OPTOMETRY RESEARCH PAPER. How to place the computer monitor: measurements of vertical zones of clear vision with presbyopic corrections

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1 C L I N I C A L A N D E X P E R I M E N T A L OPTOMETRY RESEARCH PAPER How to place the computer monitor: measurements of vertical zones of clear vision with presbyopic corrections Clin Exp Optom 2015; 98: Mirjam König Dipl-Ing (FH) Claudia Haensel BSc Wolfgang Jaschinski Dr-Ing Leibniz Research Centre for Working Environment and Human Factors, Dortmund, Germany jaschinski@ifado.de Submitted: 6 June 2014 Revised: 23 October 2014 Accepted for publication: 25 November 2014 DOI: /cxo Background: This study intended to measure the near and far points of clear vision as a function of the inclination of the line of sight with comfortable head posture. Measurements with different lenses for presbyopic correction were made to suggest comfortable positions of the monitor for computer work. Method: An inclined optometer was built, including a concave mirror to shift a visual target of constant angular size from near to infinity (proposed by Reiner). The optometer could be inclined vertically from horizontal to 50 degrees downward to vary the inclination of the line of sight. Measurements were made with a comfortable head position adjusted on a headrest. Results: The near and far points were plotted both in the unit one/metre as a function of eye inclination (optometric diagram) and also as positions from the eyes in workspace co-ordinates (workplace diagram). First, individual examples of plots of the vertical zones of clear vision at the workplace are shown. Second, the group mean data of 22 observers with newly prescribed lenses showed that the vertical zones of clear vision for general purpose progressive addition lenses (PALs) reach infinity and are flatter, while computer vision PALs lead to more steep vertical zones ending at intermediate distances. Third, the mean results of three samples from our laboratory were compared with respect to general purpose PALs, which are most frequently used by presbyopic people. Conclusions: The diagrams of the vertical zones of clear vision for different spectacles provide information on the ergonomic vertical position of computer monitors for clear vision with a comfortable head position. The grand mean of general purpose PALs suggests that the upper edge of the monitor should be at least approximately 15 cm below eye level at a typical viewing distance of approximately 75 cm. Higher monitor positions are possible with computer vision PALs. Key words: head posture, monitor position, presbyopia, progressive addition lenses INTRODUCTION Presbyopic observers need lenses that feature additional positive refraction for near vision to compensate for the reduced accommodative capacity. These lenses may be single vision lenses, bifocal lenses or progressive addition lenses (PALs); these last may be designed optically for distance and near vision (general purpose PALs) or for computer vision and near vision (computer vision PALs). PALs are widely used 1 becauseoftheiradvantageofa gradual increase in the near vision addition with lowered eye inclination; however, this benefit of a gradual near vision addition for any viewing distance is connected to two types of limitations of PALs that are inevitable for physical reasons. The first limitation refers to the horizontal direction: clear vision for intermediate and near vision is only possible within a central vertical progressive zone, while astigmatic distortions are present in the left and right peripheral visual fields. As a consequence, the patterns of horizontal eye and head movements are modified. 2 5 The second limitation of PALs refers to the vertical direction and is the topic of the present study, namely, to use the required addition for a given viewing distance, the inclination of the eye and the head must be adjusted, so that thevisuallineintersectsthelensatthepoint with the required near vision addition. If the monitor is placed relatively high, observers may incline the head backwards to use a stronger addition power in the lower part of the lens, which may lead to uncomfortable head posturesandhigherloadonthetrapezius neck muscle. 6 As an alternative, computer vision PALs are optically designed to include the addition required for the monitor viewing distance in the upper part of the lens, so that clear vision with comfortable head posture is possible for intermediate and near targets. 7,8 In optometry, the effect of a presbyopic correction is described as the distribution of the near addition power within the lens dimension. From an applied, ergonomic point of view, knowing the corresponding viewing distances and gaze angles, where clear vision is provided with a particular lens would be helpful. Thus, the user should know the vertical zone of clear vision of his/her spectacles to place the monitor at a position with clear vision and comfortable head posture Therefore, the present study proposes measurements of the vertical zones of clear vision. Research on this topic is very limited. Von Buol 12 measured the horizontal and vertical dimensions of the zones of clear vision for some PALs, with an extensive test 244

2 procedure and showed a tube-like space of clear vision, which extends from the low and near positions relative to the eyes to a high and distant position. Vassilieff and Dain 13 calculated the zones of clear vision in space for bifocal lenses from the near addition in the lower part of the lens and accommodative ability; depending on head inclination, a bipartite field of clear vision was shown and superimposed onto the dimensions of a computer workstation. These calculations 13 assumed the screen at a fixed standard position of 50 cm viewing distance and -20 degrees gaze angle relative to the horizontal (as typical in these early days of computer workplace technology). In these conditions, wearers of bifocal lenses up to the age of 50 years were able to see the monitor clearly but users older than 55 years would experience problems. The latter users tended to tip their head back to view the monitor in these previous standard ergonomic conditions. 14 The present study focuses on the vertical dimensions of the zones of clear vision of progressive lenses because they are most frequently used in presbyopia and visual and musculoskeletal complaints in the neck may occur, if the monitor is not placed within the zone of clear vision, particularly with general purpose PALs. These zones of clear vision depend on the lens characteristics and head inclination. They are relevant for the resulting ergonomic placement of computer screens. Pilot studies have applied simple test methods and samples with habitual PALs The present study uses an elaborate instrument (the inclined optometer ) to test a sample with newly prescribed PALs (for general purpose and for computer vision). This research proposes a graphical presentation of the vertical zones of clear vision in workplace dimensions and provides data regarding groups of observers, which may be helpful to place computer monitors ergonomically. optometer) is shown in Figure 1A. The subject looks through the semi-transparent mirror into the concave mirror, back through the semi-transparent mirror and via the full mirror ontothetargetthatappearsatopticalinfinity, when located in the focal plane of the concave mirror. The target appears closer when moved toward the concave mirror. To ensure a large range of eye inclinations, we disassembled a Binoptometer 1 and rearranged the optical parts in a new housing that can be tilted in steps of 10 degrees in the range from zero to -50 degrees downward (Figure 1C). The visual target (as shown) can be mechanically shifted over a 25 cm range and, due to the mirror arrangement, METHOD The apparatus (inclined optometer) The basis of the inclined optometer is the Binoptometer 1 (vision screening test of Oculus, Wetzlar, Germany) that originates from Reiner 18 and includes a concave mirror (dimensions: mm;aradiusof the curvature of 661 mm) to present binocular targets from near to infinity within the housing of the device. The optical principle of the Binoptometer 1 (as realised in the inclined Figure 1. (A) Optical principle of the Binoptometer (Oculus, Wetzlar, Germany) following Reiner (1980) using a concave mirror. (B) Design of the inclined optometer, including the principle of the Binoptometer: a concave mirror is used to present a visual target of constant angular size from infinity to 33 cm, when shifting the target over a range of 25 cm. (C) Stepwise vertical rotation of the optometer around the eye position to allow for vertical eye inclination of up to 50 degree. 245

3 it appears at a constant angular size at any distance from 33 cm (or 3.00 D) to infinity (0.00 D) and beyond to a hyperopic state of D (Figure 1B). To test the vertical zones of clear vision for office working conditions, the mechanical rotation axis of the inclined optometer was arranged in agreement with the centre of the eyes when the observer adopts a comfortable body and head posture at an office desk. To control for this eye position, two small cameras were installed in the tilt axis to display the position of the two eyes on control monitors. The device comprised rests for the chin and forehead, which were very flexible and could be adjusted to reach the individual with a comfortable head position as previously determined. The inclined optometer was used on a motor-adjustable table to easily fit different body sizes. Test procedure At the start of a session, the subject was seated on an office chair adjusted to his/her body size. The individually comfortable head inclination was determined as follows: 9,10 the subject closed the eyes (in order not to fixate on any visual target) and moved the head upward and downward until he/she found the head position most comfortable for the neck, shoulder and back. The resulting head inclination was measured with a goniometer with an attached pendulum, which determined the anglebetweentheeye-earlineandthehorizontal in space. The eye-ear line connects the corner of the eye and the tragus at the ear (Figure 1C). The participant was requested not to change the head posture when the table with the Inclined Optometer was moved toward him/her and the headrest was adjusted to fit to the head in a comfortable posture. After these adjustments, the comfortable posture was assessed with the goniometer. The zones of clear vision were then measured with the participants wearing their spectacles. At each eye inclination angle α (that is, 0, -10, -20, -30, -40 and -50 degrees downward), the near point p near (cm) was measured by moving the target slowly from a blurred near position to longer distances, until clear vision was reported and the far point p far (cm) was measured by moving the target from a blurred distant position to shorter distances until clear vision was reported. Subjects Part 1 of this study was performed to test the procedures, provide preliminary results and test the repeatability of measurements. Three observers were tested with their habitual lenses, that is, a single vision lens for computer use, a computer vision PAL and a general purpose PAL (Figure 2). Part 2 investigated a group of experienced PAL wearers who belong to a sample of 23 subjects in a parallel field study; 19 for the present analyses, the data set of one subject was incomplete, resulting in 22 remaining subjects for the present study with the inclined optometer (mean and SD of ages were 55 ± 4, range: 46 to 61 years, 12 females). The refractive error (spherical refraction) was ± 2.46 D (range D to D) and the cylindrical refraction was ± 0.70 D (range from plano to D). The additional power for near vision was 2.03 ± 0.41 D (range from 1.00 D to 2.50 D). Table 1 shows a list of individual refractions and visual functions. Two types of lenses were compared: Zeiss officelens Individual (computer vision PAL) and Zeiss progressive Individual 2 (general purpose PAL). For Zeiss officelens Individual, participants could choose the maximum intermediate distance (MID) between one and four metres and the power in the upper part of the lens was calculated accordingly. The MID-values ranged from 200 to 400 cm (Table 1). The mean and SD of the corresponding distance refraction were 0.33 ± 0.09 D, with an average maximum intermediatedistanceof3.03metres.thegeometrical profiles of near vision addition of these lenses are illustrated in a parallel study. 19 All subjects received a subjective refraction of both eyes by acertified optometrist (co-author MK), using a letter chart at six metres distance and trial lenses. Visual acuity was at least 0.0 logmar in each eye, with optical correction if necessary. The lenses were fitted with the procedures of the system RV Terminal (Zeiss). The methods applied in this study were approved by the ethics board of the Leibniz Research Centre of Working Environment and Human Factors. Participants signed an informed consent; the study followed the tenets of the Declaration of Helsinki. RESULTS Part 1: Different individual presbyopic corrections for computer work Figure 2 shows the vertical zones of clear vision for three individual examples of different types of lenses: a single vision lens for computer use, a computer vision PAL and a general purpose PAL. The left graphs (Figure 2A) are referred to as optometric diagrams because they show the optometric properties of the lenses in terms of near points (black curves, p near ) and far points (grey curves, p far ) of clear vision, depending on eye inclination α (degree); the unit one/ metre is used to numerically represent the dioptric power of the near vision addition and the refraction at the far point. The right graphs (Figure 2B) are referred to as workplace diagrams. They show the same near and far points as in (Figure 2A); however, these data are given in centimetres in real space as horizontal positions (p cos α) and vertical positions (p sin α) relative to the eye position at the origin of the co-ordinate system. Thus, these are calculated geometrically from the eye inclination angle α and the near and far points (cm). The bundle of rays in Figure 2B illustrates the angles of eye inclination as adjusted in the inclined optometer (Figure 1) and corresponds to the horizontal lines in Figure 2A. Data of three lenses from three different observers are presented, indicated by the numbers 1, 2 and 3. The three examples in Figure 2 include a re-test on a separate day, which revealed a reasonable agreement between these two measurements. For single vision lenses, the near and far points hardly change with eye inclination, which is indicated by approximately vertical lines in the optometric diagram in Figure 2(A1). This form of vertical lines in the optometric diagram leads to nearly concentric arcs in the corresponding workplace diagram Figure 2(B1). The zone of clear vision extends between viewing distances of approximately 60 and 80 cm for the present observer and spectacle. For the computer vision PAL in Figure 2(A2), the optometric diagram shows a far point at a horizontal eye inclination of approximately 0.7 D. This amount is an effect of the corresponding near-vision addition arranged in the upper part of the lens, which blurs distant targets in the horizontal gaze direction. The workplace diagram in Figure 2 (B2) shows a zone of clear vision, which appears bent in a sickle shape. With the general purpose PAL in Figure 2(A3), a far point near zero dioptre shows that the glasses are appropriate for distance vision in the horizontal gaze direction, that is, the distance refraction is correct (given the lack of a residual cylindrical component). Increasing the downward 246

4 Figure 2. Zones of clear vision are illustrated in two pairwise diagrams (left and right) that include the same data. The left graphs (A) are referred to as optometric diagrams because they show the optometric properties of the lenses in terms of near (black curves) and far points (grey curves) of clear vision depending on eye inclination (degree). The unit one/metre is used to numerically correspond to the dioptric power of the near vision addition and the refraction at the far point. The right graphs (B) are referred to as workplace diagrams because they show the same near and far points as in (A) but in centimetres in real space as horizontal and vertical positions (cm) from the eye position at the origin of the co-ordinate system. Three different types of lenses (1, 2, 3) of three observers are shown: single vision lens for computer vision of a 56-year-old presbyope (A1, B1), progressive addition lens (PAL) (Rodenstock: Impression N 50 5, 8 22) for the computer vision of a 56-year-old presbyope (A2, B2), general purpose PAL (Rupp + Hubrach: HP Evolis 1,6) of a 55-year-old presbyope (A3, B3). The optometric data of these lenses are indicated. Two repeated measurements of different days are shown. inclination increases the near-vision addition and shifts the near and far points to higher dioptric values. The corresponding workplace diagram in Figure 2(B3) shows a zone of clear vision that is practically limited only by the near point curve because the far point curve lies mostly beyond typical office workplace dimensions. Part 2: Group means for computer vision PALs and general purpose PALs For the group of 22 PAL wearers, the vertical zones of clear vision were measured with two types of newly prescribed PALs, that is, each observerwastestedwithbothageneralpurpose PAL and a computer vision PAL. In these measurements, the eye-ear line of the comfortable head inclination was 15.8 ± 5.5 degrees (range 4.4 to 28.7); two sessions were conducted on separate days, in which the head rest in the second session was mechanically adjusted to the comfortable head inclination measured in the first session. As a result, the head inclination highly correlated between the two sessions (r = 0.91). The mean results of the vertical zones of clear vision for the two types of lenses are shown in Figure 3. The separate presentation of the two sessions shows a good agreement. These graphs show gaze inclinations up to -30 degrees, where data were available of all participants. In some cases, small frames prevented the measurement of near and far points with lower gaze inclinations. Two effects can be observed in the optometric diagrams in Figure 3A. The mean accommodative capacity of this sample of subjects is demonstrated by the shift in the near point curves by approximately 1.00 D compared to the far point curves. Furthermore, both the near and far point curves of the computer vision PALs are shifted to higher dioptric values than those of the general purpose PALs. On average, this difference was 0.31 D, which agrees well with the mean difference in the distance refraction between these lenses. This value was 0.33 D due to the maximum intermediate distance of the computer vision PALs. This difference between the lens types was significant for the far point curves (F(1,21) = 47.4, p < , repeated measures analysis of variance) and for the near point curves (F(1,21) = 65.3, p < ). The average standard deviation was 0.26 D for the far points and 0.47 D for the near points. If these near and far points (in one/metre) are re-plotted in centimetres, the corresponding curves in the workplace diagrams differ in a way that can be relevant to the ergonomic placement of computer monitors. The range 247

5 Right eye Left eye Subject Gender Age (years) Sphere Cylinder Spherical equivalent Axis (deg) Addition Sphere Cylinder Spherical equivalent Axis (deg) Addition Distance binocular acuity (decimal unit) Maximum intermediate distance (cm) for computer vision PALs A F B F C M D M E M F F G M H F I F J F K M L M M F N F O F P F Q M R M S M T F U F V M Table 1. Optometric description of the 22 participants 248

6 Figure 3. The vertical zone of clear vision as an average of 22 observers wearing either a computer vision progressive addition lens (PAL) or a general purpose PAL. The left graph (A) shows the optometric diagram with near and far points in the unit one/metre as a function of eye inclination (degree), while the right graph (B) shows the workplace diagram with near and far points plotted in a co-ordinate system showing horizontal and vertical positions (cm) relative to eye position (at the origin). To illustrate inter-individual variability, the pair of two dotted lines in (B1) and (B2) plots a range of ± one standard deviation. The monitor positions as used by these participants during their daily office work are included in the workplace diagrams. The thick grey line illustrates the gaze inclination. For general purpose PALs, the upper part of the monitor is above the near point curve and thus appears blurred. between the dotted lines indicates the interindividual range of the near points that corresponds to the standard deviation (in one/metre). The depth of the vertical zones of clear vision is expected to depend on the stage of presbyopia: older observers have a more distant near point (without wearing near additions) and required larger near additions. To demonstrate this effect in the present sample, we formed three subgroups with larger, intermediate and smaller accommodative powers (1.34 ± 0.19 D; 0.92 ± 0.13 D; 0.34 ± 0.21 D), as measured by the horizontal near point with general purpose PALs. Figure 4 shows the results. As expected these three groups showed similar far point curves, in the optometric diagrams, while the near point curves are shifted according to the accommodative power. The ergonomic implications, which result from the workplace diagrams are more interesting. At low gaze inclinations, the three curves tend to approach because the near addition was prescribed to provide clear vision at a 40 cm reading distance for all subjects when using the lower progressive zone. At higher gaze inclinations, the near point curve is transiently shifted to longer viewing distances; this effect increases as the accommodative power declines. For a small accommodative power, clear vision is possible only near the table plane, so that it becomes difficult to have the complete monitor within the zone of clear vision. Part 3: Grand mean for general purpose PALs To determine whether these mean results of the vertical zones of clear vision are robust and may be representative for other lens designs, we compared the mean results of three samples of our laboratory with respect to general purpose PALs, which are most frequently used in presbyopia. Figure 5 summarises the findings of three samples, where a comfortable head inclination (measured with closed eyes) was always applied. 1. Twenty-two participants of the present study(meanage:55±4years)withrecently described general purpose PALs of a defined optical design (Zeiss Individual 2) were tested with the inclined optometer. 2. Twenty-five users (mean age: 53 ± 5 years) of habitual general purpose PALs with unspecified optical designs were tested in field studies. 20,21 Near and far points were tested with the inclined optometer. 3. Twelve users (mean age: 55 ± 3 years) of habitual general purpose PALs with unspecified optical designs were tested with the inclined slide. 15 This approach uses a simpler device, in which a visual target is shifted in free space (not using an optometer principle); only near point measurements are available. These three groups showed similar curves in the optometric diagram. The far point curves in the optometric diagram (Figure 5A) do not include an accommodative response because the mean horizontal far point was closed to optical infinity (0 D). Therefore, the far point curves reflect the gradual increase in the near vision addition in the PALs, as the eyes are inclined. The near point curves include the accommodative response of these groups, which was approximately 1.00 D. For a general conclusion for workplace designs, the grand mean near point curve (in grey dashes) of all 59 cases was re-plotted into the workplace diagram in Figure 5B. DISCUSSION In Part 1 of this study, we tested different observers with single vision lenses for computer use, general purpose PALs and computer vision PALs to demonstrate the different forms 249

7 Eye inclination (deg) (A1) Optometric diagram Far and near points (1/metre) Far points Computer vision PAL Near points Grey = far points Black = near points Vertical position (cm) (B1) Workplace diagram Horizontal position (cm) Near points Eye inclination (deg) (A2) Far and near points (1/metre) Far points General purpose PAL Near points Vertical position (cm) (B2) Horizontal position (cm) Near points Figure 4. The vertical zone of clear vision with participants wearing computer vision progressive addition lenses (PALs) (upper graphs) or general purpose PALs (lower graphs). The left graphs (A) show the optometric diagrams with near and far points in the unit one/metre as a function of eye inclination (degree), while the right graphs (B) show the workplace diagrams with near and far points plotted in a co-ordinate system showing horizontal and vertical positions (cm) relative to eye position (at the origin). To illustrate the effect of the stage of presbyopia, the sample was divided into three subgroups with seven, right and seven participants, depending on the near point without addition power (tested with general purpose PALs at horizontal gaze direction): this was 0.34, 0.92 and 1.34 (one/metre) in the subgroups, respectively. Figure 5. The left graph shows the optometric diagram (A) with mean near and far points (one/metre) as a function of eye inclination (degree) for general purpose progressive addition lenses (PALs) in three samples of different studies (see text): 1. the present group of 22 subjects (dots) wearing newly prescribed general purpose PALs of a defined optical design (Zeiss Individual 2), 2. a group of 25 subjects (squares) wearing habitual PALs of different, non-specified optical designs and, 3. a group of 12 subjects (triangles) wearing habitual PALs of different, non-specified optical designs (only near points available). The average near point curve (in grey dashes) was replotted from the optometric diagram into the workplace diagram (B) relative to the eye position (at the origin) to demonstrate the vertical zone of clear vision where the full monitor can be viewed clearly with comfortable head inclination. Three possible monitor positions, a typical range of comfortable gaze inclination (-10 to -30 degrees) and the average table level are included. of vertical zones of clear vision at the workplace expected from the optical design of these glasses. We found a good agreement between repeated measurements, as shown in the examples in Figure 2. The optical parameters of presbyopic corrections differ between individuals based on the individual amounts of residual accommodative capacity and the lens design. Accordingly, the vertical zones of clear vision differ between subjects. The depth of focus depending on the actual pupil size and the perceptual criterion of the observer for the transition between clear and blurred vision may constitute further residual influences. Because individual measurements of vertical zones of clear vision are typically not performed, giving average workplace recommendations based on the mean results of groups like those measured in Part 2 may be useful for practical purposes. The diagrams in Figure 3 of the mean values of 22 observers with recently prescribed PALs show more distant zones of clear vision (workplace diagram) for general purpose PALs than for computer vision PALs. Consequently, these zones determine where the computer monitor may be placed for clear vision with comfortable head posture. In a parallel field study, 19 we investigated the experiences of the 22 participants from Part 2 with these lenses and know their everyday monitor position during office work. They kept a similar, rather conventional mean monitor position relative to the eyes, as shown in Figure 3B, when they wore these newly prescribed computer vision PALs or general purpose PALs. Thus, the screen was completely within the vertical zone of clear 250

8 vision with computer vision PALs; however, the upper part of the monitor was above the near point curve with general purpose PALs. Thus, this part would have appeared blurred when the head inclination was not changed. The subjects might either have accepted this blurring or needed to incline the head (and thus the near point curve) more upward to view the upper part of the screen clearly with general purpose PALs. The stage of presbyopia has an effect on the vertical zones of clear vision: the more the accommodative capacity is reduced, the more distant is the near point (without near addition) and the more distant and lower is the vertical zone of clear vision (Figure 4). For a more general conclusion about general purpose PALs, Figure 5 shows the grand mean of the results from three studies. The curves in the optometric diagram (Figure 5A) were similar in the three samples, meaning that the type of optical design seems not to be critical, at least for group means. The dashed grey average near point curve in the workplace diagram (Figure 5B) shows that high monitor positions are possible at longer viewing distances; the closer the monitor, the lower it should be placed and the more it should be inclined backwards to allow for a nearly perpendicular view on the monitor. Figure 5B illustrates three possible monitor positions. Within this range, the flexibility of the average user to choose a comfortable gaze inclination is limited: the eyes may adopt an individual vertical gaze inclination to the centre of the monitor within the range of approximately -10 to -30 degrees relative to the horizontal (average approximately -20 degrees). These gaze inclinations are in the same range as previous findings of visual and musculoskeletal research The following rule of thumb can be suggested: for a conventional viewing distance of approximately 75 cm, the upper edge of the monitor should be at least approximately 15 cm below eye level, which corresponds to a gaze inclination of at least approximately -20 degrees to the centre of the screen (for a monitor height of approximately 30 cm). This rule of thumb serves only as a rough guide based on mean group data. For a particular user, the individual conditions need to be considered with respect to the optical design of the lens, remaining accommodative capacity, the comfortable head inclination, occupational tasks and individual preferences. In the present study of 22 participants (Part 2), the comfortable head inclination was tested at the beginning of the first session and kept fixed in a headrest for both sessions to ensure constant conditions; however, the head and body posture will vary slightly in real work situations, which is physiologically reasonable to avoid continuous static muscle load. The variability of head inclination can be estimated from an office field study: 25 the standard deviation of differences between repeated measures was approximately four degrees, meaning that the head inclination varied over a range of ± 4.0 degrees in 67 per cent of all cases (in the range of ± one SD). As a consequence, the location of the vertical zones of clear vision will also vary to this extent. Thus, the limits of these zones in the workplace should not be understood as a fixed sharp borderline; rather, a typical variability due to changes in the head posture of a few degrees should be realistic. The present results can also have implications for the patterns of vertical eye and head movements. When the monitor is placed within the vertical zones of clear vision of PALs, the monitor remains clear only when the eyes are moved vertically without requiring head movements. This is the way a young observer moves the eyes for vertical gaze shift; thus, this movement represents the natural pattern of behaviour, at least within the range of comfortable vertical eye movements. If no accommodative capacity remains in the later phase of presbyopia and the zone of clear vision is thus limited to the depth of focus around the far point curve, a rather low and backward inclined monitor position remains (Figure 4B). The monitor can be seen clearly over a certain vertical extent by eye movements alone and keeping the head in a comfortable posture, including with general purpose PALs. This adjustment of the monitor is an advantage compared to the more unfavourable condition, in which the monitor is placed in a conventional higher and more upright position. The point in the lens with the appropriate near vision power must be shifted across the monitor by vertical head movements to vertically scan the monitor. The present results suggest that general purpose PALs can provide comfortable and clearvisionintheverticaldirectiononamonitor that is placed sufficiently low and inclined backward and at the correct distance; however, the suitability of general purpose PALs for computer work may also depend on the horizontal field of clear view, which is limited by the progressive zone. The limited horizontal width of clear vision affects the patterns of the co-ordination of eye and head movements when reading pages of text. 2 4 Furthermore, the performance in the occupational task may be impaired. Selenow and colleagues 28 compared different visual tasks in a laboratory study and showed that general purpose PALs had a significant seven per cent disadvantage compared to single vision lenses in a task that involved 30 degree horizontal gaze shifts from a screen to paper document, while no differences occurred in other reading or visual tasks that were presented on a single monitor. Thus, possible performance decreases may depend on the actual task at work. Aparalleloffice study 19 compared general purpose PALs and computer vision PALs. In quasi-experimental conditions over four weeks, complaints were lower with computer vision PALs on average; after another eight weeks of free use of these lenses, 44 per cent of the participants preferred computer vision PALs, while the other 56 per cent preferred general purpose PALs. This finding suggests that individual and/or occupational factors play a role in the subjective estimation of the width of progressive zones. The conclusions of the present study are based on laboratory optometric measurements. Thus, the applicability of these conclusions in real working conditions, where many intervening factors can play a role, including variability in the head and body posture and limitations in ergonomic flexibility, should be discussed. In fact, the following field studies yielded results that are compatible with the vertical zones of clear vision as measured with the inclined optometer. Allie and colleagues 26,27 conducted two office studies with freely adjustable monitor positions. 1. Ten users of habitual PALs preferred a gaze inclination of approximately -20 degrees and a monitor inclination of approximately -14 degrees, when provided with freely adjustable monitors. 2. Twenty-four presbyopic users of multifocal lenses adopted a lower gaze inclination (-21.4 degrees) than 23 pre-presbyopes (-13.7 degrees). Accordingly, the multifocal wearers used a stronger backward inclination of the monitor (-16.4 versus -9.6 degrees); the multifocal lenses group comprised 23 general purpose PALs (personal communication). In a small field study, 15 eight participants with general purpose PALs used a low monitor position over years. At a comfortable head inclination of approximately 15 degrees (eye-ear line), the eyes were inclined by 20 to 40 degrees, partly with special low monitor support arms. In a further field study, 20 the monitors (mostly on flexible support 251

9 arms) were lowered after measurements with the inclined optometer from a gaze inclination of to degrees. On average, the 15 users of general purpose PALs accepted this change and reported fewer musculoskeletal complaints. In conclusion, the users of general purpose PALs in these ergonomic field studies preferred monitor positions that corresponded to the vertical zones of clear vision measured in the present study. Another principal consequence of the present research refers to the combination of optometric and ergonomic aspects when optimising computer work for presbyopic users. The earlier conventional procedure, stated for example, by Horgen and colleagues 6 was as follows: the firststepshouldbeergonomics evaluation and adjustment of the workplace and the last step is optometric measurements and care. Clearly, some basic ergonomic settings should be fixed first, such as the height of the chair and table for proper sitting posture; however, regarding the monitor position relative to the eyes, which is important for presbyopia corrections, the conventional procedure may be reconsidered for several reasons. 1. Guidelines for monitor position recommend considerable ranges, for example, 50 to 100 cm for the viewing distance and -15 to -35 degrees for gaze inclination. 29,30 Thus, the ergonomist or user may be uncertain as to how to precisely place the monitor. If an average value is applied, some users may not accept this position, perhaps because the solution may not be favourable for the physiological disposition of the individual Ergonomic guidelines and procedures originate from times when cathode ray tube computer monitors with large housings have been used that were difficult to place according to physiological requirements. Instead, essentially standard positions were used, which were often rather high on normal office desks. Accordingly, presbyopic corrections could only be fitted to this somewhat pre-existing monitor position. These limitations can be overcome today. Flat screens are generally available. If mounted on flexible support arms, they can easily be placed at any position on the desk, which allows the individual physiological functions to be taken into account. These functions include the muscular eye-head-neck system for gaze inclination and oculomotor functions for the viewing distance The vertical zone of clear vision is an additional individual visual parameter, which is a function of the lens to correct presbyopia. These lens types may be selected based on optometric parameters, the occupational task and the preferences of the user, as suggested by the following workplace examples. If a large monitor or several monitors must be carefully observed at a rather fixed viewing distance and no other tasks are involved, a single vision lens may be appropriate and can provide an advantage in task performance. 28 If a computer user must also engage in other tasks, including reading paper documents and conversations with other people and wishes to have clear distance vision in a large office or outside, a general purpose PAL seems to be useful. If vision is limited to a smaller office, a computer vision PAL that provides a large clear field of view and clear vision within the office is appropriate. These three examples illustrate that the type of lens must be selected primarily based on the user s task and that the monitor position must be arranged accordingly. Furthermore, the individual pattern of eye and head movements, 2 5 the preference of clear distance vision or the practical issue of not changing glasses for distance vision, specifically for driving, could play a role. These arguments suggest that ergonomic and optometric aspects strongly depend on each other; thus, optimising computer work for presbyopic users requires a multidisciplinary approach. 29,33,34 Friedrich and colleagues 5 suggest that the design of a workplace should be considered or maybe optimised, while selecting the optimal lens type. This does not necessarily mean that the spectacles need to be adapted to the existing workplace. Long 35 proposes that the communication between optometrists and ergonomists can be improved by developing information sharing documents. These documents could include the vertical zones of clear vision, as suggested by the present research. These recent developments in visual ergonomics are taken into account in a webbased information tool ( that suggests a three-step procedure for typical office conditions: 1. the adjustment of the basic ergonomic conditions, such as illumination, chair and table 2. the choice of the individually favourable lens type for presbyopic correction according to the optometric requirements, occupational tasks and individual preferences of the employee and 3. the arrangement of the monitor position relative to the eyes corresponding to the vertical zone of clear vision (an exception may be workplaces, where the monitor is placed at a certain position, so that lenses must be fitted accordingly). This three-step procedure may help to find an individual optimum of both the visual-optometric and musculoskeletal conditions. Practical application Although the forms of the zones of clear vision are expected from the optical lens parameters, the quantitative workplace diagrams constitute an important tool to understand the workplace implications of each individual lens. These graphical illustrations help to illustrate the optical function of the lenses not in terms of the conventional gradual distribution of near addition dioptres within the lens dimension but as dimensions of clear vision in workplace co-ordinates. Non-optometrists often do not know what presbyopia is and the implications of the optical characteristics of lenses for reading conditions and computer work. Thus, education by the optometrist is required. 36 For presbyopic computer users, the illustration of the individual zones of clear vision can be very helpful, as we experienced in the present and other studies. 20 In fact, individual measurements of vertical zones of clear vision for each lens prescription may be an unrealistically high effort. As a compromise, the user may be informed of the general properties of vertical zones of clear vision with his/her type of lens (as provided in the present study) and may explore the most favourable monitor position with a comfortable head position by trial and error, if the monitor can freely be moved with appropriate mechanical supports. ACKNOWLEDGEMENTS The authors thank the Dortmund financial administration where Part 2 of the study was conducted, the team including C Reiffen and U Lobisch for technical support and the anonymous reviewers for their constructive comments. Part of this study was included in the Bachelor thesis of the co-author Claudia Haensel, (student at Fachhochschule Jena, 252

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