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Fabrication of Microlens Array and Its Application: A Review

  • Shenzhen Technology Unversity


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
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
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
(, which permits unrestricted use, distribution, and reproduction in any medium,
provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license,
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
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
and inter-
ommatidial angle
can be expressed as follows[24]:
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
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
can be obtained from the height (h), the radius of curva-
ture (
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
Figure 2 Artificial compound eye optical systems with planar
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
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
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
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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... The SCPs and MLAs are fabricated together as polymer structures on glass substrate through two-photon polymerization lithography (TPL) (see results section on Autostereoscopic Effect), though the MLAs can also be fabricated separately through photolithography and thermal reflow (see results section on Moiré Effect). TPL enables design flexibility and rapid prototyping of complex 3D nanostructures [9][10][11][12], whereas photolithography and thermal reflow enables parallel large-scale fabrication of MLAs [13][14][15]. For recycling, the substrate can be immersed in an organic solvent to dissolve the polymer structures and erase previous information. ...
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. ...
Full-text available
A microlens array (MLA) is a fundamental optical element, which has been widely applied in the fields of imaging sensing, 3D display, and lighting source. However, it is still a challenge to design the MLAs simultaneously satisfying small size, wide field of view, and high image quality. Herein, a novel type of concave lens array on an aspheric convex substrate (CLAACs) is presented, which is composed of an aspheric substrate and a spherical concave subeye array. The facilely designed method of the CLAACs is described and its geometric model is also established by a numerical example. Furthermore, a fabrication method, which is directly machining the CLAACs on PMMA material, is proposed. To realize the ultra-precision machining of the lens, tool path planning is carried out before fabricating. The profile, surface quality, and imaging performance of the fabricated lens are then characterized to reveal its optical capabilities. The results show that the proposed method can realize the rapid design and fabrication of lenses flexibly and efficiently. The fabricated CLAACs exhibit excellent morphology uniformity, high imaging quality, and focusing performance. The study provides a feasible solution for the design and fabrication of such lens arrays with complex discontinuous surfaces.
Ultraprecision mold machining combined with precision glass molding is the most promising technology for the mass production of glass microlens arrays (MLAs). However, fabricating an MLA with a high aspect-ratio (AR) (ratio of height to aperture), submicron scale profile accuracy, nanoscale surface finish, and good consistency remains a considerable challenge. A local spiral diamond milling (LSDM) method is proposed to generate a high AR MLA on a nickel-phosphorous (Ni-P) plated mold. A spiral toolpath generation algorithm that considers the tool edge radius and performs profile error (PV) compensation is developed. Each lenslet at any local position can be precisely machined by controlling the servo motions of the three translational axes. Then, the entire MLA can be generated by shifting the toolpath to go through the centers of all lenslets according to the MLA layout. The tool parameters are optimized to avoid local and global tool interference during machining. The generated toolpath provides steady slide axis movements to avoid dramatic changes in cutting speed and acceleration. An MLA with an AR of 0.166 is fabricated. The results show that the fabricated mold and the molded lenses have a profile accuracy (PV) of less than 1 μm and a surface roughness (Ra) of less than 14 nm. The cross-sectional profiles indicate that the fabricated MLA has high machining consistency.
The consumer demand for emerging technologies such as augmented reality (AR), autopilot, and three-dimensional (3D) internet has rapidly promoted the application of novel optical display devices in innovative industries. However, the micro/nanomanufacturing of high-resolution optical display devices is the primary issue restricting their development. The manufacturing technology of micro/nanostructures, methods of display mechanisms, display materials, and mass production of display devices are major technical obstacles. To comprehensively understand the latest state-of-the-art and trigger new technological breakthroughs, this study reviews the recent research progress of master molds produced using nanoimprint technology for new optical devices, particularly AR glasses, new-generation light-emitting diode car lighting, and naked-eye 3D display mechanisms, and their manufacturing techniques of master molds. The focus is on the relationships among the manufacturing process, microstructure, and display of a new optical device. Nanoimprint master molds are reviewed for the manufacturing and application of new optical devices, and the challenges and prospects of the new optical device diffraction grating nanoimprint technology are discussed.
Light field 3D display has attracted great attention as the hardware interface for a virtual 3D application. The design and fabrication of light-modulating structures directly affect the authenticity of the reconstructed 3D image. The critical challenges in 3D display, such as visual fatigue, limited motion parallax, and insufficient light efficiency, are closely related to the inaccurate phase or angular reconstruction of the virtual light field. While the designs of light field modulators are widely introduced in all kinds of studies, the importance of fabrication for 3D display is usually under-estimated. Large format, small feature size, and high precision in shape are the three critical requirements of fabrication for 3D display. In this paper, we focus on the fabrication technologies for light-modulating structures. Fabrication tools for both microstructures and nanostructures are introduced. The fabrication capability, critical challenges and its applications in light field 3D display are discussed. Finally, the future development of 3D display related fabrication technologies are highlighted.
To improve the efficiency and consistency of machined microlens array using single-point diamond turning technology, a theoretical model of surface form error is proposed in this paper. Then, a compensation method for this model is studied. In the proposed tool equivalent tilt angle model, the microlens array is regarded as a freeform surface. The corresponding curvature radius of the surface at each cutting point along the cutting direction is calculated by establishing a slow slide servo cutting model. In the spatial form error model, the assumption is that surface form error has a linear relationship with z-axis maximum speed vz. An empirical linear equation is obtained and verified, with a maximum deviation of 0.4 μm. Then, after machining, the surface form error is measured and processed using on-machine measurement. The theoretical and measured surface form errors are consistent. The surface form error is compensated in the machining program. The peak-to-valley value is reduced from 5.4 to 0.6 μm after compensation. Findings show that the single-point diamond turning and compensation method for the microlens array presented in this paper can predict the surface form error and significantly improve machining accuracy and consistency.
In this paper, a tunable zoom bifocal liquid lens based on selective wettability is proposed. This lens consists of internal and external immiscible coaxial droplets surrounded by immiscible ambient liquid. Since curvatures and refractive indexes of the internal and external droplets are different, the system forms a long focus and a short focus, respectively. By applying different voltages, the curvatures of the internal and external droplets change exerting continuous movement of a long/short focal point in a certain range. To verify the feasibility and practicability of this concept, a prototype of the bifocal compound lens is fabricated in experiment, and the modulation ability of its long/short focal length is detected. The short focal length of our proposed lens varies from 15.46 mm to 17.47 mm, while the relative long focal length ranges from 96.25 mm to 70.31 mm driven by 200 V.
Inspired by the compound eyes of insects, many multi-aperture optical imaging systems have been proposed to improve the imaging quality, e.g., to yield a high-resolution image or an image with a large field-of-view. Previous research has reviewed existing multi-aperture optical imaging systems, but few papers emphasize the light field acquisition model which is essential to bridge the gap between configuration design and application. In this paper, we review typical multi-aperture optical imaging systems (i.e., artificial compound eye, light field camera, and camera array), and then summarize general mathematical light field acquisition models for different configurations. These mathematical models provide methods for calculating the key indexes of a specific multi-aperture optical imaging system, such as the field-of-view and sub-image overlap ratio. The mathematical tools simplify the quantitative design and evaluation of imaging systems for researchers.
Objective: Brain-machine interfaces (BMIs) have the potential to restore motor function but are currently limited by electrode count and long-term recording stability. These challenges may be solved through the use of free-floating "motes" which wirelessly transmit recorded neural signals, if power consumption can be kept within safe levels when scaling to thousands of motes. Here, we evaluated a pulse-interval modulation (PIM) communication scheme for infrared (IR)-based motes that aims to reduce the wireless data rate and system power consumption. Approach: To test PIM's ability to efficiently communicate neural information, we simulated the communication scheme in a real-time closed-loop BMI with non-human primates. Additionally, we performed circuit simulations of an IR-based 1000-mote system to calculate communication accuracy and total power consumption. Main results: We found that PIM at 1kb/s per channel maintained strong correlations with true firing rate and matched online BMI performance of a traditional wired system. Closed-loop BMI tests suggest that lags as small as 30 ms can have significant performance effects. Finally, unlike other IR communication schemes, PIM is feasible in terms of power, and neural data can accurately be recovered on a receiver using 3mW for 1000 channels. Significance: These results suggest that PIM-based communication could significantly reduce power usage of wireless motes to enable higher channel-counts for high-performance BMIs.
Full-text available
In most animal species, vision is mediated by compound eyes, which offer lower resolution than vertebrate single-lens eyes, but significantly larger fields of view with negligible distortion and spherical aberration, and high temporal resolution in a tiny package. Compound eyes are ideally suited for fast panoramic motion perception. Engineering a miniature artificial compound eye is challenging, because it requires accurate alignment of the photoreceptive and optical components on a curved surface. Here we describe a novel design method for biomimetic compound eyes featuring a panoramic, undistorted field of view in a very thin package. The design consists of three planar layers of separately produced arrays, namely, a microlens array, a neuromorphic photodetector array and a flexible printed circuit board, that are stacked, cut and curved to produce a mechanically flexible imager. Following this method, we have prototyped and characterized an artificial compound eye bearing a hemispherical field of view with embedded and programmable low-power signal processing, high temporal resolution, and local adaptation to illumination. The prototyped artificial compound eye possesses several characteristics similar to the eye of the fruit fly Drosophila and other arthropod species. This design method opens up new vistas for a broad range of applications where wide field motion detection is at a premium, such as collision-free navigation of terrestrial and aerospace vehicles, and for the experimental testing of insect vision theories.
Full-text available
In this work, we demonstrate a simple yet flexible method to fabricate biomimetic compound eye (BCE) arrays in polymers massively by using a novel hierarchic reflow method. In this method, the subtle control on the thermal reflow of different polymeric materials shows that a hierarchic reflow process in polymers can be realized. As a result, ommatidia with a spherical microlens form can be formed on a convex curved surface with a sub-millimeter size. By using the developed method, BCEs with a field of view from 34.211 to 107.951, ommatidia with a diameter ranging from 5.02 mm to 47.81 mm and numerical aperture ranging from 0.09 to 0.45 have been successfully obtained, which shows the great flexibility and tunability of the fabrication method. The optical tests show that the fabricated BCE has a near diffraction-limited optical performance, which may find application in highly compact endoscopic or surveillance devices as well as in wide-angle illuminating systems. The method developed in this work is flexible and compatible with the standard semiconductor manufacturing process so that it can be adapted to manufacture other useful photonic components with hierarchic microstructures massively and cost-effectively.
Full-text available
In this work, we demonstrate a simple yet flexible method to fabricate biomimetic compound eye (BCE) arrays in polymers massively by using a novel hierarchic reflow method. In this method, the subtle control on the thermal reflow of different polymeric materials shows that a hierarchic reflow process in polymers can be realized. As a result, ommatidia with a spherical microlens form can be formed on a convex curved surface with a sub-millimeter size. By using the developed method, BCEs with a field of view from 34.21° to 107.95°, ommatidia with a diameter ranging from 5.02 μm to 47.81 μm and numerical aperture ranging from 0.09 to 0.45 have been successfully obtained, which shows the great flexibility and tunability of the fabrication method. The optical tests show that the fabricated BCE has a near diffraction-limited optical performance, which may find application in highly compact endoscopic or surveillance devices as well as in wide-angle illuminating systems. The method developed in this work is flexible and compatible with the standard semiconductor manufacturing process so that it can be adapted to manufacture other useful photonic components with hierarchic microstructures massively and cost-effectively.
Full-text available
We demonstrate a simple route to fabricate closed-packed infrared (IR) silicon microlens arrays (MLAs) based on femtosecond laser irradiation assisted by wet etching method. The fabricated MLAs show high fill factor, smooth surface and good uniformity. They can be used as optical devices for IR applications. The exposure and etching parameters are optimized to obtain reproducible microlens with hexagonal and rectangular arrangements. The surface roughness of the concave MLAs is only 56 nm. This presented method is a maskless process and can flexibly change the size, shape and the fill factor of the MLAs by controlling the experimental parameters. The concave MLAs on silicon can work in IR region and can be used for IR sensors and imaging applications.
In fast or slow tool servo (F-/STS) diamond turning of micro-lens arrays (MLAs), the inherent non-smooth servo motion will lead to undesired tool vibrations, and it can significantly deteriorate the quality of the machined surface. Starting from a mathematical explanation of the underlying mechanism for vibration suppression, a quasi-elliptical tool servo (QETS) technique and the corresponding optimal toolpath determination algorithm are proposed to overcome the inherent defects in F-/STS turning of MLAs. As for the QETS, the inherent non-smooth servo motion in the F-/STS is proposed to be decomposed into two smooth quasi-harmonic motions along the cutting and servo motion directions, which then constructs the quasi-elliptical trajectory. Taking advantage of the smooth nature of the two decomposed motions, the undesired tool vibrations induced by the motion non-smoothness in the F-/STS can be significantly eliminated, accordingly facilitating the generation of MLAs with homogeneous and smooth surfaces. Finally, the new concept is verified through numerical simulation of the tool motion and experimental demonstration by turning a typical hexagonal aspheric MLA.
Compound eye lens is the key component in light field system. A novel design and manufacturing method is proposed. The parameters of compound eye lens are optimized based on different cameras so that the quality of the image is improved; the manufacturing process mainly includes modified ultra-precision turning and mould injection. Besides, the synthetic aperture and digital refocusing related to light field imaging are discussed. The results of experiments show that the high accuracy and repeatability of compound eye lens arrays are achieved by utilizing the proposed method and the core function of light field image is also achieved.
A method to fabricate microlens arrays in polymer substrates via hot embossing is presented in this paper. The partial-filling hot embossing technique of micro cavities on a mold insert through surface tension and capillary action is proven to be an effective means of limiting imperfections on the surface of microlens arrays. The effects of the investigated process parameters including temperature, embossing pressure, and holding time are analyzed via Taguchi method to identify effective processing conditions for microlens arrays of varying heights and diameters. Signal-to-noise (S/N) ratios are calculated for the focal length of the fabricated microlens arrays to identify key individual parameters and their interactions for a streamlined fabrication process. Experimental data indicates that the holding time in the embossing process has the most significant impact on lens focal length followed by embossing temperature and pressure. This study identifies a reliable means of microlens production and demonstrates the effects of varying process parameters in the partial-filling method of micro hot embossing for the production of lens arrays.
An electron beam pattern generator with variable shaped beam has been used to fabricate different types of surface corrugated micro-optical components. For the creation of computer generated holograms this variable shaped beam method is much more efficient compared to a point beam method. It is demonstrated that a big variety of surface corrugated structures can be obtained by a variable dose writing process. Using this method different types of both Fresnel and refractive lenses and lens arrays and also gratings for integrated optical applications have been fabricated. Using the Ag+-Na+ ion exchange in special optical glasses, one- and two-dimensional refractive index profiles of definite shape have been realized. It has been shown that good quality one-dimensional profiles can be used for micro-cylindrical lens fabrication. Such lenses are applicable for high power laser diode collimation. Two-dimensional index gradients with periodical structure we used for transmission phase grating fabrication. Such gratings have been successfully applied to beam division purposes.