8.4: Distinguished Paper: A Full Color Eyewear Display using ... - Sony

8.4 / H. Mukawa
8.4: Distinguished Paper: A Full Color Eyewear Display using Holographic
Planar Waveguides
Hiroshi Mukawa, Katsuyuki Akutsu, Ikuo Matsumura, Satoshi Nakano,
Takuji Yoshida, Mieko Kuwahara, Kazuma Aiki, Masataka Ogawa
Information Technology Laboratories, Sony Corporation, Tokyo, Japan
Abstract
A full color eyewear display with over 85% see-through
transmittance and 2500cd/m 2 brightness was developed. Very low
color crosstalk, less than 0.008 Δu’v’ uniformity and 120% NTSC
color gamut were achieved. Waveguides with two in- and outcoupling
hologram elements enabled simple configuration with
side-mounted microdisplays.
1. Introduction
An eyewear display comes to be an important device along with
two technological environment changes, i.e. (1) widespread use of
mobile video contents such as digital broadcasts and video
download services via a cellular phone network or wireless
internet, (2) miniaturization of mobile video devices such as MP4
players and mobile TV devices whose weights and sizes are
essentially determined by the size of those displays. An eyewear
display has mainly three advantages beyond a direct-viewing
mobile display; (1) hands-free, (2) high privacy nature and (3)
larger screen size. Among several proposed eyewear displays [1,
2], those with see-through capability [3] are especially important
for mobile uses since they are safer and also easier to be accepted
in public. It was, however, challenging to configure compact and
high quality image viewing eyewear displays with clear seethrough
capabilities.
Using a volume hologram or grating as an optical combiner in
front of the eyes on a waveguide is a possible approach [4, 5, 6],
since it could also minimize the size of optics. On the other hand,
it shows a significant color dispersion and Bragg wavelength
dependency on an incident angle from a light source, and, in
general, causes color crosstalks and uniformity degradation when
transmitting full-color images through the waveguide. In the
paper, we propose a double-waveguide structure using reflection
volume holograms to eliminate the color crosstalks. To achieve
adequate color uniformity for displaying full color images, we
designed an optimum incident angle of image rays on the
hologram. Further, to fine tune the uniformity, a drive signal
management scheme was employed on a microdisplay.
2. Image quality improvement
2.1 Elimination of color crosstalk
Figure 1 illustrates a basic structure of holographic planar
waveguide of the eyewear display. The waveguide has an incoupling
and an out-coupling reflection volume holograms, those
having exactly the same fringe pattern and are placed mirror
symmetrically. We employed reflection volume holograms since
their diffraction bandwidths are much smaller than those of
transmission holograms and could potentially eliminate color
crosstalk. Each of these holograms has three, Red, Green and
Blue (R, G and B), hologram layers to transmit full color images
through the waveguide.
However, with the structure shown in figure 1, R, G and B color
rays would potentially be diffracted by different color hologram
layers between the in-coupling and out-coupling holograms. As
grating periods of R, G and B hologram layers are different from
each other, those combinations would cause ghost images. For
example, the B ray diffracted by the out-coupling G hologram and
the G ray diffracted by the out-coupling B hologram would
deviate from intended directions as shown in figure 1.
Out-coupling hologram
R
B
ISSN/008-0966X/08/3901-0089-$1.00 © 2008 SID
G
Collimator
Crosstalk between B and G
(Ghost image) Optical engine
Eye
Microdisplay
In-coupling hologram
Figure 1: A basic structure of the holographic planar
waveguide having reflection volume holograms
Diffraction efficiency characteristics of the in-coupling hologram
with three R, G and B layers are shown in figure 2. The
rectangles of white line indicate the input R, G and B LED spectra
from a collimator. The black line quadrangles indicate the spectral
ranges coupled into the waveguide. Figure 3 shows the diffraction
efficiency of the out-coupling hologram. Black line quadrangles
indicate the R, G and B spectra diffracted by the in-coupling
hologram and reached to the out-coupling hologram. Note that
the spectra diffracted by the in-coupling hologram (quadrangles in
figure 3) become broader compared to the ones before diffracted
by the in-coupling hologram (quadrangles in figure 2) due to color
dispersions of the holographic diffractions.
Figure 4 shows calculated diffraction efficiencies of the outcoupling
B and R hologram layers. The spectrum diffracted by the
in-coupling G hologram layer interferes with the diffraction
bandwidths of the out-coupling B and R hologram layers and has
approximately 1.5% efficiencies. Consequently, the extra
diffractions of B and R spectra by the G hologram cause color
crosstalks and ghost images. To eliminate the crosstalks, we
separated the waveguide into two, one for G and the other for B
and R. The color crosstalks between the B spectrum and the R
hologram layer or the R spectrum and the B hologram layer were
less than 0.2% as shown in figure 4.
B
SID 08 DIGEST • 89
R
G
B
R
G

8.4 / H. Mukawa
Wavelength (μm)
Figure 2: Diffraction efficiencies of R, G and B hologram
layers of the in-coupling hologram. The thicknesses and index
modulations of each hologram layer were approximately 5μm
and 0. 04 respectively.
Wavelength Wavelength (μm) (μm)
Input blue spectrum
A
Coupled in red spectrum
Input green spectrum
Coupled in blue spectrum
Coupled in green spectrum
A
Input red spectrum
Incident angle to the in-coupling hologram (degree)
Incident angle to the out-coupling hologram (degree)
Figure 3: Diffraction efficiencies of R, G and B hologram
layers of the out-coupling hologram. The thicknesses and
index modulations of each hologram layer were
approximately 2μm and 0.035 respectively.
2.2 Improvement of color uniformity
Another issue to achieve high color image quality is the
uniformity through out the field of view (FOV). A Bragg
diffraction wavelength (Bragg λ) varies by an incident angle of a
ray to a hologram. Therefore, each R, G and B color spectrum
profile changes depending on the specific angle (the field viewing
angle) within the FOV. The peak wavelengths of R, G and B
spectra become shorter at the in-coupling hologram side of the
field. To reduce the spectrum shift within the entire FOV, we
employed two approaches. One is to have oblique in and out
optical axes layout on the waveguide and the other is to employ a
drive signal management of the microdisplay.
90 • SID 08 DIGEST
Diffraction efficiency (%)
Diffraction efficiency (%)
Diffraction efficiency (%)
0.2%
Blue diffraction
efficiency
Blue spectrum Green spectrum Red spectrum
Wavelength (μm)
Red diffraction
efficiency
B spectrum diffracted
by the R hologram
G spectrum diffracted
by the B hologram
G spectrum diffracted
by the R hologram
R spectrum diffracted
by the B hologram
Figure 4: Diffraction efficiencies of the out-coupling B and R
hologram layers at an incident angle of -50°. (A cross section
A-A in Figure. 3) The incident R, G and B spectral ranges
on the out-coupling hologram are also shown.
2.2.1 Oblique optical axis layout on the incoupling
hologram
Amount of a Bragg λ shift (the difference between the Bragg
wavelengths at both ends of a given FOV) depends on a designed
light incident angle on a hologram. When a ray C enters the
hologram, a Bragg wavelength λc is given by the Bragg condition,
λc = 2 n Λ Sin θB (1)
θB is an angle between the ray C and a fringe of the hologram,
Λ is a fringe period and n is an index of the hologram as shown in
figure 5. A λc change by θB could be obtained by differentiating
the equation (1) to derive equation (2).
δλc/δθB = 2 n Λ Cos θB (2)
Bragg condition also satisfies the following equation (3).
θB = (θs –θr)/2 (3)
θr is an incident angle on a hologram and θs is a diffraction angle
of the ray C.
λC
C
λRGB
θB
Λ
θr
θs
n
Figure 5: A geometry of the Bragg condition

From the equation (2) and (3), the δλc/δθ B could be minimized
when θs –θr was close to 180°. It indicates that the Bragg λ shift
could be reduced if the incident ray on the in-coupling hologram
was slanted towards diffracted rays. Consequently, we slanted
the optical axis of the microdisplay and collimator (the optical
engine) by 10° against the in-coupling hologram, as shown in
figure 6.
Out-coupling hologram
R
B
G
R
B
G
Collimator
Optical engine
Eye Microdisplay
Figure 6: An oblique layout between the waveguide and
the optical engine
As a result, the Bragg λ disparity between both ends of the FOV
reduced from 45nm to 37nm and the color gamut improved from
76% to 120% compared to the NTSC standard as shown in figure
7. The improved color gamut was confirmed within the common
area of color reproduction areas for -8° through +8° field viewing
angle in an u’v’ diagram. We used R, G and B LEDs to illuminate
the microdisplay and each of the peak wavelengths was 640nm,
525nm and 450nm respectively.
v´
0.5
0.4
0.3
0.2
0.1
(1) 0˚ incident angle
In-coupling hologram
10°
Common area=76 % of NTSC gamut
0
0 0.1 0.2 0.3 0.4 0.5 0.6
u´
NTSC
sRGB
R
B
G
v´
0.5
0.4
0.3
0.2
0.1
(2) 10˚ incident angle
8.4 / H. Mukawa
Figure 7: Color gamut at two different incident angles
on the in-coupling hologram
2.2.2 Drive signal management of microdisplay
The final approach to improve color uniformity is controlling a
drive signal of the microdisplay. If there is no color signal
management, a white position in the diagram moves more than
0.045 Δu’v’, depending on a field viewing angle from -8° through
+8 ° as shown in figure 8. The scheme is similar to that of
projection displays. Our system requires only one-dimensional
control along the diffraction direction, while projection displays
usually require two-dimensional control. The basic signal process
is to multiply compensation matrices by original drive signals (Ri,
G i, B i) to obtain new signals (R’ i, G’ i, B’ i) which could display
uniform chromaticity at any field viewing angle as expressed by
the equation (4) .
⎛ R'
i ⎞
⎜ ⎟
⎜G'
i⎟
⎜ ⎟
⎝ B'
i ⎠
=
⎛αR
⎜
⎜ βR
⎜
⎝ γR
− i
− i
− i
α
β
γ
G − i
G − i
G − i
α
β
γ
B − i
B − i
B − i
⎞
⎟
⎟
⎟
⎠
⎛ Ri
⎞
⎜ ⎟
⎜Gi
⎟
⎜ ⎟
⎝ Bi
⎠
αR − i is a compensation coefficient which is multiplied by
original drive signal Ri to obtain compensated red drive signal R’ i
β is one which is multiplied by original drive signal R i
and R − i
Common area =120% of NTSC gamut
0
0 0.1 0.2 0.3 0.4 0.5 0.6
u´
to obtain compensated green drive signal G’ i, for example. The “i”
means the specific position in the displayed image along the
diffraction direction.
(4)
NTSC
sRGB
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8.4 / H. Mukawa
The experimental data of how the color uniformity was improved
by our drive signal management is also shown in figure 8. Δu’v’
decreased from over 0.045 to less than 0.008 which was sufficient
for eyewear displays.
Δu'v'
0.05
0.04
0.03
0.02
0.01
0
-8 -6 -4 -2 0 2 4 6 8
Field angle (deg ree)
Figure 8: Color differences before and after the drive
signal management
3. Conclusion
We have developed a see-through full color eyewear display using
holographic elements on planar waveguides (Figure 9). 120%
NTSC color gamut, less than 0.008 Δu’v’ color uniformity and
over 2500cd/m 2 brightness were achieved with a 20° diagonal
FOV. The advantages of the display are over 85% see-through
transmittance and simply-structured waveguides suitable for
eyewearable devices. The waveguide was a 1.4mm thick glass
substrate with volume reflection holograms placed in mirror
symmetry. In the future, glass substrates could be replaced with
plastic substrates to make them lighter and more robust to expand
applicable fields.
92 • SID 08 DIGEST
before compensation
after compensation
Figure 9: A photograph of the prototype
4. References
[1] J. E. Melzer and K. Moffit, “Head Mounted Displays”,
(McGraw Hill, 1997), ISBN 0070418195
[2] H. Mukawa et al., “Novel Virtual Image Optics for
Reflective Microdisplays”, 2000 IDRC Digest, pp96-99,
2000
[3] Y. Amitai, “A Two-Dimensional Aperture Expander for
Ultra-Compact, High-Performance Head-Worn Displays”,
2005 SID International Symposium Digest, pp360-363, 2005
[4] Y. Amitai, S. Reinhom and A.A.Friesem, “Visor-display
design based on planar holographic optics”, Appl. Opt. 34,
1352-6, 1995
[5] I. Kasai et al., “Actually wearable see-through display using
HOE”, ODF 2000, pp117-120, 2000
[6] T. Levola, “Diffractive optical components for near to
eye displays”, 2006 SID International Symposium
Digest, pp64-67, 2006