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Demonstration of a multiview projection display using decentered microlens arrays |
Optics Express, Vol. 18, Issue 25, pp. 26092-26106 (2010)
http://dx.doi.org/10.1364/OE.18.026092
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▸ Article Outline
– Abstract
1. Introduction
2. Image steering projection screen
3. System architecture
4. Implementation of a 27-view system
5. Discussion
6. Conclusion
– References and links
– Abstract
1. Introduction
2. Image steering projection screen
3. System architecture
4. Implementation of a 27-view system
5. Discussion
6. Conclusion
– References and links
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Abstract
In this work we present a prototype multiview projection display that combines high-spatial and high-angular resolution with low complexity, compact form factor and potentially low-cost design. The system consists of a single projector and an image steering projection screen. It is based on beam steering using decentered microlens arrays in the projection screen and time-sequential rear-projection of the view images. The prototype has a 25 in. screen, a total of 27 viewing zones with XGA resolution and a horizontal field of view of 30°.
© 2010 Optical Society of America
1. Introduction
Multiview displays show many view images of an object to perceive depth
[1]. Unlike stereoscopic
displays [2] they do not
require special eyeglasses. This is made possible by generating many viewing zones:
a region in which only one view image is visible. These viewing zones are
sufficiently narrow in order that the left and right eye see a different image. The
parallax between both images is used to perceive depth. In addition, motion parallax
depth cues are available. When the viewer moves from left to right, the eyes will
pass adjacent viewing zones and see the object from a different direction. This is
often referred to as look-around capability and can be experienced by multiple
viewers at the same time. Multiview displays have the potential to be as realistic
as holograms. For this, the viewing zones have to be extremely narrow in order that
more than one view image is incident on each eye pupil [3, 4, 5]. This solves the
accommodation-vergence conflict that causes visual discomfort in many stereoscopic
and autostereoscopic systems. This type of multiview displays is often called super
multiview.
J. Son and B. Javidi, “Three-dimensional imaging methods based on multiview images,” J. Disp. Technol. 1, 125–140 (2005). [CrossRef]
Y. Takaki, “High-density directional display for generating natural three-dimensional images,” Proc. IEEE 94, 654–663 (2006). [CrossRef]
The view images can either be simultaneously or time-sequentially generated by a flat
panel display, one or many projectors or a combination of both. Firstly, a
lenticular sheet or parallax barrier can be added to a flat panel display
[6, 7, 8]. This approach
provides a compact solution using widespread technology but with limited
performance. The view images are spatially multiplexed over the display surface and
shown at the same time. Because of this, the spatial resolution of the object will
decrease when the number of viewing zones is increased. By slanting the viewing
optics, the resolution loss is divided over the horizontal and vertical directions.
So, a full HD panel (1920x1080 pixels) will enable a 9-view system with a spatial
resolution of 640x360 pixels. Other issues are the reduced image brightness of
parallax systems, the occurrence of many copies of the view images, crosstalk
between viewing zones and the non-uniform angular distribution of the viewing zones.
In 2009 Takaki [9] proposed
and demonstrated a slanted sub-pixel arrangement instead of a slanted lenticular
sheet that has a uniform angular distribution without crosstalk. Secondly,
projectors can be used to create multiview displays. One single projector with a
configurable slit aperture in the projection lens [10] or a directional illumination of the light
modulator [11] can realize
this. All view images are time-sequentially modulated by the same device. Therefore,
a high-speed projector is necessary. Kanebako [11] reported a digital light processing (DLP)
projection system with directional illumination of the micromirrors by 15 monochrome
light emitting diodes (LEDs). This approach can be interpreted as image steering at
the light modulator level. DLP technology is considered because of its
high-frame-rate capability [12]. 50 Hz monochrome operation with 15 viewing zones was achieved
in 5-bit gray scale. To show full-color images, the number of viewing zones or the
number of gray scales have to decrease. The main advantage of these single-projector
systems is that the spatial resolution of the view images is the same as that of the
light modulator irrespective of the number of viewing zones and that the system can
be potentially low cost. On the other hand, the brightness of the images decreases
when the number of viewing zones is increased because the useful étendue of
each view image is limited to the total system étendue divided by the number
of viewing zones. Multiview projection systems also exist that use the same number
of projectors [13] or light
modulators [5] than viewing
zones. This has the advantage that the light modulators do not have to operate
time-sequentially and so high-resolution low-speed light modulators can be
considered. Takaki [5]
reported a 64-view system using 64 transparent color liquid crystal light
modulators. Kikuta [14]
reported a 128-view system using 128 reflective liquid crystal light modulators.
Such systems are large, complex and expensive, but the high number of viewing zones
can enable super multiview operation. A final approach combines the spatial
multiplexing of flat panel displays with multiple projection systems. Takaki
[15] reported a system
using 16 liquid crystal displays with each 16 viewing zones enabled by a lenticular
sheet. All images are combined to form a 256-view system with a lower cost than a
256-projector system, but with reduced spatial resolution.
O. Willemsen, S. de Zwart, M. Hiddink, D. de Boer, and M. Krijn, “Multi-view 3D displays,” SID Symposium Digest 38, 1154–1157 (2007). [CrossRef]
M. Krijn, S. de Zwart, D. de Boer, O. Willemsen, and M. Sluijter, “2-D/3-D displays based on switchable lenticulars,” J. Soc. Inf. Display 16, 847–855 (2008). [CrossRef]
B. Lee, H. Hong, J. Park, H. Park, H. Shin, and I. Jung, “Multiview autostereoscopic display of 36view using an ultra-high resolution LCD,” in Stereoscopic Displays and Virtual Reality Systems XIV , A. Woods, N. Dodgson, J. Merritt, M. Bolas, and I. McDowall, eds., Proc. SPIE-IS&T Electronic Imaging 6490, 64900S (2007).
J. Son, V. Komar, Y. Chun, S. Sabo, V. Mayorov, L. Balasny, S. Belyaev, M. Semin, M. Krutik, and H. Jeon, “A multiview 3-D imaging system with full color capabilities,” in Stereoscopic Displays and Virtual Reality Systems V , M. Bolas, S. Fisher, and J. Merritt, eds., Proc. SPIE 3295, 218–223 (1998).
Y. Takaki, “High-density directional display for generating natural three-dimensional images,” Proc. IEEE 94, 654–663 (2006). [CrossRef]
Y. Takaki, “High-density directional display for generating natural three-dimensional images,” Proc. IEEE 94, 654–663 (2006). [CrossRef]
Y. Takaki and N. Nago, “Multi-projection of lenticular displays to construct a 256-view super multi-view display,” Opt. Express 18, 8824–8835 (2010). [CrossRef] [PubMed]
When designing a multiview display one has to take into consideration the desired
spatial and angular resolution, brightness, system dimensions, operational speed and
cost. We want to develop a multiview system with high spatial resolution. Therefore,
we consider a projection approach. Additionally, the system should be potentially
low cost and compact. For this reason, we use only one projector that
time-sequentially modulates the view images. We also want to show high-brightness
images with high-angular resolution. This requirement is not compatible with the
discussed state-of-the-art single-projector systems because the brightness of the
view images decreases with the number of viewing zones. We have developed an image
steering projection screen that time-sequentially steers images into different
viewing zones. Each image is created using the entire system étendue of the
projector. In this way, the image brightness does not decrease with the number of
viewing zones. The screen is based on the concept of beam steering using decentered
microlens arrays [16]. This
concept has strictly been applied for the steering of coherent light in telescopes
[17], infrared
countermeasures, optical switches and laser radar.
E. Watson, “Analysis of beam steering with decentered microlens arrays,” Opt. Eng. 32, 2665–2670 (1993). [CrossRef]
J. Duparré, D. Radtke, and P. Dannberg, “Implementation of field lens arrays in beam-deflecting microlens array telescopes,” Appl. Opt. 43, 4854–4861 (2004). [CrossRef] [PubMed]
In this paper, we will discuss a prototype multiview display using a single projector
and an image steering projection screen. Moving microlens arrays have already been
reported in 3D displays [18].
To our knowledge, this is the first time that decentered microlens arrays are
implemented in the projection screen of such systems. In Sec. 2 we overview the
working principle of our multiview display. In Sec. 3 we discuss the design and
implementation of the projector and the image steering projection screen. In Sec. 4
we calculate the number of viewing zones the system can show and experimentally
characterize the prototype system. Finally, we discuss system modifications and
enhancements in Sec. 5 and conclude in Sec. 6.
J. Jang and B. Javidi, “Improved viewing resolution of three-dimensional integral imaging by use of nonstationary micro-optics,” Opt. Letters 27, 324–326 (2002). [CrossRef]
2. Image steering projection screen
2.1. Beam steering
The multiview display is based on time-sequentially projecting the view images
onto a screen that steers each image into a different direction. This concept is
illustrated in Fig. 1. The projector is
located behind the projection screen because the image steering results from
refraction of light propagating through a microlens system implemented in the
screen. This microlens system consists of three microlens arrays as is shown in
Fig. 2. To explain its operating
principle, let us consider one beam steering element which consists of three
microlenses one after the other. The light is steered in different directions by
equally changing the decenter δ between the first lens
and the second lens, and the first lens and the third lens. The second lens is
placed at the back focal plane of the first lens and at the front focal plane of
the third lens. Ideally, the first lens focuses a collimated light beam to a
point at the second lens which is again transformed to a collimated light beam
by the third lens. If the second and third lens are decentered with respect to
the first lens, the focused light point at the second lens will appear as an
off-axis point for the third lens, and the direction of the collimated light
beam will change. The second lens is necessary to ensure a high fill factor of
the third lens at large steering angles and to avoid crosstalk between adjacent
beam steering elements which leads to undesired beam directions [16]. We consider that the second
lens has the same optical properties as the third lens.
E. Watson, “Analysis of beam steering with decentered microlens arrays,” Opt. Eng. 32, 2665–2670 (1993). [CrossRef]
2.2. Screen dimensions
The microlens arrays cover the entire screen surface in order to steer the entire
image. In total, there are as many beam steering elements on the screen as
pixels in the image. Each beam steering element changes the propagation
direction of one pixel. We consider a projector with DLP technology
[12]. This choice is
based on the high-frame-rate capability of DLP projection systems which is
crucial for time-sequential multiview systems [11]. For example, 20 viewing zones and a
refresh rate of 50 frames per second necessitate a frame rate of 1000 images per
second. Furthermore, we consider a projection architecture with only one digital
micromirror device (DMD) light modulator to limit the cost of the multiview
display. This implies that the spatial light modulator has to color-sequentially
modulate the images. Such high frame rates are not possible with transmissive or
reflective liquid-crystal-based spatial light modulators [19, 20]. DLP projection systems rely on electrostatic actuation
of micromirrors by voltage differences between the mirror and the underlying
memory cell. Each pixel on the screen is modulated by a single square-shaped
micromirror. It can switch between two orientation states by tilting 12°
about hidden diagonal hinges. The switching time is about 5.5
μs (DMD Discovery 3000). Another 8
μs are needed for the micromirror to reach a stable
state. When a micromirror is in the on state, light will be reflected to the
projection lens and a bright pixel is seen on the projection screen. When a
micromirror is in the off state, light will be reflected to a light absorber and
the pixel is dark. By varying the on and off time, during each image frame, gray
levels are obtained.
S. Hashimoto, O. Akimoto, H. Ishikawa, T. Kiyomiya, T. Togawa, T. Isozaki, H. Abe, M. Nakai, H. Terakawa, H. Horikiri, T. Ishii, and M. Kogure, “SXRD (Silicon X-tal Reflective Display); a new display device for projection displays,” SID Symposium Digest 36, 1362–1365 (2005). [CrossRef]
We use a DMD light modulator with XGA resolution (1024 x 768 pixels). The
projection setup is shown in Fig. 3. The
micromirrors have a pitch of 13.68 μm and a fill factor
of 85%. They are imaged by the projection lens and form pixels with a
pitch of 500 μm on the projection screen. The width of
the image is 51.2 cm and the height is 38.4 cm. Consequently, the microlens
arrays have to cover a surface with a 25 in. diagonal (1 in. = 2.54
cm).
2.3. Microlens specifications
We have designed the microlenses to enable horizontal parallax images. For this,
we use vertically-oriented cylindrical microlenses in combination with
horizontal decenter. The microlenses have a footprint of 500
μm which is equal to the pixel pitch. We consider a
maximal decenter of 225 μm to steer the light
(90% of half the footprint of the microlenses). We work with two lens
sheets: one which contains the first microlens array and one which contains both
the second and the third microlens array on opposite sides of the lens sheet.
The lens sheets are made out of glass with polymer microlenses on its surface
[21]. In front of the
first lens sheet, we place a large Fresnel lens to collimate the light rays of
the projected image. Behind the second lens sheet, we place a diffuser to spread
the steered light beam in the vertical direction.
The microlenses are optimized to obtain a broad steering range and minimal
divergence of the steered light beam. The latter is essential to achieve many
viewing zones. For this, we used the nonsequential ray-tracing program ASAP from
Breault Research Organization. The input light distribution of each microlens on
the first lens sheet has a square-shaped spatial distribution of 461
μm x 461 μm and a
cone-shaped angular distribution with a half-angle of 0.328°. The latter
corresponds to the illumination light cone of the DMD light modulator divided by
the magnification of the projection lens (étendue conservation). In
Fig. 4, we show the optimized design
of the lens sheets. Both lens sheets are separated by a distance equal to 1 mm.
The first lens sheet is 400 μm thick and the second
lens sheet 1200 μm. Their thickness is sufficient to
provide stability for the considered screen size. When the decenter is zero, the
light beam propagates straight on. When the decenter is maximal (225
μm), the light beam is steered 15.4°. For
negative decenter values, the magnitude of the steering angle is the same as for
positive decenter values, but the direction is opposite.
Fig. 4 Horizontal cross section of one beam steering element on both lens
sheets. One pixel of the image is traced through the lens system with no
decenter (top) and with maximal decenter (bottom).
The numerical values of the steering angle and the divergence of the outgoing
light beam for different horizontal decenter values are shown in Fig. 5. We see that the steering angle
linearly increases as a function of the decenter and that the beam divergence
slightly increases for larger steering angles. The latter is caused by system
aberrations when the focused light spot at the second lens moves away from the
optical axis of the second and third lens for larger decenter values. On
average, the full-angle divergence of the light beam is 1°.
3. System architecture
3.1. DLP projector with LED light sources
The projector, using only one DMD light modulator, has to time-sequentially
modulate the three color components of all view images. For this, the DMD light
modulator is consecutively illuminated by red, green and blue (RGB) light. Due
to the required high frame rate of the projector, we decided to use high-power
LEDs as projection light sources because they can be rapidly switched on and
off, and because they are available in the primary colors of the display
[22, 23, 24].
Y. Cheng, F. Tsai, A. Wang, and H. Chiu, “High brightness LED based projector with NTSC 120% wide color gamut,” SID Symposium Digest 41, 976–978 (2010). [CrossRef]
F. Fournier and J. Rolland, “Design methodology for high brightness projectors,” J. Disp. Technol. 4, 86–91 (2008). [CrossRef]
We have reengineered the optical module of a 0.7 in. XGA DMD Discovery 3000
development kit, for RGB LED illumination [25]. We designed a compact illumination
system that collects and combines the light of one red, green and blue PT54
PhlatLight LED [26]. This
light output is made uniform by a rod integrator and then imaged onto the DMD
light modulator. The projector has a light output of 80 ANSI lumen and a
contrast ratio of 680:1. The LEDs have an average power consumption of 20 W.
S. Roelandt, L. Bogaert, Y. Meuret, A. Avci, H. De Smet, and H. Thienpont, “Color uniformity in compact LED illumination for DMD projectors,” in Optics, Photonics, and Digital Technologies for Multimedia Applications , P. Schelkens, T. Ebrahimi, G. Cristóbal, F. Truchetet, and P. Saarikko, eds., Proc. SPIE 7723, 77230X (2010).
C. Hoepfner, “PhlatLight photonic lattice LEDs for RPTV light engines,” SID Symposium Digest 37, 1808–1811 (2006). [CrossRef]
3.2. Vibrating projection screen
We want to operate the multiview system at a refresh rate of 50 Hz to avoid
flickering images. This implicates that 50 images are directed into each viewing
zone per second. For this, the second lens sheet is moved from the outmost right
position (225 μm) to the outmost left position
(−225 μm) while the view images are
time-sequentially projected and steered into the respective viewing zone. Next,
the second lens sheet is moved back to its starting position and the view images
are shown in opposite order. This process is repeated 25 times per second to
achieve a 50 Hz refresh rate.
The mechanism that moves the second lens sheet, back and forth, has to operate at
a frequency of 25 Hz and a travel range of 450 μm.
Furthermore, it has to be sufficiently powerful (push-pull force) and it has to
move with high accuracy and repeatability. The 1.2 mm thick glass lens sheet
weighs 730 g and it is held in an aluminum frame that weighs 350 g.
Additionally, the vertical diffuser is attached to the moving lens sheet which
increases the weight with 215 g. We considered using a linear translation stage
with a servo motor, but the required mechanical transmission to enable the
push-pull force did not allow 25 Hz operation. This is also valid for
piezoelectric actuators. Furthermore, the relative large travel range would
require a very long piezoelectric stack. We solved the force-frequency issue by
implementing the screen motion as a mass-spring system that oscillates at its
natural frequency. In this way, a low-power motor can be used because the motion
is self-sustaining.
To create the mass-spring system, leaf springs are attached on both sides of the
aluminum frame and connected to a fixed structure which is also used to position
the first lens sheet. This is shown in Fig.
6. The spring constant of these leaf springs is chosen in order that
the natural frequency of the screen equals the desired oscillation frequency of
25 Hz. We use a DC-motor that operates at 1500 rpm. It has a magnetic encoder to
determine the screen position during the motion. An excenter is connected to the
motor shaft which initiates the oscillating motion. The excenter is in contact
with a cantilever which has a fixed connection to the aluminum frame of the
second lens sheet. During half the rotation of the motor shaft, the excenter
pushes the screen away (225 μm to −225
μm decenter). During the other half of the
rotation, a set of two springs in the motor setup ensures that the excenter
remains in contact with the cantilever while the screen returns to its start
position (−225 μm to 225
μm decenter). In this way, the amplitude of the
oscillation is perfectly 450 μm. When the motor is
powered off, the motor springs ensure that the screen stops at the start
position. The screen motion is sinusoidal with both a horizontal and vertical
component. The horizontal component is the desired decenter between both lens
sheets that controls the beam steering direction. The vertical component does
not impact the beam steering because the lens sheets consist of
vertically-oriented cylindrical microlenses.
The screen structure has several alignment features to position both lens sheets
with respect to each other. Firstly, it is possible to change the orientation of
the second lens sheet in order that the cylindrical lenses are parallel to those
on the first lens sheet. Secondly, the distance between both lens sheets can be
adjusted. Finally, the horizontal starting position and the amplitude of the
motion can be fine-tuned.
4. Implementation of a 27-view system
4.1. Number of viewing zones
The image steering projection screen moves continuously back and forth while
images are color-sequentially and view-sequentially projected onto it. Because
the red, green and blue color components of each view image are not shown at the
same time, their steering direction will be different. This has to be kept
minimal in order to perceive the three colored light beams as one light beam
with the correct color information; the steering range of each viewing zone has
to be about the same size as the divergence of the steered light beam. If the
steering range is much larger, the viewing zone will be split into three
separate parts with different colors and the eye will not see the correct color
information.
We used the ALP-3 high speed controller board to access the DMD light modulator.
The number of image frames it supports per second is given in Table 1. We see that in 8-bit gray scale a total of
250 image frames can be modulated per second. Three image frames are needed to
modulate one full-color image. To show one viewing zone, a total of 150 image
frames are needed. This corresponds to the refresh rate of 50 Hz and full-color
operation. Because of this, the DMD projector can only show one viewing zone
with 8-bit images. By decreasing the bit depth, the modulation time becomes
smaller and the number of image frames increases. 5-bit images already enable 9
viewing zones, but within a steering range of approximately 30° and a
full-angle divergence of 1° this is not sufficient to obtain a uniform
angular distribution. Therefore, we further decreased the bit depth to 2-bit
which enables a maximal number of 35 viewing zones.
Table 1 Overview of the number of image frames the DMD can modulate and the
number of viewing zones this corresponds to.
| Gray scale (bit) | Max. switching rate (frames/sec) | Number of viewing zones |
|---|---|---|
| 1 | 13333 | 88 |
| 2 | 5319 | 35 |
| 3 | 3546 | 23 |
| 4 | 2315 | 15 |
| 5 | 1462 | 9 |
| 6 | 864 | 5 |
| 7 | 476 | 3 |
| 8 | 250 | 1 |
The sinusoidal motion of the screen has to be taken into account to determine the
number of viewing zones. In Fig. 7, we
show the calculated angular distribution of 35 images modulated in a time period
of 20 ms (50 Hz, 225 μm to −225
μm decenter). We see that the steering angle
changes sinusoidally. This is because the change in decenter, during the fixed
time period to modulate each 2-bit full-color image, is not constant, but
sinusoidal. In the beginning of the motion (225 μm
decenter), the velocity of the screen is minimal resulting in a small steering
range (0.06°). The steering range then gradually increases while the
velocity of the screen increases until halfway the motion (0
μm decenter) where the steering range is maximal
(1.36°). Next, the steering range decreases until the end of the motion
(−225 μm decenter). This process is repeated
while the screen returns to its start position. Thus, the steering range of the
images with large steering angles (begin and end) is smaller than the images
with small steering angles (middle). For images 1 to 4 and 32 to 35, the
steering range is less than half the divergence of the steered light beam. These
images will largely overlap and the image information will be distorted by
crosstalk. This can be solved by repeating the image information during
consecutive images and thereby combining several images into one viewing zone
with sufficient steering range. We will show the same image information during
images 1 to 4, 5 and 6, 30 and 31 and, 32 to 35. In this way, we obtain 27
viewing zones with a minimal steering range of 0.75° and a maximal
steering range of 1.36°. On average, the steering range is 1.12°
which is slightly more than the beam divergence.
4.2. Synchronization and timing
The screen has to be perfectly synchronized with the projector to steer the image
information into the correct direction. For this, we have implemented a
synchronization module based on a field-programmable gate array (FPGA) that
instructs the projector to show two sets of view images each time the screen is
in its start position (25 Hz). Each set consists of 35 2-bit full-color images
(105 image frames). The first set of images is time-sequentially directed into
viewing zone 1 to 27 (225 μm to −225
μm decenter), the second set of images into viewing
zone 27 to 1 (−225 μm to 225
μm decenter).
Every 190 μs an image frame is modulated. For this, the
FPGA triggers the DMD to show the next image frame and turns on the LED with the
color of the image information. The DMD is configured to modulate the image
frame in 189 μs. During the first 114
μs, the two bit planes of the image frame are
modulated. The remaining 75 μs is used to upload the
first bit plane of the next image frame in the memory cells beneath the
micromirrors. The LED illuminates the DMD during the first 114
μs. We remark that the 1
μs difference between the image frame time and the
image modulation time is used by the DMD controller board to detect the next
rising edge of the trigger signal. The prototype system is shown in Fig. 8
Fig. 8 Picture of the prototype system. The projector is located on the left and
the image steering projection screen on the right. The 3D images are
seen on the other side of the projection screen (
Media
1).
4.3. Experimental characterization
We have demonstrated the multiview projection display for still images. We
created a 27-view image set of a virtual object taken from different camera
positions that correspond with the direction of the viewing zones. The viewer
perceives a 3D image without special eyeglasses from a viewing distance between
1 m and 4 m with look-around capability. For smaller viewing distances, the
entire image is not visible because of the limited steering range. For larger
viewing distances, only two-dimensional images are seen because the viewing
zones are expanded to the extent that both eyes see the same image.
To characterize the operation of the entire system, we measured the angular
distribution of the viewing zones. For this, we projected a black image with a
white spot in the middle, and investigated the position of the steered light
spot at a viewing distance of 1 m. The spot is only a few pixels wide in order
not to contribute to the width of the viewing zones at the detection plane. In
Fig. 9, we show a picture of the
steered light spot using an image set that consists of the black image with the
white spot in all odd viewing-zone positions and a plain black image in all even
viewing-zone positions. The picture is taken with a shutter speed of 1 s and
therefore consists of 50 traces of the image steering. Each light spot
corresponds to one viewing zone with odd numbering. It can be interpreted as the
print of all directions in which light is steered during the time that the red,
green and blue color components of that viewing zone are modulated. The angular
distribution of the viewing zones is shown in Fig. 10. Both subfigures are based on a different image set; in
Fig. 10(a) we examine the odd and
even viewing zones separately and in Fig.
10(b) we use the black image with the white spot for all viewing
zones to examine the uniformity of the steered light throughout the steering
range. We find that the intensity of the first and last viewing zone, and the
second and second last viewing zone are respectively four and two times larger
than the other viewing zones. This can be explained by the fact that in these
viewing zones, respectively, four and two images are used to form the viewing
zone (Section 4.1). Therefore, the intensity is higher than in the other viewing
zones that appear by steering one image only. The global U-shaped angular
intensity distribution also shows spikes of about 10%. These correspond
to the angular intensity distributions of the viewing zones that do not form a
flat intensity distribution when summed. To achieve this, the angular intensity
distribution of each viewing zone should be a flat-top distribution. We also
find that the steering range corresponds to the simulated range of the lens
sheets (Section 2.3) and that the color components of each viewing zone do not
perfectly overlap: red is visible on the left and blue on the right. Because of
the overlap of adjacent viewing zones, we still perceive the correct color
information.
4.4. Image artifacts
Every view image is repeated 50 times per second, yet the viewer does not
perceive flicker-free images from all viewing positions. We find that in the
center of the viewing space the refresh rate is 50 Hz whereas toward the sides
the refresh rate decreases to 25 Hz. This can be explained by considering the
time between two images that are directed by the screen into the same viewing
zone. This is shown in Fig. 11. Because
both directions of the sinusoidal-screen motion are used to show images, the
time between two consecutive images is not the same for all viewing zones. For
the outmost viewing zones (1 and 27), the images follow immediately one after
the other. The eye integrates these images and only sees one image each 40 ms.
Therefore, the refresh rate is perceived to be 25 Hz instead of 50 Hz. On the
other hand, in the center (viewing zone 14), the time between two consecutive
images is exactly 20 ms and the refresh rate is perceived correctly (50 Hz).
Fig. 11 The time between consecutive images in viewing zones 1, 14 and 27 (225
μm, 0 μm and
−225 μm decenter, respectively).
The images that are visible on the vibrating screen are distorted by
Moiré patterns as is visible in Fig.
12. These image artifacts result from the optical sampling of the
images, projected onto the screen, by the periodic elements in the screen. These
are the Fresnel lens, the vertically-oriented cylindrical microlenses and the
holographic vertical diffuser. They can cause radial, horizontal and vertical
sampling, respectively. The pitch of the Fresnel lens structures is about five
times smaller than the pixel pitch. Therefore, the radial sampling frequency is
high enough to avoid aliasing. On the other hand, the pitch of the cylindrical
lenses is designed to be the same as the pixel pitch. Small variations of the
lens pitch and the pixel pitch introduce Moiré patterns. The pitch of
the holographic diffuser is much smaller than the pixel pitch and does not
contribute to the Moiré patterns.
5. Discussion
We have demonstrated a multiview display capable of showing horizontal parallax
images by means of image steering. To progress toward higher angular resolution or
larger field of view or horizontal and vertical parallax images combined, even more
images will have to be modulated. This requires modifications and enhancements of
the DMD projector as well as the image steering projection screen.
Showing more images is limited with the current DMD projector because it would imply
a further decrease of the bit depth of the images from 2 bit to 1 bit. A possible
solution is to use three DMD light modulators instead of only one. The color
components of the view images can then be shown at the same time instead of color
sequentially. This has the advantage that each DMD light modulator has to modulate
three times less images compared to a single DMD light modulator approach and that
the color components of each viewing zone will perfectly overlap. In our
demonstrator system, this approach would enable 4-bit images, an improvement of the
number of gray scales with a factor four. Also, the brightness of the images would
increase. Nevertheless, the increase of the number of images is not sufficient to
allow systems with the envisioned multiview characteristics. This will also require
a significant increase of the performance of the DMD controller board, the DMD
driver electronics and the DMD chip itself. Furthermore, innovative modulation
techniques can be used to maximize the frame rate. For example, by intensity
modulation of the LED illumination in order to show the same bit depth in a shorter
time. Other spatial light modulators such as liquid-crystal-on-silicon devices
cannot currently be considered because they are not fast enough. On the other hand,
in most cases, they do have the benefit of being an analog technology and thereby
providing full bit depth.
Higher angular resolution will require the viewing zones to be narrower. For this, it
will be necessary to decrease the divergence of the steered light beam. This is
possible when the étendue of the light processed by each microlens
decreases. A DMD projector based on laser illumination could be appropriate for
this. A larger field of view will require microlenses with a smaller f-number and
additional vertical parallax will necessitate microlenses with a square footprint
and also vertical screen motion.
The optical and mechanical implementation of the image steering projection screen can
be improved in several ways. We experienced that it is difficult to set the distance
between the lens sheets to 1 mm over the entire screen surface. Deviations result in
larger beam divergence and consequently the view images overlap. This can be
improved by increasing the necessary distance between both lens sheets in order to
make the relative errors smaller. Also, we experienced that the lens sheets are
fragile and that they can slightly be bent. This can be improved by making them
thicker. The Moiré patterns can be eliminated by making the pitch of the
microlenses at least two times smaller than the pixel pitch. Furthermore,
antireflection coatings will improve the transmission efficiency of the
rear-projection screen; currently, the transmission is 66% (Fresnel lens
92%, lens sheet 1 92%, lens sheet 2 92% and diffuser
85%). The mechanical part of the screen can be improved by adding a feedback
control system to compensate for frequency fluctuations of the motor. Also, the
oscillation frequency has to be increased in order to obtain flicker-free images for
the entire field of view. A first approach is to double the oscillation frequency of
the screen (25 Hz to 50 Hz) and to show images only in one direction of the motion
(225 μm to −225 μm
decenter). This has the advantage that the refresh rate of all view images will be
equal to 50 Hz, but the disadvantage that the brightness of the system will decrease
with 50% because only half of the screen motion is used to show the images.
Another approach is to continue showing the images during both directions of the
motion. Because of this, the refresh rate will not be the same for all viewing
zones, but it will be high enough to ensure flicker-free operation for the entire
field of view (outmost viewing zones 50 Hz and center viewing zone 100 Hz). We
remark that both approaches, again, increase the number of images that have to be
shown. The angular spread of the viewing zones can be decreased by implementing a
stepwise motion instead of a continuous motion. The screen moves to its new position
and then images are projected onto it. Because of this, the angular spread of each
viewing zone will be equal to the divergence of the steered light beam. Also, the
color components of each viewing zone will perfectly overlap. Finally, the entire
system can be made more compact by folding the optical path from the projector to
the screen.
6. Conclusion
We have implemented the concept of beam steering using decentered microlens arrays
for multiview displays. We designed and demonstrated a rear-projection system using
a single projector and an image steering projection screen. The latter consists of
two lens sheets covered with vertically-oriented cylindrical microlenses. The
horizontal decenter between the microlenses on both lens sheets determines the
horizontal steering direction. The view images of a 3D object are time-sequentially
projected onto the screen and steered into the respective viewing zone by changing
the relative position of the lens sheets.
The screen has a diagonal of 25 in. and contains microlenses with a footprint of 500
μm which corresponds to the pitch of the pixels in the
images that are projected onto it. The decenter between the microlenses ranges
between 225 μm and −225
μm. This enables beam steering from 15° to
−15° with a full-angle beam divergence of 1°. The projector
shows all view images in full-color at a refresh rate of 50 Hz. To accomplish this,
we have designed a DLP projector with LED light sources. The moving lens sheet in
the projection screen is implemented as a mass-spring system that oscillates at 25
Hz. The resulting prototype shows a total of 27 viewing zones with XGA resolution
within a horizontal field of view of 30°. The bit depth of the images is
reduced to 2 bit. This is required to modulate the high number of image frames.
The demonstrated system can show high-brightness multiview images with high-angular
resolution using a single projector. The spatial resolution of the images is
determined by the projector. The system has the potential to progress toward super
multiview with both horizontal and vertical parallax by significantly increasing the
frame-rate capability of the DLP projector and by adapting the projection screen to
steer narrower beams of light.
Acknowledgments
This work is supported by the Research Foundation -
Flanders (FWO-Vlaanderen). We also acknowledge the
Industrial Research Funding (IOF),
the Methusalem and Hercules foundations,
VUB-GOA, and the OZR of the Vrije Universiteit Brussel. Lawrence
Bogaert is indebted to FWO-Vlaanderen for an Aspirant grant.
References and links
J. Son and B. Javidi, “Three-dimensional imaging methods based on multiview images,” J. Disp. Technol. 1, 125–140 (2005). [CrossRef] | |
L. Lipton, “Digital stereoscopic cinema: the 21st century,” in Stereoscopic Displays and Applications XIX , A. Woods, N. Holliman, and J. Merritt, eds., Proc. SPIE-IS&T Electronic Imaging 6803, 68030W (2008). | |
Y. Kajiki, H. Yoshikawa, and T. Honda, “Hologram-like video images by 45-view stereoscopic display,” in Stereoscopic Displays and Virtual Reality Systems IV , S. Fisher, J. Merritt, and M. Bolas, eds., Proc. SPIE 3012, 154–166 (1997). | |
T. Honda and M. Shimomatsu, “Natural 3-D display system by a fan-like array of projection optics,” in Three-Dimensional TV, Video, and Display , B. Javidi and F. Okano, eds., Proc. SPIE 4864, 96–103 (2002). | |
Y. Takaki, “High-density directional display for generating natural three-dimensional images,” Proc. IEEE 94, 654–663 (2006). [CrossRef] | |
O. Willemsen, S. de Zwart, M. Hiddink, D. de Boer, and M. Krijn, “Multi-view 3D displays,” SID Symposium Digest 38, 1154–1157 (2007). [CrossRef] | |
M. Krijn, S. de Zwart, D. de Boer, O. Willemsen, and M. Sluijter, “2-D/3-D displays based on switchable lenticulars,” J. Soc. Inf. Display 16, 847–855 (2008). [CrossRef] | |
B. Lee, H. Hong, J. Park, H. Park, H. Shin, and I. Jung, “Multiview autostereoscopic display of 36view using an ultra-high resolution LCD,” in Stereoscopic Displays and Virtual Reality Systems XIV , A. Woods, N. Dodgson, J. Merritt, M. Bolas, and I. McDowall, eds., Proc. SPIE-IS&T Electronic Imaging 6490, 64900S (2007). | |
Y. Takaki, O. Yokoyama, and G. Hamagishi, “Flat panel display with slanted pixel arrangement for 16-view display,” in Stereoscopic Displays and Applications XX, A. Woods, N. Holliman, and J. Merrittl, eds., Proc. SPIE-IS&T Electronic Imaging 7237, 723708 (2009). | |
J. Son, V. Komar, Y. Chun, S. Sabo, V. Mayorov, L. Balasny, S. Belyaev, M. Semin, M. Krutik, and H. Jeon, “A multiview 3-D imaging system with full color capabilities,” in Stereoscopic Displays and Virtual Reality Systems V , M. Bolas, S. Fisher, and J. Merritt, eds., Proc. SPIE 3295, 218–223 (1998). | |
T. Kanebako and Y. Takaki, “Time-multiplexing display module for high-density directional display,” in Stereoscopic Displays and Applications XIX , A. Woods, N. Holliman, and J. Merritt, eds., Proc. SPIE-IS&T Electronic Imaging 6803, 68030P (2008). | |
L. Hornbeck, “Digital light processing for high-brightness high-resolution applications,” in Projection Displays III , M. Wu, ed., Proc. SPIE 3013, 27–40 (1997). | |
T. Balogh, “The HoloVizio system,” in Stereoscopic Displays and Virtual Reality Systems XIII , A. Woods, N. Dodgson, J. Merritt, M. Bolas, and I. McDowall, eds., Proc. SPIE-IS&T Electronic Imaging 6055, 60550U (2006). | |
K. Kikuta and Y. Takaki, “Development of SVGA resolution 128-directional display,” in Stereoscopic Displays and Virtual Reality Systems XIV , A. Woods, N. Dodgson, J. Merritt, M. Bolas, and I. McDowall, eds., Proc. SPIE-IS&T Electronic Imaging 6490, 64900U (2007). | |
Y. Takaki and N. Nago, “Multi-projection of lenticular displays to construct a 256-view super multi-view display,” Opt. Express 18, 8824–8835 (2010). [CrossRef] [PubMed] | |
E. Watson, “Analysis of beam steering with decentered microlens arrays,” Opt. Eng. 32, 2665–2670 (1993). [CrossRef] | |
J. Duparré, D. Radtke, and P. Dannberg, “Implementation of field lens arrays in beam-deflecting microlens array telescopes,” Appl. Opt. 43, 4854–4861 (2004). [CrossRef] [PubMed] | |
J. Jang and B. Javidi, “Improved viewing resolution of three-dimensional integral imaging by use of nonstationary micro-optics,” Opt. Letters 27, 324–326 (2002). [CrossRef] | |
W. Bleha and R. Sterling, “D-ILA technology for high resolution projection displays,” in Cockpit Displays X , D. Hopper, ed., Proc. SPIE 5080, 239–249 (2003). | |
S. Hashimoto, O. Akimoto, H. Ishikawa, T. Kiyomiya, T. Togawa, T. Isozaki, H. Abe, M. Nakai, H. Terakawa, H. Horikiri, T. Ishii, and M. Kogure, “SXRD (Silicon X-tal Reflective Display); a new display device for projection displays,” SID Symposium Digest 36, 1362–1365 (2005). [CrossRef] | |
E. Wolterink and K. Demeyer, “Waferoptics mass volume production and reliability,” in Micro-Optics 2010 , H. Thienpont, P. V. Daele, J. Mohr, and H. Zappe, eds., Proc. SPIE 7716, 771614 (2010). | |
Y. Cheng, F. Tsai, A. Wang, and H. Chiu, “High brightness LED based projector with NTSC 120% wide color gamut,” SID Symposium Digest 41, 976–978 (2010). [CrossRef] | |
F. Fournier and J. Rolland, “Design methodology for high brightness projectors,” J. Disp. Technol. 4, 86–91 (2008). [CrossRef] | |
E. Geißler, “Meeting the challenges of developing LED-based projection displays,” in Photonics in Multimedia , A. Tervonen, M. Kujawinska, W. IJzerman, and H. De Smet, eds., Proc. SPIE 6196, 619601 (2006). | |
S. Roelandt, L. Bogaert, Y. Meuret, A. Avci, H. De Smet, and H. Thienpont, “Color uniformity in compact LED illumination for DMD projectors,” in Optics, Photonics, and Digital Technologies for Multimedia Applications , P. Schelkens, T. Ebrahimi, G. Cristóbal, F. Truchetet, and P. Saarikko, eds., Proc. SPIE 7723, 77230X (2010). | |
C. Hoepfner, “PhlatLight photonic lattice LEDs for RPTV light engines,” SID Symposium Digest 37, 1808–1811 (2006). [CrossRef] |
OCIS Codes
(100.6890) Image processing : Three-dimensional image processing
(110.0110) Imaging systems : Imaging systems
(120.2040) Instrumentation, measurement, and metrology : Displays
(220.0220) Optical design and fabrication : Optical design and fabrication
(350.3950) Other areas of optics : Micro-optics
ToC Category:
Imaging Systems
History
Original Manuscript: October 8, 2010
Revised Manuscript: November 23, 2010
Manuscript Accepted: November 23, 2010
Published: November 30, 2010
Citation
Lawrence Bogaert, Youri Meuret, Stijn Roelandt, Aykut Avci, Herbert De Smet, and Hugo Thienpont, "Demonstration of a multiview projection display using decentered microlens arrays," Opt. Express 18, 26092-26106 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-25-26092
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References
- J. Son, and B. Javidi, "Three-dimensional imaging methods based on multiview images," J. Disp. Technol. 1, 125-140 (2005). [CrossRef]
- L. Lipton, "Digital stereoscopic cinema: the 21st century," in Stereoscopic Displays and Applications XIX, A. Woods, N. Holliman, and J. Merritt, eds., Proc. SPIE-IS&T Electronic Imaging 6803, 68030W (2008).
- Y. Kajiki, H. Yoshikawa, and T. Honda, "Hologram-like video images by 45-view stereoscopic display," in Stereoscopic Displays and Virtual Reality Systems IV, S. Fisher, J. Merritt, and M. Bolas, eds., Proc. SPIE 3012, 154-166 (1997).
- T. Honda, and M. Shimomatsu, "Natural 3-D display system by a fan-like array of projection optics," in Three-Dimensional TV, Video, and Display, B. Javidi and F. Okano, eds., Proc. SPIE 4864, 96-103 (2002).
- Y. Takaki, "High-density directional display for generating natural three-dimensional images," Proc. IEEE 94, 654-663 (2006). [CrossRef]
- O. Willemsen, S. de Zwart, M. Hiddink, D. de Boer, and M. Krijn, "Multi-view 3D displays," SID Symposium Digest 38, 1154-1157 (2007). [CrossRef]
- M. Krijn, S. de Zwart, D. de Boer, O. Willemsen, and M. Sluijter, "2-D/3-D displays based on switchable lenticulars," J. Soc. Inf. Disp. 16, 847-855 (2008). [CrossRef]
- B. Lee, H. Hong, J. Park, H. Park, H. Shin, and I. Jung, "Multiview autostereoscopic display of 36view using an ultra-high resolution LCD," in Stereoscopic Displays and Virtual Reality Systems XIV, A. Woods, N. Dodgson, J. Merritt, M. Bolas, and I. McDowall, eds., Proc. SPIE-IS&T Electronic Imaging 6490, 64900S (2007).
- Y. Takaki, O. Yokoyama, and G. Hamagishi, "Flat panel display with slanted pixel arrangement for 16-view display," in Stereoscopic Displays and Applications XX, A. Woods, N. Holliman, and J. Merrittl, eds., Proc. SPIE-IS&T Electronic Imaging 7237, 723708 (2009).
- J. Son, V. Komar, Y. Chun, S. Sabo, V. Mayorov, L. Balasny, S. Belyaev, M. Semin, M. Krutik, and H. Jeon, "A multiview 3-D imaging system with full color capabilities," in Stereoscopic Displays and Virtual Reality Systems V, M. Bolas, S. Fisher, and J. Merritt, eds., Proc. SPIE 3295, 218-223 (1998).
- T. Kanebako, and Y. Takaki, "Time-multiplexing display module for high-density directional display," in Stereoscopic Displays and Applications XIX, A. Woods, N. Holliman, and J. Merritt, eds., Proc. SPIE-IS&T Electronic Imaging 6803, 68030P (2008).
- L. Hornbeck, "Digital light processing for high-brightness high-resolution applications," in Projection Displays III, M. Wu, ed., Proc. SPIE 3013, 27-40 (1997).
- T. Balogh, "The HoloVizio system," in Stereoscopic Displays and Virtual Reality Systems XIII, A. Woods, N. Dodgson, J. Merritt, M. Bolas, and I. McDowall, eds., Proc. SPIE-IS&T Electronic Imaging 6055, 60550U (2006).
- K. Kikuta, and Y. Takaki, "Development of SVGA resolution 128-directional display," in Stereoscopic Displays and Virtual Reality Systems XIV, A. Woods, N. Dodgson, J. Merritt, M. Bolas, and I. McDowall, eds., Proc. SPIE-IS&T Electronic Imaging 6490, 64900U (2007).
- Y. Takaki, and N. Nago, "Multi-projection of lenticular displays to construct a 256-view super multi-view display," Opt. Express 18, 8824-8835 (2010). [CrossRef] [PubMed]
- E. Watson, "Analysis of beam steering with decentered microlens arrays," Opt. Eng. 32, 2665-2670 (1993). [CrossRef]
- J. Duparré, D. Radtke, and P. Dannberg, "Implementation of fiel lens arrays in beam-deflectin microlens array telescopes," Appl. Opt. 43, 4854-4861 (2004). [CrossRef] [PubMed]
- J. Jang, and B. Javidi, "Improved viewing resolution of three-dimensional integral imaging by use of nonstationary micro-optics," Opt. Lett. 27, 324-326 (2002). [CrossRef]
- W. Bleha, and R. Sterling, "D-ILA technology for high resolution projection displays," in Cockpit Displays X, D. Hopper, ed., Proc. SPIE 5080, 239-249 (2003).
- S. Hashimoto, O. Akimoto, H. Ishikawa, T. Kiyomiya, T. Togawa, T. Isozaki, H. Abe, M. Nakai, H. Terakawa, H. Horikiri, T. Ishii, and M. Kogure, "SXRD (Silicon X-tal Reflect ve Display); a new display device for projection displays," SID Symposium Digest 36, 1362-1365 (2005). [CrossRef]
- E. Wolterink, and K. Demeyer, "Waferoptics mass volume production and reliability," in Micro-Optics 2010, H. Thienpont, P. V. Daele, J. Mohr, and H. Zappe, eds., Proc. SPIE 7716, 771614 (2010).
- Y. Cheng, F. Tsai, A. Wang, and H. Chiu, "High brightness LED based projector with NTSC 120% wide color gamut," SID Symposium Digest 41, 976-978 (2010). [CrossRef]
- F. Fournier, and J. Rolland, "Design methodology for high brightness projectors," J. Disp. Technol. 4, 86-91 (2008). [CrossRef]
- E. Geißler, "Meeting the challenges of developing LED-based projection displays," in Photonics in Multimedia, A. Tervonen, M. Kujawinska, W. IJzerman, and H. De Smet, eds., Proc. SPIE 6196, 619601 (2006).
- S. Roelandt, L. Bogaert, Y. Meuret, A. Avci, H. De Smet, and H. Thienpont, "Color uniformity in compact LED illumination for DMD projectors," in Optics, Photonics, and Digital Technologies for Multimedia Applications, P. Schelkens, T. Ebrahimi, G. Cristóbal, F. Truchetet, and P. Saarikko, eds., Proc. SPIE 7723, 77230X (2010).
- C. Hoepfner, "PhlatLight photonic lattice LEDs for RPTV light engines," SID Symposium Digest 37, 1808-1811 (2006). [CrossRef]
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