Physiological effects of intermittently illuminated textual displays

By KATSUO NISHIYAMA, ULRICH BRAUNINGER, HILDEKE DE BOER, ROBERT GIERER and ETIENNE GRANDJEAN

Department of Hygienc and Ergonomics, Federal Institute of Technology, CH-8092 Zurich, Switzerland

Keywords: Visual display; Visual load; Eye fatiguc; Flicker fusion frequency; Reversed presentation of displays.

Optical equipment was developed to simulate visual display terminals (VDTs) with reversed prcsentation (bright background and dark characters). The decay time and mean luminancc were kept constant, while {lie oscillating frequcncy varicd, During one experiment suhjects had to read text aloud for 6Otnin. Heforc and after Ihe reading, near point distance, critical flicker fusion frequency (CFF), visual acuity, stereo depth perception, lateral heterophoria and contrast sensitivity were measur- ed. The 30 subjects had then to report their subjective reactions on a questionnaire. The following frequencies werc used: 0,30,60,90,180 Hz. In addition subjects read a normally printed text with a luminance of 75 or 150cd/m2. The main finding was that of a strong decrcase of the CFF after exposure to 30 and 60 Hz. It is suggested that for reversed prcsentation the refresh rate should bc higher than 90 Hz.

1. Introduction Visual complaints of VDT operators are associated with luminance contrasts

between screen surface, source documents and other surfaces (Liiubli et al. 1981). It might therefore be assumed that a reversed presentation with dark characters on a bright surface (positive presentation) would strongly decrease excessive contrasts with source documents. For this reason, some manufacturers havc produced VDTs with a positive presentation of characters. However, until now the experience with these deviccs has been controversial. Some operators complain that the bright screen is flickering. preferring thc bright characters on a dark background (negative present- ation). On the other hand, Radl (1980) showed that the presentation mode of dark characters on a bright background provided better readability and higher visual comfort than bright characters on a dark background. Furthermore, Bauer and Cavonius (1980) observed that both the error rate and the time required to identify letters on a television terminal were significantly reduced when positive presentation was used.

There are important individual differences in sensitivity to flicker. Size and brightness of the target and the nature of the wave-form influence the critical flicker fusion frequency (CFF) (see Kelly 1970). The refresh rates of nearly all makes of VDTs are today independent of the frequency of thc power distribution network, and the majority have refresh rates of 50 or 60FIz. Bauer (1984) measured the CFF of 30 subjects using a bright screen background (80 cd/m2) with a rather f a t phosphor and observed a range between 73 and 93 Hz. Using 28 subjects, Gyr et al. (1984) recordcd CFF levels between 45 and 55 Hz using simulated VDT equipment with bright characters on a dark background. This discrepancy is the main justification for the present study which aims to examine thc effects on visual functions and subjective feelings of various oscillating frequencies of a bright terminal screen background.

K. Nishiyama et al.

2. Methods 2.1. Development of an apparatus generating oscillating luminances

An apparatus was developed to expose subjects to different oscillating luminances. The device generated either bright characters on a dark screen or dark characters on a bright screen. The luminances of both could be continuously set from a constant light up to a frequency modulated light of 180 Hz or more. A DC powered halogen lamp was used as a light source, its luminance was adjustable by the voltage control. With a modified slide projector as shown in figure 1, the light beam was concentrated with a system of lenses toward a chopper disc (see figure 2) located in the focus of the final projection lenses. From here the light beam was projected onto a screen with a height of 170rnm and a width of 210mm. The chopper disc was driven by a servo-amplifier system. The chopper disc generated oscillating luminances very similar to those of VDTs, with a decay time of 4 ms to drop down to 10% of the peak luminance (see figure 3).

Text slides with sharpness and contrast characteristics similar to those of a good printed test were prepared. The hue of the screen background was light grey. The projected text was taken from a book.on ergonomics of the home; therefore the interest and difficulty levels were the same in all experiments.

2.2. Subjects Twenty-eight students and two clerical workers acted as subjects. Their ages ranged

from 17 to 40 years; 22 were less than 25 years old. Half of the subjects were female. The sub.jects were screened for visual defects and only those with normal vision without correction were selected. The screening test consisted of the following measurements:

(l) Visual acuity in far and near vision, stereo depth perception in far vision, colour recognition in far vision and vertical and lateral heterophoria in far and near vision were measured by the Titmus Vision Tester and the subjects were evaluated according to the 'Job Standard for use with the Titmus Vision Tester'.

(2) Contrast sensitivity was measured by the Rodenstock-Nyk t omcter and only sub.jects with a value less than or equal to 2.0 in f i r wsmn participated.

( 3 l CFF figures were measured with an apparatus developed by Gierer et at. (1 98 1): a subject participated if the CFF was between 38 and 53 Hz.

The instruments for screening tests were the same as those used in the experiments (see 5 2.4.).

2.3. Experimental condition In five experimental conditions dark characters were displayed on a bright screen

background, and in one condition the subjects had to read a normal printed text. The five screen backgrounds had a constant luminance (0 Hz) and oscillating luminances of 30, 60, 90 and 180 Hz, respectively. Mean luminance (Lm) was set at 75 cd/m2 or 150 cd/m2. The experimental room was illuminated by DC powered incandescent light with 2001x (vertical) and 3001x (horizontal) at the screen centre. The oscillating characteristics are shown in figure 3,

The sequence of the six trials was randomized for each subject. Each trial was done on a different day and lasted 60 min. Provided that the subject did not complain of ill

Elfbcls of inrerrnirtenfb illuminated textual displays ,

\ Parabol mirror

Figure 1

/ Heat shield

i Servo m tor

Optical system of a simulated video display terminal.

Figure 2. Chopper discs with cuttings generating different oscillating luminances.

K. Nishiyarna et al.

Figure 3. Characteristics of the luminance decay. Horizontal axis: Time in ms. Vertical axis: Luminance in cd/m2.

health or fatigue, each trial was carried out to completion. During the trial the subject had to read text aloud at his or her preferred reading speed, at a visual distance of about 60 cm.

2.4. Measurements Before, during and after each trial the following visual functions were measured by

the apparaus mentioned above.

Near point distance: Measured with a widely used device, No. 427 R.A.T. Gauge produced by Clement Clarke International Ltd. This device has a 50 cm-long rod with a movable target holder and scales from which the distance of the target from the observer's eyes can be read. During testing the rod was held by the observer in a standard orientation. The observer determined the near point with binocular vision by moving the target from 50cm towards himself or herself. CFF: Measured with an apparatus developed by Gierer et al. (1 98 1). The size of flickering field was 40 min and the shape of waveform of the flickering light was rectangular. A halogen lamp (Type 64425 Osram) was used as a light source, while a chopper disc, driven by a servo-amplifier system, interrupted the light beam and produced the flickering light.

&ffect o f intermittently illuminated textual displays 1147

(3) Binocular visual acuity is determined by the subject's ability to locate the solid (unbroken) ring based on the Landolt Ring principle in each of the numbered signs. The level of difliculty ranges from 201200 to 20/13 for far vision and from 141140 to 14/10 for near vision in Snellen equivalents.

(4) Stereo depth perception in far vision: The relative difficulty of each of the nine steps in this series is determined by the angular disparity of the 'key ring in each group. This level of difficulty is expressed as a percentage of theoretical maximum stereopsis according to the Shepard-Fry formula. Stereopsis depends on combining the images received by each eye into one image. The recognition of the relative distances between oneself and different objects, with ability to orient one's position in relation to them, is the basis of depth perception. .

(5) Lateral heterophoria in far and near vision: Heterophoria is the deviation of a visual axis from that of the other eye when fusing is prevented. The relative posture of the eyes in the vertical or lateral plane is measured with all stimuli to binocular fixation eliminated. In the visual field the target is seen by the right eye, a series of red dashes by the left. The deviation of the target through which the red line passes provides a measure of vertical or lateral balance. Vision Tester (Titmus, Zeiss, u.s.A.) was used for functions (3) (4) and (5). Far vision is taken at 20ft (6.1 m) and near vision at 14 in. (35-6 cm).

(6) Contrast sensitivity in binocular and far vision (4m) with the Rodenstock- Nyktometer: The dark target size is 10 min and the bright surrounding field size is 3'. The index of contrast sensitivity Z is defined by the following equation (and ranges from 1 to 10 in steps)

z= 10 log [10(1 - LzlLu)]

where Lz is the luminance ol the target (0.032 cd/m2 00.03 cd/rn2), and Lu is the luminance of the surrounding field.

Eye blink rate was also measured during the experiment; the frequency of blinking was counted for a period of two minutes on two separate occasions.

Before and after the experimenf the subjects were asked to rate on a semantic 7- point scale ten subjective symptoms (tired eyes, prickling or itching eyes, blurring eyes, blurred sight, doubled image, blurred view from near to far, flickering, headache, eye pain and tears, all of which were observed in a previous field study by Laubli et al. (1981)).

Tn a few cases subjects felt exhausted and extremely tired, and so the experiment was prematurely ended. Apart from these cases, each experiment was carried out in the following way:

Time: 0 min-10 rnin: Adaptation to the lighting condition of the experimental room

l0 min-25 min: Full test battery and rating scales. 25 min-55 min: First session of reading task. 55 min-60 min: Test batteries (l), (2) and (3). 60 min-90 min: Second session of reading task. 90min-l05 min: Full test battery and rating scales.

Thus the subjects had to read twice for a period of 30min each.

1148 K . Nishiyama et al.

The first series of experimental trials was conducted with only ten subjects in order to compare the effects of two mean luminances of the screen (which were 75 and 150 cd/m").

A second series of trials was camed out with another group of 20 subjects, but only with the lower background luminance of 75 cd/m2. The results of 30 subjects could therefore by analysed for the background luminance of 75 cd/m2.

2.5. Statistical analyses The distribution of each variable for the visual functions was examined by a

Kolmogorov-Smirnov one-sample test. If necessary, the data were transformed to obtain normal distributions. The data of the visual functions or subjective symptoms measured before, during and after the exposure were examined by a t-test for paired samples or by the Wilcoxon matched pairs ranked-signs test. For the variables showing significant differences by the above tests, the temporary shifts were determined by subtracting the value observed before the exposure from the value observed during or after the exposure. The distributions of the temporary shifts were also examined by the Kolmogorov-Smirnov one-sample test. If the distribution was normal, the effect of the frequency of luminance oscillation was examined by a multiple factorial analysis of variance. If a distribution of the temporary shift was non-normal the effects of luminances or oscillating frequencies were examined by the non-parametric tests for each pair of the experimental conditions. Furthermore, the visual functions which showed significance effects, related to the background luminance or the oscillating frequency, were examined for each pair of experimental conditions by the analysis of variance (ANOVA). Mter confirming the significance by ANOVA, the difference between all possible pairs of experimental conditions were examined by the paired t-test and Duncan's multiple range test while Newman-Keuls tests were done for a posteriori contrasts in order to compare all possible pairs of group means.

The subjective symptoms showing significant effects were also examined by the same non-parametric tests in order to disclose which luminance or oscillating frequency was more effective. Furthermore, the ratio of subjective symptoms, showing an increase of complaints, was calculated for each condition and subject.

The relationship between variables showing significant effects of the oscillating frequencies was examined either by Pearson's analysis or by a non-parametric correlation.

The statistical Package for the Social Sciences (SPSS), a system of computer programs (see Nie et al. 19751, was used for all of thc above analyses.

3. Results 3.1. Eflects of screen luminances

After 58 trials with 10 subjects had been completed, the results were analysed in order to check whether a differential effect of the two background luminances of 75 and 150 cd/m2 could be demonstrated. In general these analyses did not reveal essential differences between the results of the 75 cd/m2 condition and those of the 150 cd/m2 condition. There was only one exception: the lateral heterophoria in far vision was signdicantly more pronounced (in the direction of an esophoric S W ) after the experiment with 150 cd/m2 than after that with 75 cd/m2. Since a screen luminance of 75 cd/m2 seemed to be more realistic and more similar to the luminance expected from VDTs. all the following trials with 20 more subjects were carried out with a background screen luminance of 75 cd/m2.

Effects of' intermit tent ly illuminated textual displays 1149

3.2. Eflects of the various oscillating frequencies 3.2.1. Results of the visual tests. A preliminary statistical analysis with the Kolmogorov-Smirnov one-sample test showed that the data of near point distance, of CFF and of heterophoria were not non-normally distributed (p < 0.05). The measures of contrast sensitivity indexed as mentioned above were inverse-transformed. The frequency of blinks in 2 min were transformed to the logarithm of mean interblink interval to achieve normality. The comparison of scores before and after the reading tasks was significant only at 30 and 60 Hz (p < 0.05). The results of the experiments with 30 and 60 Hz are shown in table 1.

Table 1 shows that the CFF is decreased much more with reversed contrast at 30 and 60 Hz than after conventionally printed text. Both visual acuity (near) and contrast sensitivity are decreased to a small extent, though the differences with the printed text are not significant. All other differences are not consistent and only a few are weakly significant.

Another statistical approach was investigated: the temporary shifts of CFF, near point distance and heterophoria were also examined by the analysis of variance, since these data were not non-normal.

The mean decrease of CFF for all 30 subjects is represented in figure 4. The results disclosed significant effects of the different oscillating frequcncies on CFF (p < 0.001) and heterophoria in near vision (p < 0-01). Furthermore it showed that the decrease of CFF became significantly larger with the duration of the reading period by ANOVA in which the covariate was the time of exposure. The oscillation frequency of 60 Hz caused a small but significantly greater decrease of CFF than with 0 Hz, 180 Hz and with the printed text by the paired t-test. The Newman-Keuls test showed only a significant difference between the condition of 30 Hz and the others ( p < 0.05). However, the result

. of Duncan's multiple range test showed a significant difference between the conditon of 30 Hz, that of 60 Hz and the others (p<0-10). These results showed that 60 Hz was reasonably effective. The subjects perceived the flicker under 30 Hz but not with 60 Hz or more.

Table 1. The mean shifts of physiological functions after the reading tasks with the printed text and the screen background oscillating frequencies of 30 and 60 Hz.

30 Hz oscillating 60 Hz oscillating Printed text screen screen

Physiological functions A (Hz) Shift p A(Hz) Shift p A(Hz) Shift p

Critical fusion frequency 46.3 -2-5 *** 46.2 -8-8 *** 46.9 -4.1 *** '

Near point distance (cm) 13.2 + O n l 13-0 +O-4 12.9 +0.3 * Visual acuity (far)? 1-23 -0.05 1-23 -0-06 * 1.20 -0.05 Visual acuity (near)f 1.26 0.00 1-28 +0.05 *** 1.27 -0.04 *** Heterophoria (near) -0.80 -0.3 - 1.22 +O-4 * -0.09 -0-3 Contrast sensitivity$ 0-59 -0.027 0-61 -0-039 * 0.60 -0-054 ** Blink interval•˜ 1.00 -0.046 1-01 -0-102 * 0.92 +0-019

* p <0-05, ** p<0-01. *** p<0.001; l-tailed probability by t-test for normal distributions and by Wilcoxon matched test for non-normally distributed samples. A = Mean values before the reading task.

J. Visual acuity is evaluated by Snellen factor. 1 Contrast sensitivity = Luminance of characters/Luminance of screen background. $Blink interval =log (120/frequency of blinks in 2 min).

K. Nishiyama et al.

3 ' F fl 1-1: 30 rnin exposure

: 60 min exposure

Text Figure 4. The mean decrease of CFF for different oscillating frequencies of the screen

background. *** p < 0.001, ** p < 0.01, * p c 0.05 by the paired t-test between the oscillating conditions and the control conditions with 0 Hz or the printed text. Horizontal axis: Oscillating frequency in Hz. Vertical axis: Decrease of CFF in Hz.

The mean decrease of CFF suggested that the CFF decreased monotonically with reading time. Stepwise multiple regression analysis was used to summarize this relationship using l//'. llfand l/ Jf as possible representations of the decrease in CFF, with oscillating frequencyfi Goodness of fit was greater when the relative decrease in CFF was used as the dependent variable instead of the absolute value of CFF. The analysis was'computed for the 30 rnin exposure data and again for the 60 min exposure data, yielding regression equations

Y= - 0.541 X 103/('JZ - 1.67) (multiple R = 0.65; p < 0.001)

and

Y= - 0.538 X 1O3/(f2 - 2.80) (multiple R = 0.57; p < 0.001)

respectively, where Y is the predicted relative decrease in CFF. These two functions are shown in figure 5. In this figure the upper limits of the 95% confidence interval for the mean decrease of CFF are drawn for the two control conditions with OHz and the printed texl. These limits cross the regression curves at an oscillating frequency of less that 90 Hz, so that a bright screen surface with an oscillating frequency of more than 90 Hz is estimated to have no more adverse effect on CFF than the control conditions.

The determination of lateral heterophoria gave the results shown in table 2. It can be seen that the mean changes after a reading time of 60 min are exophoric shifts for the oscillating frequencies of 30 and 90 Hz and esophoric shifts for 60, l80 and 0 Hz and the printed text. These exophoric and esophoric shifts were significantly different at the 5% level by Duncan's multiple range in a posteriori contrast tests. The variance was large, so that only the difference between 30 Hz and the control conditions of 0 Hz and with the printed text was significant at the 5% level by the paired t-test.

Eficts of intermittently illuminated textual displays

A f t e r - 30 min (control)

Figure 5 . Predicted decrease of CFF by the equations obtained from the regression analysis. Y, Predicted relative decrease of CFF: X, Exposed oscillating frequency of screen back- ground. Horizontal axis: Oscillating frequency in Hz. Vertical axis: Predicted relative decrease of CFF in %.

The measurements of visual acuity in near vision after the reading task disclose a very small decrease with the oscillating frequencies of 30 and 60 Hz. The mean values are shown in table 2. An analysis using the Kruskal-Wallis test showed that the effects of different oscillating frequencies were significant at the 5% level. There was, however. no significant difference in these values between 30 Hz or 60 Hz and the printed text or 0 Hz. Therefore the results shown in table 2 can only be considered as suggestive.

All other physiological parameters, such as near point distance, stereo depth perception. contrast sensitivity and blinking rate, did not reveal a significant change compared to their OHz or printed text conditions.

3.2.2 Results of rating subjective symptoms. The results of the scale questionnaire were analysed with non-parametric tests by comparing the differences between scores beforc and after the reading tasks. The statistical analysis of each complaint before the reading tasks failed to show significant differences between experimental conditions.

The median values for the self-rated symptoms 'fatigued eyes' and 'eye pain' after the reading task are given in table 2. These results show that the increase of the degree of 'fatigued eyes' as well as the increase of 'eye pain' is more pronounced in the conditions

1152 K. Nishiyama et al.

Table 2. The significant effects of different oscillating conditions of screen background on the shift of some visual functions and subjective symptoms after 6Ornin of exposure.

Oscillating frequency Visual functions and Printed 6 groups

subjective symptoms 3OHz 60Hz 90Hz 180Hz OHz text test

Heterophoria (near) 035* -0.25 0.10 -0.18 -0.17* -0.25" * Visual acuity (near) -0.05 -0.04 -0.00 0-02 -001 -0.00 * 'Fatigued eyes' 2.7" 1.5* 1.8" 1-8" 1.4' 1.4' 'Eye pain' 1.5" 0.3* O.3* 0-3" 0.3" 0.3* Ratio of the increased

subjective symptoms 0.39* 0.29** 0.30 0.30 0.27** 0-32"

* p<0.05, ** pcO.01; probability tests by Duncan's multiplc range in a posteriori contrast tests and the paired t-test for the lateral heterophoria in near vision, by Kruskal-Wallis test for visual acuity in near vision, by Mann-Whitney U-test (2 dependent samples) for 'fatigued eyes' and 'eye pain' and by Wilcoxon matched pairs test for the ratio of the increased subjective symptoms. Shift of lateral heterophoria is evaluated by the difference of dioptres which positive value mean exophoric shift occurred; shift of visual acuity is evaluated by the difference of Snellen factor; the value of subjective symptoms present the median.

with 30 Hz. In both cases complaints at 30 Hz gave significantly higher ratings than at all other conditions (p < 0.01 by Mann-Whitney U-test).

An index was then calculated by the following equation: Index of increase of complaints = number of increased subjective symptoms/number of available answers for 10 subjective symptom questions X 100%. This index shows the rate of the increased subjective symptoms by the reading task. These results are shown in table 2. The index is significantly higher at 30 Hz than at all other experimental conditions (p < 0.01 for a printed text and 180 Hz and p < 0.05 for 0 Hz and 60 Hz by Wilcoxon matched pair test).

The rank correlation coefficients between visual tests and reported subjective symptoms were analysed. No significant relationship could be obtained regardless of the tested parameters.

4. Discussion The experiments with a 60min reading task disclose that a bright screen surface

with an oscillating frequency of 30Hz impairs some visual functions and increases subjective feelings of tired eyes and eye pain. Except with respect to the CFF, the oscillating frequency of 60 Hz has only small and non-significant effects. The result of a stepwise multiple regression analysis for the relative decrease in CFF shows that a refresh rate higher than 90 Hz produces the same decrease in CFF as a printed text and the non-oscillating screen (0 Hz). As mentioned before, Bauer (1984) has observed that bright screens and dark characters cause CFF figures of between 73 and 93 Hz. The author concluded that the screen with bright background 'has to be flicker-free', which is achievable for 99% of all observers if an image repetition rate of 95 Hz is chosen for raster without interlacing and 'fast' phosphor. Our results indicate a similar conclusion: on the average only a small effect on CFF is detectable at 60 Hz, with no adverse effects at 90Hz or more.

Eflecb of intermittently illuminated textual displays 1153

One important question remains open: Are the results of a l-hour reading test transferable to the situation in practice with working conditions lasting months and years at VDTs? Clearly they are not, but they do indicate that care is needed when choosing refresh rate and phosphor of bright screen terminals.

Acknowledgment This work was supported by the Eidgen~ssische Stiftung zur Forderung Schweize-

rischer Volkswirtschaft.

On a mis au point un systeme optique pour simuler des terminaux a ecran avec presentation inversk (fond clair et caracthres noirs). La duree de la remanence et la luminance moyenne ont ete maintenues constantes, alors que la frequence de clignotement de l'irnage variait. Au cours d'une des experiences, les sujets devaient lire a voix haute un texte pendant 60 minutes. Avant et apres la lecture, on a mesure la distance du point proximum, la frequence critique de fusion du papillotement, l'acuite visuelle, la perception sterioscopique. I'heterophorie laterale et la sensibilite de contraste. Les 30 sujets devaient ensuite exprimer leurs reactions subjectives a l'aide d'un questionnaire. On a utilise les frequences suivantes: 0, 30, 60, 90 et 180Hz. En outre, les sujets disqosaient, pour la lecture, d'un texte imprime normalement, avec une luminance de 75 ou 150 d i m - . Le resultat principal etait une forte baisse de la F.C.E. apres I'exposition a 30 et 60 Hz. On recommande, une frequence de recyclage d'image supirieurc a 90Hz, dans le cas d'une presentation inversee.

Es wurden optische Gerate entwickelt, urn die inverse Darstellung (heller Hintergrund, dunkle Zeilen) auf Bildschirmen zu simulieren. Die Nachleuchtzeit und die rnittlere Helligkeit wurden konstant gehalten, wahrend die Bildfrequenz variiert wurde. Bei einer Testreihe rnul3ten Versuchspersonen einen Text iiber einen Zeitraum von 1 Stunde hinweg, laut lesen. Vor und nach dem Lesen wurden kiirzeste Sehentfernung, Flimmerverschmelzungsfrequenz, Sehscharfe. raumliches Sehvermogen, seitliches Schielen (Heterophoric) und Kontrastemptindlichkeit- gemessen. Die 30 Versuchspersonen m ul3ten ihre subjektiven Empfindungen auf einem Fragebogen angeben. Folgende Frequenzen wurden getestet: 0,30,60,90 und 180 Hz. Zusatzlich b e n die Versuchspersonen einen normal dargestellten Text bei einer Leuchtdichte von 75 oder 150cd/m2. Das wichtigste Ergebnis war, da8 die Flimmerverschmelzungsfrequenz nach Textdarstellungen (mit 30 und 60Hz) sehr stark abfallt. Es wird vermutet, daD fiir inverse Darstellungen die Bildfrequenz grijI3er als 90 Hz sein sollte.

References BAUER, D., 1984, What causes flicker in bright background VDTs and how to cure it? In

Ergonomics and Health Aspects in Modern Ofices (Edited by E. GRANDJEAN and E. VICLIANI) (London: TAYLOR & FRANCIS LTD.), pp. 364-370.

BAUER? D., and CAVONNS, C. R., 1980, Improving the legibility of visual display units through contrast reversal. Tn Ergonomic Aspects of Visual Display Terminals (Edited by E . GRANDJEAN and E. VIGLIANI) (London: TAYLOR & FRANCIS LTD.), pp. 137-142.

GIEKEK, R., MARTIN, E., BASCHEKA, P., and GKANDJIIAN, E.. 1981, Ein Geriit zur Bestimmung der Flimmerverschmelzungsfrequenz des Auges. Zeitschrijtfir Arbeitswissenschaji~ 35,45-47.

Gm, ST., NISHIYAMA, K., LAUBLI, T., and GRANDJEAN, E:: 1984, The effects of various refresh rates in positive and negative displays. In Ergonomics and Health Aspects in Modern Ofices (Edited by E. GRANDJEAN and E. VIGLIANI) (London: TAYLOR & FRANCIS LTD.), pp. 359-363.

KELLY, D. H., 1970, Flicker, Lb Sensory Phq).siology. 7, 273-301. LAURLI, TH.. H ~ ~ K T I K C ; . W., and GRANDJEAN, E., 1981. Postural and visual loads at VDT

workpiaces. 11 Lighting conditions and visual impairments. Ergonomics, 24, 933-944.

1154 Efects qf intermitterttly illtminated textual displays

NIE, N. H., HULL C. H., JENKINS, J. G., STEINIIKDNNER_ K., and BENT- D. H., 1975, SPSS Statistics! package fir social sciences, second edition (New York: MCGRAW-HILL).

RADL, G. W.. 1980, Experimental investigations for optimal presentation-mode and colours of symbols on the CRT-screen. Tn Ergonomic Aspects of Visual Display Terminals (Edited by E. GRANDJEAN and E. VIGLIANI) (London: TAYLOR & FRANCIS LTD.), pp. 127-135.

Manuscript received 22 November 1983. Final revision receivcd and acceptcd 17 March 1986.