ikg A017 displays Taylor & Francis Cyr,S., Nishiyama, The effects of var i ous refresh rates i n pos i t i ve and negat i ve

Transcription

1 A017 ikg The effects of var i ous refresh rates i n pos i t i ve and negat i ve displays. Ergonomics and health in modern off ices Taylor & Francis Cyr,S., Nishiyama, K., Gierer,R., Lau bl i,t. & Crandjean,E.

2 Ergonomics and Health in Modern Offices Edited by Etienne Grandjean Insrirut l;ir Hygiene und Arbei~physiologie, Zurich, Switzerland Proceedings of the International Scientific Conference on Ergonomic and Health Aspects in Modern Offices, held in Turin, Italy on 7-9 November 1983 under the auspices of the International Ergonomics Association, the Permanent Commission and International Association of Occupational Health, the University of Turin, the Sociera italiana di Medicina del Lavoro c di lgiene Industriale and the Regione Piemonte Taylor C Francis ond don and Philadelphia

3 POSITIVE/NEGATIVE AND COLOURED DISPLAYS The Effects of Various ~efresh Rates in Positive and Negative Displays (Introductory paper) S. GYR, K. NISHIYAMA, R. GIERER, T. LAUBLI AND E. GRANDJEAN Department of Hygiene and Ergonomics, ETH-Center. CH-8092 Zurich, Switzerland For most VDTs the refresh rates are 50 Hz in Europe and 60 Hz in USA and Japan. At present little is known about the effect of the frequency of oscillating luminance on visual functions and subjective feeling. The present study deals with the effect of various refresh rates on visual functions and subjective feeling: it is a summary of two studies described elsewhere in detail (Nishiyama et al., 1984; Laubli et al., 1984). 2. Methods An apparatus generating oscillating luminances of various frequencies on a screen was developed. The apparatus is described elsewhere by Nishiyama et al. (1984). The apparatus is based on the priple of a light ray projected on a screen of 170 x 210 mm2. Chopper discs generate oscillating lulninances between 0 and180 Hz. With a modified slide projector a text with dark characters on a bright screen or with bright characters on a dark screen can be produced. The oscillating luminances have a decay time of about 4 ms to drop to 10% of the peak luminances. Thirty subjects were engaged in the bright screen and 29 in the dark screen (bright characters) experiments. The subjects had to read aloud the text from the screen. The results of the reading task lasting one hour are presented here. Before and after the reading task the following visual functions were measured: critical fusion frequency (CFF), near point distance, visual acuity (near and far) and lateral heterophoria (near and far). Furthermore, the subjective feelings were recorded in questionnaires; in the bright screen experiments a semantic seven step scale was used and in the dark screen experiments a non-semantic stepless scale. Five frequencies of oscillating luminance and a printed text, which are indicated in Figure 1. were used in the experiments.

4 3 60 S. Cyr, K. Nishtpma, R. Geierer, T. Laubli and E. Grandjean 3. Results Figure 1 shows the result of the CFF for the bright and dark screen experiments. The ordinate indicates the mean shift of the CFF after a reading time of one hour and the abscissa indicates the six experimental conditions. This figure reveals-for the bright as well as for the dark screen: 1. A significant decrease for each experimental condition. For the bright and the dark screen the decrease is clearly at its highest level for the 30 Hz condition. 2. A significant difference between the decrease for the 0 Hz condition and the ones for the 30 and 60 Hz conditions; the decrease is higher for the 30 and 60 Hz conditions than for the 0 Hz condition. There is no significant difference between the bright screen and the dark screen experiments. FIGURE I. Mean skij) (decrease} oj the criticul f isiott jequency ajer a readifis rin~e of Frequency of oscil latina luminance (Hz) Printed text me horrr. t Dark screen (29 subjects) 0 Bright screen (30 subjects) " = W i l coxon o c 0.05, compared with 0 Hz Figure 2 shows for the symptom 'I have tired eyes' the mean shift for each experimental condition. This figure reveals a significant rease for the bright and dark screen for each experimental condition and a significant difference between the rease for the O Hz condition and the one for the 30 Hz condition; the decrease is higher for the 30 Hz condition than for the 0 Hz condition. Figure 3 shows the results of the various items related to possible visual complaints. This figure reveals a similar result for the bright and dark screen: the highest rease is in gene;al observed for the 30 Hz condition and the lowest one for the printed text. For the ten items the following index was calculated: number of reased symptoms (items) divided by the total number of items. Figure 4 shows a significant higher index value for the dark screen for the 30 and 60 Hz conditions than for the 0 Hz condition.

5 The Efects of Various R f i ~ Rates h in Positive and Negative Di~php 361 FIGURE 2. Mean shifi (rease) of the symptom 'I have tired eyes' 4cr a reading time of one hour. B r i g h t screen (30 subjects) D a r k screen (29 su~jects) Printed Printed text text Freo~ency of osc:llatinq lummance (Hz) FIGURE 3. Mean shi/r (rease) of ten symptoms related 10 subjective J'eclinf afier a reading rime of one hour. 11 red eyes I tch~ns eves Eurn~ns eves Blurr I:W Image Double maqe 5w I I nq I mase 3 r I a h t screen D a r k screen (36 sublects), (29 subjects) -... G $3 l.9 1 n c r e a ; e o f r a t i The significant changes at the 30 and 60 Hz conditions compared with the 0 Hz condition are recorded in Table 1. The results do not reveal a clear picture except that they show a higher idence of eye troubles with the dark than with the bright screen but this difference cannot be considered as relevant.

6 362 S. Gyr, K. Nishiyarna. R. Geierer, T Laubli and E. Grandjean F:(:uRE 4. Mean index often symptoms related to subjecrive jieling qier a reading time of one hour. $ 3 r 1 g h t screen (30 ~~blecti) D a r t screen (29 subjects) Fresuencv of oscillating luminance (Hz),, 0 Pr~nteo Printed text text : " = Wllcoxon p compared with 0 HZ TABLE I. Variables rhotvinq 2 siyntj;ianr (Wikoxon p < 0.05) higher decrease (drc) respmively reuse (h) Jir rhe 30 und 60 Hr cotrdirions rhan &r [he 0 Hz condition after a reading lime of one hour. 30 Hz 60 Hz Screen Bright3 Darka Bright Dark Critical fusion frequency dec dec dec dec Near point distance Visual acuity near dec Tired eyes Itching eyes Headache Eyepain Index (subjective leeling) ~nc a Bnght and dark are related to the background 4. Discussion The results of the present study reveal a higher decrease of the CFF figures and a slightly higher rease of some symptoms related to subjective feeling for the 30 and, to a smaller extent, for the 60 Hz conditions than for the 0 Hz condition. It can be deduced that oscillating luminances with 30 Hz and, to a smaller extent those with 60 Hz are associated with

7 The EjJects OJ Various R fresh Rms in Positive and Negative Displays a certain amount of eye-strain. The results. however. give no clear picture of the nature of this strain. In this relation a result of 39 subjects engaged in the study with the dark screen might be of interest: the CFF of the bright characters was determined when subjects focused on the screen centre and when looking at an edge of the screen. The mean CFF was 48 respectively 49 Hz. For eight subjects the CFF was within 50 and 60 Hz and one subject had a threshold above 60 Hz. Bauer et al. (1983) determined the CFF on 31 subjects exposed to a bright screen. The measured CFF lay between about 55 and 85 Hz. Another recent publication of Isensee and Bennett (1983) reveals that subjects viewing a bright screen with a 60 HI oscillation notice a higher sensitivity to flicker than when viewing a dark screen (bright characters). Taking the results of the present study as well as those mentioned in the literature it is concluded that VDTs using phosphors with usual decay times should be provided with refresh rates of more than 60 Hr. VDTs with dark characters on a bright screen should be equipped with a refresh rate of at least 100 Hz. Acknowledgement The research reported in this paper has-been supported by the Swiss National Science Foundation, Grant No / References Bauer, D.. Bonacker. M. and Cavonius. C.R., 1983, Frame repetition rate for flicker-free wewing af bright VDT screens. Display, January, Isensee, S.H. and Bennett, C.A., 1983, The perception of flicker and glare on computer CRT displays. Humon Fmors, 25(2), Laubli, T., Gyr, S., Nishyama, K.. Gierer. R., Brauninger, U. and Grandjean, E , Effects of various refresh rates in a VDT reading task (bright characters on dark screen), in the press. Nishiyama. K.. Brauninger, U., de Boer, Hildeke, Gierer, R. and Grandjean, E., 1984, Physiological effects of oscillating luminances in the reversed display of subjects, in the press.