numerical aperture (NA), surface rough-
ness and array uniformity.
e numerical aperture is calculated as [24]:
where n is the refractive index of the medium between
the object and the microlens. e half aperture angle α
(3)
can be obtained from the height (h), the radius of curva-
ture (
Ru
):
With the increase of the value of NA, the resolution
and magnification of the microlens also increases. e
contact angle θ is equal to half aperture angle α and the
F-number F# is defined as [25]:
Surface roughness is another important parameter in
evaluating the optical performance of a lens. And it is
strongly affected by the fabrication process. Optics with
large surface roughness may suffer from scattering issues,
decreasing the efficiency of the contrast and light collec-
tion [26]. e array uniformity is of great importance in
the imaging system, especially when the area of micro-
lens array is large, and light retrace is needed for further
process. e array uniformity can be described by the
standard deviation of the height and the radius of curva-
ture [26].
3 Fabrication Methods
e fabrication methods for microlens arrays are cat-
egorized into direct methods and indirect methods. e
direct method does not need to fabricate a mask or a
mold insert with concave 3D microstructures. e shape
of microlens is usually formed based on the surface ten-
sion effect when the material is in a thermoplastic state
or liquid state resulting a super smooth surface (arith-
metic average roughness Ra less than 1 nm) [2729].
More importantly, these methods involve simple and
cost-effective processes, which are preferred in industry.
However, it is still very difficult to control the microlens
precision because the geometry of the microlens is only
determined by the controlling parameters such as tem-
perature, wettability, pressure and process time. e indi-
rect methods need to fabricate the mold with concave
microlenses and produce the final lenses by replication
technologies, such as hot embossing, compact molding
and injection molding. Using the indirect method, the
shape of microlens array can be well-controlled but the
process is complex.
3.1 Direct Fabrication Methods
3.1.1 Thermal Reflow Method
e thermal reflow method has been used to produce
microlens arrays in the last few decades [3034]. e
fabrication process is depict in Figure 4 [30]: first, the
photoresist layer is coated onto the substrate to let the
UV-light thrust through the mask which has circular
(4)
α
=arccos
R
u
h
Ru.
(5)
F
#
=
1
2n·sinα
.
Figure 2 Artificial compound eye optical systems with planar
structure
Figure 3 Geometry parameters of microlens array
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Yuan et al. Chin. J. Mech. Eng. (2018) 31:16
array patterns. Second, the photo-resist layer is devel-
oped and the cylindrical isolated-islands are then gener-
ated. ird, the isolated-islands are heated to a certain
temperature, then the cylindrical island will turn into a
spherical structure due to the effect of surface tension.
In this method, an optical separator is not needed and
the microlens array is fabricated by the common MEMS
technique. However, the contact angle is hard to control
because it is only affected by the wettability of both the
material on the substrate and the surrounded air around
the microlens, rather than the size of the isolated-islands.
3.1.2 Microplastic Embossing Method
e microplastic embossing method is a low-cost and
highly efficient technique, which was developed to the
fabrication of plastic microlens [29, 3537]. e par-
tial-filling technology [37] based on the microplastic
embossing process does not require a mold insert with
the desired microlens geometry, and thereby alleviates
the surface defects induced by contact between mold
insert and injected materials. Based on this method, a
silicon mold insert of circular openings with a few hun-
dred micrometers is first fabricated by a deep reactive
ion etching process. en a polymer substrate is placed
between the heating plates and the silicon mold insert,
as shown in Figure5. e microlenses are constructed by
applying an external pressure at an elevated temperature
above the glass transition point for a given time. Finally,
the newly formed microlens is cooled down by decreas-
ing the temperature as slowly as possible to reduce the
thermal stress and attendant replication errors. e
temperature-dependent viscosity and surface tension are
the major factors in the fabrication process, determining
both the height and radius of the microlens. e applied
pressure has a linear relationship with the height of the
microlens but has little effect on the radius. e process-
ing temperature is capable of affecting both the radius
and height in a complicated relationship. e microplas-
tic embossing method is considered as a one-step mold-
ing process, and is not influenced by the quality of the
mold surface [35].
3.1.3 Microdroplet Jetting Method
Microdroplet jetting, also known as ink jet printing,
is another direct method for fabricating microlenses
[3842]. Figure 6 shows a schematic diagram of the
working principle of this method [39]. e droplets of a
UV-polymerizable liquid are ejected from a nozzle to a
substrate. When the droplets reach this substrate, they
are exposed to UV light and converted to a solid state
with a super smooth shape. In order to obtain a micro-
lens with large a NA (more than 0.4), the substrate is
treated by Nano texturing and fluorodecyltrichlorosilane
(PFTS) or C4F8 coating [3941]. is method is suitable
for fabricating microlens arrays in a large area rapidly at
room temperature. However, it is hard to control the con-
sistency and feature size of the microlens.
3.2 Indirect Method
In contrast with direct methods for the fabrication of
compound eye microlens arrays, indirect methods need
Figure 4 Fabrication process of microlens array based on the ther‑
mal reflow method
Figure 5 Fabrication process of microlens array based on microplas‑
tic hot embossing
Figure 6 Fabrication process of a microlens array based on micro‑
droplet jetting
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Yuan et al. Chin. J. Mech. Eng. (2018) 31:16
a mold with a concave spherical microlens. e final
microlens array is produced by replication technologies
such as injection molding, hot embossing or UV mold-
ing. e key to the indirect methods is how to generate
concave microlenses with precise geometry. e poten-
tial technologies are divided into two categories, i.e.,
MEMS based technologies and ultraprecision machining
technologies.
3.2.1 MEMS Based Methods
e standard MEMS methods utilizes photolithogra-
phy to generate patterns on the mask layer and chemi-
cal reactions to etching the curvature of microlens onto
the substrate [40, 4348]. Albero et al. [43] proposed a
novel microlens wafer-scale fabrication method based
on isotropic wet etching technology, shown in Figure7.
First, the protection layers of SiO2, Si3N4 and NiCr are
coated on the substrate. en the patterns are generated
though photolithography, reactive ion etching (RIE) and
the HF solution step by step. After that, the wafers are
immersed in the isotropic etch solution to generate the
concave microlenses. Finally, the mold is finished after
the removal of the mask layer. A wide range of lens geom-
etries and lens arrays with good surface smoothness, high
uniformity and repeatability can be achieved using on
this method. However, this approach requires expensive
equipment and complicated procedures to produce the
mask on the wafer. Besides, the control precision of the
mask fabrication technology must be improved to meet
the requirements in the miniaturization of lenses.
To overcome this problem, direct writing technologies
have been developed. For example, femtosecond laser
wet etching was developed in which the patterns are
directly generated on the wafer by laser processing. en,
in the wet etching process, the concave microlens are
formed as the induced region has a higher selective etch-
ing rate than the other parts. Other examples of direct
writing techniques includes focused ion beam writing
[47, 48] and electron beams writing [49, 50].
Direct laser writing can be used to produce patterns on
a spherical substrate. Curved artificial compound eyes
(CACEs) are realized by this method [51]. However, the
optical system is hard to be miniaturized because it needs
overlapping of the concave and the convex bulk lenses
(the radius of the spherical lenses is 40 mm). Another
approach to fabricate the CACEs is by transforming the
2D microlens-pattern films into a 3D shape by the nega-
tive pressure deformation process [5255]. However,
the uniformity of the microlenses need to be improved
because in the negative pressure deformation process,
some microlens at the marginal areas may be damaged
[55].
3.2.2 Ultraprecision Machining Methods
Ultraprecision machining technologies, such as diamond
micro-milling and single point diamond turning (SPDT)
are effective methods to fabricate microstructures and
nanostructures with good uniformity in a large area [18,
21, 5658]. Such ultraprecision technologies are inte-
grated into the process chain for mass fabrication of
microlens arrays. Ball-ending milling usually utilizes
half-arc single crystal diamond tools to removal metallic
materials [56, 57]. Metallic materials such as OFHC-Cu,
AlMg3 and NiP can be processed. e achieved surface
roughness (Ra) is as low as 5 nm. However, this method
needs to machine the microlenses one by one, which
severely extends the operation time and increases the
cost. To our knowledge, the bottom of the each concave
microlens is affected by the alignment error (around 1
μm) between the vertex of the cutting edge and the spin-
dle axis. With regard to this, the single point diamond
turning (SPDT) method is performed to produce micro-
lens arrays with high quality by a slow slide servo [18, 19,
58] or a fast tool servo [21, 59, 60].
e procedures of single point diamond turning of
microlens arrays on a flat surface were described by
Zhang etal. [20]. A design model is first generated based
on the ideal structures of microlens array. en the ideal
tool path is calculated based on the design model and
cutting parameters such as spindle speed (S) and feedrate
(F) based on the cylindrical coordinate method [18, 19].
After that, the final tool path is modified by considering
alignment error, tool radius error and squareness error.
Two translation axes (X and Z) and a rotational spin-
dle axis (C) are precisely controlled to generate the 3D
structures on the cylindrical end face of the workpiece.
e form errors of the microlens array influence the
Figure 7 Fabrication process by standard MEMS technologies
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Yuan et al. Chin. J. Mech. Eng. (2018) 31:16
machining errors. However, the surface fluctuation (Fig-
ure8) is probably noticeable on the cutting parts around
the inflection points of the path, which reduces the opti-
cal performance severely. e surface fluctuation appears
due to the limit of the machine dynamic response. With
regard to this, a new fabrication method is proposed to
cut the microlens on a metal sheet mounted on the side
face of a cylindrical basement, as shown in Figure 9.
After the turning process, the metal sheet is striped off
and inserted in the injection mold. en the microlenses
are produced by microinjection molding process. Based
on this method, the surface fluctuation is reduced and a
smoother surface is obtained, with the surface roughness
(Ra) less than 10 nm and the PV-value less than 0.150
µm [61]. One of the drawbacks of this method is that,
in order to avoid the interference between the clearance
face of diamond tool and the machined concave micro-
lens, a small F number (less than 4) is difficult to achieve.
It is expected that the surface finish will be further
improved by adaptive tool path planning, considering the
dynamic response of the machine tool. Some improve-
ment has been achieved by developing the quasi-elliptical
tool servo (QETS) technique when producing a single
microlens on the mold [62].
Ultraprecision machining was also applied to fabricate
3D compound eye lens arrays [63, 64]. It was reported
that 601 individual compound eye microlenses (aperture
of 0.58 mm) and the related microprisms were produced
in a 20 mm diameter area, providing a large light devia-
tion angle of 18.43° and maximal FOV of 180°, if the entire
hemispherical surface is fabricated with microlenses. e
microprism array and microlens array were precisely fab-
ricated on a curved and a flat surface respectively, with
a combination of single point diamond turning, diamond
broaching and micromilling processes [63]. However, the
intensity of the microlens on the hemispherical surface is
low, therefore the measured FOV is much smaller than
the maximum theoretical value.
4 Applications
Microlens arrays can be integrated into a light field cam-
era to achieve the function of “take photo and then focus”
[65, 66] and obtain an image with both large FOV and
aperture. Figure10 shows the current light field of cam-
eras, including the camera arrays developed by Stanford
[67, 68]; Integrated lens array designed by the Adobe
company [69]; light field camera and microscope devel-
oped by Lytro [25, 70]; 3D light field camera produced by
Raytrix [71] and compound eye camera module devel-
oped by the Toshiba company [72]. Compared with the
camera array based devices, the microlens based devices
have more potential in the market because of their light
and compact structure. Such microlens array is mounted
between the main lens and the light sensor, changing the
light path (Figure11). Figure12 shows the experimental
setup and the image acquired by the camera, having a
compound eye microlens array fabricated by single point
diamond turning and injection molding. Figure 12(b)
shows the images processed by the digital refocus algo-
rithm. To our knowledge, although 3D images can be
obtained, however, the resolution of image obtained from
Figure 8 Surface fluctuation on the cutting parts around the inflec‑
tion points of path measured by Laser Interferometers
Figure 9 Turning of microlens array on the mental sheet
Figure 10 Light field cameras [61]
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Yuan et al. Chin. J. Mech. Eng. (2018) 31:16
current light field camera (less than 1200×900 pixels) is
much lower than the single-lens reflex camera.
5 Conclusions and Outlook
In this paper, the most important fabrication technolo-
gies for microlens arrays are reviewed and compared.
By reviewing the advantages and disadvantages of those
technologies, the following conclusions are drawn.
(1) e direct fabrication technologies, including the
thermal reflow method, microplastic embossing and
microdroplet jetting method, are simple and low-cost
processes which are suitable for mass industrial pro-
duction, but it is very difficult to control the accuracy
of the microlenses shape based on the direct fabrica-
tion methods.
(2) Compared with MEMS based technologies, ultra-
precision machining is more suitable in terms of
producing microlens array on a mold with good uni-
formity in a large area. e shape of microlens can be
well controlled by ultraprecision machining. Further
improvement is needed to reduce the surface fluc-
tuation and surface roughness by optimizing the tool
path considering the dynamics of machine tools.
(3) Both the MEMS based technologies and ultrapreci-
sion machining are able to fabricate curved com-
pound eye microlenses, but the production quality
needs to improve.
(4) 3D imaging systems inserted with microlens array
can be used to capture the light field information,
but the spatial resolution is much lower compared
with that of photos captured by 2D camera. ese 3D
imaging systems may be applied in the situation that
the high spatial resolution is not required.
Authors’ contributions
WY carried out the studies in the reviews of the principles of compound eye
and the fabrication methods. He wrote the draft. LL investigated the applica‑
tions of microlens array and she also contributed to the review of indirect
fabrication method including molding process. WL and CC shared many fun‑
damental ideas in the ultraprecision machining technologies, thermal reflow
method and UV molding technology. CC conducted proof reading and made
some critical revisions. All authors read and approved the final manuscript.
Author details
1 State Key Laboratory of Ultra‑precision Machining Technology, Partner Labo‑
ratory in the Hong Kong Polytechnic University, Hong Kong 999077, China.
2 Shenzhen Branch of State Key Laboratory of Ultra‑precision Machining
Technology, PolyU Shenzhen Research Institute, Shenzhen 518000, China.
Authors’ Information
Wei Yuan born in 1990, is currently a PhD candidate at State Key Laboratory
of Ultra-precision Machining Technology, the Hong Kong Polytechnic University,
China. He received his bachelor degree from Hefei University of Technology,
China and master degree from The Chinese University of Hong Kong, China. His
interests include metal cutting theory, ultraprecision machining processing
and robotics. Li‑Hua Li born in 1981, is current a research assistant at State Key
Laboratory of Ultra-precision Machining Technology, the Hong Kong Polytechnic
University, China. She earned her PhD degree in measurement science and
technology from Tsinghua University, China, in 2012. Dr. Li’s research focuses
on the study of design theory of optical elements, fabrication and measure‑
ment technology of optics. Wing‑Bun Lee born in 1951, is currently the Head
of State Key Laboratory of Ultra-precision Machining Technology, the Hong Kong
Polytechnic University, China. His teaching and research interests include
advanced manufacturing technology, materials processing, ultra‑precision
machining, manufacturing strategy and knowledge management systems.
Chang‑Yuen Chan born in 1965, is current the project manager at State Key
Laboratory of Ultra-precision Machining Technology, the Hong Kong Polytechnic
University, China. He earned his PhD degree in mechanical engineering from
Hong Kong University, China, in 1995. Dr. Chan’s research focuses on the study
of ultraprecision machining technology and 3D imaging processing.
Figure 11 3D camera with compound eye microlens insert
Figure 12 3D light field camera (a) experiment setup; (b) the image
processed by digital refocus algorithm [61]
Page 8 of 9
Yuan et al. Chin. J. Mech. Eng. (2018) 31:16
Acknowledgements
Supported by Shenzhen Science, Technology and Innovation Commission of
China (Grant No. JCYJ20150630115257902), the Research Grants Council of the
Hong Kong Special Administrative Region of China (Grant No. ITS/339/13FX),
and Research Committee of The Hong Kong Polytechnic University, China
(Grant No. RUK0).
Competing Interests
The authors declare that they have no competing interests.
Ethics approval and consent to participate
Not applicable.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in pub‑
lished maps and institutional affiliations.
Received: 19 October 2016 Accepted: 14 January 2018
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Structural colour printing is a sustainable printing technology that avoids the use of environmentally harmful dyes or pigments. Structural colours are produced from the interaction of light with nanostructures through physical mechanisms such as scattering, instead of selective absorption caused by chemical properties. Because of these physical mechanisms, structural colours are durable and non-fading, which find them potential applications in artworks and security prints. However, structural colour prints have a fixed two-dimensional image appearance that limits the perception of three-dimensional information. An effective solution is to combine structural colour prints with microlens arrays. This solution enables three-dimensional information to be perceived by the naked eyes under white light illumination without the use of specialized optics. As microlens arrays are compact and lightweight, they can be easily integrated with structural colour prints. Here, we demonstrate that the combination of structural colour prints with microlens arrays produces dynamic moiré and autostereoscopic effects that can be used to display-three-dimensional information. To achieve these effects, we fabricate polymer structures on glass substrate using straightforward fabrication methods (primarily, two-photon polymerization lithography). Furthermore, these structures can be dissolved using organic solvents for recycling the substrate. We believe that this work contributes towards displaying a colourful and three-dimensional world with a sustainable future.
... (3) Combined with the convex lens to realize achromatic aberrations, whereas the literature on the design and fabrication of the MCLAs on the curved substrate, especially the single-use of this lens as an optical device has not been reported. Furthermore, the development of injection molding [25], UV-cured printing [26], laser ablation [27], grayscale lithography [28], 3D printing method [29], ultra-precision diamond turning [30], and other processing techniques [31][32][33][34][35] have also laid the foundation for exploring and investigating new lenses. ...
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