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Effects of refresh rates of a simulated CRT display with bright characters on a dark screen
International Journal of Industrial Ergonomics. 1 (1986) 9-20 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
9
EFFECTS OF REFRESH RATES OF A SIMULATED CRT DISPLAY WITH BRIGHT CHARACTERS ON A DARK SCREEN Th. L~lubli, St. Gyr, K. Nishiyama, R. Gierer and E. Grandjean Department of Hygiene and Ergonomics, Swiss Federal Institute of Technology, Zurich (Switzerland)
(Received October 10, 1985; accepted in revised form march 14. 1986)
ABSTRACT Twenty-nine subjects carried out a 3 hour reading task at a simulated CRT. In the apparatus chopper discs generated oscillating luminances of bright characters of 30, 60, 90, 180 and 0 Hz. A further control experiment was conducted with a printed text. The oscillating luminances had an approximate decay time of 4 ms (to 10% of peak luminance). The character luminance was kept constant (75 c d / m2). In the simulated C R T all characters oscillated simultaneously. Before, during and after each reading task, the following parameters were determined: Near point distance, critical flicker frequency (CFF), visual acuity, heterophorias, contrast sensitivity, eye blinking rate, and subjective symptoms of
discomfort. The reading speed was continuously recorded Furthermore the CFF at the simulated CR T was determined The range of individual CFF at the simulated CR T was found between 40 and 56 Hz. It is concluded that most models of VDTs have today refresh rates lying in a critical range of CFF. In every condition the reading task was found to hate an effect on visual fimctions and caused visual discomfort. Differentiated results among the six conditions were only observed for CFF and visual discomfort. The risible flicker (30 Hz) produced the strongest effects while 180 Hz, 0 Hz and the printed text were associated with relatively small effects.
INTRODUCTION
cially those with high oscillation degrees. It is well known that a visible flickering light is annoying, producing a strong decrease of critical flicker frequency (CFF) (Rey and Rey, 1965). Less clear is the knowledge about effects of oscillating fluorescent lamps; Zacharia and Bitterman (1953) compared the effects of fluorescent lighting with constant and with oscillating luminance (120 cycles per second (Hz)). The oscillating light did not
Many field studies on VDT operators revealed a high incidence of eye complaints (L~iubli and Grandjean, 1984). Although the interpretation of these complaints is still controversial, L~iubli et al. (1981) observed a higher incidence of eye complaints and of lowered visual acuity in a group of operators working with VDTs of poor quality, espe0169-8141/86/$03.50
© 1986 Elsevier Science Publishers B.V.
10 influence the speed of reading while the C F F dropped. Five of 20 subjects could distinguish the two lighting conditions and gave preference to the constant luminance. When fluorescent lighting oscillating at 100 Hz was introduced on a large scale in European offices, some operators complained about eye pains and headaches. That is why lighting technology in Europe subsequently introduced lamps with two phase-shifted fluorescent tubes, distinctly reducing the degree of oscillation of the light source. Unfortunately the effects of oscillating fluorescent tubes were never studied systematically and the question of their importance remains open. The possibility of the existence of adverse effects due to unperceived frequency oscillations in lighting must therefore still be considered as a hypothesis. Today most VDTs have a refresh rate of 50 or 60 Hz. This range seems to be slightly above or very near the visible flicker of CRTs under normal lighting conditions. These considerations induced us to construct a simulated CRT, generating optionally various oscillating frequencies of bright characters and to study their effects on subjects.
were perforated in such a way that they created oscillating luminances that were similar in shape to those of VDTs (decay time of 4 ms to drop to 10% of the peak luminance). Text slides with sharpness and contrast characteristics similar to those of a well printed text were prepared and projected on the screen. Characters were bright and the background was dark.
Experimental conditions The following six conditions were selected: (1) Oscillating luminance of 30 Hz. This is a clearly visible flicker and such an oscillation frequency is perceived as disturbing. (2) Oscillating luminance of 60 Hz. This is just above flicker fusion, that means it is perceived as a constant luminance. (3) Oscillating luminance of 90 Hz. This is clearly above the flicker fusion but the human visual system is able to register changes of luminances at this speed under specific conditions (Kelly, 1964). (4) Oscillating luminance of 180 Hz. It is assumed that the visual system is not able to react on such a fast oscillation. It is a control condition.
METHODS 30 H|
60 H:
O00
The simulated CRT
400
The apparatus was described by Nishiyama et al., (1986) and only the principle of its operation will be described here. A slide projector was modified so that the luminance from a DC powered halogen lamp was adjustable in a proper range by a voltage control. The light beam was projected on a screen of 170 × 210 mm. A chopper disk was located in the focus of the final projection lens. The chopper disk was driven by a servoamplifier system and the light could be continuously varied from a constant light up to a frequency modulated light of 180 Hz. The chopper disks
200
0
¢ au
c
2()
40
0
20
40
(Time ms)
180 H t
gOHz 600
400,
200
0
"
20
4o
0
210
4'0 ( T i m e m s )
Fig. 1. Characteristicsof the luminance oscillation.Mean luminance and decay time are kept constant.
11 (5) Constant fight ( = oscillating luminance 0 H-). A control. (6) Printed text. This is a linkage to the known condition with dark characters on white paper. The applied oscillating characteristics have been measured by an equipment described by Fellmann et al. (1982) and are shown in Fig. 1. In the conditions 1 to 4 (characterized by luminance oscillations) the mean luminance of characters (75 c d / m 2) and the decay time (4 ms to drop down to 10% of the peak luminance) were kept constant. In the 0 Hz condition the character luminance was also 75 c d / m 2. The peak luminance decreased with increasing oscillation frequency, to keep the mean luminance constant. The printed test was fixed directly on the screen and was consequently in a vertical position. The size of its characters was about the size of the projected ones. In each condition the experimental room was illuminated by a DC powered incandescent light with 200 Ix (vertical) and 300 Ix (horizontal) at the screen centre. D e p e n d e n t variables
6. Lateral heterophorla in near vision
"Vision Tester" by Titmus, Zeiss, USA was used for 3, 4, 5 and 6. " F a r vision" was 6.1 m and "near vision" was 35.6 cm. 7. Contrast sensitivity in far vision
It was measured with an apparatus that is normally used to evaluate the visual capacity to drive a car at night (" Rodenstock Nyktometer"). To get a partial adaptation to dark vision, subjects had to look into the completely dark apparatus for three minutes. Afterwards test targets of low contrasts but of a non-critical size (corresponding to a visual acuity of 0.1) were presented. The test symbol was a black disk with a little cam sticking out on a brighter surrounding surface. The subject was asked to tell the position of the cam. The luminance of the surface was decreased by eight steps so that the contrast-ratio between disk and surrounding decreased logarithmically from 1 : 23.5 to 1 : 1.14. 8. Subjective feelings of discomfort
Ten questions concerning eye discomfort which were observed in a previous field study (Liaubli et al., 1981) and three questions about fatigue were arranged into three groups.
1. Near point distance
The mean value of three measurements, taken with a widely used device (Clement Clarke Ltd.) were registered. 2. Critical flicker frequency (CFF)
This was measured by an apparatus and procedure described by Gierer et al. (1981). Each measurement was repeated three times and mean values of these three evaluations were used for the analysis. 3. Binocular visual acuity in far vision
8a. Eye pain, eye fatigue, flickering view. On three stepless scales subjects had to adjust the levels according to the discomfort of eye pain, eye fatigue and flickering view by moving a sliding pointer on each of the three scales. The range of each scale was between " n o feeling at all" and "unbearable". The full range of the scale was set at 100 points and each final position of the printer was related to this level. The adjusted levels were electronically recorded and checked by the supervisor. After each trial the pointer was automatically reset at the zero level.
4. Lateral heterophoria in far vision 5. Binocular visual acuity In near vlsion
8b. Fatigue, bored, tired. On three stepless scales of a questionnaire subjects had to mark
12 their levels of fatigue, boredom and tiredness. The range of each scale was between " n o feelings at all" and "unbearable". The full range of the scale was set at 100 points and each mark was related to this level. 8c. Burning eyes, headache, itching eyes, tears, blurred vision, near, double image. On a second sheet seven questions concerning eye troubles were asked using the kind of stepless scales described in group "8b".
9. Eye blinking rate The experimentalist sat by the side of the subject and counted the blinking of the right eye during two minutes. Subjects did not known the aim of the intensive observations. 10. Reading performance The time between the changes of slides (approximately 2 min) was electronically recorded. The number of lines read per min was calculated for each slide, and the reading performance at each slide was noted at time intervals of approximately 2 rain during each session. 11. CFF levels at the simulated CRT In the end of the 60 Hz condition the 60 Hz chopper disk was used to determine the C F F levels of each subject focussing the bright characters. The experimentalist assessed the C F F by three ascending and three descending procedures and the individual mean values were recorded. Procedure
Each subject participated in the six experimental conditions (within subjects design). The sequence of the conditions was random. Experiments were conducted in the morning or afternoon, but a single subject was only tested once a day. Before each session subjects were asked about their health, eye troubles, headaches, consumption of drugs or al-
cohols. In such cases experiments were postponed. Each subject read the presented text aloud at a visual distance of about 60 cm under the six sets of conditions. All subjects read the identical text in the logical sequence. It was taken from a text book of ergonomics. Subjects reading very fast, needed two or three sessions to read the whole text. When the text was finished it had to be read again; consequently the text was read twice or three times in the course of the six sessions, depending on the individual reading speed. The time schedule of each session was as follows: (numbers in parentheses refer to the test-numbers in the section "dependent variables"). 10 min. Adaptation to the lighting conditions. Questions (set 8a, 8b and 8c). Full test battery (beginning with test one and ending with test 7) followed by the three groups of questions (8a, 8b, 8c). 15 min. Reading task. After five minutes reading, the eye blinking rate was recorded (9). Questions (set 8a). 15 min. Reading task. C F F (2) and questions of sets 8a and 8b. 15 min. Reading task. Questions (set 8a). 15 min. Reading task. Questions (set 8a, 8b, 8c) and test battery (1, 2,3,4,5,6). 15 min. Reading task. Questions (set 8a). 15 min. Reading task. C F F (2) and questions (set 8a and 8b).
13 15 rain. Reading task. Questions (set 8a). 15 min. Reading task. Questions (set 8a, 8b, 8c) and test battery (1, 2, 3, 4, 5, 6). 15 rain. Reading task. Questions (set 8a). 15 min. Reading task. C F F (2) and questions (set 8a and 8b).
15 min. Reading task. Questions (set 8a). 15 rain. Reading task. During the last five minutes of reading the eye blinking rate (9) was recorded. Questions (set 8a, 8b, 8c) and full test battery (1, 2, 3, 4, 5, 6, 7). Questions (set 8a, 8b, 8c). The total reading time therefore lasted 180 rain. and the full session lasted about four hours.
Subjects A total number of 29 subjects were divided in three subgroups: (A) 13 young subjects between 19 and 26 years with normal vision (four male and nine female); (B) 7 young subjects between 23 and 33 years wearing glasses (three male and four female); (C) 9 elderly subjects between 49 and 59 years (two male and seven female). Each subject was previously examined at the ophthalmic hospital of the University in order to assess their visual characteristics and had to fulfill some visual standards to be accepted for the experiments. In group B and C these preliminary tests were done after prescribing any glasses that were required. The limits of acceptance were set as follows: Visual acuity for far and near vision with
both eyes should be above 0.8 Snellen equivalents (tested by the Titmus vision tester). Lateral heterophoria (exophoric or esophoric scores) should not exceed three prism diopters in near or far vision and vertical heterophoria (far vision) should not exceed 0.5 prism diopters. Subjects should not suffer from subjective eye troubles. C F F figures had to lie within 38-55 Hz. In the contrast sensitivity test the two easiest figures had to be recognized. In group A subjects had normal vision. Group B consisted of six myopic and one hyperopic subjects. Five subjects also suffered from astigmatism. In group C the nine subjects suffered from presbyopia which in 6 cases was combined with a slight hyperopia or a slight astigmatism. They were provided with special glasses, to get an optimal correction for the visual distance of 60 cm. A tenth subject was eliminated from the study. She started in the 30 Hz condition that was fixed at random. After a few minutes of reading she complained about dizziness and general feelings of discomfort and was unwilling to go on with the experiment or to take part in the other conditions.
RESULTS The CFF measured on the oscillating bright characters The results of 28 subjects focussing the screen centre are reported as a cumulative distribution in Fig. 2. (In one case the experimentalist did not measure it by mistake). The range of individual values lies between 40 and 56 Hz with a mean figure of 48 Hz. When the subjects were focussing slightly outside the screen, the C F F figures were a little higher. When considering these results, the following remarks must be taken into account: The C F F measurements were conducted after the reading task sessions, when lowered levels must be expected.
14
28
Comparison of the three subgroups
100 %
ee
The initial values, measured before the beginning of each experimental session, are reported in Table 1 for each of the subgroups. It is well known that CFF, visual acuity. and near point are changing with age, which is confirmed in Table 1. In the present groups, age is also associated with the other measured parameters. The young subjects with glasses show no relevant differences compared with the young colleagues without glasses. The shifts of each of the visual functions measured before and after the 3 hours reading task were calculated for the three subgroups. The most important shifts were observed in the 30 Hz condition which are compared with the 0 Hz condition in Table 2. The results show that for the three subgroups the mean shifts are of the same order of magnitude within the respective conditions. The only significant difference is found with lateral heterophoria in the 30 Hz condition. Here young subjects with glasses show a more pronounced shift towards esophoria than young subjects without glasses or elderly
20 50 % ta (J
tO
J~ o•
e°
0
O~
° 0~4o critical
45 flicker
5o
55
f r e q u e n c y of c h a r a c t e r s
( Hz )
Fig. 2. Cumulative distribution of the critical flicker frequency of 28 subjects focussing bright characters on the centre of the screen of the CRT simulator.
Subjects with very high or very low C F F values were not accepted to the experiments. In the present apparatus the full text of the screen is oscillating simultaneously while in a C R T the electron beam runs over the screen line by line. We do not know whether this is of importance for the eyes focussing single bright characters.
TABLE 1 Initial values of the measured visual functions for the three subgroups, n = number of measurements Visual function
critical flicker frequency (Hz)
A Young subjects without glasses (n = 78)
B Young subjects with glasses (n = 42)
C Elderly subjects with glasses (n = 54)
mean
mean
mean
46.3
s 3.3
45.3
s 3.5
42.7
s 4.8
visual acuity far (Snellen equivalents)
1.25
0.12
1.21
0.14
1.13
0.16
visual acuity near (Snellen equivalents)
1.31
0.10
1.26
0.12
1.15
0.21
near point (diopters)
8.5
1.8
9.2
1.7
3.0
0.6
contrast sensitivity (arbitrary units)
5.6
1.0
6.1
1.5
6.6
1.2
blink rate (blinks/rain)
48
40
27
21
26
28
lat. heterophoria far (prism diopters )
7.8
1.1
8.1
2.3
9.0
1.1
lat. heterophoria near (prism diopters)
7.6
2.4
8.5
2.0
11.2
2.4
15 TABLE 2 Shifts of visual functions after the 3 hours reading tasks for the three groups in the conditions of 30 Hz and of 0 Hz. n -- number of subjects Visual function
30 Hz condition
0 Hz condition
A young subj. without glasses (n =12)
B young subj. with glasses (n=7)
C elderly subj. with glasses (n=9)
A young subj. without glasses (n=13)
B young subj. with glasses (n=7)
C elderly subj. with glasses (n=9)
mean
mean
mean
mean
mean
mean
s
s
s
s
s
s
critical flicker frequency (Hz)
-4.1
1.7
-3.9
2.1
-4.0
1.4
-1.8
1.8
-2.1
1.5
-2.6
1.5
visual acuity far (Snellenequivalents)
-0.09
0.18
-0.03
0.08
-0.09
0.11
-0.05
0.11
-0.09
0.12
-0.06
0.11
visual acuity near (Snellenequivalents)
-0.04
0.07
-0.06
0.14
-0.04
0.07
-0.09
0.13
-0.06
0.08
-0.04
0.10
near point (diopters)
-0.6
0.9
-0.0
1.0
-0.1
0.4
-0.0
0.7
+0.2
1.0
-0.1
0.2
contrast sensitivity (arbitrary units)
-0.5
0.8
-0.1
0.9
-0.5
0.9
-0.5
1.1
-0.4
1.0
-0.2
0.8
blink rate ( b l i n k s / m i n )
10
25
4
19
lateral heterophoria far (prism diopters)
- 0.3
0.8
- 0.4
1.3
0.1
0.7
- 0.1
0.8
0.5
0.9
- 0.2
0.5
lateral heterophoria near (prism diopters) - 0.1
0.5
- 0.9
0.8
0.6
1.1
- 0.6
0.8
- 0.3
1.0
- 0.1
1.6
subjects with glasses ( p < 0.05 t-Test). The same analysis was conducted with the results of the questionnaires dealing with the subjective symptoms: The initial values are very low for each item and reveal neither eye complaints nor fatigue before the sessions. A mean increase of subjective symptoms after the reading tasks is clear for seven items but again no important difference appears among the three subgroups. Only the subgroup of young subjects without glasses has slightly, but not significantly, more symptoms than the two other groups. From the whole analysis of the three subgroups it can be concluded that in our experiments the three groups react about in the same way to the reading tasks. Since the three groups are rather small, a generalization is not possible, however it is certainly justified
4
10
2
13
- 1
15
0
11
to consider the three groups as one overall group of twenty nine subjects for further analysis. The effects of oscillating frequencies on visual functions The figures of the mean shifts of the measured visual functions are reported in Table 3. One of the young subjects did not finish the session with an oscillation of 30 Hz. She judged it to be unbearable. She was dropped from the analysis of the 30 Hz condition but included in the other five conditions. Consequently the figures of discomfort are underestimated in the 30 Hz condition. The paired comparisons between "before-measurements" and "after reading task measurements" for each of the experimental conditions give the following results:
16
TABLE 3 S h i f t s o f v i s u a l f u n c t i o n s o f all 29 s u b j e c t s a f t e r t h e 3 h o u r s r e a d i n g t a s k s Visual function
unit
30 H z
60 H z
90 H z
180 H z
0 Hz
Printed text
CFF V i s u a l a c u i t y far Visual acuity near Contrast sensitivity Blink rate Near point
Hz Snellen equivalents Snellen equivalents Arbitrary units Blinks per min. Diopters
+ -
4.0 0.05 0.05 0.4 2.5 0.3
(1) The C F F reveals a marked decrease in each condition; all changes are significant ( p < 0.01 with Wilcoxon test). (2) The visual acuity "far" and "near" show very small but still mostly significant decreases in each condition. (3) The contrast sensitivity is slightly decreased in each condition, but only significant on the p < 0.05 level for 30, 60, and 180 Hz. (4) The blinking rate is increased in each condition, but only at 30 Hz significant with p < 0.05. (5) The near point reveals no change in any of the six conditions. (6) The lateral heterophorias exhibit a small but irrelevant esophoric shift in most conditions. A further analysis shows that the physiological reactions do not depend significantly
+
2.6 0.1 0.05 0.5 1.4 0
+ -
2.8 0.1 0.1 0.5 2.0 0.1
+ -
1.9 0.05 0.05 0.5 1.7 0.1
+
2.1 0.05 0.05 0.3 1.8 0
+
on the experimental condition with the exception of the CFF. Only the near point (accommodation) and the blink rate are slightly higher at the 30 Hz condition that at the printed text condition. An analysis of variance was conducted with all values of the CFF, which is reported in Table 4. Subjects are treated as levels of a factor and consequently there is one observation in each cell. All sources of variation other than main effects are considered to be part of the experimental error. Consequently the computed statistics are too conservative (Winer, 1971). The results reveal a great individual variance of the CFF; nevertheless the factors "time" and "experimental conditions" exhibit a highly significant influence. Based on this analysis of variance, the differences be-
TABLE 4 A n a l y s i s o f v a r i a n c e o f t h e critical flicker f r e q u e n c y M a i n effects
Sum of squares
Mean square
F
Sigmificance
28 6
54 339 2 886
1941 481
521.3 129.2
0.001 0.001
5
1504
301
80.8
0.001
2
40
20
5.4
0.004
Total main effects Residual
41 3 585
58 610 13 346
1430 3.7
384.0
0.001
Total
3 626
71957
19.8
Individual differences Time Experimental conditions Repetition of measurements
DF
2.2 0.1 0.1 0.1 1.1 0
17
TABLE 5 Paired comparison (t-Test) of the CFF shifts after 3 h reading task for the 6 experimental conditions. * * = p < 0.01; * = p < 0.05 30 HZ 6 0 HZ
**
9 0 HZ
.o
180 HZ
**
0 HZ
*"
Printed text
6OHz 9OHz 180 HZ 0 HZ
The effects on reading performances
*"
tween the 5 experimental conditions were tested for the shifts after 3 hours reading task by the paired t-test. The results, reported in Table 5, show that the 30 Hz figures are significantly different from all other conditions, while the 60 and 90 Hz figures differ from the 180 Hz with p < 0.05. In Fig. 3 the mean CFF shifts are represented for each time step during the six experimental sessions. In all conditions a clear decrease is already seen after 30 min and the greatest drop seems Duration of reading t a s k s ( m i n ) 0
to be reached after 120 min. Furthermore, the figure reveals a separation in 3 groups: The strongest drop is generated by the 30 Hz condition; the smallest decrease is seen in the three control conditions, while the 60 and 90 Hz, which belong to the invisible oscillations, produce an intermediate decrease of CFF.
60 t
i
120 •
.
180 t
The reading performances expressed as read lines per minute did not reveal any relevant differences among the six experimental conditions. The mean values are about 11 lines per min (110 words per min). This rather low reading speed may be the result of having to read aloud. Looking at all conditions, the reading performance is Slightly higher at the first (0-15 min) and the last time interval (166-180 min). The effects on visual comfort
The self-rated assessments were not normally distributed. The mean differences of the final self-ratings minus the initial ones were calculated and the significance of the shifts analysed with Wilcoxon test. The re-
30 Hz
•
6 0 Hz
-)(- 9 0 H z
• 180 Hz •
0 Hz
'4- print, t e z t N
TABLE 6
Differences of the increases of complaints at the end of the experiments for each experimental condition. Variables that showed a significant difference are tabulated (Wilcoxon-Test, p < 0.05)
= 2 3 0 Hz U
6 0 Hz
o
9 0 HZ
"
A. B. E B.E
180 HZ
3
9 0 HZ
A.B.E
0 HZ
S
A , B. E. C. D. G
0 HZ B.D
4
CaDtions : (n • 29 )
Fig. 3. Mean shifts of the critical flicker frequency of 28 subjects for each of the 6 experimental conditions.
180 HZ
A. B , E
Printed Text
IE
60Hz
A
B
eye fatigue
eye Dain burning eyes D headache E itching eyes G flickering C
S. E
E
B
18 Mean shift of complaints (arbitrary units) 0
10
20
tO0
30
eye fatigue eye pain burning eyes headache
the 30 Hz condition two persons were disturbed so much that they refused to continue the reading task; they were not taken into consideration for the calculation of the shifts in the 30 Hz condition. The increase of eye complaints
itching eyes tears
flickering blurred
vision:
near far double images
//
•
60
Hz
*
90
Hz
e 180
Hz
e
Hz
+
0
print.text
Fig. 4. Mean shifts of complaints at the end of the experiments of 29 subjects for each of the 6 experimental conditions.
suits are represented in Fig. 4 and Table 6. It must be pointed out that all shifts were significant; this means that the 3 hour reading tasks increased the symptoms of discomfort in each of the experimental conditions including "printed text" and "0 Hz". The symptoms reported in Fig. 4 were separated into three groups, according to a factor analysis used in a prior field study (Liubli et al., 1981). The factor analysis of the present study did not lead to a consistent grouping of the symptoms. For instance the symptoms "tired" (K) and "sleepy" (L) were mostly highly correlated with "eye fatigue" (A), but not however in every case. Nevertheless it seemed reasonable to represent the symptoms in the two groups shown in Fig. 4. The shifts of the symptoms "fatigued", "tired" and "bored" were of equal size in all conditions. From all results it can be deduced that the increases of "eye fatigue", "eye pain" and "itching eyes" are most pronounced at the 30 Hz condition and lowest in the printed text condition. In the other conditions the assessments covered an intermediate range. One observation must be pointed out: In
Affirmative answers to all 13 questions about eye discomfort or fatigue showed a continuous increase throughout the experiment. As an example the development of feelings of eye pain is represented in Fig. 5. The mean shift compared to the initial value is indicated for each experimental condition. From the results of the 13 questions, eye pains showed the best discriminating power among the 6 conditions. Relations between eye functions and subjective feelings
To get some indications about relations between physiological and subjective changes during the reading task, Pearson correlation coefficients were computed. The shifts of subjective feelings as well as of visual acuity (far and near), near point and blinkrate were not normally distributed so that the correlation coefficients and the tests of significance
I00' r legend: • 3G cycles per second "-. 6 0 H Z .~ * 90 .~ •
"~
o ..:
J0
(Hz)
~ /
6'0
~
/
7
/
9'0 l~C 150 180 duration of reading task (min)
Fig. 5. Mean shifts of eye pains of 29 subjects for each of the 6 experimental conditions.
19 TABLE 7 Pearson correlations between shifts of eye functions and subjective feelings in the 30 Hz condition (n = 27). Pearson correlation coefficients greater than 0.37 (p < 0.05) are tabulated
eye fatigue eye pain burning eyes headache itching eyes tears flickering blurred vision far blurred vision near double images
CFF
contrast sensitivity
0.5 0.5 0.6 0.5 0.4 0.4
0.5 0.4 0.4 0.4 0.4 0.5 0.5 0.5
should be taken as rough estimates of relations. Only the C F F and contrast sensitivity measurements revealed clear relationships with the subjective ratings. The correlation coefficients for these two physiological functions are reported in Table 7; only the 30 Hz condition with the most pronounced shifts was taken into consideration. It is concluded that a greater drop of C F F might be associated with a greater increase of eye discomfort. This kind of relationship was already observed in other experiments (e.g. Weber et al., 1973). The relationship between the contrast sensitivity and the eye discomfort is more difficult to understand.
DISCUSSION As mentioned above the visible flicker of a light source generates strong discomfort. One of the most striking effects is the immediate drop of the C F F which is most pronounced at levels of 20 to 30 Hz (Rey and Rey, 1965). It is generally accepted, that the screens of VDTs should be free of visible flicker. At the experimental screen (with an ap-
proximate decay time of 4 ms to drop to 10% of the peak luminance), the present experiments revealed C F F levels between 40 and 56 Hz. Bauer (1983) determined the C F F for a large population with a reversed presentation of a C R T and a background luminance of 80 c d / m 2. The range of the measured C F F was between 55 and 87 Hz. (mean: 73 Hz). If the results of Kelly (1964) are taken into consideration, one can expect C F F levels above 5 0 - 8 0 Hz for V D T screens. Most of the V D T models have refresh rates of 50 or 60 Hz with relatively short phosphor decay times. F r o m this point of view it must be concluded that these refresh rates are in a critical range of C F F . Some restrictions related to our experimental conditions must, however, be pointed out. The lighting conditions of the simulated C R T screen are not identical with a real VDT. AT a CRT, the electron beam wanders down line by line, while in our experiment, the total screen illumination is turned on and off simultaneously. We have kept the mean luminance constant and this means that the decreasing refresh rate is combined with an increased peak luminance (See Fig. 1). It is therefore theoretically not clear whether we evaluated the refresh rate or the peak luminance. The studies of Kelly (1964) give an answer to this question: The C F F depends on the amplitude of the groundwave divided by the mean luminance. In our experiments this quotient measured over a surface of 5 × 7 cm varied from 0.36 to 0.46 only. We therefore assume that for the eyes the refresh rate is the determining factor and not the very short peak luminance. In every condition the reading task, lasting 3 hours, had physiological effects on visual functions and caused visual discomfort. Besides this, a certain distinction among the six conditions was observed with the parameters "visual discomfort" and ' C F F " . The visual discomfort was strong with visible flicker (30
20 Hz) and low with the printed text. T h e remaining c o n d i t i o n s of invisible flicker (including the 0 Hz) did not reveal differentiating effects o n discomfort. H o w e v e r , the C F F revealed m o r e conclusive effects: T h e visible flicker caused a very strong d r o p o f the thresholds. T h e three control c o n d i t i o n s (180 Hz, 0 Hz and printed text) p r o d u c e d a smaller d r o p of the thresholds, while 60 a n d 90 H z had an i n t e r m e d i a t e effect. T h e r e f o r e the q u e s t i o n arises as to w h e t h e r flicker frequencies slightly a b o v e the perceived flicker threshold might have some physiological effects, but at the present time this c o n s i d e r a tion is not m u c h m o r e than a hypothesis.
ACKNOWLEDGEMENT T h e research r e p o r t e d in this p a p e r has b e e n s u p p o r t e d b y the Swiss N a t i o n a l Science F o u n d a t i o n , G r a n t No. 3.810.81.
REFERENCES Bauer, D.. Bonacker, M. and Cavonius C.R., 1983. Frame repetition rate for Flicker-free viewing of bright VDT screens. Displays, January: 31-33.
Gierer, R., Martin, E.. Baschera, P. and Grandjean, E., 1981. Ein Ger~it zur Bestimmung der Flimmerverschmelzungsfrequenz des Auges. Z. Arbeitswissenschaft, 35: 45-47. Kelly, D.H.. 1964. Line waves and flicker fusion. Documenta Ophthalmologica, 18: 16-35. L~iubli, Th. and Grandjean, E., 1984. The magic of control groups in VDT field studies. In: Grandjean, E. (Ed.), Ergonomics and Health in Modern Offices. Taylor and Francis. London. Laubli, Th., Hianting, W. and Grandjean, E., 1981. Postural and visual loads at VDT workplaces, II. Lighting conditions and visual impairments. Ergonomics, 24: 933-944. Nishiyama, K., Br~iuninger, U., de Boer, H., Gierer, R. and Grandjean, E., 1986. Physiological effects of oscillating luminances in the reversed display of VDTs. To be published in Ergonomics. Rey, P. and Rey, J.P., 1965. Effect of an intermittent light stimulation on the critical fusion frequency. Ergonomics, 8: 173-180. Weber, A., Jermini, C. and Grandjean, E., 1973. Beziehung zwischen objektiven und subjektiven Messmethoden bei experimentell erzeugter "Ermiadung". Z. Pr~iventivmedizin, 18: 279-283. Winer. B.J., 1971. Statistical Principals in Experimental Design. McGraw-Hill Inc., New York. Zacharia, A. and Bitterman, M.E., 1953. The effect of fluorescent flicker on visual efficiency. Applied Psychology, 36: 413-416.
9
EFFECTS OF REFRESH RATES OF A SIMULATED CRT DISPLAY WITH BRIGHT CHARACTERS ON A DARK SCREEN Th. L~lubli, St. Gyr, K. Nishiyama, R. Gierer and E. Grandjean Department of Hygiene and Ergonomics, Swiss Federal Institute of Technology, Zurich (Switzerland)
(Received October 10, 1985; accepted in revised form march 14. 1986)
ABSTRACT Twenty-nine subjects carried out a 3 hour reading task at a simulated CRT. In the apparatus chopper discs generated oscillating luminances of bright characters of 30, 60, 90, 180 and 0 Hz. A further control experiment was conducted with a printed text. The oscillating luminances had an approximate decay time of 4 ms (to 10% of peak luminance). The character luminance was kept constant (75 c d / m2). In the simulated C R T all characters oscillated simultaneously. Before, during and after each reading task, the following parameters were determined: Near point distance, critical flicker frequency (CFF), visual acuity, heterophorias, contrast sensitivity, eye blinking rate, and subjective symptoms of
discomfort. The reading speed was continuously recorded Furthermore the CFF at the simulated CR T was determined The range of individual CFF at the simulated CR T was found between 40 and 56 Hz. It is concluded that most models of VDTs have today refresh rates lying in a critical range of CFF. In every condition the reading task was found to hate an effect on visual fimctions and caused visual discomfort. Differentiated results among the six conditions were only observed for CFF and visual discomfort. The risible flicker (30 Hz) produced the strongest effects while 180 Hz, 0 Hz and the printed text were associated with relatively small effects.
INTRODUCTION
cially those with high oscillation degrees. It is well known that a visible flickering light is annoying, producing a strong decrease of critical flicker frequency (CFF) (Rey and Rey, 1965). Less clear is the knowledge about effects of oscillating fluorescent lamps; Zacharia and Bitterman (1953) compared the effects of fluorescent lighting with constant and with oscillating luminance (120 cycles per second (Hz)). The oscillating light did not
Many field studies on VDT operators revealed a high incidence of eye complaints (L~iubli and Grandjean, 1984). Although the interpretation of these complaints is still controversial, L~iubli et al. (1981) observed a higher incidence of eye complaints and of lowered visual acuity in a group of operators working with VDTs of poor quality, espe0169-8141/86/$03.50
© 1986 Elsevier Science Publishers B.V.
10 influence the speed of reading while the C F F dropped. Five of 20 subjects could distinguish the two lighting conditions and gave preference to the constant luminance. When fluorescent lighting oscillating at 100 Hz was introduced on a large scale in European offices, some operators complained about eye pains and headaches. That is why lighting technology in Europe subsequently introduced lamps with two phase-shifted fluorescent tubes, distinctly reducing the degree of oscillation of the light source. Unfortunately the effects of oscillating fluorescent tubes were never studied systematically and the question of their importance remains open. The possibility of the existence of adverse effects due to unperceived frequency oscillations in lighting must therefore still be considered as a hypothesis. Today most VDTs have a refresh rate of 50 or 60 Hz. This range seems to be slightly above or very near the visible flicker of CRTs under normal lighting conditions. These considerations induced us to construct a simulated CRT, generating optionally various oscillating frequencies of bright characters and to study their effects on subjects.
were perforated in such a way that they created oscillating luminances that were similar in shape to those of VDTs (decay time of 4 ms to drop to 10% of the peak luminance). Text slides with sharpness and contrast characteristics similar to those of a well printed text were prepared and projected on the screen. Characters were bright and the background was dark.
Experimental conditions The following six conditions were selected: (1) Oscillating luminance of 30 Hz. This is a clearly visible flicker and such an oscillation frequency is perceived as disturbing. (2) Oscillating luminance of 60 Hz. This is just above flicker fusion, that means it is perceived as a constant luminance. (3) Oscillating luminance of 90 Hz. This is clearly above the flicker fusion but the human visual system is able to register changes of luminances at this speed under specific conditions (Kelly, 1964). (4) Oscillating luminance of 180 Hz. It is assumed that the visual system is not able to react on such a fast oscillation. It is a control condition.
METHODS 30 H|
60 H:
O00
The simulated CRT
400
The apparatus was described by Nishiyama et al., (1986) and only the principle of its operation will be described here. A slide projector was modified so that the luminance from a DC powered halogen lamp was adjustable in a proper range by a voltage control. The light beam was projected on a screen of 170 × 210 mm. A chopper disk was located in the focus of the final projection lens. The chopper disk was driven by a servoamplifier system and the light could be continuously varied from a constant light up to a frequency modulated light of 180 Hz. The chopper disks
200
0
¢ au
c
2()
40
0
20
40
(Time ms)
180 H t
gOHz 600
400,
200
0
"
20
4o
0
210
4'0 ( T i m e m s )
Fig. 1. Characteristicsof the luminance oscillation.Mean luminance and decay time are kept constant.
11 (5) Constant fight ( = oscillating luminance 0 H-). A control. (6) Printed text. This is a linkage to the known condition with dark characters on white paper. The applied oscillating characteristics have been measured by an equipment described by Fellmann et al. (1982) and are shown in Fig. 1. In the conditions 1 to 4 (characterized by luminance oscillations) the mean luminance of characters (75 c d / m 2) and the decay time (4 ms to drop down to 10% of the peak luminance) were kept constant. In the 0 Hz condition the character luminance was also 75 c d / m 2. The peak luminance decreased with increasing oscillation frequency, to keep the mean luminance constant. The printed test was fixed directly on the screen and was consequently in a vertical position. The size of its characters was about the size of the projected ones. In each condition the experimental room was illuminated by a DC powered incandescent light with 200 Ix (vertical) and 300 Ix (horizontal) at the screen centre. D e p e n d e n t variables
6. Lateral heterophorla in near vision
"Vision Tester" by Titmus, Zeiss, USA was used for 3, 4, 5 and 6. " F a r vision" was 6.1 m and "near vision" was 35.6 cm. 7. Contrast sensitivity in far vision
It was measured with an apparatus that is normally used to evaluate the visual capacity to drive a car at night (" Rodenstock Nyktometer"). To get a partial adaptation to dark vision, subjects had to look into the completely dark apparatus for three minutes. Afterwards test targets of low contrasts but of a non-critical size (corresponding to a visual acuity of 0.1) were presented. The test symbol was a black disk with a little cam sticking out on a brighter surrounding surface. The subject was asked to tell the position of the cam. The luminance of the surface was decreased by eight steps so that the contrast-ratio between disk and surrounding decreased logarithmically from 1 : 23.5 to 1 : 1.14. 8. Subjective feelings of discomfort
Ten questions concerning eye discomfort which were observed in a previous field study (Liaubli et al., 1981) and three questions about fatigue were arranged into three groups.
1. Near point distance
The mean value of three measurements, taken with a widely used device (Clement Clarke Ltd.) were registered. 2. Critical flicker frequency (CFF)
This was measured by an apparatus and procedure described by Gierer et al. (1981). Each measurement was repeated three times and mean values of these three evaluations were used for the analysis. 3. Binocular visual acuity in far vision
8a. Eye pain, eye fatigue, flickering view. On three stepless scales subjects had to adjust the levels according to the discomfort of eye pain, eye fatigue and flickering view by moving a sliding pointer on each of the three scales. The range of each scale was between " n o feeling at all" and "unbearable". The full range of the scale was set at 100 points and each final position of the printer was related to this level. The adjusted levels were electronically recorded and checked by the supervisor. After each trial the pointer was automatically reset at the zero level.
4. Lateral heterophoria in far vision 5. Binocular visual acuity In near vlsion
8b. Fatigue, bored, tired. On three stepless scales of a questionnaire subjects had to mark
12 their levels of fatigue, boredom and tiredness. The range of each scale was between " n o feelings at all" and "unbearable". The full range of the scale was set at 100 points and each mark was related to this level. 8c. Burning eyes, headache, itching eyes, tears, blurred vision, near, double image. On a second sheet seven questions concerning eye troubles were asked using the kind of stepless scales described in group "8b".
9. Eye blinking rate The experimentalist sat by the side of the subject and counted the blinking of the right eye during two minutes. Subjects did not known the aim of the intensive observations. 10. Reading performance The time between the changes of slides (approximately 2 min) was electronically recorded. The number of lines read per min was calculated for each slide, and the reading performance at each slide was noted at time intervals of approximately 2 rain during each session. 11. CFF levels at the simulated CRT In the end of the 60 Hz condition the 60 Hz chopper disk was used to determine the C F F levels of each subject focussing the bright characters. The experimentalist assessed the C F F by three ascending and three descending procedures and the individual mean values were recorded. Procedure
Each subject participated in the six experimental conditions (within subjects design). The sequence of the conditions was random. Experiments were conducted in the morning or afternoon, but a single subject was only tested once a day. Before each session subjects were asked about their health, eye troubles, headaches, consumption of drugs or al-
cohols. In such cases experiments were postponed. Each subject read the presented text aloud at a visual distance of about 60 cm under the six sets of conditions. All subjects read the identical text in the logical sequence. It was taken from a text book of ergonomics. Subjects reading very fast, needed two or three sessions to read the whole text. When the text was finished it had to be read again; consequently the text was read twice or three times in the course of the six sessions, depending on the individual reading speed. The time schedule of each session was as follows: (numbers in parentheses refer to the test-numbers in the section "dependent variables"). 10 min. Adaptation to the lighting conditions. Questions (set 8a, 8b and 8c). Full test battery (beginning with test one and ending with test 7) followed by the three groups of questions (8a, 8b, 8c). 15 min. Reading task. After five minutes reading, the eye blinking rate was recorded (9). Questions (set 8a). 15 min. Reading task. C F F (2) and questions of sets 8a and 8b. 15 min. Reading task. Questions (set 8a). 15 min. Reading task. Questions (set 8a, 8b, 8c) and test battery (1, 2,3,4,5,6). 15 min. Reading task. Questions (set 8a). 15 min. Reading task. C F F (2) and questions (set 8a and 8b).
13 15 rain. Reading task. Questions (set 8a). 15 min. Reading task. Questions (set 8a, 8b, 8c) and test battery (1, 2, 3, 4, 5, 6). 15 rain. Reading task. Questions (set 8a). 15 min. Reading task. C F F (2) and questions (set 8a and 8b).
15 min. Reading task. Questions (set 8a). 15 rain. Reading task. During the last five minutes of reading the eye blinking rate (9) was recorded. Questions (set 8a, 8b, 8c) and full test battery (1, 2, 3, 4, 5, 6, 7). Questions (set 8a, 8b, 8c). The total reading time therefore lasted 180 rain. and the full session lasted about four hours.
Subjects A total number of 29 subjects were divided in three subgroups: (A) 13 young subjects between 19 and 26 years with normal vision (four male and nine female); (B) 7 young subjects between 23 and 33 years wearing glasses (three male and four female); (C) 9 elderly subjects between 49 and 59 years (two male and seven female). Each subject was previously examined at the ophthalmic hospital of the University in order to assess their visual characteristics and had to fulfill some visual standards to be accepted for the experiments. In group B and C these preliminary tests were done after prescribing any glasses that were required. The limits of acceptance were set as follows: Visual acuity for far and near vision with
both eyes should be above 0.8 Snellen equivalents (tested by the Titmus vision tester). Lateral heterophoria (exophoric or esophoric scores) should not exceed three prism diopters in near or far vision and vertical heterophoria (far vision) should not exceed 0.5 prism diopters. Subjects should not suffer from subjective eye troubles. C F F figures had to lie within 38-55 Hz. In the contrast sensitivity test the two easiest figures had to be recognized. In group A subjects had normal vision. Group B consisted of six myopic and one hyperopic subjects. Five subjects also suffered from astigmatism. In group C the nine subjects suffered from presbyopia which in 6 cases was combined with a slight hyperopia or a slight astigmatism. They were provided with special glasses, to get an optimal correction for the visual distance of 60 cm. A tenth subject was eliminated from the study. She started in the 30 Hz condition that was fixed at random. After a few minutes of reading she complained about dizziness and general feelings of discomfort and was unwilling to go on with the experiment or to take part in the other conditions.
RESULTS The CFF measured on the oscillating bright characters The results of 28 subjects focussing the screen centre are reported as a cumulative distribution in Fig. 2. (In one case the experimentalist did not measure it by mistake). The range of individual values lies between 40 and 56 Hz with a mean figure of 48 Hz. When the subjects were focussing slightly outside the screen, the C F F figures were a little higher. When considering these results, the following remarks must be taken into account: The C F F measurements were conducted after the reading task sessions, when lowered levels must be expected.
14
28
Comparison of the three subgroups
100 %
ee
The initial values, measured before the beginning of each experimental session, are reported in Table 1 for each of the subgroups. It is well known that CFF, visual acuity. and near point are changing with age, which is confirmed in Table 1. In the present groups, age is also associated with the other measured parameters. The young subjects with glasses show no relevant differences compared with the young colleagues without glasses. The shifts of each of the visual functions measured before and after the 3 hours reading task were calculated for the three subgroups. The most important shifts were observed in the 30 Hz condition which are compared with the 0 Hz condition in Table 2. The results show that for the three subgroups the mean shifts are of the same order of magnitude within the respective conditions. The only significant difference is found with lateral heterophoria in the 30 Hz condition. Here young subjects with glasses show a more pronounced shift towards esophoria than young subjects without glasses or elderly
20 50 % ta (J
tO
J~ o•
e°
0
O~
° 0~4o critical
45 flicker
5o
55
f r e q u e n c y of c h a r a c t e r s
( Hz )
Fig. 2. Cumulative distribution of the critical flicker frequency of 28 subjects focussing bright characters on the centre of the screen of the CRT simulator.
Subjects with very high or very low C F F values were not accepted to the experiments. In the present apparatus the full text of the screen is oscillating simultaneously while in a C R T the electron beam runs over the screen line by line. We do not know whether this is of importance for the eyes focussing single bright characters.
TABLE 1 Initial values of the measured visual functions for the three subgroups, n = number of measurements Visual function
critical flicker frequency (Hz)
A Young subjects without glasses (n = 78)
B Young subjects with glasses (n = 42)
C Elderly subjects with glasses (n = 54)
mean
mean
mean
46.3
s 3.3
45.3
s 3.5
42.7
s 4.8
visual acuity far (Snellen equivalents)
1.25
0.12
1.21
0.14
1.13
0.16
visual acuity near (Snellen equivalents)
1.31
0.10
1.26
0.12
1.15
0.21
near point (diopters)
8.5
1.8
9.2
1.7
3.0
0.6
contrast sensitivity (arbitrary units)
5.6
1.0
6.1
1.5
6.6
1.2
blink rate (blinks/rain)
48
40
27
21
26
28
lat. heterophoria far (prism diopters )
7.8
1.1
8.1
2.3
9.0
1.1
lat. heterophoria near (prism diopters)
7.6
2.4
8.5
2.0
11.2
2.4
15 TABLE 2 Shifts of visual functions after the 3 hours reading tasks for the three groups in the conditions of 30 Hz and of 0 Hz. n -- number of subjects Visual function
30 Hz condition
0 Hz condition
A young subj. without glasses (n =12)
B young subj. with glasses (n=7)
C elderly subj. with glasses (n=9)
A young subj. without glasses (n=13)
B young subj. with glasses (n=7)
C elderly subj. with glasses (n=9)
mean
mean
mean
mean
mean
mean
s
s
s
s
s
s
critical flicker frequency (Hz)
-4.1
1.7
-3.9
2.1
-4.0
1.4
-1.8
1.8
-2.1
1.5
-2.6
1.5
visual acuity far (Snellenequivalents)
-0.09
0.18
-0.03
0.08
-0.09
0.11
-0.05
0.11
-0.09
0.12
-0.06
0.11
visual acuity near (Snellenequivalents)
-0.04
0.07
-0.06
0.14
-0.04
0.07
-0.09
0.13
-0.06
0.08
-0.04
0.10
near point (diopters)
-0.6
0.9
-0.0
1.0
-0.1
0.4
-0.0
0.7
+0.2
1.0
-0.1
0.2
contrast sensitivity (arbitrary units)
-0.5
0.8
-0.1
0.9
-0.5
0.9
-0.5
1.1
-0.4
1.0
-0.2
0.8
blink rate ( b l i n k s / m i n )
10
25
4
19
lateral heterophoria far (prism diopters)
- 0.3
0.8
- 0.4
1.3
0.1
0.7
- 0.1
0.8
0.5
0.9
- 0.2
0.5
lateral heterophoria near (prism diopters) - 0.1
0.5
- 0.9
0.8
0.6
1.1
- 0.6
0.8
- 0.3
1.0
- 0.1
1.6
subjects with glasses ( p < 0.05 t-Test). The same analysis was conducted with the results of the questionnaires dealing with the subjective symptoms: The initial values are very low for each item and reveal neither eye complaints nor fatigue before the sessions. A mean increase of subjective symptoms after the reading tasks is clear for seven items but again no important difference appears among the three subgroups. Only the subgroup of young subjects without glasses has slightly, but not significantly, more symptoms than the two other groups. From the whole analysis of the three subgroups it can be concluded that in our experiments the three groups react about in the same way to the reading tasks. Since the three groups are rather small, a generalization is not possible, however it is certainly justified
4
10
2
13
- 1
15
0
11
to consider the three groups as one overall group of twenty nine subjects for further analysis. The effects of oscillating frequencies on visual functions The figures of the mean shifts of the measured visual functions are reported in Table 3. One of the young subjects did not finish the session with an oscillation of 30 Hz. She judged it to be unbearable. She was dropped from the analysis of the 30 Hz condition but included in the other five conditions. Consequently the figures of discomfort are underestimated in the 30 Hz condition. The paired comparisons between "before-measurements" and "after reading task measurements" for each of the experimental conditions give the following results:
16
TABLE 3 S h i f t s o f v i s u a l f u n c t i o n s o f all 29 s u b j e c t s a f t e r t h e 3 h o u r s r e a d i n g t a s k s Visual function
unit
30 H z
60 H z
90 H z
180 H z
0 Hz
Printed text
CFF V i s u a l a c u i t y far Visual acuity near Contrast sensitivity Blink rate Near point
Hz Snellen equivalents Snellen equivalents Arbitrary units Blinks per min. Diopters
+ -
4.0 0.05 0.05 0.4 2.5 0.3
(1) The C F F reveals a marked decrease in each condition; all changes are significant ( p < 0.01 with Wilcoxon test). (2) The visual acuity "far" and "near" show very small but still mostly significant decreases in each condition. (3) The contrast sensitivity is slightly decreased in each condition, but only significant on the p < 0.05 level for 30, 60, and 180 Hz. (4) The blinking rate is increased in each condition, but only at 30 Hz significant with p < 0.05. (5) The near point reveals no change in any of the six conditions. (6) The lateral heterophorias exhibit a small but irrelevant esophoric shift in most conditions. A further analysis shows that the physiological reactions do not depend significantly
+
2.6 0.1 0.05 0.5 1.4 0
+ -
2.8 0.1 0.1 0.5 2.0 0.1
+ -
1.9 0.05 0.05 0.5 1.7 0.1
+
2.1 0.05 0.05 0.3 1.8 0
+
on the experimental condition with the exception of the CFF. Only the near point (accommodation) and the blink rate are slightly higher at the 30 Hz condition that at the printed text condition. An analysis of variance was conducted with all values of the CFF, which is reported in Table 4. Subjects are treated as levels of a factor and consequently there is one observation in each cell. All sources of variation other than main effects are considered to be part of the experimental error. Consequently the computed statistics are too conservative (Winer, 1971). The results reveal a great individual variance of the CFF; nevertheless the factors "time" and "experimental conditions" exhibit a highly significant influence. Based on this analysis of variance, the differences be-
TABLE 4 A n a l y s i s o f v a r i a n c e o f t h e critical flicker f r e q u e n c y M a i n effects
Sum of squares
Mean square
F
Sigmificance
28 6
54 339 2 886
1941 481
521.3 129.2
0.001 0.001
5
1504
301
80.8
0.001
2
40
20
5.4
0.004
Total main effects Residual
41 3 585
58 610 13 346
1430 3.7
384.0
0.001
Total
3 626
71957
19.8
Individual differences Time Experimental conditions Repetition of measurements
DF
2.2 0.1 0.1 0.1 1.1 0
17
TABLE 5 Paired comparison (t-Test) of the CFF shifts after 3 h reading task for the 6 experimental conditions. * * = p < 0.01; * = p < 0.05 30 HZ 6 0 HZ
**
9 0 HZ
.o
180 HZ
**
0 HZ
*"
Printed text
6OHz 9OHz 180 HZ 0 HZ
The effects on reading performances
*"
tween the 5 experimental conditions were tested for the shifts after 3 hours reading task by the paired t-test. The results, reported in Table 5, show that the 30 Hz figures are significantly different from all other conditions, while the 60 and 90 Hz figures differ from the 180 Hz with p < 0.05. In Fig. 3 the mean CFF shifts are represented for each time step during the six experimental sessions. In all conditions a clear decrease is already seen after 30 min and the greatest drop seems Duration of reading t a s k s ( m i n ) 0
to be reached after 120 min. Furthermore, the figure reveals a separation in 3 groups: The strongest drop is generated by the 30 Hz condition; the smallest decrease is seen in the three control conditions, while the 60 and 90 Hz, which belong to the invisible oscillations, produce an intermediate decrease of CFF.
60 t
i
120 •
.
180 t
The reading performances expressed as read lines per minute did not reveal any relevant differences among the six experimental conditions. The mean values are about 11 lines per min (110 words per min). This rather low reading speed may be the result of having to read aloud. Looking at all conditions, the reading performance is Slightly higher at the first (0-15 min) and the last time interval (166-180 min). The effects on visual comfort
The self-rated assessments were not normally distributed. The mean differences of the final self-ratings minus the initial ones were calculated and the significance of the shifts analysed with Wilcoxon test. The re-
30 Hz
•
6 0 Hz
-)(- 9 0 H z
• 180 Hz •
0 Hz
'4- print, t e z t N
TABLE 6
Differences of the increases of complaints at the end of the experiments for each experimental condition. Variables that showed a significant difference are tabulated (Wilcoxon-Test, p < 0.05)
= 2 3 0 Hz U
6 0 Hz
o
9 0 HZ
"
A. B. E B.E
180 HZ
3
9 0 HZ
A.B.E
0 HZ
S
A , B. E. C. D. G
0 HZ B.D
4
CaDtions : (n • 29 )
Fig. 3. Mean shifts of the critical flicker frequency of 28 subjects for each of the 6 experimental conditions.
180 HZ
A. B , E
Printed Text
IE
60Hz
A
B
eye fatigue
eye Dain burning eyes D headache E itching eyes G flickering C
S. E
E
B
18 Mean shift of complaints (arbitrary units) 0
10
20
tO0
30
eye fatigue eye pain burning eyes headache
the 30 Hz condition two persons were disturbed so much that they refused to continue the reading task; they were not taken into consideration for the calculation of the shifts in the 30 Hz condition. The increase of eye complaints
itching eyes tears
flickering blurred
vision:
near far double images
//
•
60
Hz
*
90
Hz
e 180
Hz
e
Hz
+
0
print.text
Fig. 4. Mean shifts of complaints at the end of the experiments of 29 subjects for each of the 6 experimental conditions.
suits are represented in Fig. 4 and Table 6. It must be pointed out that all shifts were significant; this means that the 3 hour reading tasks increased the symptoms of discomfort in each of the experimental conditions including "printed text" and "0 Hz". The symptoms reported in Fig. 4 were separated into three groups, according to a factor analysis used in a prior field study (Liubli et al., 1981). The factor analysis of the present study did not lead to a consistent grouping of the symptoms. For instance the symptoms "tired" (K) and "sleepy" (L) were mostly highly correlated with "eye fatigue" (A), but not however in every case. Nevertheless it seemed reasonable to represent the symptoms in the two groups shown in Fig. 4. The shifts of the symptoms "fatigued", "tired" and "bored" were of equal size in all conditions. From all results it can be deduced that the increases of "eye fatigue", "eye pain" and "itching eyes" are most pronounced at the 30 Hz condition and lowest in the printed text condition. In the other conditions the assessments covered an intermediate range. One observation must be pointed out: In
Affirmative answers to all 13 questions about eye discomfort or fatigue showed a continuous increase throughout the experiment. As an example the development of feelings of eye pain is represented in Fig. 5. The mean shift compared to the initial value is indicated for each experimental condition. From the results of the 13 questions, eye pains showed the best discriminating power among the 6 conditions. Relations between eye functions and subjective feelings
To get some indications about relations between physiological and subjective changes during the reading task, Pearson correlation coefficients were computed. The shifts of subjective feelings as well as of visual acuity (far and near), near point and blinkrate were not normally distributed so that the correlation coefficients and the tests of significance
I00' r legend: • 3G cycles per second "-. 6 0 H Z .~ * 90 .~ •
"~
o ..:
J0
(Hz)
~ /
6'0
~
/
7
/
9'0 l~C 150 180 duration of reading task (min)
Fig. 5. Mean shifts of eye pains of 29 subjects for each of the 6 experimental conditions.
19 TABLE 7 Pearson correlations between shifts of eye functions and subjective feelings in the 30 Hz condition (n = 27). Pearson correlation coefficients greater than 0.37 (p < 0.05) are tabulated
eye fatigue eye pain burning eyes headache itching eyes tears flickering blurred vision far blurred vision near double images
CFF
contrast sensitivity
0.5 0.5 0.6 0.5 0.4 0.4
0.5 0.4 0.4 0.4 0.4 0.5 0.5 0.5
should be taken as rough estimates of relations. Only the C F F and contrast sensitivity measurements revealed clear relationships with the subjective ratings. The correlation coefficients for these two physiological functions are reported in Table 7; only the 30 Hz condition with the most pronounced shifts was taken into consideration. It is concluded that a greater drop of C F F might be associated with a greater increase of eye discomfort. This kind of relationship was already observed in other experiments (e.g. Weber et al., 1973). The relationship between the contrast sensitivity and the eye discomfort is more difficult to understand.
DISCUSSION As mentioned above the visible flicker of a light source generates strong discomfort. One of the most striking effects is the immediate drop of the C F F which is most pronounced at levels of 20 to 30 Hz (Rey and Rey, 1965). It is generally accepted, that the screens of VDTs should be free of visible flicker. At the experimental screen (with an ap-
proximate decay time of 4 ms to drop to 10% of the peak luminance), the present experiments revealed C F F levels between 40 and 56 Hz. Bauer (1983) determined the C F F for a large population with a reversed presentation of a C R T and a background luminance of 80 c d / m 2. The range of the measured C F F was between 55 and 87 Hz. (mean: 73 Hz). If the results of Kelly (1964) are taken into consideration, one can expect C F F levels above 5 0 - 8 0 Hz for V D T screens. Most of the V D T models have refresh rates of 50 or 60 Hz with relatively short phosphor decay times. F r o m this point of view it must be concluded that these refresh rates are in a critical range of C F F . Some restrictions related to our experimental conditions must, however, be pointed out. The lighting conditions of the simulated C R T screen are not identical with a real VDT. AT a CRT, the electron beam wanders down line by line, while in our experiment, the total screen illumination is turned on and off simultaneously. We have kept the mean luminance constant and this means that the decreasing refresh rate is combined with an increased peak luminance (See Fig. 1). It is therefore theoretically not clear whether we evaluated the refresh rate or the peak luminance. The studies of Kelly (1964) give an answer to this question: The C F F depends on the amplitude of the groundwave divided by the mean luminance. In our experiments this quotient measured over a surface of 5 × 7 cm varied from 0.36 to 0.46 only. We therefore assume that for the eyes the refresh rate is the determining factor and not the very short peak luminance. In every condition the reading task, lasting 3 hours, had physiological effects on visual functions and caused visual discomfort. Besides this, a certain distinction among the six conditions was observed with the parameters "visual discomfort" and ' C F F " . The visual discomfort was strong with visible flicker (30
20 Hz) and low with the printed text. T h e remaining c o n d i t i o n s of invisible flicker (including the 0 Hz) did not reveal differentiating effects o n discomfort. H o w e v e r , the C F F revealed m o r e conclusive effects: T h e visible flicker caused a very strong d r o p o f the thresholds. T h e three control c o n d i t i o n s (180 Hz, 0 Hz and printed text) p r o d u c e d a smaller d r o p of the thresholds, while 60 a n d 90 H z had an i n t e r m e d i a t e effect. T h e r e f o r e the q u e s t i o n arises as to w h e t h e r flicker frequencies slightly a b o v e the perceived flicker threshold might have some physiological effects, but at the present time this c o n s i d e r a tion is not m u c h m o r e than a hypothesis.
ACKNOWLEDGEMENT T h e research r e p o r t e d in this p a p e r has b e e n s u p p o r t e d b y the Swiss N a t i o n a l Science F o u n d a t i o n , G r a n t No. 3.810.81.
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Gierer, R., Martin, E.. Baschera, P. and Grandjean, E., 1981. Ein Ger~it zur Bestimmung der Flimmerverschmelzungsfrequenz des Auges. Z. Arbeitswissenschaft, 35: 45-47. Kelly, D.H.. 1964. Line waves and flicker fusion. Documenta Ophthalmologica, 18: 16-35. L~iubli, Th. and Grandjean, E., 1984. The magic of control groups in VDT field studies. In: Grandjean, E. (Ed.), Ergonomics and Health in Modern Offices. Taylor and Francis. London. Laubli, Th., Hianting, W. and Grandjean, E., 1981. Postural and visual loads at VDT workplaces, II. Lighting conditions and visual impairments. Ergonomics, 24: 933-944. Nishiyama, K., Br~iuninger, U., de Boer, H., Gierer, R. and Grandjean, E., 1986. Physiological effects of oscillating luminances in the reversed display of VDTs. To be published in Ergonomics. Rey, P. and Rey, J.P., 1965. Effect of an intermittent light stimulation on the critical fusion frequency. Ergonomics, 8: 173-180. Weber, A., Jermini, C. and Grandjean, E., 1973. Beziehung zwischen objektiven und subjektiven Messmethoden bei experimentell erzeugter "Ermiadung". Z. Pr~iventivmedizin, 18: 279-283. Winer. B.J., 1971. Statistical Principals in Experimental Design. McGraw-Hill Inc., New York. Zacharia, A. and Bitterman, M.E., 1953. The effect of fluorescent flicker on visual efficiency. Applied Psychology, 36: 413-416.