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Japanese Psychological Research
2000, Volume 42, No. 4, 213–221
Special Issue: Motion perception, eye movements, and orientation in real and virtual space
Visual ergonomics of head-mounted displays
MARINO MENOZZI
Institute of Hygiene and Applied Physiology, Swiss Federal Institute
of Technology, CH-8092 Zürich, Switzerland
Abstract: Head-mounted displays (HMDs) are increasingly used in the field and in the laboratory. Prolonged use of HMDs requires their ergonomics to be optimized in order to reduce
discomfort. Among factors causing visual load are the physical/optical properties of eyepieces
used in HMDs. Many HMD eyepieces are adjusted manually to fit the individual requirements
of the user. Proper adjustment of the eyepieces requires some knowledge of visual ergonomics. Additionally, in order to enable comfortable viewing, restrictions on the visual information presented in HMDs, such as the motion of objects in virtual depth, must be considered.
Visual performance and therefore visual load depends on the display technology used in HMDs.
Key words: head-mounted displays (HMDs), visual accommodation, vergence, resting position.
Head-mounted displays (HMDs) are becoming
increasingly popular, and the number of applications based on HMDs is rising in many
different fields, such as medicine, robotics,
and leisure. HMDs enable the simulation of an
observer’s visual environment. They are therefore a key element of “virtual reality.” Depending on the technique used in combination with
HMDs, the simulation can include the stereoscopic presentation of images to give the perception of depth. HMDs play an important role
in applications where the visual information
from the observer’s real world is combined
with artificial visual information. This kind of
application, so-called “augmented reality,” is
set to become particularly important in combination with tasks like the maintenance of
complex technical systems or surgery. The
integration of the displays into a helmet enables the system to account for the direction of
the head in space, thereby offering the possibility of simulating the whole of the observer’s
surroundings.
Usually, the optical set-up in HMD is optimized to simulate reality as closely as possible.
In order to achieve this aim, the resolution of
the images presented and angles subtended by
the images are maximized. These requirements
are still a technical challenge. The typical interpixel distance on the small, high-resolution
displays used in HMDs is about 14 µm, which
is more than an order of magnitude smaller
than on the typical computer monitors used for
office work. Nonetheless, an interpixel distance
of another one or two orders of magnitude
smaller would be required to give a photorealistic representation of visual information
in HMDs.
In order to give a large field of view, small
displays have to be placed close to the observer’s eye. In order to view the information
presented on the display clearly, an eyepiece is
introduced between the eye and the display.
A large variety of HMDs are available on
the market. Books have been published summarizing the various technical principles behind
HMD technology (Melzer & Moffitt, 1997;
Velger, 1998).
It can be expected that different technical
set-ups exert a different load on the visual
© 2000 Japanese Psychological Association. Published by Blackwell Publishers Ltd, 108 Cowley Road,
Oxford OX4 1JF, UK and 350 Main Street, Malden, MA 02148, USA.
214
M. Menozzi
system. The ability of different HMDs to fit
the needs of the visual system cannot be determined by analyzing the literature alone. Reports
in the literature of the human factors involved
are almost exclusively based on evaluations
carried out using a single device (Hasebe,
Oyamada, Ukai, Toda, & Bando, 1996). There
are also occasional reports of investigations of
the suitability of a single device for accomplishing a variety of different visual tasks. In
some studies (Edwards & Demczuk, 1993;
Näpflin & Menozzi, 1998) the visual load of
tasks carried out using HMDs is compared
with the visual load of similar tasks carried out
under natural viewing conditions or carried out
by means of a two-dimensional display unit.
Factors responsible for visual load can roughly
be subdivided in two categories: technical
factors and task-specific factors. The technical
factors causing visual load are mainly due to a
mismatch between the properties of the technical elements in HMDs and the needs of
perception. The low contrast of displayed
images in HMDs degrades visual acuity and
may therefore cause visual load. Among the
symptoms caused by technical factors in stereoscopic HMDs are the defocusing of objects
(Miyashita & Uchida, 1990) and stress of the
accommodation-vergence system (Menozzi,
Näpflin, & Krueger, 1991; Okuyama & Miyao,
1998). Task-specific factors causing visual load
for users of HMDs are due to a mismatch of
the tasks carried out and cognitive demands.
In this paper I focus on the role of two technical factors, namely the eyepiece and the display used in the HMD. Furthermore, I discuss
the role of stereoscopic depth as a factor
causing discomfort for the users of HMDs.
Impact of optical errors on the
visual ergonomics of HMDs
In HMDs, the display is presented close to the
eye. The actual distance may vary between
5 cm and 10 cm. Since the nearest distance to
the eye at which things can be seen clearly is
about 10–15 cm in very young persons and
about 1 m or more in older persons, an optical
system needs to be introduced between the
© Japanese Psychological Association 2000.
observer’s eye and the display. This optical
system shows the image of the display at a
distance, usually at infinity, to allow a normal,
so-called emmetropic eye, to focus the image
properly. Given the closeness of the display, an
optical system consisting of lenses with a high
refractive power is required. The optical power
required of the system depends mainly on the
distance of the display to the optical system.
Since this distance must be smaller than the
distance between the observer’s eye and the
display, the power often has to exceed 10 or
20 D.
Given the high power of the optical system,
physical/optical errors play an important role
in the quality of the perceived image. These
errors are spherical and chromatic aberration,
coma and prismatic effects. The errors could
be corrected by complex optics, but these are
expensive and so are not incorporated within
most HMDs.
The optical systems are generally assembled
within the eyepieces. Most HMDs are provided
with eyepieces, to allow adjustment for any
spherical refractive errors of the eye (myopia/
hyperopia) as well as for variations in interpupil distance. While these two adjustments
are required to establish the proper viewing
conditions with HMDs, visual load may result
if the eyepieces are not adjusted correctly.
If the lateral separation between the two eyepieces is not matched to the interpupil distance,
the observer’s line of gaze will not coincide
with the optical axis of the eyepiece. Figure 1
illustrates the optical effects when the observer’s
line of gaze departs from the optical axis of
the eyepiece. In order to avoid confusion, a
particular case of off-axis vision is represented
in which the line of gaze is parallel to the optical
axis of the eyepiece. Furthermore, in order
to demonstrate the optical effects clearly, the
line of gaze has been replaced by a bundle of
parallel rays, simulating the course of incoming
rays, which is required for a clear vision when
the eye is accommodated to infinity.
As can be seen in Figure 1, the peripheral
bundles (hatched) are focused at a point closer
to the lens than is the case for the bundle imaged by the center of the lens (gray). This shift
[
Z
Y
O
N
M
Visual ergonomics of head-mounted displays
Display
h
O
Observer
Eyepiece
G
F
Figure 1. Schematic representation of the effects
of spherical aberration and the prismatic
effect, which are present in the periphery of the eyepiece (lens). O = optical
axis of the eyepiece. h = distance between center and periphery of the lens.
F = location of focal plane of the lens.
G = location of focal plane for off-axis
rays (hatched bundle).
in focus is caused by spherical aberration.
Spherical aberration is a function of the
distance (h) between the center of the lens and
the position where the ray hits the lens. In
addition to the effects of spherical aberration,
the whole bundle is excessively converged
towards the optical axis due to the prismatic
effect of the lens.
Spherical aberration can in part be compensated by accommodation of the eye or by
adjusting the power of the eyepiece. Compensation of prismatic error requires an adaptation
of vergence. With high-power lenses, prismatic
error can be significant. The prismatic error can
be calculated using Prentice’s formula (Long,
1991), which states that the prismatic error in
cm/m is equal to the product of the power of
the lens in diopters (D) times the amount of
off-axis shift of the line of gaze in cm. Therefore, if a 20 D lens is used and the misalignment
between the line of gaze and the optical axis of
the eyepiece is about 1 mm, the prismatic effect
is 2 cm/m. This means that the line of gaze is
deviated by 2 cm at a viewing distance of 1 m,
which is equivalent to about 1.1º.
Since the optics of the eyepiece can be
compared to a convex lens, the line of gaze is
always deviated towards the optical axis of
the eyepiece. If both eyes fixate the same point,
as is the case in augmented reality, a fusional
215
effort is required to make the line of gaze of
both eyes coincide at the fixation point. There
is little information in the literature on the load
on the visual system caused by fusional effort.
It is stated that comfortable vision requires
fusion not to exceed a given proportion of
fusional limits. Arbitrary estimates of this
proportion are one-third (Percival’s rule) and
one-half (Sheard’s rule) (Alpern, 1969; Goss,
1995). The significance of prismatic errors can
be estimated by comparing the error to the
size of the range of fusion enabling comfortable vision defined by Percival’s or Sheard’s
rule. Norm data about fusional limits vary
among different authors. Kramer (1949), for
example, states that the range of fusion is about
16–20 cm/m for convergence and about 6–8
cm/m for divergence at a viewing distance of
6 m. Goss (1995) reviewed the literature and
came to the conclusion that the range of fusion
is about 10 cm/m in convergence and 7 cm/m in
divergence for a viewing distance of 6 m. Even
though the numbers differ, it can be concluded
that a convergent fusional movement requires
less effort than a divergent movement.
A misalignment of the line of gaze appears
if the distance between the two eyepieces is
not set equal to the interpupil distance. The
following example illustrates that, independently of which fusional limit is considered,
small errors in the adjustment of the distance
of the two eyepieces lead to uncomfortable viewing, due to prismatic effects. If the
distance between the eyepieces has been set to
2 mm less than the interpupil distance (1 mm
per eye), a divergent fusional effort of 4 cm/m
is required for proper binocular vision. This
value exceeds the tolerance for comfortable
vision and is actually close to the limit of the
fusional range.
As has been shown by Nagata (1996),
fusional limits are reduced by an increase in the
geometrical complexity of the area surrounding the fixation point. If complex images, such
as photo-realistic scenes, are presented in a
HMD, the mismatch between the distance of
the eyepieces and the interpupil distance by,
say, 2 mm might require a fusional effort which
is unachievable.
© Japanese Psychological Association 2000.
216
M. Menozzi
If HMDs are used in virtual reality as
opposed to augmented reality applications,
misalignment of the eyepieces is much less
critical, provided that separate displays are
used for each eye. Under this condition, the
prismatic effects compensate for the need to
adjust the lateral distance between the two
displays for varying interpupil distances. In such
a system the induced heterophoria is small,
about 0.5 cm/m per 1 mm mismatch between
the interpupil distance and the lateral separation of the optical center of the eyepieces.
Besides horizontal misalignment, a vertical
misalignment of the eyepieces in HMDs is also
possible. The optical axis of the two eyepieces
defines a plane. Vertical prismatic effects appear
when the lines of gaze are not in the same
plane. The size of this prismatic effect can
be estimated by considering the tilt of the line
connecting the two centers of the two eyes with
respect to the plane. Assuming eyepieces with
a power of 20 D and a normal interpupil
distance of 65 mm, a total prismatic effect (two
eyes) of 4 cm/m is caused if the connecting line
is tilted by 2º. Vertical fusional limits are about
one-fifth of horizontal limits (Goss, 1995).
Vertical misalignment of HMD is therefore
critical to visual load.
Resting position of
accommodation and vergence
Many HMDs are equipped with eyepieces permitting manual adjustment of the focal length
in order to enable compensation for spherical
refractive errors. Therefore, at least for individuals without astigmatism, an HMD can be
used without the need for corrective glasses.
The proper adjustment of the focal length is
at least a twofold problem. Because the eye is
able to meet varying needs for refractive power,
the focal length of the eyepieces can be set
to any value, imaging the screen in a range
between the near point and far point of accommodation. From studies of visual load with
close-up work (see Watten, 1994, for a review)
we know that prolonged visual tasks requiring
a high degree of accommodation can produce
complaints of asthenopia. Therefore the focal
© Japanese Psychological Association 2000.
length of the eyepieces should be set to a value
enabling focusing of the image without the need
for a high degree of accommodation. The load
associated with accommodation can hardly be
estimated if the eye is not maximally accommodated. It is almost impossible to adjust the
focal power of an eyepiece by relying on the
perceived load on the accommodation system.
By definition, the least accommodation is
achieved at zero diopters, that is, when the
normal eye focuses objects at infinity. This
value, however, does not correspond to the
minimum load on the accommodation system.
Minimum load is present when the eye is
accommodated to focus an object close to the
far point (Menozzi, 1999). The minimum load
for accommodation probably corresponds to
the so-called resting position or dark focus of
accommodation, but consideration of the resting position of accommodation is difficult, since
it depends on variables such as mental workload, drug intake, time of day, the optical
properties of the stimulus and age (Korge &
Krueger, 1984; Krueger, 1985; Owens & WolfKelly, 1987; Post, Johnson, & Owens, 1985;
Winn, Gilmartin, Mortimer, & Edwards, 1991).
Roughly, the mean value of the resting position
for accommodation is at about 1.5 D for young
and about 1 D for elderly people (Krueger,
1985). Therefore, reducing by 1 D the power of
the eyepiece needed for clearly viewing objects
at infinity can help reduce the load for accommodation. Due to the ability of the eye to
compensate for a myopic correction by means
of accommodation, the procedure for setting
the eyepiece in focus at infinity should start by
installing the lens of the highest positive power
available. The power is then decreased little by
little until the image is in focus.
As with accommodation, the resting position
of vergence (dark vergence) depends on a
variety of factors over which there is little control. Under conditions of dark vergence, the
fixation point is at a distance of about 50–100 cm
from the eye (Conrady, Krueger, & Zülch, 1987).
Conrady et al. (1987) showed that the eyepieces of binocular microscopes were best
adjusted to enable vergence settings close to
dark vergence. It is reasonable to assume
Visual ergonomics of head-mounted displays
that this finding also applies to settings of the
eyepieces in HMDs. Therefore, the lateral
distance between the eyepieces should be
adjusted to be less than the interpupil distance.
It is important to emphasize that persons
using HMDs must be made aware that the
adjustment of the eyepieces is crucial for visual
comfort and that the most comfortable viewing
condition is achieved when gazing at a distance
of about 1 m. Therefore, the optimal setting
requires the optical power of the eyepieces to
be reduced by 1 D and the distance between
the two eyepieces to be reduced by 1 mm compared with settings for viewing at infinity.
Linkage of accommodation and
vergence
In binocular HMDs, two images, one for each
eye, are presented. Therefore, stereoscopic
images can be displayed, enabling the presentation of objects in depth. Objects presented at
different virtual distances from the observer’s
eye may require different settings for vergence.
The difference in vergence between the fixation
of a point at infinity and the fixation of a point
at 40 cm is about 4.6º, which is about 8.1 cm/m
per eye, if an interpupil distance of 65 mm is
assumed. Accommodation does not require
different settings if objects are presented at
different depths.
Vergence and accommodation are crosslinked mechanisms (Krueger, 1968; Schor,
Alexandr, Cormack, & Stevensen, 1992). Figure 2 shows a simplified schematic diagram of
the accommodation and vergence mechanisms.
Accommodation and vergence are driven
by two separate mechanisms. The input for
both mechanisms are retinal images as well as
cognitive data regarding the distance of perceived objects. The two mechanisms exchange
this distance information. It is therefore possible
to drive accommodation with information fed
by the vergence system and to control vergence
using information retrieved from the accommodation system. When visual tasks are carried
out using HMDs, the information in the two
systems concerning the distance of objects may
be incompatible, since accommodation should
217
Cognition
Retina
Error
detection
Extraocular
muscles
Ciliary
muscles
Computation
Computation
Vergence
Error
detection
Accommodation
Figure 2. Simplified diagram showing the crosslinking of accommodation and vergence.
remain at a fixed value, independent of the
virtual distance. Accommodation control competes with the cross-linked input in order to
avoid or minimize retinal blur. Depending on
the amount of incompatible information between
accommodation and the cross-linked input,
and depending on individual factors, the stress
of accommodation in dealing with conflicting
information may cause asthenopia. Most probably this is related to an individual’s ability to
drive accommodation and vergence separately,
via an ability to ignore the cross-linked input.
The ability to cope with conflicting information
from the cross-link is determined by the size of
the so-called zone of clear single binocular
vision (ZCSBV). Research on asthenopia caused
by an incompatibility of distance information is
not yet at the point to allow statements on how
to avoid it. As a first attempt at estimating the
limits of this incompatibility, we again might
consider Percival’s or Sheard’s rules, mentioned
above. If accommodation is set at a given value,
comfortable vision requires the variation of
vergence not to exceed the ZCSBV. If accommodation is at 0 D and Percival’s rule (onethird) is applied to Morgan’s norms on the
fusional limit of convergence, which is 10 cm/m
(Goss, 1995), maximum vergence for comfortable vision is 3.3 cm/m. Therefore, stereoscopic
objects should not be presented closer than
about 2 m (1.3 m if Sheard’s rule is considered)
for subjects with a normal interpupil distance
© Japanese Psychological Association 2000.
M. Menozzi
© Japanese Psychological Association 2000.
–1.2
Accommodation (D)
of 65 mm. This limit would greatly restrict the
range of depth presented using HMDs.
As pointed out by Nagata (1996), binocular
fusion depends, among other things, on the
geometrical complexity of the area surrounding the target to be fused. Nagata found a
decrease in fusional limit if this area surrounding the fixation object is changed from dark
to bright. He also found a reduction in the
fusional limit if the blurred surrounding area
was made sharp. Morgan’s normative data
were set up using fixation targets with a rather
simple surround. With HMDs displaying a
complex surround, we therefore may predict a
reduced range for a clear binocular vision. In
Nagata’s experiment, fusional limits dropped
to about one-third if a complex area surrounding the fixation target was used instead of a
simple one. This reduces the virtual depth
within which objects can be presented without
greatly loading the visual system to a range
within which stereoscopic presentation is
meaningless.
The presentation of objects moving in depth
is an additional factor to be considered in
HMDs. As in the static case discussed above,
the vergence mechanism feeds information to
the accommodation mechanism under dynamic
conditions, that is, during vergence eye movements, as shown in Figure 3. The amount of
information transfer has been shown to depend
on the velocity of vergence (Menozzi, 1989).
Figure 3 shows a continuous record of accommodation and vergence during a task in
which the degree of vergence required was
varied over time, in a triangular function. The
amount of accommodation required was fixed
at 2 D. As can be seen in Figure 3, changes in
accommodation follow changes in vergence. If
the time course of accommodation is approximated using the triangular function, we can
estimate the deviation of accommodation
from its ideal value for clear vision. In the case
illustrated in Figure 3 the deviation has an
amplitude (A) of about .4 D. The amount by
which the accommodation deviates from its
ideal value depends on the frequency of the
triangular motion. Figure 4 shows deviation of
accommodation as a function of the frequency
Linear regression
–2.0
A
–2.8
1
3
5
Time (s)
1
3
5
Time (s)
Vergence
(relative units)
218
Figure 3. Accommodation and vergence during
stimulation of vergence using a target
requiring a triangular change of vergence
over time. Transfer of information from
the vergence system to the accommodation system can be characterized
by assessing the amplitude, A, which
denotes the amount by which the actual
accommodation departs from the accommodation required (in this case –2D).
of the triangular motion of the vergence. The
amplitude by means of which vergence was
stimulated was 86.4′ (minutes of arc), which is
equivalent to 2.5 cm/m per eye. Thus, 2.5 cm/m
is the amount of vergence required to shift the
fixation point from infinity to a viewing distance
of about 130 cm or from 130 cm to about 65 cm.
The transfer of information from the
vergence system to the accommodation system
is like a band-pass filter. Under conditions of
a fast or a very slow change of vergence, little
or no information is transferred from the
vergence system to the accommodation system.
A maximum transfer of information is found
when the mirror used to control the vergence is
tilted at an angular speed of about 5–15′/s,
corresponding to a frequency of .06–.17 Hz.
Visual ergonomics of head-mounted displays
.7
Amplitude of accommodation
Amplitude of mirror = 34.2'
.5
.3
.1
0
0
20
40
60
80
100
Angular speed (minutes of arc/s)
Figure 4. Modulation transfer function reporting
the information transfer from the vergence system to the accommodation
system, expressed as the maximum
amount of accommodation departing
from the stimulus value (2 D). The frequency refers to the triangular movement of the vergence (Menozzi, 1989).
In this experiment, vergence was controlled by the tilt of a mirror.
Unfortunately, we do not know yet to what
extent this dynamic stress of accommodation
has the potential to cause asthenopia. Possibly,
people with a low-gain cross-link, as is the
case in presbyopia, are less susceptible to the
phenomenon.
Impact of display technology
on eye movements
Intermittent luminance of stimuli has been
shown to have a significant impact on eye
movements (Kennedy, Brysbaert, & Murray,
1998; Krummenacher, 1996; Neary & Wilkins,
1989). Prolongation of the duration of saccades
as well as corrective saccades are found in
subjects gazing at objects presented on cathode
ray tubes (CRTs). The refresh rate as well as
the persistence of the phosphor have been
shown to be correlated with this phenomenon.
We may therefore expect that the visual load
on users of HMDs is in part caused by the
temporal characteristics of the luminance of
the pixels comprising the displays.
219
Because of the rapid development of liquid
crystal display (LCD) technology, in terms of
both optical features and price, LCDs have
come to compete with CRTs. Today, many
HMDs are equipped with LCDs. The time
course of the luminance of pixels in LCDs
differs from that of CRTs. We may therefore
expect the two technologies to have a different
impact on eye movements, on visual performance and on load on the visual system.
Recently we carried out two experiments to
compare the effects of the two technologies on
visual performance. In a pilot study, 10 subjects
carried out a search task using an LCD and
using a CRT monitor. A 10.4-inch STN LCD
monitor with a resolution of 640 × 480 pixels
and a 14-inch CRT monitor with a resolution of
800 × 600 pixels and a frame rate of 60 Hz were
used in the study. The task was to detect a letter
“F” (target) among series of “E”s (distractors).
Letters subtended an equal angle on both
monitors. Time and error rate were recorded as
a measure of performance. Further details are
given by Menozzi (1999). The main result of
the pilot study was a decrease in the error rate
by about 30% when an LCD was used.
We then carried out a more detailed study to
compare reaction time and error rate on the
same task as described above, while using an
LCD monitor and CRT monitors with varying
refresh rates. As can be seen from Figure 5, the
error rate increased with increasing refresh
rate of the CRT monitors. The error rate with
the LCD was lower than the rates with the
CRTs. The variation in error rate was statistically significant. A similar result was found
for reaction times. The details of this study will
be presented in a forthcoming paper.
Some practical advice
Much further research and development are
needed in order to make HMDs more ergonomic. Most available HMDs are equipped
with features enabling the optics to be adjusted
to meet the requirements of the individual
user. Experience suggests insufficient knowledge on the part of the user is one of the
principal factors responsible for a high visual
© Japanese Psychological Association 2000.
M. Menozzi
220
Total errors, 10,000 trials, 24 subjects
Total errors
160
120
80
40
0
48 Hz
60 Hz
75 Hz
Display technology
LCD
Reaction time (s)
Median reaction time, 10,000 trials, 24 subjects
8.6
8.4
8.2
8.0
7.8
48 Hz
60 Hz
75 Hz
Display technology
LCD
Figure 5. Total errors and median of reaction
times assessed using 24 subjects and
four different monitors.
load in tasks carried out using HMDs. Advice
on how to fit HMD optics to individual needs is
outlined below:
1.
Set the distance between the eyepieces
equal to the interpupil distance.
2. Select the power of the eyepieces needed
to focus the display images at infinity.
3. Align the exit pupil of the eyepieces with
the pupils of the eye.
4. Then account for the resting position of
accommodation and vergence by reducing the power by 1 D and by reducing the
distance between the eye pieces by about
1 mm.
5. Avoid slow stereoscopic motion in depth.
6. Preferably use LCD instead of CRT
displays.
The procedure for fitting starts with an
assessment of individual interpupil distance for
gazing at an object at infinity. An easy way to
accomplish this is to assess one’s own interpupil distance while gazing into a mirror placed
© Japanese Psychological Association 2000.
in the frontoparallel plane at a short distance
(about 20 cm) from the eyes. A ruler is placed
on the mirror so that the position of each
center point of the pupil can be read off. It is
important to make sure that position of the
head remains fixed during these readings and,
in order to prevent any errors of parallax, the
left eye should be closed while reading position
of the right eye and vice versa. This value for
the interpupil distance is then used to adjust
the separation of the eyepieces.
HMDs should allow the user to wear corrective lenses in combination with the eyepieces,
to reduce visual load. Refractive errors like
astigmatisms cannot be compensated by means
of adjustment of the eyepieces.
The following procedure facilitates the adjustment of the power of the eyepiece for
the clear viewing of displays at infinity. The
procedure requires the observer’s eyes to be
corrected for far vision. Starting from the highest available positive power of the eyepieces,
the power is subsequently decreased until a
clear image of the display is perceived for
the first time. This procedure is carried out
monocularly.
A proper alignment of the optical axis of
the eye and eyepiece can in part be achieved
by centering the exit pupil of the eyepieces to
the user’s pupils. Provided the exit pupil is not
too small, centering can be accomplished by
maximizing perceived luminance of the display
while shifting the eyepiece in the frontoparallel plane.
If during prolonged vision with HMDs
asthenopia is experienced, accounting for the
resting position of accommodation and vergence
may help to reduce the visual load. Since it
is almost impossible to account for the resting
position of accommodation and vergence under
field conditions, we suggest this is done by
simply reducing the power of the eyepiece by
about 1 D and by decreasing distance between
the eyepieces by about 1 mm. These changes
will shift the initial settings of the HMD from
infinity to a viewing distance of about 1 m.
Especially in tasks where HMDs are used to
track or search objects, the use of LCDs is a
better choice than CRT technology.
Visual ergonomics of head-mounted displays
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(Received Nov. 22, 1999; accepted May 13, 2000)
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