change of a liquid lens will also induce aberration itself due to

clamped edge effect. When injected volume of 0.1 ml, we can

experimentally deduce that aberrations of Z3/Z5 induced by

MEMS DM at a magnitude of 0.486 lm/�1.472 lm, fluidic

lenses can only marginally improved to �0.245 lm/0.305 lm.

ACKNOWLEDGMENTS

This work was financially supported in partial by the National

Science Council of Taiwan for their funding support under NSC

99-2911-M-008-001.

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microlenses activated by stimuli-responsive hydrogels, Nature 442

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miniature cameras, Appl Phys Lett 85 (2004), 1128–1130.

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Tabiryan, Adaptive liquid lens actuated by photo-polymer, Opt

Express 17 (2009), 17590–17595.

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by combinative convex/concave interfaces of refractive-index-mis-

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VC 2012 Wiley Periodicals, Inc.

INTEGRAL IMAGING SYSTEMFEATURING A SEAMLESSLY EXTENDEDDEPTH OF FIELD

Kai Xu, Yong-Seok Hwang, and Sang-Shin LeeDepartment of Electronic Engineering, Kwangwoon University,139-701 Wolgye-Dong, Nowon-Gu, Seoul 139-701,Republic of Korea; Corresponding author: slee@kw.ac.kr

Received 7 September 2011

ABSTRACT: An integral imaging display system, incorporating three

birefringence lens arrays (BLAs) paired with a half-wave plate (HWP),is proposed to accomplish a seamlessly extended depth of field. TheBLAs were built by injecting three different types of liquid crystal into

the space between the lens array substrate and glass substrate, and theyact either as a concave lens, convex lens, or a transmissive plain

homogeneous medium, depending on the light polarization. The HWP is

used to adaptively control the polarization of the light impinging oneach of the BLAs, to ensure that one of them acts as a concave/convexlens, whereas the others act as a transmissive plain medium. The three

BLAs were appropriately aligned to extend the entire depth of fieldsubstantially. Consequently, the feasibility of the proposed display

system was verified theoretically and experimentally. VC 2012 Wiley

Periodicals, Inc. Microwave Opt Technol Lett 54:1705–1711, 2012;

View this article online at wileyonlinelibrary.com. DOI 10.1002/

mop.26896

Key words: three-dimensional display; liquid crystal; integral imaging;

imaging

1. INTRODUCTION

Three dimensional (3D) display technologies have fully matured

and have been commercialized in various fields, such as movies,

advertisements, and exhibitions. Special glasses are indispensa-

ble in most of the present 3D display systems, allowing the right

and left eyes to obtain different viewpoints. To overcome this

limitation, various 3D displaying methodology requiring no

glasses, including the integral imaging (InIm) display [1], the

holographic display, the lenticular-based autostereoscopic dis-

play, and the pallarax barrier-based autostereoscopic display,

have been extensively researched. The InIm and holographic

displays, which naturally present real 3D images with continu-

ous perspective, are classified as volumetric 3D techniques.

Recently, ample attention has been paid to InIm as an authentic

3D display due to its simple implementation compared to holo-

gram based displays [2–8].

The InIm technique is composed of two processes, the

pickup process acquiring the 3D information from an object and

the reconstruction process reproducing the object. In the case of

the pickup process, the ray information illuminated from the

object is stored in recording optoelectronic devices, such as a

CCD, passing through a lens array, producing an elemental

image array (EIA). For the reconstruction process, the 3D object

is reconstructed by integrating the EIA, which is loaded onto

flat panel displays such as LCDs via a similar lens array. The

InIm display has attracted considerable attention by enhancing

the resolution, depth of field, and viewing zone including the

capability of offering viewing points in a row within a certain

field view. However, it still requires further improvement in

terms of the depth of field, resolution, and viewing width. To

extend the depth of field, various approaches [9–13] were pur-

sued based on such devices as birefringence lenses [14–18].

In this article, an InIm display system, incorporating three

birefringence lens arrays (BLAs) paired with half-wave plates

(HWPs), designed to provide a seamlessly extended depth of

field is presented. The three types of BLAs were built by filling

three different types of liquid crystals into the region between

the base lens array substrate and the glass substrate; they act ei-

ther as a concave lens, a convex lens, or just a transmissive

plain medium, depending on the incident light polarization. The

HWPs are used to properly control the polarization of the light

impinging on each of the BLAs, making sure that one of them

serves as a concave/convex lens, whereas the other BLAs act as

a transmissive plain medium. The depth of field corresponding

to each of these BLAs is serially combined to enlarge the entire

depth of field. The integral images with these combinations of

different depths of field can be seamlessly reconstructed without

degrading the depth resolution. Finally, the feasibility of

the proposed display system was confirmed through

experimentation.

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 7, July 2012 1705

2. STRUCTURE AND DESIGN OF THE PROPOSED InImSYSTEM

The configuration of the proposed InIm system using the three

BLAs is shown in Figure 1(a). Each BLA is produced by inject-

ing a nematic LC in between a lens array substrate and an in-

dium-tin-oxide (ITO) glass plate. The three BLAs, labeled as

BLA I, II, and III, are made using different LCs: E-7, ZLI-4119,

and ZLI-3119, respectively. An HWP is placed in front of each

of the BLAs, and rotate the polarization of the incoming light to

be aligned on the optic axis which plays the role of a concave

lens, a convex lens, or a glass plate. As shown in Figure 1(b), the

performance of the BLAs is determined by the difference in the

refractive index between the LC (no for the ordinary axis and nefor the extraordinary axis) and the base lens substrate. The focal

Figure 2 The characteristics of the three BLAs as (a) a concave lens, (b) a convex lens, and (c) a transmissive plain glass substrate. [Color figure can

be viewed in the online issue, which is available at wileyonlinelibrary.com]

Figure 1 (a) The InIm display system based on multiple BLAs in conjunction with HWPs and (b) the polarization dependent behavior of the BLAs.

[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

1706 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 7, July 2012 DOI 10.1002/mop

length of the BLAs is expressed as fi ¼ 1= nl � np� �

R, where nl iseither the no or ne of the LC; np and R are the refractive index

and the radius of the curvature of the base lens, respectively.

The BLA acts as a concave lens for the case of nl > np, a con-

vex lens for nl < np, and a homogeneous plain medium for nl ¼np, as shown in Figures 2(a), 2(b), and 2(c), respectively. In this

work, both BLA I and III are devised to function as concave

lenses, and BLA II as a convex lens. The polarization of the light

impinging on each of the three BLAs is adaptively controlled to

ensure that only one of them acts as a lens whereas the others act

as a plain medium. The reconstructed images can be located within

the continuous depth of field region of the BLAs, by determining

the combination of the BLAs. To achieve a seamlessly extended

region, the depth of field is derived from the focal length of the

BLAs, which are determined by the pixel size of the EI, the width

of the elemental lens, and the radius of the curvature of the lens.

The characteristics of the light polarization for the proposed

InIm system were indicated by means of the Jones vector. Three

different situations are considered depending on the arrangement

of the HWPs and BLAs, as shown in Figure 3. The HWPs and

BLAs are preferentially arranged to minimize the amount of

polarization rotation for the impinging beam. The incoming light

is polarized in the horizontal direction; and the extraordinary

axes of BLA I and II are set in the vertical direction, whereas

that of BLA III is set in the horizontal direction.

For the case shown in Figure 3(a), the optical axes of HWP I

and III are set in the diagonal direction with an angle of 45�,whereas the optical axis of HWP II is set in the horizontal direc-

tion. In this combination, only BLA I acts as a concave lens,

because the incoming light on BLA I experiences the difference in

the refractive index between the base lens array and the LC. For

the case depicted in Figure 3(b), where the optical axis is arranged

horizontally for all of the HWPs, only BLA II acts as a convex

lens. For the case described in Figure 3(c), the optical axis of the

HWP in front of BLA II is set in the diagonal direction with an

angle of 45�, whereas that of the other HWPs is set in the horizon-

tal direction. Here, only BLA III functions as a concave lens.

A 3D analysis based on ray tracing was performed to verify

the operation of the proposed InIm system with the assistance of

a commercially available simulator LightToolsVR . As shown in

Figure 4(a), a computational pick-up was used to generate an

EIA. Then, we carried out the numerical and optical reconstruc-

tion using the EIA, under a similar situation to the proposed sys-

tem. The EI matches the optical elemental lens, with a footprint

of 4 � 3 mm2 and the number of EIs is 10 � 13 (column �row). With that in the optical set up, the focal length of the ele-

mental lens of the base lens array is 38 mm in the computa-

tional set up. Three different objects were used: Object I is a

cube containing a character ‘‘3,’’ whose dimensions are 30 � 30

� 30 mm3, Object II is a die including a character ‘‘4" with a

footprint of 20 � 20 � 20 mm3, and Object III is a similar type

of cube with a character number ‘‘7,’’ with a dimension of 10 �10 � 10 mm3. Objects I, II, and III are located at a distance of

75, 95, and 120 mm away from the lens array plane, respec-

tively. As displayed in Figures 4(b), 4(c) and 4(d), the corre-

sponding reconstructed image plane for Objects I, II, and III is

placed at 370, 470, and 670 mm, respectively. The depth of field

for Objects I, II, and III is 50, 70, and 90 mm, respectively. It is

approximately defined by the range, beyond which the recon-

structed image is blurred. Through BLA III with a positive focal

Figure 3 The polarization characteristics for: (a) BLA I acting as a concave lens array, (b) BLA II acting as a convex lens array, and (c) BLA III act-

ing as a concave lens array. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 7, July 2012 1707

length, a real reconstruction was also achieved. For BLA I and

II, having a negative focal length, however, a virtual reconstruc-

tion was not obtained. For BLA III, the reconstruction plane for

Objects I, II, and III was placed at 510, 610, and 810 mm,

respectively. Here, considering the two reconstructions based on

the base lens array and BLA III, the reconstruction plane of

Object II based on the BLA III belongs to the depth range of

Object I based on the base lens array. It is believed that the

reconstruction of the Object I and II is clearly fulfilled through

the proposed InIm system. As a consequence, it is numerically

confirmed that the proposed system is useful for accomplishing

a seamlessly extended depth of field.

3. EXPERIMENTAL RESULTS

The experiment setup for the proposed InIm display system is

shown in Figure 5. A projector, used as a light source, is con-

nected to a laptop to load the EIA (8 � 8) displayed in Figures

6(a) and 6(b), which are used for reconstructing a real image

and a virtual image, respectively. One of the EIs was selected in

accordance with the function of the lens, either convex or con-

cave, which is dependent on the combination of the three BLAs.

The camera lenses and the collimating lens set are used to

expand the polarized light, coming from the projector, and to

achieve a collimated beam, which steadily propagates through

the BLAs. Here, the distance between the camera lens 1 and 2

is 40 mm, and the gap between the two lenses, comprising the

collimating lens set, is about 200 mm. The HWP, in front of

each of the BLAs, serves to rotate the polarization as desired. It

is noted that the size of the EI transported by the outgoing light

should match the size of the lenslet of the lens array. The opera-

tion of the three BLAs is polarization dependent, as mentioned

above: the refractive index (ne) for the extraordinary axis of the

E-7 LC used for BLA I and the ZLI-3119 LC used for BLA III

is 1.746 and 1.621, respectively, whereas for the ordinary axis,

their refractive index (no), being 1.521 and 1.520, is close to

that of the base lens substrate (np ¼ 1.523).Therefore, for the

incident light polarized along the extraordinary axis, BLA I and

III act as a concave lens. Meanwhile, as the no ¼ 1.471of the

ZLI-4119 LC used for BLA II is not equal to that of the base

lens substrate, instead the ne value of 1.531 is similar to that of

the base lens substrate, the BLA II functions as a convex lens

for the light polarized along the ordinary axis.

Figure 6 The EIs for: (a) the real reconstruction image and (b) the

virtual reconstruction image

Figure 4 (a) The EIA, (b) the reconstruction of the center plane for

Object I, located at 370 mm, (c) the reconstruction of the center plane

for Object II, located at 470 mm, and (d) the reconstruction of the center

plane for Object III, located at 670 mm. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com]

Figure 5 The experimental setup for the proposed InIm display sys-

tem. [Color figure can be viewed in the online issue, which is available

at wileyonlinelibrary.com]

1708 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 7, July 2012 DOI 10.1002/mop

We fabricated three different types of planoconvex BLAs using

a 10 � 12lens array, consisting of an elemental lens with a foot-

print of 4 � 3 mm2. BLA I, II, and III were made by combining

the ZLI-4119 LC, E-7 LC, and ZLI-3119 layers with a base lens

array through a typical LCD fabrication processes. The alignment

membrane was formed by spin coating a RN-1702 polyimide film

on top of the lens array. It was heated on a hot plate, so that the

alignment was uniformly acquired by rubbing the membrane in a

certain direction. A 10-lm-thick spacer was then distributed on

the surface of the ITO glass substrate. The height of the lens array

and the thickness of the LC layer were about 500 and 10lm at

the center, respectively. The base lens array was glued to the ITO

substrate in an antiparallel direction. The BLAs were filled with

the ZLI-4119, E-7 LC, and ZLI 3119 LC via the capillary effect.

The estimated focal length of BLA I and III is –89 and –205

mm, respectively, whereas that of BLA II is þ385 mm. The ra-

dius of the curvature of the base lens is 20mm. For the analysis

of the optical reconstruction, the diameter for the circle of confu-

sion limit for a single lens was determined to be15 lm, consider-

ing the pixel size of the LC panel used for the project display

system. The size of the elemental lens was 4 mm. When the dis-

tance from the lens to the center of an object under interest is

120 mm, for BLA I the depth of field was calculated to be 21.3

mm, roughly ranging from 110.3 to 131.5 mm. For BLA II, the

depth of field was 6.2 mm, ranging from 116.9 to 123.0 mm; for

BLA III, it was 14.3 mm, ranging from 113.2 to 127.5 mm. It is

found that a shorter focal length results in a larger depth of field.

For the proposed InIm system, the diameter for the circle of

confusion at the reconstruction plane needs to be defined as the

size of the pixels, which are superposed through the elemental

lenses, having a pixel corresponding to a specific point on the

object. This is because the superposed pixels have a minimum

size of 15 lm at one reconstruction plane. In this work, the circle

of the confusion limit was determined to be 250 lm, which can

be discriminated as a pixel by human eyes. The depth of field for

the InIm system is larger than the depth defined above.

To examine the depth of field, the position of the recon-

structed image of a single object was first observed. Three

objects are then subsequently reconstructed at different loca-

tions depending on the lens array. For the first case, the recon-

structed image is shown in Figure 7. The image restored by

virtue of a base lens array is displayed in Figure 7(a), when

the camera lens and the collimating lens are adjusted properly.

Figures 7(b), 7(c) and 7(d) show the images rebuilt through

the BLA I, II, and III, respectively. The light from the project

was adjusted to be linearly polarized. When white light is illu-

minated by the projector, colored images such as green or red

are acquired. It is noted that the coloring of the reconstructed

images, which are manifested in Figures 7(b)–7(d), can be

ascribed to the fact that the state of the linear polarization of

the light, coming out of the project, is dependent on the color.

In this work, the polarization was controlled with the help of

the HWPs, as discussed above. The real image, reconstructed by

BLA II, was observed to be positioned at þ1300 mm as shown

in Figure 7(c), and the distance between the EI and the BLA

was estimated to be about 400 mm according to the lens equa-

tion. The integral image seen in Figure 7(a) is reasonably clear,

whereas the images given in Figures 7(b)–7(d) are slightly

blurred; the image shown in Figure 7(c) is accompanied by a

ghost image, which could be eliminated by enlarging the beam

size to increase the number of EIs available.

In addition, we attempted to optically reconstruct three objects,

which are placed at different positions. The configuration of the

setup is similar to that of the first experiment except for the

adjusted gap between the collimating lens and the camera lens.

The reconstructed images, based on the base lens array, BLA I

and II, are shown in Figure 8. Because of the diffraction induced

blurring with the lens set, it was difficult to measure the depth of

field precisely. Thus, the depth of range is estimated from the

readability. The seamless depth of field can be constructed using

the InIm system incorporating multiple BLAs, where the regions,

accounting for the depth of fields, overlap each other.

The results of the reconstruction by means of the base lens

array are shown in Figure 8(a). The reconstruction planes for

Objects I, II, and III are placed at 225, 270, and 340 mm, respec-

tively, when the depth of field was 50, 75, and 75 mm, respec-

tively. The reconstruction based on the BLA I (E-7 LC) is shown

Figure 7 The images reconstructed using: (a) the base lens array, (b)

BLA I with E-7 LC, (c) BLA II with ZLI-4119 LC, and (d) BLA III

with ZLI-3119 LC. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 7, July 2012 1709

in Figure 8(b). The reconstruction planes for Objects I, II, and III

were placed at 176, 205, and 234 mm, respectively, while the

depth of field is 17, 14, and 14 mm, respectively. The reconstruc-

tion for BLA II (ZLI-4119 LC) is shown in Figure 8(c). The

reconstruction planes for the Object I, II, and III were positioned

at 182, 227, and 286 mm, respectively, while the depth of field is

21, 29, and 31mm, respectively. These results are listed in Table 1.

Each of the operating lens arrays is selectively chosen to

restore just one of the three objects reconstructed. And through

the proposed time-sequential method three objects are

TABLE 1 Optical Reconstruction Location of Objects I, II, IIIfor the Base Lens Array, BLA I and BLA II

Base Lens Array BLA I with E-7

BLA II with

ZLI-4119

Object Left Center Right Left Center Right Left Center Right

I 200 225 250 172 182 193 169 176 186

II 235 275 310 204 227 233 200 205 214

III 310 340 385 269 286 300 229 234 243

Figure 8 The reconstructed images of Objects I, II, and III using: (a) the base lens array, (b) BLA I with E-7 LC, and (c) BLA II with ZLI-4119 LC.

[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

1710 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 7, July 2012 DOI 10.1002/mop

reconstructed concurrently. For example, the base lens array may

be designed to be responsible for the reconstruction of Object III,

the E-7 BLA for Object II, and the ZLI-4119 BLA for Object I.

As a consequence, the fact that the images can be successfully

restored at different positions proves the feasibility of the proposed

InIm system, enabling a seamlessly extended depth of field.

The reconstruction of occluded objects is discussed here:

Both the occluding object and the partially occluded object can

be efficiently restored within limits, imposed by the circle of

confusion, when the gap between them is smaller than the depth

of field of the lens. A clear reconstructed image is also attained

when the BLA is sufficiently separated from the occluding and

occluded objects by as far as the hyper focal length. The esti-

mated viewing angle for the proposed InIm system, using a

base lens array with a focal length of 38 mm and a width of 4

mm, is about 2.8�. However, this work has been focused on the

seamless extension of the depth of field.

4. CONCLUSIONS

In summary, an InIm system, exploiting three different types of

BLAs combined with a group of HWPs, was presented demon-

strating a seamlessly extended depth of field. The light polariza-

tion was adaptively controlled by the HWPs to alter the role of

the BLAs. The depth of field was found to be efficiently

enlarged by serially combining the depth of fields, available

from each of the three BLAs.

ACKNOWLEDGMENTS

This work was supported by the National Research Foundation of

Korea (NRF) grant funded by the Korea government (MEST) (No.

2011-0017901 and 2011-0030821), by Business for Cooperative

R&D between Industry, Academy, and Research Institute funded by

Korea Small and Medium Business Administration in 2011 (Grant

No. 00043574), and a research grant fromKwangwoonUniv. in 2012.

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VC 2012 Wiley Periodicals, Inc.

LOW-PROFILE TETRA WIRE-PATCHANTENNA FOR AUTOMOTIVEAPPLICATIONS

L. Lizzi, F. Ferrero, J. Ribero, and R. StarajCREMANT/LEAT, Universit�e de Nice-Sophia Antipolis, CNRS 250,rue Albert Einstein, 06560, Valbonne, France; Correspondingauthor: leonardo.lizzi@unice.fr

Received 7 September 2011

ABSTRACT: A wire-patch antenna operating over the terrestrialtrunked radio emergency and public safety standard is presented. Itexploits the use of shorting wires to operate below its fundamental

cavity resonance mode. Thanks to low-cost, low-profile, solidity, ease offabrication, and dipole-like radiation pattern properties, it results to be

particularly suitable for automotive applications. VC 2012 Wiley

Periodicals, Inc. Microwave Opt Technol Lett 54:1711–1714, 2012;

View this article online at wileyonlinelibrary.com. DOI 10.1002/

mop.26895

Key words: wire-patch antennas; low-profile antennas; TETRA;automotive applications

1. INTRODUCTION

In the last years, there has been a constantly increasing demand of

vehicles equipped with on-board communication devices. In partic-

ular, emergency and public safety vehicles must be able to always

receive (and transmit) information to provide a fast and effective

service. In this framework, non-negligible research efforts have

been devoted to the study and design of innovative automotive

antenna systems. The most important requirement for this kind of

antennas is generally low-cost since they have to be suitable for

mass production [1]. In addition, desirable characteristics are low-

profile, solidity, radome protection, and dipolar-like radiation pat-

tern enabling the communication in any azimuthal direction.

To fulfill all these requirements, a possible solution is repre-

sented by wire-patch antennas, i.e., microstrip antennas that,

thanks to the use of one or more wires connecting the radiating

element to the ground plane, can be used below its well-known

classical fundamental mode [2]. These antennas enable the

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 7, July 2012 1711