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Formation of multiple aerial LED signs in multiple lanes formed with AIRR by use of two beam splitters

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We propose multiple aerial imaging system by use of two beam splitters in the optical system of aerial imaging by retro-reflection (AIRR). The AIRR optical system consists of a light source, a beam splitter, and a retro-reflector. Use of two-layered beam splitters enables us to show multiple aerial signs to drivers in multiple driving lanes. The purpose of this paper is to confirm feasibility that an aerial display by use of AIRR for a novel traffic information provision. First, we explain an optical configuration to show aerial images to multiple road lanes, a driving lane and an overtaking lane with AIRR by use of two beam splitters. Next, we have developed a prototype optical system by use of two large beam splitters and a large high-brightness LED sign that is designed to be used for actual road displays. The light source was an LED display with a height of 450 mm and a width of 390 mm, which shows a character. We observed aerial images by naked eyes as experiments. And we discuss the issues for practical application that the experiments have revealed.
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Optical Review
https://doi.org/10.1007/s10043-022-00771-y
SPECIAL SECTION: REGULAR PAPER
Formation ofmultiple aerial LED signs inmultiple lanes formed
withAIRR byuse oftwo beam splitters
ShinyaSakane1,2· DaikiKudo2· NaoyaMukojima2· MasakiYasugi2 · ShiroSuyama2· HirotsuguYamamoto2
Received: 30 June 2022 / Accepted: 29 September 2022
© The Author(s) 2022
Abstract
We propose multiple aerial imaging system by use of two beam splitters in the optical system of aerial imaging by retro-
reflection (AIRR). The AIRR optical system consists of a light source, a beam splitter, and a retro-reflector. Use of two-
layered beam splitters enables us to show multiple aerial signs to drivers in multiple driving lanes. The purpose of this paper
is to confirm feasibility that an aerial display by use of AIRR for a novel traffic information provision. First, we explain an
optical configuration to show aerial images to multiple road lanes, a driving lane and an overtaking lane with AIRR by use of
two beam splitters. Next, we have developed a prototype optical system by use of two large beam splitters and a large high-
brightness LED sign that is designed to be used for actual road displays. The light source was an LED display with a height
of 450mm and a width of 390mm, which shows a character. We observed aerial images by naked eyes as experiments. And
we discuss the issues for practical application that the experiments have revealed.
Keywords Retro-reflection· Aerial display· Multiple image formation· Transport infrastructure
1 Introduction
Traffic information display is variable display devices
installed on roads to provide information on roads. As road
signs, they helped prevent traffic accidents [1]. There are sev-
eral issues with these traffic information displays. The first,
the risk of falling accidents. As is true of all road structures,
there are the danger of falling accidents due to aging [2].
Traffic infrastructure has been changing to prevent accidents
[3]. Road manager do not want to place heavy electrical
equipment overhead for driver's safety reasons. The second
is the evolution of technology. Self-driving technology is
being developed [4], and cities are becoming “smart city” [5,
6]. In addition, "flying cars" are currently being researched
[7, 8], and will be put to practical use in the near future.
At that time, traffic information display installed overhead
on roads will obstruct the path of vehicles. It is predicted
that the method of providing information will change in the
future. Therefore, traffic information display must change
in future. We propose a new information provision technol-
ogy for transportation infrastructure based on aerial imaging
technology. This technology enables the display of images
in the air by imaging image rays on an empty space. An
advantage of this method is that it can be displayed in front
of the driver's eyes without physical contact with the moving
vehicle. We consider that this would allow information to be
provided without obstructing the path of the flying car. The
other is that only the display can be seen on the road, without
the display unit overhead. This will eliminate the risk of fall-
ing accidents. Several methods have been proposed for aerial
imaging technology [9, 10]. Among them, we decide aerial
imaging by retro-reflection (AIRR) [11]. The advantage of
AIRR is that it has excellent scalability for larger screens.
Furthermore, the AIRR aerial display has wide horizontal
and vertical viewing angles, making it suitable for trans-
portation infrastructure applications in that multiple users
can view the aerial image simultaneously. Moreover, the
AIRR can improve brightness using by polarization [12].
Using the AIRR, we have developed a secure aerial display
with a three-layered screen [13], an aerial display with depth
Laser Display andLighting Conference (LDC’ 22), Yokohama, Japan
* Shinya Sakane
SAKANE_sinya@seiwa.co.jp
Hirotsugu Yamamoto
hirotsugu@yamamotolab.science
1 Seiwa Electric Mfg. Co., Ltd., Joyo, Japan
2 Utsunomiya University, Utsunomiya, Japan
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combined with Depth-Fused 3D [14, 15], a walk-through
aerial display in three different directions [16], an immer-
sive aerial display surrounding the user [17], the AIRR
thinner by combining laminated mirror optics [18], and a
novel steganography method to show an aerial image that is
formed with AIRR by use of dual transparent balls made of
acrylic resin [19]. Various applied technologies for AIRR
are developed in this way. But there is still no aerial display
of a high-brightness, large-screen LED sign used in actual
transportation infrastructure.
In this paper, we propose multiple aerial imaging system
by use of two beam splitters as one of the applications of
AIRR to display on multiple lanes. AIRR is composed of
three elements: a light source, a beam splitter, and a ret-
roreflective element. In this technology, two beam splitters
are installed in parallel and multiple reflections are made
between the beam splitters to form multiple aerial images
from a single light source. This allows drivers in different
lanes to see the same display in the same aerial image. This
paper reports on the development of a prototype for an actual
road display. A large-sized aerial LED sign display is fab-
ricated to confirm the actual multiplexing of aerial images
and imaginary images, and to discuss issues for its practical
application.
2 Principles offorming multiple aerial
images withAIRR byuse oftwo beam
splitters
The principle of AIRR is shown Fig.1. The conventional
AIRR consists of three elements: a light source, a retro-
reflector, and a beam splitter. The light emitted from the
light source is divided into transmitted light and reflected
light by the beam splitter, and the light reflected by the beam
splitter is reflected in a direction along the incident direction
by the retro-reflector. The retro-reflected light converges to
the position that is plane-symmetrical of the light source
with respect to the beam splitter.
In this paper, two beam splitters are used as the beam
splitter, as shown in Fig.2. The beam splitters are installed
in parallel. The aim is to multiple reflect light between the
beam splitters to form a number of aerial images [18]. The
first aerial image (i) is formed by the reflected light of the
beam splitter 1. The second aerial image (ii) is formed by the
reflected light of the beam splitter 2. The light ray that return
to the beam splitter 2 are retro-reflected and formed as an
aerial image (iii). The mirrored images (virtual images) are
also formed by the beam splitters. (a), (b) and (c) in Fig.3
are the imaginary images created by the beam splitter1 and
2, respectively. Note that multiple aerial (real) images and
mirrored (virtual) images are formed after multiple reflec-
tions between the beam splitters.
Next, we propose an optical configuration to show aerial
images to multiple road lanes, a driving lane and an overtak-
ing lane, as shown in Fig.4. We design that (i) and (iii) are
visible in separate lanes, and (ii) is formed exactly between
the lanes. The major advantages of providing traffic infor-
mation by aerial display are that there is no physical contact
and the viewing time is extended by displaying the informa-
tion in front of the driver's eyes. To avoid contact between
the cars and the display, the proposed method is set up in
a location outside of the road, and only the aerial images
are displayed on the road. Next, we explain the vanishing
distance and make a numerical comparison between the con-
ventional method and the proposed method. The purpose of
the comparison is to discuss the advantages of having it in
front without any physical contact. The vanishing distance
is the distance that a driver cannot see the sign in the course
of driving. If the vanishing distance is shorter, the driver will
Fig. 1 Principle of AIRR (Aerial Imaging by Retro-Reflection)
Fig. 2 Principle of multiple aerial images formed with AIRR by use
of two beam splitters. (i), (ii) and (iii) denote the aerial images
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have more time to see the display. In other words, more time
for the driver to recognize the display. First, we consider the
vanishing distance for conventional traffic information dis-
play. Figure5 shows specifications of the display for instal-
lation over the road head [20]. If the display is located as
shown in the figure, the vanishing distance of traffic informa-
tion display L can be determined by the following equation.
(1)
L
=
h
1
tan𝛼
,
where h1 is the distance from the driver's eye line to the out-
ermost edge of the display. h0 is the driver's eye level. It is
determined to be 1.2m. The vanishing point is the position
where the display is no longer visible. α is the angle between
the direction line at the vanishing point and the outermost
line of the display. For overhead display, the angle is deter-
mined as α = 7 degrees. The standard installation height of
the display is 5.0m. If h1 = 3.8m, the vanishing distance
L is 30.9m according to Eq.(1). The vertical length of the
display at this time assumes 0.5m.
Next, we consider the vanishing distance for aerial image
with AIRR. If the aerial image is formed as shown in the
Fig.6, the vanishing distance of aerial image LA can be
determined by the following equation.
where S is vertical length of aerial image, θi is angle of
light ray. When the vertical length of the beam splitter
and the retro-reflector are enough, subscript i is 1, and are
not enough, subscript i is 2. For example, if S = 0.5m and
θ1 = 7.5 degrees, the vanishing distance LA is 1.9m accord-
ing to Eq.(2). Calculation of Eq.(1) gave the vanishing
distance of the traffic information display as L = 30.9m.
Results of calculations, the vanishing distance of aerial
image will be shorter than traffic information display, as
shown in Fig.5. If both displays were visible from 100m
away, the visibility distance would be longer for the aerial
display. Thus, driver visibility time increases as vanishing
distance decreases. This is the advantage of aerial image. We
also discuss the enough vertical length of the retro-reflector
to meet the condition for Fig.6. The length depends on the
(2)
L
A=
S
2tan𝜃
i
,
Fig. 3 Principle of multiple vertical images formed with AIRR by use
of two beam splitters. a, b and c show the virtual images
Fig. 4 Forming multiple aerial signs imaging from a single light
source for two driving lanes. (i), (ii), and (iii) denote the aerial images
Fig. 5 Vanishing distance for traffic information display
Fig. 6 Vanishing distance for aerial image with AIRR
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distance of the light ray. Light emitted from the light source
is first reflected by the beam splitter. At this time, the length
of the beam splitter SBS required is as shown in the follow-
ing equation.
where LBS is distance between light source and beam splitter.
Due to the principle of AIRR, the position of aerial image
is plane-symmetrical of the light source with respect to the
beam splitter. Therefore, LBS is also distance between aerial
image and beam splitter. The reflected light by beam split-
ter is reflected by retro-reflector. The length of the retro-
reflector SRR required is as shown in the following equation.
where LRR is distance between beam splitter and retro-
reflector. Thus, we need to prepare the beam splitter more
than length SBS and the retro-reflector more than length SRR
to satisfy Eq.(2). Also, if sufficient length is available, the
Eq.(2) holds true even for multiple image formation. For
example, if S = 0.5 m, θ1 = 7.5 degrees, LBS = 1.4 m and
LRR = 1.2m, the length of the beam splitter SBS is about
0.87m according to Eq.(3), and the length of the retro-
reflector SRR is about 1.18m according to Eq.(4). Even if
the length is sufficient, if the installation position of each
component is misaligned with the axis of the light source,
the appearance of the aerial image changes. Therefore, as
shown in Fig.6, the height of each component must be
aligned. Next, we discuss when the length of the retro-
reflector SRR is not enough. In this case, θi = θ2. θ2 can be
determined by the following equation.
where S’RR is the length of the retro-reflector not enough.
The gap between S'RR and the length SRR needed to show the
whole is Δx (= SRR–S'RR). This allows us to think of θ2 as a
function of θ1. Figure7 is a graph comparing the case with
and without sufficient length. The graph is considered with
S = 0.5m, LBS = 1.4m and LRR = 1.2m. When the length
is not long enough, the vanishing distance required to see
the whole image is greater. But, as the angle of the light
source increases, the gap in vanishing distance disappears.
The approach to vanishing distance in the horizontal direc-
tion can be the same as in the vertical direction. In addition,
we discuss the visibility area of the aerial image. Figure8 is
a picture of an aerial image forming in the center of a road.
The length of the beam splitter and retro-reflector here is
assumed to be sufficient for aerial image formation. To see
the entire aerial image from any position in the lane, the light
rays at the edge of the aerial image have to reach the edge of
(3)
SBS =S+2LBStan𝜃1,
(4)
S
RR =S+2
(
LBS +LRR
)
tan𝜃
1,
(5)
𝜃
2=tan1
(
S
RR S
2(L
BS
+L
RR
)
),
the lane, as shown in the Fig.8. The required distance Lv for
visibility can be obtained by the following equation.
where W is the length of the lane width. The road width is
basically designed at 3.5m per lane. For example, if θ1 = 7.5
degrees and LA = 1.9m, Lv is 15.2m according to Eq.(6).
The aerial image can be seen from any position on the lane
at or above this distance.
3 Experiments byuse alarge LED sign
We experimented with a prototype of a large aerial LED
sign display based on an actual roadway. The purpose of
the experiment was to confirm that multiple images could
be formed, as shown Fig.4. Figures9 and 10 show the
configuration of our developed prototype system by use
(6)
L
v=LA+
W
2tan𝜃
1
,
Fig. 7 Relationship between angle of light source and vanishing dis-
tance according to Eq.(2)
Fig. 8 Relationship between viewing angle of aerial image and lane
width
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of a large LED sign. For the light source, we used an LED
display unit that shows the Kanji "stop" with a height of
450mm × a width of 390mm (Fig.11). This is the text
size used on actual roads. The used LED was 5mm round
and red (OptoSupply: OS5RKA5111A). LED half-value
angle was 15 degrees (θ1 = 7.5 degrees). The LEDs were
mounted at a pitch of 10mm horizontally and vertically,
in accordance with the standard for road signs. The center
height was set at 1.2m, which is the driver's eye level. The
beam splitter was made of 900mm × 1800mm tempered
glass with a film (38% transmittance, 46% reflectance) on
one side to increase the reflectance. For the beam splitter,
a holder as shown in Fig.12 was fabricated, and casters
were attached for easy mobility. The retro-reflector was
used 1400mm × 1200 mm RF-AN. The retro-reflector
also was attached to the base with holes drilled where the
LEDs were placed. The angle of retro-reflector is shifted to
reduce surface reflections [21]. This is to avoid the appear-
ance of ghost images caused by surface reflections. The
formation positions for respective aerial image are: (i) is
about 1.4m from the beam splitter1, (ii) is about 2.2m
from the beam splitter 2, and (iii) is about 3.8m from the
beam splitter 2. As experiment method, we observed aerial
images by naked eyes moving between point A and B.
Point A is on the angle of the flat object to the beam split-
ter. Point B is on the angle of the front of the light source.
The distance between aerial images is about 0.4m. The
road width is basically designed at 3.5m per lane. This
distance should be 1.75m when forming multiple lanes.
However, this experiment did not provide the ideal display
on multiple lanes due to lack of length for the beam splitter
Fig. 9 Configuration of our prototype multiple aerial LED signs. (i),
(ii), and (iii) denote the aerial images. a and b denote the two virtual
images. A and B denote observation positions
Fig. 10 Cross-sectional view of prototype multiple aerial LED signs
Fig. 11 LED display used for the prototype. The left figure shows the
LED display when turned on, and the right figure shows the LED dis-
play when turned off
Fig. 12 Holder of beam splitter
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and retro-reflector, as shown in Fig.4. This experiment is
about a quarter of the actual conditions.
The experiment showed that formation of multiple real
and virtual images were confirmed at plural viewing posi-
tions. Figure13 shows the aerial images viewed at point
A. As a result, three aerial images formed in a different
position could be confirmed with naked eyes. Figure14
shows images viewed at point B. One aerial image and
two virtual images were clearly visible with naked eyes.
In this case, the overlap of the light source, the imaginary
image, and the aerial image formed a depth perception,
and the appearance of the aerial image seemed to be more
three-dimensional. We discuss the vanishing distance for
this experiment. This distance can be thought of as the dis-
tance from the light source to the beam splitter (LBS) when
considering the vanishing distance of each aerial image.
The distance between the beam splitter and retro-reflector
(LRR) for each aerial image was as follows (i) 1.2m, (ii)
and (iii) 2.0m. For each aerial image, the length of the
beam splitter and retro-reflector was checked for suffi-
ciency using Eqs. (3 and 4). As a result, the length was suf-
ficient only for aerial image (i). The vanishing distance for
aerial image (i) is 1.7m according to Eq.(2) (S = 0.45m,
θ1 = 7.5 degrees). Figure15 is a photograph of the aerial
image before and after the vanishing point. Beyond the
vanishing point, the entire aerial image is obscured. For
(ii) and (ii), the vanishing distance was more than 1.7m
due to a lack of a beam splitter and retro-reflector length.
4 Theoretical model development
We consider a theoretical model for light distribution in
aerial image. To study this, we first examined the angular
dependence of the retro-reflectance. Experimental setups to
measure the angle dependence of retro-reflector are shown in
Fig.16. A collimated laser diode (LD) was used as the light
source. The retro-reflector was used RF-AN. Align the LD,
beam splitter, and retro-reflector in line. The retro-reflector
is rotated and illuminance of the aerial image is measured
with illuminance meter. The experimental results are shown
in Fig.17. The light distribution of the aerial image can
be obtained from this result and the light distribution of
the light source. Figure18 is a graph comparing the half-
value angle of the light source, LED, at 15 degrees and at
60 degrees. We use the estimated light source distribution.
The results show that when the light distribution angle of
the light source is narrow, it matches the light distribution
of the aerial image, but when it is wide, the light distribution
of the aerial image is narrower than that of the light source.
When considering the vanishing distance of an aerial image,
Fig. 13 Aerial image with a large device when the viewing position is
point A. As a result, three aerial images which (i), (ii), and (iii) could
be confirmed by the naked eye
Fig. 14 Aerial image with a large device when the viewing position is
point B. As a result, one aerial image and two virtual images could be
confirmed by the naked eye
Fig. 15 The aerial image before and after the vanishing point. The
left figure is the aerial image at about 2.2m from the formation posi-
tion, and the right figure is the aerial image at about 1.6m from the
formation position
Fig. 16 Experimental setups to measure the angle dependence of
retro-reflector
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the light distribution angle of the light source can be used
in the case of a pincer angle, but in the case of a wide angle,
it is necessary to consider the light distribution of the aerial
image.
5 Discussion
Aerial display by use of AIRR can provide traffic infor-
mation. We discuss the issues that the experiments have
revealed. The first issue is that improvement of brightness
and sharpness for outdoor use. Brightness and sharpness
are important factors in display perception. We will discuss
brightness first. Conventional traffic information displays
have a brightness index to convey information to drivers
at a distance [22]. The brightness standard is defined for
each color. In the case of red, the luminance is 1600cd/
m2. These standards are set so that drivers can see them
from a distance of about 100m. To use an aerial image as
a road sign, the luminance must be 1600cd/m2 or higher.
Currently, we estimate that only the aerial image (i) meets
this standard. We also consider the luminance contrast with
the background to be an important factor. Conventional traf-
fic information displays show text on a black surface. In
aerial images, the background is other. It is possible that
changes in these conditions make it harder to perceive light.
Therefore, we consider luminance more than the standard
value will be required. The reason for the lack of bright-
ness is the low amount of light used as an aerial image. The
amount of light used for the aerial image is considered to be
less than half that of the light source. In particular, in the
case of this optical system, the transmitted light divided by
the beam splitter is not used for the aerial image at all. It is
necessary to design an optical system and equipment that
can make good use of this wasted portion of light. Next, we
discuss improvement of sharpness. The aerial image was
generally blurred. We considered that this is due to the fact
that the light rays are not fully converged at the image forma-
tion position. In previous AIRR experiments, a decrease in
sharpness due to the spread of retro-reflector light has been
reported. The main cause of this is diffraction at the retro-
reflector, and the development of a retroreflective element
that reduces diffraction is one of the issues to be addressed.
The second issue is that Formation of aerial images at
long floating distances. When the three aerial images were
compared, the third aerial image was the darkest and blurred.
Due to the principle of this method, the later the aerial
image is formed, the longer the floating distance of the ray
becomes. This suggests that the brightness and sharpness of
the image may be degraded by the floating distance of the
ray. In fact, it was confirmed that the clarity was improved by
moving the light source and retro-reflector closer to the beam
splitter. However, the position where the aerial image of the
AIRR is formed is symmetrical in the plane with respect to
the beam splitter. For practical use as a traffic information
display, the system must be levitated over a longer distance
Fig. 17 Relationship between incident angle to retro-reflector and
illuminance intensity of aerial image
Fig. 18 Comparison of light source LED and aerial image light distribution. The left figure shows LED light distribution of 15 degrees, the right
figure shows LED light distribution of 60 degrees
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than this prototype. The most important issue is to form a
highly visible aerial image with a long floating distance,
which is included by our future works. We believe that if
conditions such as brightness, etc. are met, it is possible to
use aerial image (iii) or more images. Some conditions may
cause unwanted aerial images to be shown. It is possible to
eliminate this problem by changing the conditions of the
components such as retro-reflector and installation angles.
In this study, for simplicity, we performed simulation under
the condition that there are no other cars in front. However,
it is possible that the distance between cars on highways
becomes narrower during congestion and formed aerial dis-
play may be hidden by preceding cars. We will consider
aerial display on congested roads as a future work.
6 Conclusion
We have succeeded in forming multiple aerial images from
a single light source by introducing two beam splitters in
AIRR. We have realized a prototype aerial display by use
of a large LED sign which can be used for the actual road-
way. Therefore, we confirmed Aerial display by use of AIRR
can provide traffic information. Multiple aerial and virtual
signs that draw attention of viewers, i.e., drivers in practical
installment, were presented for multiple viewing positions.
Authors' contributions S.S. contributed for this paper as 1st author.
They designed and conducted the experiments, analyzed the data and
wrote the original draft. D.K and N.M. analyzed the data and edited
the manuscript. M.Y, S.S and H.Y designed the experiments and edited
the manuscript.
Declarations
Conflict of interests The authors declare no conflicts of interest associ-
ated with this manuscript.
Open Access This article is licensed under a Creative Commons Attri-
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