Air Mounted Eyepiece:
Optical See-Through HMD Design with Aerial Optical Functions
Kazuki Otao

Yuta Itoh

Kazuki Takazawa

University of Tsukuba
Pixie Dust Technologies, Inc.
kazuki.otao@pixiedusttech.com

University of Tsukuba
Pixie Dust Technologies, Inc.
atuyoti@pixiedusttech.com

University of Tsukuba
Pixie Dust Technologies, Inc.
kazuki.takazawa@pixiedusttech.com(a)Hiroyuki Osone

Yoichi Ochiai

University of Tsukuba
Pixie Dust Technologies, Inc.
soneo@pixiedusttech.com

University of Tsukuba
Pixie Dust Technologies, Inc.
wizard@slis.tsukuba.ac.jp(b) Displayed

Display

Image

(c)

See-throughBackgroundPhysical
Lens

VirtualLens

Graphics

Optical
See-Through

Eye

Aerial
Image

TMD

(d)

PhysicalObjectFigure 1: (a) Concept of a near-eye image formation and virtual lens. (b) System overview using TMD instead of conventional
optical elements. (c) Obtained image of a prototype display with aerial graphics and see-through background. (d) Our fabricated
HMD enables a wide field-of-view, near-eye, and see-through.

ABSTRACT
We propose a novel method of implementing an optical see-through(OST) head-mounted display (HMD) with a wide viewing angle
and high resolution for augmented reality, called an Air Mounted
Eyepiece (AME). In past years, many optical elements, such astransmissive liquid-crystal display (LCD), half-mirror, and waveguide have been adopted for OST-HMD. To achieve the AME design,
we employ an off-the-shelf HMD and Transmissive Mirror Device(TMD), which is used in aerial real-imaging systems, instead of
conventional optical elements. In the proposed method, we present“virtual lens,” which has the same function as the HMD lens in frontof the eyes. By using TMD, it is possible to shorten the opticallength between the virtual lens and the eye. Therefore, the aerial
lens provides an immersive image with see-through capability. In
this paper, we describe a detailed design method of TMD-based
HMD, and compare it to previous half mirror-based HMD and convex mirror-based HMD. Then, we construct a fabricated prototypeof the OST-HMD using TMD. We aim to contribute to the fieldof human-computer interaction and the research on eyepiece interfaces by discussing the advantages and the limitations throughsimulations and experiments.

CCS CONCEPTS• Hardware → Displays and imagers;

KEYWORDS
Augmented Reality, Near-Eye Display, Optical See-Through Display,
Transmissive Mirror Device
ACM Reference Format:
Kazuki Otao, Yuta Itoh, Kazuki Takazawa, Hiroyuki Osone, and Yoichi
Ochiai. 2018. Air Mounted Eyepiece: Optical See-Through HMD Design
with Aerial Optical Functions. In AH2018: The 9th Augmented Human International Conference, February 7–9, 2018, Seoul, Republic of Korea. ACM, New
York, NY, USA, 7 pages. https://doi.org/10.1145/3174910.3174911

1
AH2018, February 7–9, 2018, Seoul, Republic of Korea© 2018 Association for Computing Machinery.
This is the author’s version of the work. It is posted here for your personal use. Not
for redistribution. The definitive Version of Record was published in AH2018: The
9th Augmented Human International Conference, February 7–9, 2018, Seoul, Republic ofKorea, https://doi.org/10.1145/3174910.3174911.

INTRODUCTION

eXtended Reality (XR) is now a great driving force behind the development of visual display technology. Such technologies are used instationary and automotive scenarios, where highly immersive displays are desirable. An OST-type information presentation methodwith a wide viewing angle is required for this purpose. In recent
years, several studies have proposed employing optical elements,

AH2018, February 7–9, 2018, Seoul, Republic of Korea
Light from Scenery

Otao et al.

(Free-form)
Prism
TMD

HalfMirror
Light Source(a) Half Mirror-Based HMD(b) Prism-Based HMD(c) Waveguide-Based HMD(d) TMD-Based HMD(Our method)

Figure 2: Possible variations of OST imaging. (a) Half mirror see-through HMD. (b) Free-form prism see-through HMD. (c)Waveguide see-through HMD. (d) Proposed method using TMD instead conventional optical element.
such as transmitted liquid crystal display (LCD), half mirrors, freefrom optics, holographic optical element, and waveguide for neareye see-through displays. However, these optical elements havea trade-off relationship with viewing angle, eye box, luminance,
visual qualities, and distortion. Thus, the exploration of near-eye
optical elements is required in this area, due to the lack of perfect
optical elements.
The most common way of obtaining immersion is through combination of eyepiece and LCD. Almost all head-mounted displays(HMD) for virtual reality, such as Oculus Rift1 and Google CardBoard2 , employ an eyepiece to cover the peripheral vision. However,
it is very difficult to apply this method to see-through augmented
reality (AR) situations, because the eyepiece distorts the sceneryand the real environment view cannot be observed through theoptical components.
To solve this problem, we propose a novel optical see-through(OST) head-mounted display (HMD) method using Transmissive
Mirror Device (TMD). A TMD consists of micro-mirrors that canrender real images in the air by retroreflection. A dihedral corner reflector array (DCRA) is a general structure TMD (Figure 3).
There are other Nanoink printing TMDs. TMD satisfies the requirements, because see-through optical elements allow users to view
environment scenery and visual information at the same time.
In this paper, we propose a method to create the functionality ofan HMD in the air using TMD (Figure 1(a), (b)). Because the principle of TMD is retroreflective, the transferred image is represented
by a set of point-light sources. Therefore, the lens placed in the
TMD behaves as an aerial lens reproduced by the collection of pointlight sources. We call the aerial lens a “virtual lens.” By looking into
the virtual lens in the air, the user obtains the same experience aswith the virtual reality (VR) HMD without wearing goggles. We
introduce the detailed design methods of TMD-based HMD, insteadof half mirror-based HMD or convex mirror-based HMD. From asafety standpoint, it is also useful to provide a see-through andnon-wearable immersive experience with a wide viewing angle forvehicle mounting and medical applications. Additionally, it does not

1 https://www.oculus.com/rift/ (last accessed December 24th, 2017)
2 https://vr.google.com/cardboard/ (last accessed December 24th, 2017)

Image Source

Side View(Corner Reﬂector)

Top View(Transmissive)

TMD Plate

TransﬀeredReal Image

Figure 3: (a) Structure of Micro Dihedral Corner Reflector
Array employed as a TMD. (b) The TMD transfers the real
image to the plane’s symmetric position.
cause optical distortion and optical occlusion, because TMD consists of micro-mirrors. To understand the possibilities of designing
new XR optical systems, we explore the possibilities of TMD-based
HMD.
Our primary technical contributions are the following.
• We explore the possibilities of TMDs for near-eye see-throughdisplays. TMD renders real images in the air via retroreflection, which is often used for aerial imaging and aerialinteraction.
• We place the eyepiece functionality in the air by a set of pointlight sources. It provides a highly immersive experience, like
VR-HMD, and can also be adapted to existing VR content.
• We introduce the detailed design methods and implemented
a prototype of our near-eye displays. Then, we discussed
the advantages and limitations of the near-eye display using
TMD.

2 RELATED WORK
2.1 Optical See-Through Head-Mounted
Display
OST-HMD is a head-mount display that optically achieves a seethrough effect without using a camera. In recent years, OST-HMD
has shifted from conceptual research to the mass production device

Air Mounted Eyepiecel1

Half mirror

l1

AH2018, February 7–9, 2018, Seoul, Republic of Korea

d

Image

d

θ

Mirrored Image

l1

d

a

Eye

(a) Half mirror

TMD mirror
d2

d

1

a

l1

l1
θ1

θ1
θ2
Convex mirror(b) Convex mirror

l3

l2 d

d

l2
d4

θ2

Virtual lens(c) TMD mirror

Figure 4: Design method for near-eye display using various mirrors. (a) Design of half mirror, which has been used in the past.
(b) Design of convex mirror, which is currently used in conventional displays. (c) Design of TMD mirror.
market, and new models and applications have constantly beenreleased. The Figure 2 shows a comparison to the configuration ofa conventional see-through display. Transmissive LCD-based HMD
is simply implemented and inexpensive, although it restricts themagnification rate [15]. Maimone et al. realized wide viewing angle
with transparent LCD using a pinlight emitter [17]. The half mirror
see-through display has been proposed for a long time [11, 25](Figure 2 (a)). However, the half mirror-based HMD is large, with alimited viewing angle. Additionally, it reduces the light from thereal world. Convex mirrors and free-form optics improves the seethrough display viewing angle, compared to the half mirror, though
it is still popular [4, 5] (Figure 2 (b)). Moreover, consideration of
optical design is needed to cancel the real-world distortion. Holographic optical element [14, 16, 28] and waveguide [3, 21] allow us
to realize optical see-through with high resolution and compact
light weight (Figure 2 (c)).
[1, 2] refer us to another survey on the huge area of opticalsee-through near-eye displays for AR.
Although the lens is presented in front of the eye using a half
mirror or holographic optical element or waveguide, a truly wideviewing angle cannot be achieved, because the distance from theeye to the lens becomes longer. In other words, the virtual image
presented seems far away, because of the optical length. Since TMDbased HMD presents a virtual lens in front of the eye, the optical
path length is shorter, and the viewing angle remains wide (Figure 2
(d)).

2.2

Aerial Imaging System using TMD

There have been many studies for mid-air imaging with TMDs,
which are commercially available optical devices, composed ofmultiple corner reflectors [24, 27]. TMDs form a real image withretroreflection from the ray of an object. Thereby, a mid-air image
is viewable without using any special glasses.
In MARIO [10], a mid-air image freely moves in the depth rangeof 30 cm by moving the display. EnchanTable [26] is similar toan aerial image on the table surface expressing depth by moving
the display. TMD is also used in the field of volumetric display [6,7]. There is a Sunny Day Display [12], which is a study usingtransparent LCD and TMD. This method can display mid-air images
in illuminated spaces, such as in sunlight. Alternatively, Passive

Mid-air Display [13] makes it possible for users to see mid-air
images by using illumination.
A mid-air image with TMD is used in the field of interaction.
HaptoClone [18] is telepresence system. By using this system, bothpeople and objects can interact with the surrounding area. Thissystem consists of a pair of TMDs and an airborne ultrasound tactile
display. HaptoMine [19] is also an interaction system combining
TMD and ultrasound tactility.
In contrast to previous work, we explore the combination ofeyepiece interfaces and TMD. Light Field Blender [23], in which
the light field display combines lens array and TMD, is also TMDbased HMD. Alternatively, our goal is to describe the design andimplementation of the TMD-based HMD. This work is based on thecontribution of our previous work [22], and this is first an approachto implement a TMD-based HMD using an eyepiece.

3

DESIGN METHODThis section describes the design method of the near-eye display
using TMD. By using this method, a transmission-type aerial image
with a viewing angle wider than ordinary transparent HMDs canbe obtained, because the lens position can be brought closer to theeyeball. The key point is to involve the discrete-structured passive
optical elements in the design process of the visual display. It canbe explained using an LCD metaphor, which simulates continuous
scenery by employing discreet colored pixels. In this study, we
use the TMD of micro DCRA as the discrete-structured passive
optical elements. The light rays incident on the TMD are reflectedby micromirrors and passed to the opposite side. Although thisprinciple of operation is based on mirror reflections, the device isalso transmissive and deflects light.
When using an HMD as the light source, the HMD is placed at
the position where the single HMD lens is in focus with the TMD
eye position, such that they build an optical system that includes the
TMD. This allows the aerial imaging of the single lens to be placedin front of the eyeball. By looking into the aerial image presented
in front of the eyeball, the same effect as with an HMD can beobtained. Digital transformation is necessary, because the image
output through the aerial imaging of the single lens is inverted
vertically and horizontally.

AH2018, February 7–9, 2018, Seoul, Republic of Korea(a)Image Source

Half Mirror

Otao et al.

(b)

(c)
TMD
Physical Lens

Physical Lens

Light from
Scenery

Eye

Mirror Lens

Image formation

TMD and TMD See-ThroughHalf Mirror Optical See-ThroughVirtual Lens

Proposed Method (AME)

Figure 5: Optical ray tracing. (a) Simulation with half mirror optical see-through configuration provides a narrow viewing
angle. (b) Simulation with TMD see-though configuration is not suitable for near-eye display. (c) Simulation with proposed
method provides a wide viewing angle.

3.1

Viewing Angle

In figure 4, we note the parameters in the design process of the
AME and conventional methods frequently used in see-throughHMD design.
Half mirror-based HMD is presented in Figure 4 (a). A viewing
angle θ 1 of this setup is
l1
θ1
= tan
2 (a + d)
2(1)

HMD on the screen side. Note that the angle between l 3 and theeyeball must be larger than θ 2 .

3.2

TMD reproduces the function of a lens in the air using a set of point
light sources. Thus, the aerial resolution is defined by the numberof point light sources. The number of point light sources formed in
the air is given by
number of point light source =

l

1
©
ª
(a+d )
θ 1 = tan ­(2) 2 ®®
l1
1
−
(a+d
)
«
¬
where θ 1 is size of screen, a is the distance between half-mirror
and eyeball, and d is distance between half-mirror and screen. Additional lenses are sometimes inserted between the screen and thehalf mirror. However, it does not change the maximum viewing
angle, θ 1 .
To solve this problem, a convex mirror or prisms are used insteadof a half-mirror. We show this in Figure 4 (b). In this case, theviewing angle, θ 2 , is
θ2 = θ1 × a
(3)

−1 ­

where, a is the magnification ratio of a convex mirror. Note that if
a is higher, it cannot be used as a see-through type HMD, because
it distorts the see-through view.
Then, we introduce our methods by using the TMD in Figure 4(c). A view angle is
l 3 /d 2tan−1(θ 1 < θ 2 )

2

)
1−(l
/d
3
2


θ=
(4)




l 2 /d 4 tan−1
(θ
≥
θ
)
1
2

1−(l 2 /d 4 )2
where, l 2 is the size of lens, l 3 is the size of TMD, d 2 is the distance
between TMD and screen, d 4 is distance between the virtual lensand the eyeball. This method can easily be applied to self-designing
HMD in research prototyping, because the θ 2 is shown on thespecification sheet of HMDs, in many cases. So that it works as asee-through HMD, prototypers should put the HMD close to the

Resolution of Aerial Virtual Lens

l3
Tp

(5)

where Tp is the pitch size of TMD.
Therefore, by satisfying the following expression, it is possible
to hold an aerial lens with sufficient resolution.
l3
l1(e.g. resolution of LCD) 5
Lp
Tp
where Lp is the pixel pitch of LCD.

4(6)SIMULATION

To analyze the behavior of complex light rays, we simulate opticalray-tracing using Zemax OpticStudio3 . The TMD structure, withmicro DCRA, was reproduced with CAD. The pitch size of TMD is
0.3 mm. In Figure 5, we present the light ray behavior in the neareye optical system. The blue line represents optical rays comingfrom scenery light. Red, green, and yellow lines represent opticalrays emitted from the image source. The half mirror optical seethrough display provides a narrow viewing angle, because theoptical path length between the eye and mirrored lens is far. TMD
transfers the image source to the plane symmetry position. TMD
see-through configuration is not suitable for the near-eye opticalsystem, because the transferred image will be blurred. If an image
source is placed close to the TMD, we can observe the transferred
image source. However, the focal plane of the transferred imageis also close to the eye. In our proposed method, the Air Mounted
Eyepiece emulates the function of the HMD in the air. Thus, thevirtual lens is placed in front of the eye. This provides more comfort
3 https://www.zemax.com/opticstudio/ (last accessed December 24th, 2017)

Air Mounted Eyepiece

AH2018, February 7–9, 2018, Seoul, Republic of Korea

(a)

(b)

Eyepiece

Camera

LCD

TMD
Lens

LCD

Figure 6: Optical set-up for the experiment. (a) Configuration for taking the image through the lens. (b) Configuration
for taking the image through lens and TMD.

(a)

TMD

(b)

Figure 8: The fabricated prototype simply consists of an
LCD, an eyepiece, and a TMD.
Ghost Image

Figure 7: Comparison of viewing angles. (a) HoloLens. (b)Proposed system (AME).
for the near-eye displays with wider viewing angle than previous
see-through displays.

5

EXPERIMENT

We conduct an experiment to evaluate a near-eye display using
TMD. An experimental set-up, including LCD, lens, and TMD, areplaced on the optical plate (Figure 6). An Oculus Rift Development
Kit 2 (Oculus VR, LLC) HMD is used as an LCD and lens. We display
a checker pattern, 3D model, text, and existing VR content on the
LCD. The camera (i.e., iPhone 6, Apple Inc.) is placed at the diagonal
position. The shutter speed of a camera is 1/80 sec, ISO sensitivity
is 320, and focal length is 4.15 mm.
In Figure 9, we show results of the experiments. The leftmost
column is the image displayed on the LCD. The second column
is the image taken through the lens. Images of the right three are
obtained images of lens and TMD, whose pitch size are 0.2 mm,
0.3 mm, and 0.5 mm, respectively.
As can be seen from the experiment, TMD reduces the brightnessof the image. However, sufficient visual quality is obtained. Also,
the resolution becomes higher when the pitch size of the TMD issmall.
Then, we place the hololens in the same position as the TMD
optics. To compare the viewing angles to TMD optics, we takepictures under the same conditions. The result is shown in Figure 7.
Compared to HoloLens, it is confirmed that the viewing angle isvery wide. However, double-reflection ghost images caused by TMD
appear.

6

IMPLEMENTATION

We describe the design method above with regards to the conceptof the optical element of the TMD before and during the aerial

imaging of the eye. We implement the prototype OST-HMD basedon that concept, (Figure 8). An Oculus Rift Development Kit 2
is used as the HMD in the prototype. The resolution of LCD is1134 × 750 (367 × 750 per eye) and the size of LCD was 125 mm ×
71 mm. We remove the LCD and the lens from the HMD, and set it in
a 3D printed frame. A polarizing filter is used to prevent narrowingof the visual field because of the secondarily reflected light. The
TMD size of 140 mm × 116 mm and pitch size of 0.3 mm is adopted.
The weight of HMD is 403 д.
We calculate the viewing angle and resolution of the aerial image.
From Eq. (4), by substituting l 3 = 116 and d 2 = 30. The maximum
field of view is less than θ = 146◦ . It satisfies the viewing angleof Oculus Rift Development Kit 2 (i.e., 110 degree). Therefore aviewing angle of 110 degrees is obtained. From Eq. (5), the aerial
image resolution is 467 × 387 in this prototype.
This type does not require any special software to render image
sources, so that existing VR content can be applied. The obtained
image in this prototype is shown in Figure 1 (c).

7 DISCUSSION
7.1 Optical Selectivity
Since TMD is essentially a discrete mirror, it does not cause chromatic aberration, although it is difficult to diffuse or absorb light.
This is an advantage over lenses and HOE. Compared to the half
mirror, the viewing angle increases because the optical path length
becomes short. However, the resolution decreases since the mirror is discrete, although sufficient resolution is observed in theexperiment. Additionally, as the resolution is increased, the mirror
functions as a diffraction grating. With respect to the finer structure,
it is difficult to form imaging and projection systems.
As shown by the simulation result of the Figure 5 (c), a TMD
with micro DCRA reduces the luminance of the image source andbrightness from the scenery. We need to make the light source
stronger.
As an optical element that functions by forming an image in theair, it is only possible to change the light emitting position and the
input position of a light field having a function of changing input /AH2018, February 7–9, 2018, Seoul, Republic of Korea

Otao et al.

(a) Checker pattern(b) 3D model(c) Text(d) Existing VR content

Image Source

Obtained Image
through lensObtained Image
with 0.2 mm

Obtained Image
with 0.3 mm

Obtained Image
with 0.5 mm

Figure 9: Experiment to measure visual quality, luminance, and resolution. (a) Checker pattern. (b) 3D Model. (c) Text. (d)Existing VR content.
output relationship with respect to the light field. However, caution
is required when the device is used for applications, such as theultrashort pulse laser used in Lasik surgery.

7.2

method perfectly suppresses stray light, or another trade-off occurs.
Thus, a breakthrough solution is necessary.

An Undesired Image by Double Reflection

The TMD causes double reflection ghost images to appear diagonally (Figure 7 (b)). This is undesirable because it shows an unintended image to the user. To cope this problem, several solutions
are conceivable. First, a polarizing filter can remove unwanted polarized light. It is also conceivable to apply a frequency filter thatpasses or cuts a specific wavelength. Alternatively, one could installof a pinhole and align the wavefront. However, with the method of
installing the pinhole, the luminance remarkably changes. Either

7.3

TMD Selectivity

It is known that corner cube TMDs are available on the market. In
this research, we apply our process to an easily obtainable TMD.
It is easy to perform simulation calculations for this device, andthe same method can be applied to Nanoink printing TMDs. Thereis a possibility that the above problems can be solved by adopting
different structures, such as the Nanoink printing TMD.

Air Mounted Eyepiece

8 FUTURE WORK
8.1 Gaze Tracking and Head Tracking
One of the biggest advantages of our near-eye display is gazetracking [9, 20]. Because the eyeball image is transferred to the
HMD, gaze-tracking is possible without disturbing the user’s lineof sight in wide-view retinal observation. In this paper, we only
describe a design method for rendering a virtual lens in the air.
We will attempt development of an air-mounted eyepiece withgaze-tracking in future work.
Furthermore, because our HMD can render images in the air,
A non-wearable HMD is achieved, and an immersive weightlessenvironment can be realized. In this situation, head-tracking would
be enhanced by non-wearable HMD. This development will be
useful and safe for vehicle-mounting and medical applications.

8.2

Using TMD for Other Near-Eye Display

In this research, we discuss a combination of eyepiece and TMD
to obtain a wide viewing angle. However, using TMD for near-eye
display remains a possibility. For example, Huang et al. presents
a light field stereoscope, which stacks LCDs [8]. It provides lightfield retinal blur, but not optical see-through. Because TMD can
extend such a non-see-through display to a see-through display, it
is possible to adopt optics other than the eyepiece.

9

CONCLUSION

We proposed a novel HMD design that functions as an aerial virtual
optical system in front of the eye by using TMDs. This is one of thefirst challenges of TMD, which is usually used in aerial real imaging
systems, for a near-eye display. We showed a detailed design method
for our near-eye display, enabling a wide field-of-view. Throughsimulation and experiment, we discussed the possibility of our
HMDs. We are now studying for further detail of viewing angle
considering the influence of ghost image.

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