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Workstation variables and visual discomfort associated with VDTs
Applied Ergonomics 1990, 21.2, 157-161
Workstation variables and visual discomfort associated with VDTs M. Collins, B. Brown, K. Bowman and A. Carkeet Centre for Eye Research,Department of Optometry, QueenslandUniversity of Technology, 2 GeorgeSt, Brisbane, Queensland, Australia 4001 We have investigated the effects of a range of workstation factors upon the visual symptoms experienced by a group of 92 visual display terminal (VDT) users. Subjects in the study kept a diary over five consecutive working days in which they recorded the types of visual and postural symptoms which occurred and the types of work tasks being performed. Each subject's workstation was analysed for screen legibility and stability, discomfort and disability glare, and required head postures. By the use of multiple regression analysis techniques we have considered the relative contribution of these factors to the symptoms reported by the users of these workstations. Screen legibility significantly influenced the occurrence of symptoms of ocular discomfort and vertical head movements significantly affected the incidence of postural/headache symptoms. Keywords: Visual'display terminal, vision, visual discomfort, workstation, glare, posture
Introduction VDT usersfrequently report ocular discomfort (Dainoff,
et al, 1984) and this may lead to exaggerated claims of
1982), but whether they experience ocular discomfort more frequently than control subjects performing equivalent hard copy tasks ;is a matter of conjecture. However, the high rates of visual symptoms experienced by VDT users is un. doubtedly a problem in terms of user comfort and efficiency.
In this study, VDT users made detailed records of their visual symptoms when performing both VDT and non-VDT visual tasks throughout the working day. A survey was conducted of each subject's workstation to review some aspects of the lighting, VDT screen and postural demands. Multiple regression analyses were used to identify those workstation characteristics associated with visual symptoms during VDT use.
Differences in the nature of VDT and hard copy tasks introduce different visual demands on VDT users. The physical structure of characters in the VDT screen is different to that of hard copy characters. The reflective nature of the VDT screen increases glare which may cause ocular discomfort or impair legibility of the screen text. The line of sight for VDT use is inclined more towards the horizontal than it is for hard copy tasks performed at a desk, a factor which may introduce glare sources such as windows and ceiling luminaires into the user's field of vision. The VDT user's work patterns may be altered with respect to hard copy tasks; for example, where the users once regularly looked up from the typewriter to change a sheet of paper they now continuously watch the VDT screen. In many instances, the visual quality of VDT text can be superior to that of hard copy; the contrast of the VDT screen can often be adjusted by the user for maximum comfort which is not the case for hard copy. VDTs are often introduced irtto workplaces where the fighting and workstation arrangement is designed for hard copy tasks. Any resulting mismatch between visual task and visual environment may contribute to the ocular dis. comfort or visual difficulties of the VDT user. Because of the concern over numerous health issues related to VDTs, changes to work patterns and job security, it has been suggested that VDTs have become the "symbolic focus of discontent" amongst users of this equipment (Helander
adverse health effects.
Methods Subjects Ninety-eight staff from Queensland University of Technology agreed to participate in the study. These subjects were drawn from academic, administrative and clerical staff of the University. The mean age of subjects was 33.9 years, 67% of subjects were females and most subjects had used VDTs for 3 to 5 years. A wide variety of VDT types was used by subjects who participated in this study. Visual tasks involving VDT usage accounted for 48% of working time and most VDT.related tasks involved text entry. Diary protocol Subjects were asked to complete a diary over five consecutive working days, beginning on a Monday. In this diary the subjects recorded visual symptoms and various taskrelated variables, including the type of work undertaken (VDT or non-VDT tasks). Subjects made diary entries four times per day,just prior to breaks in the morning, lunch, afternoon and the end of the day. This gave data from 20 work sessions for each subject (i e, four sessions per day for five days). A more detailed description of the diary is reported in Collins et al (1989).
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Subjects recorded symptoms under three headings; ocular, visual and systemic symptoms. Ocular symptoms were defined as any incident of ocular discomfort. Visual symptoms were defined as any incident of impaired vision. Systemic symptoms were defined as any incident of headaches or postural discomfort. The diary contained lists of specific symptoms under each of the three broader symptom categories to aid the subjects in classifying correctly the symptoms experienced.
type of habitual spectacle correction used for VDT work was noted (e g, single-vision reading glasses, contact lenses or bifocals). Working distances were measured from the VDT screen and keyboard with subjects using their habitual visual correction. At the same time we measured the range of accommodation (range of clear focus) with subjects using their habitual visual correction. Subjects were asked if they were touch typists, since this factor will influence their necessity for head and eye movements during VDT tasks.
Beside each of the symptoms entered in the diary from the above three categories, subjects were asked to rate the intensity of the symptom on a scale from 1 (very mild) to 5 (very intense). Symptoms were entered into the statistical analyses using this intensity rating - for example, a symptom of 'tired eyes' with an intensity rating of 5 (severe) was scored as an ocular symptom with a score of 5.
The subjects were asked to sit at their workstations and to explain and demonstrate the normal visual tasks involved in their work with VDTs and the length of time spent performing each task during the day. The investigator's assessment of head posture was based upon this questioning and observation of the subject during various work tasks.
The diary of symptoms and work tasks was completed prior to the workstation analysis. This was done to mimimise 'sensitisation' of the subjects to any real or perceived problems with their workstation environment, prior to completing the diary.
Workstation analysis A single investigator visited each subject and conducted all workstation assessments. This served to minimise variation in the criteria adopted for subjective assessments, but does introduce the possibility of a consistent bias in this data. In a few instances the investigator was aware of the subject's symptoms before assessing the workstation. However, it is unlikely that there was any significant influence on the independence of the symptom and workstation data. VDT screen characteristics
The legibility of screen characters (assessed from the subject's normal seated position) was graded subjectively on a scale of 1 (good) to 5 (poor) by the investigator. Those factors responsible for poor character legibility were noted (e g, contrast, character size, clarity). The facility for operator contrast adjustment and the use of filters (such as 'mesh' filters) on the screen were recorded. The stability of screen characters and raster was subjectively graded on a scale of 1 (good) to 5 (poor). Those factors responsible for poor screen stability were noted (e g, flicker, swim, jitter). Discomfort and disability glare Discomfort glare is defined as glare which causes discomfort without necessarily impairing the visibility of objects, while disability glare is defined as glare which impairs the visibility of objects without necessarily causing discomfort (Australian Standard 2713-1984). The investigator subjectively graded discomfort glare for each workstation (assessed from the subject's normal seated position) on a scale of 1 (good) to 5 (poor). Those factors responsible for discomfort glare were recorded (e g, screen reflections, luminaires, windows). Disability glare was also subjectively graded on a 1 to 5 scale (assessed from the subject's normal seated position). Those factors responsible for disability glare were also noted (e g, ambient illuminance, windows, luminaires). Head posture To investigate the influence of visual factors upon the subject's head posture, we made a series of observations and measurements with the subjects at their workstations. The
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Horizontal head deviations were defined as a head turn to the right or left, and vertical deviations were defined as downward or upward head deviations. Normal hea d posture was defined as a slight downward vertical head turn (line of sight approximately 10 ° below horizontal). Horizontal and vertical head postures were graded in terms of their range. The range of head turns referred to the amount of head turn required during work, taking into account how often these head turns were necessary in performing the work (hence the range of horizontal and vertical head turns are an estimation of the amount and frequency of the head turn). The range was graded on a scale of 1 (none) to 5 (a large amount of head turn required often). The following examples illustrate this measurement technique. A number of computer programmers habitually placed their keyboards on their knees and frequently changed vertical head posture to look from the VDT screen to the keyboard and back again. These subjects were given high ratings for vertical head turn range. A number of secretaries who were touch typists and used copy holders to place hard copy beside the VDT screen had little if any head turn for long periods of the working day. These subjects were given a low rating for vertical and horizontal head turn range.
Multiple regression analyses Complete diary data were obtained for 72 subjects, with partial data for a further 20 subjects. Multiple regression was used for the data analyses, using each of the three sets of system scores (i e, ocular, visual and systemic) separately as dependent variables. The results of the workstation investigations of the total 92 subjects formed the independent variable set. All workstation variables were directly related to VDT use and not to non-VDT tasks. For this reason, the multiple regression of the workstation variables and symptom scores was based only upon symptoms which occurred during sessions predominantly involving work with VDTs, identified in the diary record. Symptom scores during 'VDT sessions' were totalled for the five days and the mean symptom score (for ocular, visual and systemic symptoms) per session calculated for each subject.
Results Use of VDT screens with poor legibility was significantly associated with higher rates of ocular symptoms (Table 1), since screen legibility entered into the equation predicting ocular symptoms with a significant positive beta weight. A similar trend, although not statistically significant, was apparent with visual symptoms (Table 2). Most VDT screens
Table 3: Results of multiple regression of workstation variables end systemic symptoms
Table 1: Results of multiple regression of workstation variables and ocular symptoms Workstation variables
Screen legibility
Beta weight
t value
0"22
2"00
Screen stability
-0"13
Discomfort glare
p
Workstation variables
Beta weight
t value
p
0"04*
Screen legibility
1 "08
0"28
- 1 "27
0"22
Screen stability
-0"16
-1 "18
0"24
-0" 13
-0"92
0"36
Discomfort glare
-0"04
-0"35
0"73
Disability glare
0"09
0"76
0"46
Disability glare
-0"01
-0"04
0"97
Touch typist
0"02
0"14
0"89
Touch typist
0"01
0"07
0"95
Horizontal head turn
0"01
0"11
0"92
Horizontal head turn
0"14
1"23
0"23
-0"08
-0"65
0"52
Vertical head turn
-0"29
-2"40
Vertical head turn
0" 13
0"02*
Note: * indicates p < 0"05
Note: * indicates p < 0"05
Table 2: Results of multiple regression of workstation variables and visual symptoms
ratings (1 good to 5 poor) of those screens with and those without Filters. Of screens surveyed, 88% had filters or special anti-reflection coatings. The mean disability glare rating of screens with filters or coatings was 2.14 and was 2.56 for those VDTs without filters or coatings, a reduction of approximately 20%.
Workstation variables
Beta weight
t value
p
Screen legibility
0"18
1"65
0"11
Screen stability
0"01
0"03
0"97
-0"26
-1 "94
0"06
0" 18
1 "59
0" 12
0" 11 -0"07
0"89 -0"62
0"38 0"54
0"01
0"43
0"97
Discomfort glare Disability glare Touch typist Horizontal head turn Vertical head turn
were judged to be of good to moderate legibility. Character design, size and contrast were of approximately equal significance in causing reduced screen legibility. The range of screen legibility across subject workstations is presented in Table 4. Results of the multiple regressions of symptom scores versus workstation variables are presented in Tables 1, 2 and 3. Screen stability did not significantly predict the occurrence of ocular, visual or systemic symptoms. The vast majority of screens were judged to have no visible instability (Table 4) and of those that showed instability, flickering of the characters was the most common cause. Discomfort glare in the user's field of vision was not sig. nificantly associated with ocular, visual or systemic symptoms. The range of discomfort glare scores across subject work. stations is presented in Table 4. The most common causes of discomfort glare were judged to be luminaires and windows in the user's field of view. Disability glare in the subject's VDT screen did not significantly enter into any of the equations predicting symptoms, although it had a relatively high positive beta weight (p = 0-12) in the equation predicting visual symptoms. Most screens had good to moderate disability glare ratings and of those screens with disability glare present, the major sources were reflections due to Iuminaires and high ambient room illuminances. The range of disability glare scores across subject workstations is presented in Table 4. To investigate the influence of the use of VDT screen filters on disability glare, we compared the disability glare
The range of horizontal head turn entered into the equation predicting systemic symptoms with a positive betaweight but did not reach significance 0~ = 0-23). The range of required horizontal head turn is presented in Table 4. Vertical head turn entered into the equation predicting systemic symptoms with a significant beta-weight (p= 0-02). That is, the greater the amount and frequency of vertical head deviation when performing tasks at the VDT, the lower the incidence of systemic symptoms. The range of vertical head turns is presented in Table 4. The ability to touch type did not enter into any of the equations predicting ocular, visual or systemic symptoms. Of the subjects in this study, 49% were touch typists. The near point of accommodation (i e, closest distance at which the eye can focus) was measured for all subjects using their habitual visual corrections. Of subjects, 62% wore no correction, 31% wore single vision spectacles or contact lenses and 7% wore bifocal spectacles. Two subjects had near points of accommodation at a greater distance
Table 4: Ratings of workstation variables across all workstations (percentage) Good 1
2
3
4
Poor 5
Screen legibility Screen stability
26 92
42 6
29 2
3 0
0 0
Discomfort glare
27
46
19
6
2
Disability glare
28
43
19
8
2
Low 1
2
3
4
High 5
24 21
22 36
38 36
8 7
8 0
Horizontal head turn Vertical head turn
Applied Ergonomics
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than their keyboard working distance (neither were touch typists); this could result in the subject bending backward to view the keyboard resulting in postural discomfort (systemic symptoms) or maintaining normal posture and reporting blurred vision (visual symptoms). Neither subject recorded any incident of systemic symptoms during the five days of the diary survey and one of the subjects noted only minor ocular symptoms during two sessions. Both of these subjects used their VDTs infrequently during the diary survey. Bifocal spectacles were worn by five subjects. The mean vertical head turn amongst the bifocal wearing group was 3.2 (1 small to 5 large), compared with the mean vertical head turn for all other subjects of 2 '3. Because subjects wearing bifocal spectacles may adopt unusual head postures to compensate for inappropriately positioned reading segment heights, we compared the incidence of systemic symptoms of neck, back and shoulder pain in the bifocal wearing group with the incidence in all other subjects. The incidence of these symptoms was derived by calculating the total frequecy x severity systemic symptom (postural discomfort and headaches) scores per diary session. For the bifocal wearing group this score was 0.76 symptom scores/ session versus 0.57 symptom scores/session for all remaining subjects.
Discussion There are many factors aside from workstation characteristics which may load to visual symptoms amongst users of VDTs. In this particular population of VDT users we have reported a number of task-related variables such as work pressure and interest, and specific VDT tasks which significantly influence the incidence of visual symptoms (Collins e t el, 1989) along with the subject's length of VDT experience (Collins et el, 1988). If, for example, all subjects in this study with poor screen stability characteristics also had high work interest and low work pressure, the effect of screen stability on visual symptoms could easily be masked by the work interest and pressure effects. The results of field studies such as this need qualification because of these uncontrolled variables. We found that poor screen legibility was significantly associated with more frequent reports of ocular (discomfort) symptoms and showed a positive, but not significant association with visual (blur) symptoms. Factors such as character contrast, size and design were considered to be the major contributors to reduced screen legibility. These findings are not unexpected and agree with the results of previous laboratory-based studies. Aspects of screen legibility such as dot matrix design, font style, character luminance and visual angle of the characters have all been shown to affect work performance measures (Brown et el, 1982; Snyder and Taylor, 1979). Stammerjolm et al (1981) have noted an association between reports of visual discomfort and screen characteristics including screen height, angle, glare and flicker. Turner (1982) reports anecdotal evidence suggesting that asthenopia (eyestrain) amongst VDT users may be caused by poor screen legibility and poor screen stability. Most workstations surveyed in this study had good discomfort and disability glare ratings and it may be the small number of workstations with poor glare ratings which has masked any significant effect of glare on visual symptoms.
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However, we did find a relatively high positive association between disability glare ratings and visual symptoms in this study. Hultgren and Knave (1974)have noted an association between discomfort glare, screen reflections and symptoms of discomfort amongst a small sample of VDT users The ability to touch-type was included as a variable in the statistical analyses to test the hypothesis that VDT users who do not touch-type are forced to perform a greater number of eye movements, accommodation (focusing) changes and head movements in carrying out keyboard tasks. This could potentially lead to differences in the rate of symptoms. Touch-typing ability entered into the multiple regression analyses with low beta weights and therefore the hypothesis was not sustained. The estimates of head posture made during this study were based upon a number of assumptions. These assumptions were: that the subject's appraisal of habitual head posture when performing VDT tasks was an accurate assessment; that the subject's estimation of the amount of time spent with that head posture was accurate; and that the VDT tasks (and head posture) performed during the diary survey period were similar to those demonstrated during the workstation analysis. Because of these numerous assumptions, the inferences drawn from the head posture data need some qualification. Our finding of a negative association between the range of vertical head turn and systemic (postural/headache) symptoms is consistent with the concept that static muscle loading leads to pain (National Research Council, 1983). In our study, those subjects with a low vertical head turn range had little if any variation in vertical head posture and reported significantly more systemic symptoms. It follows that frequent variations in vertical head posture by VDT users may lead to the greatest postural discomfort. Horizontal head turn did not enter the equation predicting systemic symptoms with a significant beta weight. Since horizontal head position does not involve substantial static muscle loading, this result can also be interpreted as being consistent with the findings on vertical head turn in this study. It appears that the five bifocal wearing subjects in this study had a higher incidence of neck, back and shoulder pain. No statistical inferences were drawn from these data because of the small number of bifocal wearers and the potentially large influence of other uncontrolled variables on their symptom scores. However, this group of subjects also displayed a higher vertical head turn score than did the remaining subjects in the study. Since a higher score on vertical head turn was associated with significantly fewer postural symptoms amongst the group as a whole in the multiple regression analysis, we speculate that it is the unusual static head postures required with bifocals when viewing VDTs which leads to symptoms rather than the frequency or range of necessary head movements. Cakir et al (1979) have reported an association between neck pain, ocular discomfort and the use of reading spectacles by VDT users. They suggest that this association results from the spectacle correction being inappropriate for the screen viewing distance, which results in the VDT user bending forward to see the screen clearly. Martin and Dain (1988) and Vassilieff and Dain (1986)have noted difficulties with the use of multi-focal spectacle designs for VDT users. These difficulties result largely from a restriction in the corrected vertical field of vision produced by the near vision segment(s) in multi-focal spectacle lenses.
In summary, we have examined the relative contribution of various workstation characteristics to the symptoms reported by users of VDTs. Of the factors investigated, legibility of text in the screen and the required vertical head turns had a significant influence on the symptoms reported.
Acknowledgements
We thank Jim Whiting for his advice on postural ergonomics, and Dale Card and Anthony Bloesch who assisted in data analysis. This work was funded by a grant from the National Occupational Health and Safety Commission 0Vorksafe Australia).
References
Australian Standard 2713-1984. 1984, Lighting and the visual environment for screen-based tasks. Standards Association of Australia, Sydney. Brown, B.S., Dismukes, K., and Rinalducci, E J . 1982, Behaviour and Inf Tech, I, 121-140. Video display terminals and vision of workers. Cakir, A., Hart, D.J., and Stewart, T.F.M. 1979, Visual display terminals. John Wiley and Sons, New York. Collins, M.J., Brown, B., and Bowman, K.J. 1988, Visual discomfort and VDTs. National Occupational Health and Safety Commission (Worksafe Australia), Centre for Eye Research, Queensland University of Technology, Brisbane.
Collins, MJ., Brown, B., Bowman, K.J., and Caird, D. 1989, Human Factors, (submitted). Task variables and visual discomfort associated with the use of VDTs. Dainoff, M.J. 1982,Behaviour and Inform Technol, 1, 141-176. Occupational stress factors in visual display terminal (VDT) operation: a review of empirical research. Helander, M.G., Biilingsley, P.A., and Schurick, J.M. 1984, An evaluation of human factors research in visual display terminals in the workplace. In: Muckler, F.A. (Ed), Human Factors Review, Human Factors Soc, Santa Monica, 55-129. Huitgren, G.V., and Knave, B. 1974, Applied Ergonomics, 5(1), 2 - 8 . Discomfort glare and disturbances from light reflections in an office landscape with CRT display terminals. Martin, D.K., and Dain, S.J. 1988, Applied Ergonomics, 19(4), 293-300. Postural modifications of VDU operators wearing bifocal spectacles, National Research Council. 1983, Video displays, work, and vision. National Academy Press, Washington, DC. Snyder, H.L., and Taylor, G.B. 1979, Human Factors, 2 1 , 4 5 7 - 4 7 1 . The sensitivity of response measures of alphanumeric legibility to variations in dot matrix display parameters. Stammerjohn, L.W., Smith, M.J., and Cohen, B.G.F. 1981, Human Factors, 23, 401-412. Evaluation of workstation design factors in VDT operations. Turner, P.J. 1982, AustJOptom, 6 5 , 5 8 - 6 4 . Visual requirements for VDU operators. Vassilieff, A., and Dain, S. 1986, Applied Ergonomics, 17(2), 82-86. Bifocal wearing and VDU operation: A review and graphical analysis.
13 th International Symposium HFT '90 HUMAN FACTORS IN TELECOMMUNICATIONS Torino [italy] - September 10-14, 1990 DEADLINES May 31, 1990: June 30, 1990: July 15, 1 9 9 0
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Applied Ergonomics
June 1990
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Workstation variables and visual discomfort associated with VDTs M. Collins, B. Brown, K. Bowman and A. Carkeet Centre for Eye Research,Department of Optometry, QueenslandUniversity of Technology, 2 GeorgeSt, Brisbane, Queensland, Australia 4001 We have investigated the effects of a range of workstation factors upon the visual symptoms experienced by a group of 92 visual display terminal (VDT) users. Subjects in the study kept a diary over five consecutive working days in which they recorded the types of visual and postural symptoms which occurred and the types of work tasks being performed. Each subject's workstation was analysed for screen legibility and stability, discomfort and disability glare, and required head postures. By the use of multiple regression analysis techniques we have considered the relative contribution of these factors to the symptoms reported by the users of these workstations. Screen legibility significantly influenced the occurrence of symptoms of ocular discomfort and vertical head movements significantly affected the incidence of postural/headache symptoms. Keywords: Visual'display terminal, vision, visual discomfort, workstation, glare, posture
Introduction VDT usersfrequently report ocular discomfort (Dainoff,
et al, 1984) and this may lead to exaggerated claims of
1982), but whether they experience ocular discomfort more frequently than control subjects performing equivalent hard copy tasks ;is a matter of conjecture. However, the high rates of visual symptoms experienced by VDT users is un. doubtedly a problem in terms of user comfort and efficiency.
In this study, VDT users made detailed records of their visual symptoms when performing both VDT and non-VDT visual tasks throughout the working day. A survey was conducted of each subject's workstation to review some aspects of the lighting, VDT screen and postural demands. Multiple regression analyses were used to identify those workstation characteristics associated with visual symptoms during VDT use.
Differences in the nature of VDT and hard copy tasks introduce different visual demands on VDT users. The physical structure of characters in the VDT screen is different to that of hard copy characters. The reflective nature of the VDT screen increases glare which may cause ocular discomfort or impair legibility of the screen text. The line of sight for VDT use is inclined more towards the horizontal than it is for hard copy tasks performed at a desk, a factor which may introduce glare sources such as windows and ceiling luminaires into the user's field of vision. The VDT user's work patterns may be altered with respect to hard copy tasks; for example, where the users once regularly looked up from the typewriter to change a sheet of paper they now continuously watch the VDT screen. In many instances, the visual quality of VDT text can be superior to that of hard copy; the contrast of the VDT screen can often be adjusted by the user for maximum comfort which is not the case for hard copy. VDTs are often introduced irtto workplaces where the fighting and workstation arrangement is designed for hard copy tasks. Any resulting mismatch between visual task and visual environment may contribute to the ocular dis. comfort or visual difficulties of the VDT user. Because of the concern over numerous health issues related to VDTs, changes to work patterns and job security, it has been suggested that VDTs have become the "symbolic focus of discontent" amongst users of this equipment (Helander
adverse health effects.
Methods Subjects Ninety-eight staff from Queensland University of Technology agreed to participate in the study. These subjects were drawn from academic, administrative and clerical staff of the University. The mean age of subjects was 33.9 years, 67% of subjects were females and most subjects had used VDTs for 3 to 5 years. A wide variety of VDT types was used by subjects who participated in this study. Visual tasks involving VDT usage accounted for 48% of working time and most VDT.related tasks involved text entry. Diary protocol Subjects were asked to complete a diary over five consecutive working days, beginning on a Monday. In this diary the subjects recorded visual symptoms and various taskrelated variables, including the type of work undertaken (VDT or non-VDT tasks). Subjects made diary entries four times per day,just prior to breaks in the morning, lunch, afternoon and the end of the day. This gave data from 20 work sessions for each subject (i e, four sessions per day for five days). A more detailed description of the diary is reported in Collins et al (1989).
0003-6870/90/02 0157-05 $03.00 © 1990 Butterworth & Co (Publishers) Ltd
Applied Ergonomics
June 1990
157
Subjects recorded symptoms under three headings; ocular, visual and systemic symptoms. Ocular symptoms were defined as any incident of ocular discomfort. Visual symptoms were defined as any incident of impaired vision. Systemic symptoms were defined as any incident of headaches or postural discomfort. The diary contained lists of specific symptoms under each of the three broader symptom categories to aid the subjects in classifying correctly the symptoms experienced.
type of habitual spectacle correction used for VDT work was noted (e g, single-vision reading glasses, contact lenses or bifocals). Working distances were measured from the VDT screen and keyboard with subjects using their habitual visual correction. At the same time we measured the range of accommodation (range of clear focus) with subjects using their habitual visual correction. Subjects were asked if they were touch typists, since this factor will influence their necessity for head and eye movements during VDT tasks.
Beside each of the symptoms entered in the diary from the above three categories, subjects were asked to rate the intensity of the symptom on a scale from 1 (very mild) to 5 (very intense). Symptoms were entered into the statistical analyses using this intensity rating - for example, a symptom of 'tired eyes' with an intensity rating of 5 (severe) was scored as an ocular symptom with a score of 5.
The subjects were asked to sit at their workstations and to explain and demonstrate the normal visual tasks involved in their work with VDTs and the length of time spent performing each task during the day. The investigator's assessment of head posture was based upon this questioning and observation of the subject during various work tasks.
The diary of symptoms and work tasks was completed prior to the workstation analysis. This was done to mimimise 'sensitisation' of the subjects to any real or perceived problems with their workstation environment, prior to completing the diary.
Workstation analysis A single investigator visited each subject and conducted all workstation assessments. This served to minimise variation in the criteria adopted for subjective assessments, but does introduce the possibility of a consistent bias in this data. In a few instances the investigator was aware of the subject's symptoms before assessing the workstation. However, it is unlikely that there was any significant influence on the independence of the symptom and workstation data. VDT screen characteristics
The legibility of screen characters (assessed from the subject's normal seated position) was graded subjectively on a scale of 1 (good) to 5 (poor) by the investigator. Those factors responsible for poor character legibility were noted (e g, contrast, character size, clarity). The facility for operator contrast adjustment and the use of filters (such as 'mesh' filters) on the screen were recorded. The stability of screen characters and raster was subjectively graded on a scale of 1 (good) to 5 (poor). Those factors responsible for poor screen stability were noted (e g, flicker, swim, jitter). Discomfort and disability glare Discomfort glare is defined as glare which causes discomfort without necessarily impairing the visibility of objects, while disability glare is defined as glare which impairs the visibility of objects without necessarily causing discomfort (Australian Standard 2713-1984). The investigator subjectively graded discomfort glare for each workstation (assessed from the subject's normal seated position) on a scale of 1 (good) to 5 (poor). Those factors responsible for discomfort glare were recorded (e g, screen reflections, luminaires, windows). Disability glare was also subjectively graded on a 1 to 5 scale (assessed from the subject's normal seated position). Those factors responsible for disability glare were also noted (e g, ambient illuminance, windows, luminaires). Head posture To investigate the influence of visual factors upon the subject's head posture, we made a series of observations and measurements with the subjects at their workstations. The
158
Applied Ergonomics
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Horizontal head deviations were defined as a head turn to the right or left, and vertical deviations were defined as downward or upward head deviations. Normal hea d posture was defined as a slight downward vertical head turn (line of sight approximately 10 ° below horizontal). Horizontal and vertical head postures were graded in terms of their range. The range of head turns referred to the amount of head turn required during work, taking into account how often these head turns were necessary in performing the work (hence the range of horizontal and vertical head turns are an estimation of the amount and frequency of the head turn). The range was graded on a scale of 1 (none) to 5 (a large amount of head turn required often). The following examples illustrate this measurement technique. A number of computer programmers habitually placed their keyboards on their knees and frequently changed vertical head posture to look from the VDT screen to the keyboard and back again. These subjects were given high ratings for vertical head turn range. A number of secretaries who were touch typists and used copy holders to place hard copy beside the VDT screen had little if any head turn for long periods of the working day. These subjects were given a low rating for vertical and horizontal head turn range.
Multiple regression analyses Complete diary data were obtained for 72 subjects, with partial data for a further 20 subjects. Multiple regression was used for the data analyses, using each of the three sets of system scores (i e, ocular, visual and systemic) separately as dependent variables. The results of the workstation investigations of the total 92 subjects formed the independent variable set. All workstation variables were directly related to VDT use and not to non-VDT tasks. For this reason, the multiple regression of the workstation variables and symptom scores was based only upon symptoms which occurred during sessions predominantly involving work with VDTs, identified in the diary record. Symptom scores during 'VDT sessions' were totalled for the five days and the mean symptom score (for ocular, visual and systemic symptoms) per session calculated for each subject.
Results Use of VDT screens with poor legibility was significantly associated with higher rates of ocular symptoms (Table 1), since screen legibility entered into the equation predicting ocular symptoms with a significant positive beta weight. A similar trend, although not statistically significant, was apparent with visual symptoms (Table 2). Most VDT screens
Table 3: Results of multiple regression of workstation variables end systemic symptoms
Table 1: Results of multiple regression of workstation variables and ocular symptoms Workstation variables
Screen legibility
Beta weight
t value
0"22
2"00
Screen stability
-0"13
Discomfort glare
p
Workstation variables
Beta weight
t value
p
0"04*
Screen legibility
1 "08
0"28
- 1 "27
0"22
Screen stability
-0"16
-1 "18
0"24
-0" 13
-0"92
0"36
Discomfort glare
-0"04
-0"35
0"73
Disability glare
0"09
0"76
0"46
Disability glare
-0"01
-0"04
0"97
Touch typist
0"02
0"14
0"89
Touch typist
0"01
0"07
0"95
Horizontal head turn
0"01
0"11
0"92
Horizontal head turn
0"14
1"23
0"23
-0"08
-0"65
0"52
Vertical head turn
-0"29
-2"40
Vertical head turn
0" 13
0"02*
Note: * indicates p < 0"05
Note: * indicates p < 0"05
Table 2: Results of multiple regression of workstation variables and visual symptoms
ratings (1 good to 5 poor) of those screens with and those without Filters. Of screens surveyed, 88% had filters or special anti-reflection coatings. The mean disability glare rating of screens with filters or coatings was 2.14 and was 2.56 for those VDTs without filters or coatings, a reduction of approximately 20%.
Workstation variables
Beta weight
t value
p
Screen legibility
0"18
1"65
0"11
Screen stability
0"01
0"03
0"97
-0"26
-1 "94
0"06
0" 18
1 "59
0" 12
0" 11 -0"07
0"89 -0"62
0"38 0"54
0"01
0"43
0"97
Discomfort glare Disability glare Touch typist Horizontal head turn Vertical head turn
were judged to be of good to moderate legibility. Character design, size and contrast were of approximately equal significance in causing reduced screen legibility. The range of screen legibility across subject workstations is presented in Table 4. Results of the multiple regressions of symptom scores versus workstation variables are presented in Tables 1, 2 and 3. Screen stability did not significantly predict the occurrence of ocular, visual or systemic symptoms. The vast majority of screens were judged to have no visible instability (Table 4) and of those that showed instability, flickering of the characters was the most common cause. Discomfort glare in the user's field of vision was not sig. nificantly associated with ocular, visual or systemic symptoms. The range of discomfort glare scores across subject work. stations is presented in Table 4. The most common causes of discomfort glare were judged to be luminaires and windows in the user's field of view. Disability glare in the subject's VDT screen did not significantly enter into any of the equations predicting symptoms, although it had a relatively high positive beta weight (p = 0-12) in the equation predicting visual symptoms. Most screens had good to moderate disability glare ratings and of those screens with disability glare present, the major sources were reflections due to Iuminaires and high ambient room illuminances. The range of disability glare scores across subject workstations is presented in Table 4. To investigate the influence of the use of VDT screen filters on disability glare, we compared the disability glare
The range of horizontal head turn entered into the equation predicting systemic symptoms with a positive betaweight but did not reach significance 0~ = 0-23). The range of required horizontal head turn is presented in Table 4. Vertical head turn entered into the equation predicting systemic symptoms with a significant beta-weight (p= 0-02). That is, the greater the amount and frequency of vertical head deviation when performing tasks at the VDT, the lower the incidence of systemic symptoms. The range of vertical head turns is presented in Table 4. The ability to touch type did not enter into any of the equations predicting ocular, visual or systemic symptoms. Of the subjects in this study, 49% were touch typists. The near point of accommodation (i e, closest distance at which the eye can focus) was measured for all subjects using their habitual visual corrections. Of subjects, 62% wore no correction, 31% wore single vision spectacles or contact lenses and 7% wore bifocal spectacles. Two subjects had near points of accommodation at a greater distance
Table 4: Ratings of workstation variables across all workstations (percentage) Good 1
2
3
4
Poor 5
Screen legibility Screen stability
26 92
42 6
29 2
3 0
0 0
Discomfort glare
27
46
19
6
2
Disability glare
28
43
19
8
2
Low 1
2
3
4
High 5
24 21
22 36
38 36
8 7
8 0
Horizontal head turn Vertical head turn
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than their keyboard working distance (neither were touch typists); this could result in the subject bending backward to view the keyboard resulting in postural discomfort (systemic symptoms) or maintaining normal posture and reporting blurred vision (visual symptoms). Neither subject recorded any incident of systemic symptoms during the five days of the diary survey and one of the subjects noted only minor ocular symptoms during two sessions. Both of these subjects used their VDTs infrequently during the diary survey. Bifocal spectacles were worn by five subjects. The mean vertical head turn amongst the bifocal wearing group was 3.2 (1 small to 5 large), compared with the mean vertical head turn for all other subjects of 2 '3. Because subjects wearing bifocal spectacles may adopt unusual head postures to compensate for inappropriately positioned reading segment heights, we compared the incidence of systemic symptoms of neck, back and shoulder pain in the bifocal wearing group with the incidence in all other subjects. The incidence of these symptoms was derived by calculating the total frequecy x severity systemic symptom (postural discomfort and headaches) scores per diary session. For the bifocal wearing group this score was 0.76 symptom scores/ session versus 0.57 symptom scores/session for all remaining subjects.
Discussion There are many factors aside from workstation characteristics which may load to visual symptoms amongst users of VDTs. In this particular population of VDT users we have reported a number of task-related variables such as work pressure and interest, and specific VDT tasks which significantly influence the incidence of visual symptoms (Collins e t el, 1989) along with the subject's length of VDT experience (Collins et el, 1988). If, for example, all subjects in this study with poor screen stability characteristics also had high work interest and low work pressure, the effect of screen stability on visual symptoms could easily be masked by the work interest and pressure effects. The results of field studies such as this need qualification because of these uncontrolled variables. We found that poor screen legibility was significantly associated with more frequent reports of ocular (discomfort) symptoms and showed a positive, but not significant association with visual (blur) symptoms. Factors such as character contrast, size and design were considered to be the major contributors to reduced screen legibility. These findings are not unexpected and agree with the results of previous laboratory-based studies. Aspects of screen legibility such as dot matrix design, font style, character luminance and visual angle of the characters have all been shown to affect work performance measures (Brown et el, 1982; Snyder and Taylor, 1979). Stammerjolm et al (1981) have noted an association between reports of visual discomfort and screen characteristics including screen height, angle, glare and flicker. Turner (1982) reports anecdotal evidence suggesting that asthenopia (eyestrain) amongst VDT users may be caused by poor screen legibility and poor screen stability. Most workstations surveyed in this study had good discomfort and disability glare ratings and it may be the small number of workstations with poor glare ratings which has masked any significant effect of glare on visual symptoms.
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However, we did find a relatively high positive association between disability glare ratings and visual symptoms in this study. Hultgren and Knave (1974)have noted an association between discomfort glare, screen reflections and symptoms of discomfort amongst a small sample of VDT users The ability to touch-type was included as a variable in the statistical analyses to test the hypothesis that VDT users who do not touch-type are forced to perform a greater number of eye movements, accommodation (focusing) changes and head movements in carrying out keyboard tasks. This could potentially lead to differences in the rate of symptoms. Touch-typing ability entered into the multiple regression analyses with low beta weights and therefore the hypothesis was not sustained. The estimates of head posture made during this study were based upon a number of assumptions. These assumptions were: that the subject's appraisal of habitual head posture when performing VDT tasks was an accurate assessment; that the subject's estimation of the amount of time spent with that head posture was accurate; and that the VDT tasks (and head posture) performed during the diary survey period were similar to those demonstrated during the workstation analysis. Because of these numerous assumptions, the inferences drawn from the head posture data need some qualification. Our finding of a negative association between the range of vertical head turn and systemic (postural/headache) symptoms is consistent with the concept that static muscle loading leads to pain (National Research Council, 1983). In our study, those subjects with a low vertical head turn range had little if any variation in vertical head posture and reported significantly more systemic symptoms. It follows that frequent variations in vertical head posture by VDT users may lead to the greatest postural discomfort. Horizontal head turn did not enter the equation predicting systemic symptoms with a significant beta weight. Since horizontal head position does not involve substantial static muscle loading, this result can also be interpreted as being consistent with the findings on vertical head turn in this study. It appears that the five bifocal wearing subjects in this study had a higher incidence of neck, back and shoulder pain. No statistical inferences were drawn from these data because of the small number of bifocal wearers and the potentially large influence of other uncontrolled variables on their symptom scores. However, this group of subjects also displayed a higher vertical head turn score than did the remaining subjects in the study. Since a higher score on vertical head turn was associated with significantly fewer postural symptoms amongst the group as a whole in the multiple regression analysis, we speculate that it is the unusual static head postures required with bifocals when viewing VDTs which leads to symptoms rather than the frequency or range of necessary head movements. Cakir et al (1979) have reported an association between neck pain, ocular discomfort and the use of reading spectacles by VDT users. They suggest that this association results from the spectacle correction being inappropriate for the screen viewing distance, which results in the VDT user bending forward to see the screen clearly. Martin and Dain (1988) and Vassilieff and Dain (1986)have noted difficulties with the use of multi-focal spectacle designs for VDT users. These difficulties result largely from a restriction in the corrected vertical field of vision produced by the near vision segment(s) in multi-focal spectacle lenses.
In summary, we have examined the relative contribution of various workstation characteristics to the symptoms reported by users of VDTs. Of the factors investigated, legibility of text in the screen and the required vertical head turns had a significant influence on the symptoms reported.
Acknowledgements
We thank Jim Whiting for his advice on postural ergonomics, and Dale Card and Anthony Bloesch who assisted in data analysis. This work was funded by a grant from the National Occupational Health and Safety Commission 0Vorksafe Australia).
References
Australian Standard 2713-1984. 1984, Lighting and the visual environment for screen-based tasks. Standards Association of Australia, Sydney. Brown, B.S., Dismukes, K., and Rinalducci, E J . 1982, Behaviour and Inf Tech, I, 121-140. Video display terminals and vision of workers. Cakir, A., Hart, D.J., and Stewart, T.F.M. 1979, Visual display terminals. John Wiley and Sons, New York. Collins, M.J., Brown, B., and Bowman, K.J. 1988, Visual discomfort and VDTs. National Occupational Health and Safety Commission (Worksafe Australia), Centre for Eye Research, Queensland University of Technology, Brisbane.
Collins, MJ., Brown, B., Bowman, K.J., and Caird, D. 1989, Human Factors, (submitted). Task variables and visual discomfort associated with the use of VDTs. Dainoff, M.J. 1982,Behaviour and Inform Technol, 1, 141-176. Occupational stress factors in visual display terminal (VDT) operation: a review of empirical research. Helander, M.G., Biilingsley, P.A., and Schurick, J.M. 1984, An evaluation of human factors research in visual display terminals in the workplace. In: Muckler, F.A. (Ed), Human Factors Review, Human Factors Soc, Santa Monica, 55-129. Huitgren, G.V., and Knave, B. 1974, Applied Ergonomics, 5(1), 2 - 8 . Discomfort glare and disturbances from light reflections in an office landscape with CRT display terminals. Martin, D.K., and Dain, S.J. 1988, Applied Ergonomics, 19(4), 293-300. Postural modifications of VDU operators wearing bifocal spectacles, National Research Council. 1983, Video displays, work, and vision. National Academy Press, Washington, DC. Snyder, H.L., and Taylor, G.B. 1979, Human Factors, 2 1 , 4 5 7 - 4 7 1 . The sensitivity of response measures of alphanumeric legibility to variations in dot matrix display parameters. Stammerjohn, L.W., Smith, M.J., and Cohen, B.G.F. 1981, Human Factors, 23, 401-412. Evaluation of workstation design factors in VDT operations. Turner, P.J. 1982, AustJOptom, 6 5 , 5 8 - 6 4 . Visual requirements for VDU operators. Vassilieff, A., and Dain, S. 1986, Applied Ergonomics, 17(2), 82-86. Bifocal wearing and VDU operation: A review and graphical analysis.
13 th International Symposium HFT '90 HUMAN FACTORS IN TELECOMMUNICATIONS Torino [italy] - September 10-14, 1990 DEADLINES May 31, 1990: June 30, 1990: July 15, 1 9 9 0
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DETAILED INFORMATION Title: Dates: Place: Host: Cost:
Thirteenth Symposium of Human Factors in Telecommunications September 10-14, 1990 Centro Congressi SEAT, Via Bertola, 34 - Torino (Italy) CSELT - Centro Studi e Laboratori Telecomunicazioni S.p.A. - Torino Registration Fee: 450.000 Lit (includes Proceedings, reception, lunches and banquet). Proceedings only: 50.000 Lit (if ordered in advance). Accomodation in various hotels in the centre of Torino.
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