CRT versus LCD: A pilot study on visual performance and

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Displays 20 (1999) 3–10
CRT versus LCD: A pilot study on visual performance and suitability of
two display technologies for use in office work
Marino Menozzi*, Urs Näpflin, Helmut Krueger
Institute of Hygiene and Applied Physiology, Swiss Federal Institute of Technology, Clausiusstrasse 25, CH - 8092 Zürich, Switzerland
Received 24 July 1998; received in revised form 22 September 1998; accepted 22 September 1998
Abstract
Cathode ray tube (CRT) display and liquid crystal display (LCD) were compared for their suitability in visual tasks. For this purpose visual
performance was assessed by means of a search task carried out using both displays with different levels of ambient light. In addition,
suitability was rated subjectively by users of visual display units (VDUs). Error frequency for search tasks carried out using LCD were
significantly smaller when compared to error frequency for tasks at CRT. LCD gave rise to 34% less errors than did CRT. Reaction time in
search task was found to be significantly shorter using LCD when tasks were carried out in darkness. Subjective rated suitability of LCD was
scored twice as high as suitability of CRT. Results indicate that LCD used in this experiment may give better viewing conditions in
comparison to CRT display. 䉷 1999 Elsevier Science B.V. All rights reserved.
Keywords: Cathode ray tube (CRT); Liquid crystal display (LCD); Visual performance
1. Introduction
Liquid crystal displays (LCD) have become more and
more popular in visual display units (VDUs) (Cladis [1],
Firester [2]). Size of actual LCD can cover the needs of
most applications running on computers. For many reasons
LCD might become more important and might replace cathode ray tube (CRT) displays in many applications. Weight
and volume of LCD are among most the important advantages when compared to CRT VDU. A 17 00 CRT typically
occupies an area of 40 cm × 45 cm (width × depth). If
according to suggested settings for work place (e.g. DIN
4549 [3]) a desk of 120 cm × 80 cm is considered, a 17 00
CRT may occupy about one-fifth of the surface of the desk.
Dimensions of LCD monitors are smaller, therefore requiring less space and facilitating handling of the monitor. From
an ecological point of view operation of an LCD is more
advantageous than that of a CRT. Owing to lower power
consumption, LCDs emit less heat, therefore causing less
problems in air conditioning at offices where many displays
run at the same time. Low power consumption also gives
LCD an advantage over CRT with regard to potential of
electromagnetic radiation for causing possible effects on
biological matter.
Notebook PCs are very popular during travel or at any job
* Corresponding author. Tel.: ⫹ 41 1 632 39 81; Fax: ⫹ 41 1 632 11 73.
requiring a high frequency of changes in location of the
work place as is the case with jobs like those of a representative or a salesman. Minimal power consumption and light
weight is a must for displays used in notebooks. Actually
there are only very few alternatives to LCD for displays for
use in notebooks. Data compiled by Nelson [4] and by
Caladis [1] demonstrate that market volume of flat panel
displays is predominantly controlled by LCD technologies.
As a result of technical progress, physical-optical quality
of LCD has immensly improved (Tannas [5]). Among
others, backlight techniques, thin film super twisted LCD
and new materials enable a better visibility of information
displayed on an LCD and make requirements for ambient
light of VDU less critical. In contrast to CRTs, LCDs have
sharp edged pixels being therefore more suited to produce
sharp edged horizontal and vertical lines. Moreover, pixels
of LCD are not subject to spatial instabilities such as jitter.
Uncontrolled external electromagnetic radiation may induce
jitter at a CRT reducing legibility of characters displayed.
Visibility of flicker may be less at LCD because of a more
favorable time-course of luminance of single pixels. Most
CRT displays are equipped with phosphors with a short
persistence-time. Light emitted by pixels of CRT can be
compared to series of single flashes causing the perception
of flicker phenomenon which is especially pronounced at
large screens with a low refresh rate (Farrell [6]). In
contrast, shape of time-course of luminance of a single
0141-9382/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved.
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M. Menozzi et al. / Displays 20 (1999) 3–10
pixel at LCD is square wave-like causing less or no perception of flicker. Flickering light is supposed to disturb control
of eye movements (Neary [7]), possibly a cause for visual
complaints. Drawbacks of use of LCD are, among others,
reduced brightness and restricted viewing angle (Nelson
[4]).
VDUs are often used in the presence of ambient light.
Ambient light is reflected on the front of the screen thereby
reducing contrast of information displayed. Contrast reduction can be controlled by measures applied on the surface of
the screen. With regards to material used, glass in CRT and
organic material in LCD, LCD may have an advantage over
CRT displays when used in bright ambient light. To our
knowledge, reflectivity of LCD has not yet been assessed
and compared to reflectivity of CRT. Activation of pixels of
LCD might change reflectivity of the display. To account for
this possibility, reflectivity should also be assessed while
information is presented on the display.
Insufficient optical quality of displays is a potential candidate for causing visual complaints (Cole [8], Jackson [9],
Krueger [10], Läubli [11]). The domain of visual complaints
is complex. Inter-individual variations in physiological and
psychological predispositions of VDU users are probably
amongst the most confounding factors in localization of
causes of the complaints. An accurate assessment of optical
quality of displays may help in identification of causes for
complaints. In accordance with the concept of strain and
stress (Hacker [12]) we may deduce that complaints are
closely related to visual performance. Good optical quality
of a display constitutes a low visual strain and facilitates
reading (Grisham [13], Legge [14]). Scientific literature
offers some papers on optical quality of VDU and visual
performance (Edwards [15], Farrell [6], Montegut [16],
Roufs [17]). MacKenzie et al. [18] compared user performance in manipulative tasks carried out using CRT or LCD.
In their experiments subjects had to select targets on the
display by using a mouse. Time between successive button
down actions of the mouse were recorded and defined as
movement time. MacKenzie et al. [18] found that LCD gave
rise to 34% longer movement times than did CRT. Saito et
al. [19] recorded several visual functions during different
tasks carried out using different displays, such as LCD,
CRT and plasma displays. Visual accommodation was
found to be faster while using CRT when compared to
tasks where LCD or plasma display was used.
Based on actual results reported in the literature it is
difficult to draw a conclusion on the suitability of a particular display technology to improve viewing conditions. We
therefore set up an experiment by means of which visual
performance was assessed using either LCD or CRT at
otherwise identical conditions.
There are many parameters contributing to an overall
visual performance. As a first attempt in investigating suitability of mentioned techniques we were interested in parameters related to visual performance at a common task at a
VDU. If office work is considered to be a common task at
VDU, search tasks may constitute a good starting point. We
investigated visual performance by assessing time for
completing a search task and the amount of errors occurring
during a search task. Suitability of a display for use in office
work is controlled by factors more than only visual performance. Field studies constitute a method accounting for a
large quantity of factors. However, a large number of
subjects must be observed over a long period of time in
order to be able to cancel out individual interferences.
Long-term observations in the field lack from constancy
of environmental variables confounding possible effects.
Mentioned limitations can partially be overcome, if suitability is assessed subjectively. Users of VDUs may judge suitability considering a multitude of factors and their impact on
stress at long-term exposure. In this study we undertook an
attempt to rank suitability of the two displays by interviewing VDU users.
2. Method
2.1. Procedure
Performance was evaluated by means of a search task in
which we recorded reaction times for detecting targets and
amount of errors which occurred during the task. The paradigm used in our task was adapted from paradigmata used to
assess visual performance in human factors in lighting (see
Boyce [20]) and from paradigmata used in basic visual
science (e. g. Fiorentini [21], Treisman [22]). Adaptations
aimed to consider particular conditions of office work. A
two-alternative forced-choice task was set up using an
uppercase letter ‘‘F’’ as target and uppercase letters ‘‘E’’
as distractors. Target and distractors were arranged in a 20
× 20 matrix with equally spaced horizontal and vertical
gaps. Target was shown in 50% of the displays. The task
consisted in scanning the display and pressing either the
‘‘yes’’ or the ‘‘no’’ button of an answer box, depending
on whether target was seen or not. Subjects were informed
after each trial on the correctness of the given answer. An
acoustic feedback was used for this purpose. The subjects
were asked to accomplish the task within a minimum time
avoiding errors.
In order to account for the fact that ambient light may
vary depending on location of workplace, experiments were
carried out at two levels of ambient light. In one condition,
further on referred to as darkness, horizontal and vertical
illumination was about 50 lux. In the other condition,
further on referred to as brightness, vertical illumination
was 250 lux while horizontal illumination was set to
550 lux. Only diffused light consisting out of indirect
daylight or artificial light reflected from the surrounding
walls was used to install described levels of ambient light.
Each subject completed the task at both displays and on both
conditions of ambient light. The four different settings, i. e. two
displays used at two different conditions of illumination, were
M. Menozzi et al. / Displays 20 (1999) 3–10
administered in random order. The experimental session
consisted out of four blocks of 40 trials each. A training
course comprising one block of 15 trials each was carried
out in brightness at each of the two displays before the
experimental session. Data obtained from trial session
were discarded. At the end of the experiment, subjects
were asked to rate subjectively how much they would
appreciate completing office work in the four experimental
conditions.
Subjects were informed on aim and procedure of the
experiment. Before starting the experiment, subjects gave
their written consent for participating in the experiment and
for using their data for scientific evaluation.
2.2. Instrumentation
A program was developed and run on a notebook
(Toshiba Satellite 100CS). This hardware and software
enabled to display a matrix (20 × 20) consisting of target
and distractors. The program enabled control of the percentage of displays containing a target and to present displays
with and without targets in random order within a sequence
of 40 displays. As mentioned earlier probability for target
was set to 50%. The location of the target was selected
randomly to be one of the 400 places of the 20 × 20 matrix.
Reaction time and answer given by the subjects were
recorded using an answer box (Von Buol, submitted [23])
which was connected to the notebook. The answer box was
equipped with a quartz driven timer. The timer was started
by a photodiode connected to the display on which the
matrix was presented. This technique enabled synchronization of the start of the timer with the switching on and off of
pixels within a particular area of the screen, avoiding therefore errors caused by buffering of data occurring in high
level programming languages and high level operating
systems. The timer was stopped by the subjects by pressing
either the ‘‘yes’’ or the ‘‘no’’ button on the answer box
depending on whether they detected a target or not. A
new display was presented three seconds after the answer
had been recorded. During these three seconds a number
indicating the number of trials completed was displayed in
the center of the display. The number served as a target for
fixation for the subjects during the intermediate period
between the displays.
The display of the notebook was used as LCD in one
condition, further on referred as LCD-task. The LCD was
a backlight dual scan STN 10.4 00 color display with a resolution of 640 pixels × 480 pixels (horizontal × vertical).
The distance between pixels was 0.3 mm in horizontal and
in vertical direction. Referring to the manual of the display
the visible area of the display was 217.2 mm × 164.4 mm.
A 14 00 CRT (Dell model D1428-HS, shadow mask color
display, no surface treatment) was connected to the external
display connector of the notebook and used in the task
where the matrix was displayed on a CRT. This task is
further on referred as CRT-task. The dot pitch of the CRT
5
was 0.28 mm. The CRT display was run at a resolution of
800 pixels × 600 pixels at 60 Hz frame rate.
Black distractors and targets were generated on a white
background. The luminance of the background was set at
57 cd/m 2 at both displays, CRT and LCD. In order to avoid
changes in light adaptation, background of the display was
also set to 57 cd/m 2 during the intermediate period. A large
black area served to estimate luminance of the black characters. Luminance measured within this black field was
1.4 cd/m 2 when measured with no ambient light, i. e., in
darkness. In brightness luminance on the screen is increased
because of reflections on the screen.
Reflection properties of the two displays were assessed in
accordance with ISO 9241-7 (1998) [24] procedure. By
means of this procedure diffuse (RD) and specular (Rs)
reflectance properties of a display were evaluated. Specular
reflectance is evaluated using an extended light-source
subtending 15⬚ (Rs(EXT)) and a small light-source subtending aproximately 0.9⬚ (Rs(SML)). In general, reflectances of
LCD used in our experiment were much smaller than the
ones of CRT. For LCD all three reflectances were found to
be less than 0.01 (RD ˆ 0.007, Rs(EXT) ⬍ 0.001, Rs(SML)
⬍ 0.001). For CRT values of reflectance were RD ˆ 0.035,
Rs(EXT) ˆ 0.037, Rs(SML) ˆ 0.005. We can therefore
estimate that for CRT, surplus luminance owing to reflection of ambient light is about 3 cd/m 2 (specular reflection).
In brightness, contrast is therefore reduced by about 10% if
expressed in term of modulation (Michelson contrast).
Contrast reduction based on diffuse reflections can be
neglected in any of the experimental conditions. As illumination was carried out using diffuse light, we expect reflections not to be a factor interfering with visual task.
Viewing distance in LCD-task was set to 50 cm. Equality
of viewing angle of characters and matrix in LCD-task and
in CRT-task was achieved by adjusting settings of CRT
monitor (magnification of image) and by adjusting viewing
distance in CRT-task. As a result of limitation in settings of
CRT, viewing distance in CRT-task had to be increased by
10 cm compared to viewing distance at LCD-task in order to
equalize the sizes of characters and matrix displayed in
LCD-task. The size of the letter E subtended 20.6 0 ×
26.8 0 (horizontal × vertical), roughly corresponding to a
12 point size letter E viewed at a distance of 50 cm. Horizontal and vertical spacing of neighboring characters were
approximately equal (about 0.6⬚). The size of the matrix was
about 10.7⬚ × 12.8⬚ (horizontal × vertical). We did not use
any rest to fix viewing distance. Subjects were asked to keep
their viewing distance fixed during the experiment.
Subjective ratings of suitability of each of the four experimental settings for office work was accomplished by asking
the subjects ‘‘How much would you like to work using this
screen on this ambient light?’’ (German: ‘‘Wie gerne
würden Sie an diesem Bildschirm bei diesen Beleuchtungsbedingungen arbeiten?’’). Subjects answered by putting a
cross on an interval scale of six steps. They were instructed
to make the position of the cross coincide with their ratings.
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M. Menozzi et al. / Displays 20 (1999) 3–10
The scale ranged from ‘‘not at all’’ (‘‘sehr ungerne’’) to
‘‘very much (‘‘sehr gerne’’). Subjects were also asked to
comment their decision by shorthand notes.
All the measurements were carried out within two days.
Measurements lasted 35–80 min (median less than 50 min)
including completion of formalities.
2.3. Subjects
A total of 10 subjects, 5 males and 5 females, participated
in this study. All subjects had a good visual acuity of at least
1.0 for near (33 cm, Landolt rings) and 1.25 for far. Subjects
age ranged from 26 to 34 years. VDU work constituted an
important part of the daily work of our subjects. Same
subjects were used to assess suitability subjectively.
2.4. Data analysis
First, reaction time and error frequency were analyzed in
order to determine whether effects of learning were present
in the data. For this purpose an analysis of variance
(ANOVA) and Pearson chi-square tests were used.
Second, we investigated whether errors were equally
distributed within the matrix displayed or whether they
occurred more frequent in a specific direction of gaze, therefore in a particular part of the matrix. For this purpose the 20
× 20 matrix was subdivided into nine sectors (3 × 3) each
defining a sub-matrix of either 7 × 7, 6 × 7 and 7 × 6
(periphery) or 6 × 6 (central) characters. By means of a
Pearson chi-square test homogeneity of distribution of error
frequency within this sectors was analyzed.
Third, the central question whether different technologies
for displays and different ambient light give rise to variations in visual performance was studied by applying
ANOVA statistics to reaction times. On account of a departure of the distribution of errors from normal distribution
error frequencies were processed by means of Wilcoxon
rank test.
Fourth, subjective ratings on suitability of settings for
office work were compared. For this purpose, position of
the cross on the rating scale mentioned before was
converted to scores and fed to a Wilcoxon rank test.
Keywords were assigned to shorthand comments made by
the subjects.
3. Results
A total of 1600 displays were presented (four sets of 40
trials, 10 subjects), 800 with and 800 without target. An
incorrect answer was given at 246 trials, i.e., at each seventh
display or at about 15% of the displays shown. Nearly all
incorrect answers, i.e., 241, resulted from missed detection
of target. About 30% of the targets shown were missed. In
trials where target was present, a correct answer was given
after 3.28 ^ 2.20 s (mean ^ standard deviation over all
subjects and all four experimental settings) after onset of the
display. Correct detection of target occurred in 559 trials. In
conditions where displays did not contain the target, a
correct answer was given 4.14 ^ 2.74 s after onset of
the display. A correct answer was given at nearly all (795)
of the 800 displays without targets.
In order to assess learning effects, blocks were numbered
according to the order they were administered. We will refer
to this number further on as block number. An ANOVA was
run with subject and block number as factors and reaction
time as independent variable. The results show that between
subjects variations are significant (F(9) ˆ 15.17, p ⬍
0.001). The same is true for the factor block number (F(3)
ˆ 3.87, p ⬍ 0.01), indicating that reaction time increases
significantly with block number. However, the model used
in ANOVA turned out to explain only about 14% of variations in reaction time. If means for reaction times over 40
trials were used instead of single reaction times in ANOVA,
learning effects disappear (F(3) ˆ 0.953, p ⬎ 0.4). Error
frequency, defined as the number of errors within one block
of 40 trials, were calculated for each subject and each block
separately. Pearson chi-square test applied on a two way
table listing error frequency within each block and for
each subject indicates no (x 2(3, 9) ˆ 28.03, p ⬎ 0.4)
systematic effect of subject or block number on error
frequency. Therefore error frequency are not subject to
learning effects.
In order to investigate whether errors preferably occurred
within a specific area of the display, error frequency of the
nine sectors defined earlier on in the text, one central square
sector surrounded by eight square sectors, were normalized.
This procedure consisted of dividing error frequency for
each sector with the number of targets presented within
the same sector. Owing to low frequency, errors of all
subjects were pooled. Normalization enabled to account
for different number of targets presented within a sector.
Differences were because of unequal size of sectors as
well as the random selection of position of target. Pearson
chi-square test indicates that frequency of errors did not
vary (x 2(2, 2) ˆ 2.66, p ⬎ 0.6) among sectors.
An ANOVA was used to determine the variation of reaction time with experimental conditions, i.e., the display
technology and the ambient light. Multivariate ANOVA
analysis included factors such as block number, settings
(CRT-task, LCD-task, darkness, brightness), subject,
presence of target (yes, no) and error of target detection.
In order to account for individual preferences for settings,
interaction between settings and subjects was also considered in statistics. Reaction time was found to be independent
of block number (F(2) ˆ 1.15, p ⬎ 0.3) and of settings
(F(1) ˆ 1.24, p ⬎ 0.2). Reaction time depends on subject
(F(9) ˆ 14.72, p ⬍ 0.001), on presence of target (F(1) ˆ
36.6, p ⬍ 0.001), on error of detection (F(1) ˆ 19.5, p ⬍
0.001) as well as on the interaction between subject and
settings (F(26) ˆ 4.39, p ⬍ 0.001). We may suppose
that failure to demonstrate any effects of settings on reaction
time may be a result of the strong effect of the factors
M. Menozzi et al. / Displays 20 (1999) 3–10
7
Table 1
Reaction time (RT) and error frequency. Experimental settings: CRT-task, LCD-task, darkness or brightness. First to third row show mean and standard
deviation for reaction time in seconds. Only differences in third row are statistically significant. Bonferroni adjusted Student’s t-test shows significant
difference of mean of CRT-task and LCD-task for dark ambient light. Fourth row denotes median and quartile of errors for each setting. Total number of
errors is reported in fifth row. Median and quartiles of pooled error frequency for CRT-task and for LCD-task are shown in the sixth row (Standard deviations
appear in parantheses)
RT mean (s) for detecting target
RT mean (s) for missing target
RT mean (s) when target present
Error frequency (median
[quartiles])
Total errors
Error frequency of pooled data
(median [quartiles])
CRT-task
Brightness
Darkness
LCD-task
Brightness
Darkness
3.02 (1.88)
4.11 (2.69)
3.52 (2.23)
7 [4;9]
3.48 (2.30)
4.70 (2.91)
3.93 (2.60)
6.5 [5;10]
3.52 (2.47)
4.07 (3.00)
3.64 (2.60)
4 [3.5;7.5]
2.91 (2.03)
4.18 (2.87)
3.25 (2.34)
5 [4;6]
71
12.5 [9;19]
74
43
8.5 [6;11]
53
presence of target, error detection of target and their interaction. We therefore carried out post hoc ANOVA on a
reduced data set in which only trials with targets were
considered. Table 1 shows mean values and standard deviations for reaction time as well as error frequency assessed
using the reduced data set.
Reaction times assessed for displays without target are
not shown. First two rows list mean and standard deviation
of reaction time for trials in which the target was detected
(first row) and in which the target was missed (second row).
None of the difference in reaction time were significant
within each of the first two rows. If an ANOVA is run
considering reaction time of all trials at which target was
present, independent of whether target was detected or
missed, reaction time turns out to depend significantly on
settings (F(3) ˆ 2.81, p ⬍ 0.05). A Bonferroni adjusted
matrix of pairwise comparison of data of the third row
reveals that in dark ambient light a significant (p ⬍
0.05) longer reaction time results for CRT-task (3.93 s)
when compared to LCD-task (3.25 s) at same ambient light.
As only few errors (5 out of 800 possible) were done at
displays without target, trials without target were not taken
into account in the analysis on error frequency. Settings
were found to exhibit a significant influence on error
Table 2
Significance of difference in error frequency. Each entry denotes p value for
difference in error frequency between two settings. Probabilities listed in
first four lines were calculated using Wilcoxon rank test whereas probability listed in the last line of the table refers to two tailed t-test statistics for
paired samples
CRT-task
LCD-task
CRT versus LCD task (pooled)
Bright
Dark
Bright
Dark
0.0011
CRT-task
Bright Dark
LCD-task
Bright Dark
1
.587
.015
.007
1
.147
1
.070
.005
1
frequency. As can be seen from Table 1, median error
frequency varied from 7 errors per block (40 trials) in the
CRT-task in brightness to 4 errors per block in the LCD-task
for same conditions of ambient light level. Significance of
difference in error frequency between the four settings is
listed in Table 2.
In general, error frequency at LCD-tasks was significantly lower when compared to error frequency at CRTtask. In brightness error frequency in CRT-task was found
to be significantly higher (p ⬍ 0.02) than error frequency at
LCD-task. The same difference applies for darkness (p ⬍
0.01). Error frequency at CRT-task on brightness are significant (p ⬍ 0.01) higher than the ones assessed at LCD-task
in darkness. In accordance with Table 2 error frequencies
within display type do not differ significantly. Errors occurring at the same display type can therefore be pooled. If total
errors of CRT-task are pooled (145) and compared to pooled
total errors of LCD-task (96), we can conclude that at LCDtask the total amount of errors is 34% less than it is at CRTtask. A paired t-test reveals that difference in pooled data is
significant at a level of p ⬍ 0.001 (t(9) ˆ 4.72) for two
sided probability (p ⬍ 0.005 if non-parametric Wilcoxon
rank test is used).
Distribution of scores for suitability of settings for office
work are plotted in Fig. 1. One subject did not answer the
two questions on suitability of settings of LCD-task.
A Wilcoxon rank test failed (p ⬎ 0.1) to show
any significant differences in scores given to the four
different settings. A comparison of pooled data
assessed at CRT-tasks with pooled data assessed at
LCD-tasks also fails to show significant effects
(Wilcoxon rank, p ⬎ 0.1; two-tailed t-test, p ⬎ 0.1).
Shorthand notes used to comment on given scores
were mostly of negative nature. Table 3 summarizes
negative remarks which were mentioned at least twice.
Interestingly two subjects found fault with sharp characters of LCD, a characteristic which usually denotes
high quality. Two subjects contested inhomogeneous
luminance of LCD.
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M. Menozzi et al. / Displays 20 (1999) 3–10
Fig. 1. Box plot of scores. Scores (median, quartiles, extreme values) were
assessed by means of the question ‘‘How much would you like to work
using this screen in this ambient light?’’ using a scale of six intervals. * ˆ
outlier.
4. Discussion
Reaction time was found to depend on learning effect.
Apparently, two training sessions before the experiment
were not enough to eliminate the effect. Learning effect
disappeared after reducing amount of data. Therefore, learning effect can be supposed to be weak and will probably not
interfere with conclusions reported at the end of this article.
Although time needed to search the display is reduced by
learning, error frequency is found to be constant throughout
the experiment. We might therefore postulate that accuracy
of detection could not be improved whereas speed of
processing result of detection was improved by training.
In our experiment speed of processing depends on motor
as well as on mental skills, both of which can be improved
by training. Given these circumstances we may conclude
that optimal viewing conditions are imperative for ergonomics if accuracy is a significant requirement in a task
because no training can make up for poor viewing conditions in order to improve accuracy of vision.
Among reasons for involuntary missing a target in our
task are, low visibility of the target and an insufficient attention. Visibility may be biased by reflections and by inhomogeneous emission characteristics of the displays. As was
shown above, it is unlikely that reflections interfere with
visibility of target. Subjective estimation of reflections
(Table 3) deviates slightly from this assumption. However,
deviations are small. Probably some subjects rated
Table 3
Shorthand remarks on suitability of settings for office work (summary)
CRT
Bright
Dark
LCD
Bright
Dark
Size too small
Flicker
Blur
Reflections
Inhomogeneous luminance
3
5
5
2
0
3
6
4
1
0
5
0
0
0
2
5
0
0
0
2
Total
14
15
7
7
possibility of specular reflections of displays based on
their personal experience rather than visible reflections
present during the experiment.
Attention is influenced by behavior, and its focus must
not necessarily be at the location of the fixation point (Treisman [22]). Experiments reported by Posner [27] showed a
redirection of focus of attention towards locations in which a
pre-attentive stimulus was shown. We may postulate that
focus of attention can be dislocated by manipulative tasks
as is the case with spatial orientation. Shebilske [25] demonstrated that a task requiring downward gaze distorted spatial
orientation, e.g., that of baseball batters. As there are many
tasks in office work requiring downward gaze and downwards focused attention, we could assume a drift of attention
towards lower half of visual fields. However, error
frequency was found to be independent of location of target
on display. If it is supposed that variations of visibility did
not cancel out effects of varying attention, we may state that
attention does not vary with direction of gaze or orientation
in space in a critical manner. We may therefore conclude
that position of information to be displayed on the screen
must not be determined in consideration with relevance of
the information to be displayed. This conclusion seems to be
somewhat in contrast to actual recommendations made in
style guides (e.g. DIN 66234 [26]).
Differences between error rates at CRT-task and at LCDtask are considerable. Total amount of errors assessed at
LCD-task was 34% lower than amount of errors assessed
at CRT-task. As mentioned in the introduction several technical factors may be responsible for causing differences in
accuracy of detection of target such as sharpness of pixels
and time characteristics of luminance. Given the low
complexity of presented distractors and targets we suggest
spatial characteristics to be of minor importance in determining visual performance. Time course of luminance
might have played a decisive role in vision so as to give
rise to more convenient viewing condition at LCD-task.
However, LCD based technology is not necessarily required
to produce a time course of luminance needed for good
visual accuracy. The same beneficial effects result by
increasing refresh rate or by installing phosphors with
long persistence in CRT. Increasing refresh rate has been
shown to facilitate reading (Montegut [16]). Technical
reasons limit refresh rate in screens with high spatial resolution and with high depth of color representation. There are
drawbacks in the use of phosphors with long persistence.
First, effects like smearing while scrolling and transient
ghost images while changing mask reduce legibility of
displayed information at CRT equipped with long-persistence phosphor. Second, there are no triplets of long-persistence phosphors enabling to generate a color space of
similar size as can be done if triplets of short-persistence
phosphors are combined.
Our experiment revealed a weak dependency of reaction
time on settings or on technology of display used. Failure to
demonstrate strong effects might be because of low
M. Menozzi et al. / Displays 20 (1999) 3–10
sensitivity of our experimental procedure. As mentioned
before, it is possible that mechanisms other than vision
such as motor behavior, played a more significant role in
determining reaction time and could have cancelled out any
effect on vision. Efficiency of target detection should be
assessed without manipulatory tasks and without requirements in time needed for highly cognitive work for decision
making. We propose to assess detection performance by
means of an experiment in which target is displayed tachistoscopically. Visual performance shall thereby be assessed
in terms of percentage of correct detection of target as a
function of duration of presentation of target. By using
this approach it has been shown elsewhere (Fiorentini
[21]) that important changes in correctness of detection
are found if duration of visibility of display is changed
from 100 to 200 ms.
No subjective preference of either display technology or
settings could be demonstrated statistically even though
frequency distribution of subjective ratings reported in
Fig. 1 suggests a possible advantage of LCD technology
for office work. We conclude that the number of subjects
used to carry out the rating is too small to demonstrate
significant effects. Summary of shorthand notes on scoring
suitability of settings (Table 3) also indicate a possible
advantage of LCD over CRT. Subjects reported only few
drawbacks of LCD. Too small size of the screen as well as
an inhomogeneous luminance were complained at LCD.
The former complaint can be accommodated by using
displays with an adequate size. Screen size plays a role in
suitability of a display for office work. However, screen size
was not subject of our investigation. Findings made here are
independent of screen size. Latter factor might have exhibited some effect on our results, i.e., that errors might appear
more frequent in specific areas within LCD. As we did not
record variation of luminance across display area we are not
able to estimate influence of inhomogeneity of luminance
on error frequency.
5. Conclusion
The fact that the use of LCD improves accuracy in detecting targets and might also improve time for visual detection
of the target is an advantage of LCD over CRT. It is therefore expected that LCD will give rise to lower visual strain
and therefore cause less visual complaints as CRT will do.
Given the broad variety in technology of LCD available on
the market we may expect that there may be even further
improvements in visual performance if other LCD are used.
Unfortunately subjective ratings did not reflect findings
made on visual performance as was demonstrated by Roufs
[17] and Edwards [15].
Acknowledgements
We thank the following people for supporting us in this
9
experiment. The computer program used for the search task
was written by Ara Hagopian. Andreas Hoffmann adapted
the program to the needs of the notebook. Andreas von Buol
provided electronics for precise assessment of reaction time.
A special thank is addressed to our subjects who kindly
participated at the experiment without being paid. Last but
not least we thank our reviewers for fruitful comments.
References
[1] Patricia E. Cladis, World view of liquid crystal flat panel displays, in:
L.E. Tannas, E. Glenn, J.W. Dorne (Eds.), Flat-panel Display Technologies, Noyes Publications, Parkridge, NJ, 1995.
[2] A.H. Firester, Active matrix technology, in: L.E. Tannas, E. Glenn,
J.W. Dorne (Eds.), Flat-panel Display Technologies, Noyes Publications, Parkridge, NJ, 1995.
[3] DIN 4549 Büromöbel, Schreibtische, Büromaschinentische und
Bildschirmtische. Masse. Deutsches Institut für Normung e. V.,
(Office furniture, desks, desks for office machines and desks for
VDUs.) German Standards Institute, November, 1982.
[4] T.J. Nelson, J.R. Wullert II, Electronic Information Display Technologies, World Scientific, Singapore, 1997.
[5] L.E. Tannas, E. Glenn, J.W. Dorne (Eds.), Flat-panel Display Technologies Noyes Publications, Parkridge, NJ, 1995.
[6] E. Farrell-Joyce, Fitting physical screen parameters to the human eye,
in: J.A.J. Roufs (Ed.), Vision and Visual Dysfunction. The Man–
Machine Interface, Mocmillan Press, Hondmills, 1991.
[7] Catherine Neary, A.J. Wilkins, Effects of phosphor on perception and
the control of eye movements, Perception 18 (1995) 257–264.
[8] B.L. Cole, Jennifer D. Maddocks, Effects of VDUs on the eyes:
Report of a 6-year epidemiological study, Optometry and Vision
Science 73 (8) (1996) 512–528.
[9] A.J. Jackson, E.S. Barnett, A.B. Stevens, M. McClure, C. Patterson,
M.J. McReynolds, Vision screening, eye examination and risk assessment of display screen users in a large regional teaching hospital,
Ophthalmics and Physiological Optics 17 (3) (1997) 187–195.
[10] H. Krueger, Visual function and monitor use, in: J.A.J. Roufs (Ed.),
Vision and Visual Dysfunction. The Man–Machine Interface,
Mocmillan Press, Hondmills, 1991.
[11] T. Läubli, W. Hünting, E. Grandjean, Postural and visual loads at
VDT workplaces. II: Lighting conditions and visual impairments,
Ergonomics 24 (1981) 933–944.
[12] W. Hacker, Allgemeine Arbeits- und Ingenieurpsychologie (General
Work- and Engineering-Psychology), 2, Huber, Bern, 1978.
[13] J.D. Grisham, Melissa M. Sheppard, Wendy U. Tran, Visual symptoms and reading performance, Optometry and Vision Science 70 (5)
(1993) 384–391.
[14] G.E. Legge, G.S. Rubin, A. Luebker, Psychophysics of reading V:
The role of contrast in normal vision, Vision Research 27 (1987)
1165–1177.
[15] C.B. Edwards, V.J. Demczuk, Legibility of visual display terminals
and an investigation of subjective image quality ratings with respect
to physical image quality measures, in: H. Luczak, A. Cakir, G. Cakir
(Eds.), Work with Display Units 92, Elsevier Science Publishers,
Amsterdam, 1993.
[16] M.J. Montegut, B. Bridgeman, J. Sykes, High refresh rate and oculomotor adaptation facilitate reading from video displays, Spatial
Vision 10 (4) (1997) 305–322.
[17] J.A.J. Roufs, M.C. Boschman, Visual comfort and performance, in:
J.A.J. Roufs (Ed.), Vision and Visual Dysfunction. The Man–
Machine Interface, Mocmillan Press, Hondmills, 1991.
[18] I.S. MacKenzie, S. Riddersma, Effects of output display and controldisplay gain on human performance in interactive systems, Behaviour
and Information Technology 13 (5) (1994) 328–337.
10
M. Menozzi et al. / Displays 20 (1999) 3–10
[19] S. Saito, S. Taptagaporn, G. Salvendy, Visual comfort using different
VDT screens, International Journal of Human–Computer Interaction
5 (4) (1993) 313–323.
[20] P.R. Boyce, Human Factors in Lighting, Applied Science Publishers,
London, 1981.
[21] Adriana Fiorentini, Differences between fovea and parafovea in visual
search, Vision Research 29 (9) (1989) 1153–1164.
[22] Anne Treisman, P. Cavanagh, B. Fischer, V.S. Ramachandran, R. von
der Heydt, Form perception and attention. Striate cortex and beyond,
in: L. Spillmann, J.S. Werner (Eds.), Visual Perception: The Neurophysiological Foundations, Academic Press, San Diego, CA, 1990.
[23] A. Von Buol, The measurement of reaction time by computers. A
simple hardware solution to provide millisecond resolution time.
(Submitted)
[24] ISO 9241 - 7 (1998): Ergonomic requirements for office work with
visual display terminal (VDTs) – Part 7: Requirements for display
with reflections. Genève, Switzerland, 1998.
[25] W.L. Shebilske, Baseball batters support an ecological efference
mediation theory of natural event perception, Acta Psychologica 63
(1986) 117–131.
[26] DIN 66234. Bildschirmarbeitsplätze. Gruppierung von Information
und Daten. Hinweise und Beispiele. Beiblatt 1 zu DIN 66234, Teil
3. Deutsches Institut für Normung e. V., März 1983. (Workplaces
with VDUs. Grouping of Information and Data. Remarks and Examples. German Standards Institute, March 1983).
[27] M.I. Posner, Chronometric Explorations of Mind, Lawrence Erlbaum
Associates, Hillsdale, NJ, 1978.
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