COL 11(3), 031201(2013) CHINESE OPTICS LETTERS March 10, 2013
Lightweight spatial-multiplexed dual focal-plane
head-mounted display using two freeform prisms
Dewen Cheng (
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)
1
, Qingfeng Wang (
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)
1
,
Yongtian Wang (
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)
1∗
, and Guofan Jin (
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)
2
1
School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China
2
State Key Laboratory of Precision Measurement Technology and Instruments, Tsinghua University, Beijing 100084, China
∗
Corresponding author: wyt@bit.edu.cn
Received August 27, 2012; accepted October 10, 2012; posted online February 6, 2013
Accommodation and convergence play critical roles in the natural viewing of three-dimensional (3D) scenes,
and these must be accurately matched to avoid visual fatigue. However, conventional stereoscopic head-
mounted displays lack the ability to adjust accommodation cues. This is because they only have a single,
fixed image plane, but the 3D virtual objects generated by a pair of stereoscopic images are displayed at
different depths, either in front or behind the focal plane. Therefore, in order to view objects clearly, the
eyes are forced to converge on those objects while maintaining accommodation fixed on the image plane.
By employing freeform optical surfaces, we design a lightweight and wearable spatial-multiplexed dual
focal-plane head-mounted display. This display can adjust the accommodation cue in accordance with the
convergence cue as well as generate the retinal blur cue. The system has great potential applications in
both scientific research and commercial market.
OCIS codes: 120.2820, 220.1250, 220.3620, 330.1400, 330.7322.
doi: 10.3788/COL201311.031201.
The accommodation and convergence discrepancy (ACD)
problem in stereoscopic display is a well-known scientific
problem in the three-dimensional (3D) display research
community
[1−6]
. This problem has rec e ived increasing
attention in recent year s
[7−18]
. The accommodation dis-
tance of human eyes changes in accordance with the con-
vergence distance when the eyes gaze at natural 3 D ob-
jects. Specifically, the angle between the lines of sight of
the two eyes and the relaxation of the muscles are com-
patible. However, the harmony between accommodatio n
and convergence is disrupted when the user observes a
3D object through a ster e oscopic display. In this case,
the eyes need to focus on the virtual screen of the dis-
play to view the image clearly; however, the virtual 3D
objects are usually located in front or behind the vir tua l
screen. When the acc ommodation and convergence dis-
tances differ, accommodation cannot be varied to match
convergence. The ACD problem becomes more serious
as the disparity range of the virtual 3 D scene increases.
The ACD problem is determined by the essential char-
acteristics of conventional stereoscopic display technolo-
gies according to many previous works
[7−18]
. Such prob-
lem is a major cause of unnatural 3D perception and
visual fatigue. Fortunately, the accommodation distance
(D
v
) can be greatly changed with minor variatio ns of the
effective focal length (f
′
) of the eyepiece or the distance
l
′
between the eyepiece and the display device according
to Eq. (1), which is given as
D
v
=
(f
′
− l
′
)
(f
′
− l
′
) × l
erf
+ f
′
× l
′
, (1)
where l
erf
is the eye relief. The items in Eq. (1) are shown
in Fig. 1. Here, l
′
is les s than f
′
but their difference is
very small, such that D
v
can b e greatly changed with
subtle variations o f f
′
or l
′
.
The key issue is genera ting multiple continuous or
discrete focal planes in a stereoscopic display; further-
more, each foca l plane must corres pond to a virtual
screen with a specified distance. Several methods have
been proposed to ge nerate dual or multiple focal planes
for stereoscopic displays. These methods can be clas-
sified into spatial-multiplexed
[7−12]
and time-multiplexed
methods
[13−21]
.
Spatial-multiplexed methods include the s tack display
and splitting method. Rolland et al.
[7]
presented a com-
prehensive inve stigation to determine the minimum num-
ber of focal planes req uired for a spatial-multiplexed
head-mounted dis play (HMD). They concluded that an
image source consisting of a stack of at least 14 planar
displays with a total thickness of 22.5 mm is required
to accommodate from 0 to 2 D. Akeley et al. devel-
oped a three focal-plane pro totype by dividing a liq-
uid crysta l display (LCD) panel into three zones located
at different physical distances with three half mirrors
[8]
.
They studied a depth-filtering rendering technique to de-
termine how the image intensity at e ach pixel should be
assigned to the focal planes , thus reducing the number
Fig. 1. Schematic diagram showing the principle of a near-eye
display.
1671-7694/2013/031201(4) 031201-1
c
2013 Chinese Optics Letters