The Vertical Horopter and the Viewing Distance at Computer Workstations

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The Vertical Horopter and the Viewing Distance at Computer Workstations

[Citation: Ankrum, D.R., Hansen, E.E. & Nemeth, K.J., 1995. The Vertical Horopter and Viewing Distance at Computer Workstations, Symbiosis of Human and Artifacts, eds. Anzai, K., Ogawa, and Mori, H., Amsterdam: Elsevier.]

D.R. Ankrum, Human Factors Research, Nova Solutions, Inc. 421 W. Industrial Ave., Effingham, IL 62401, USA

E.E. Hanson, Department of Technology, College of Engineering, Northern Illinois University, 203 Still Hall, DeKalb, IL 60155, USA

K.J. Nemeth, Center for Ergonomic Research, Department of Psychology, Miami University, 104 Benton Hall, Oxford, OH 45056, USA

Introduction

The angle of view, often referred to as the angle of incidence, is the angle formed by the line of sight and the plane of the screen. ISO 9241 [1] and ANSI HFS-100 (1988) [2] both allow the angle of view to be from zero to =/- 40 degrees. The top of the monitor may be tilted forward (negative angle of view) or backward (positive angle of view). Because increasing the angle of view reduces the visual arc of the letters, those restrictions on the angle of view limit geometric foreshortening.

Viewing Distance

The closer the viewing distance, the greater the efforts to accommodate and converge [3,4]. Jaschinksi-Kruza [5] found less eye strain at a viewing distance of 100 cm than at 50 cm. In a study by Grandjean [6], 75% of the subjects chose viewing distances of greater than 73 cm. In a non- VDT task, Owens and Wolf-Kelly [7] found a positive correlation between inward shifts in the resting point of vergence and subjective eye fatigue. It follows that workstation design should encourage greater viewing distances.

The Horopter

The horopter is the locus of points in space that appear as single images to the observer [8]. Anywhere else in space will appear as double images to the observer. The horopter varies across individuals and with target size, fixation distance and gaze angle.

Horizontally, the horopter is curved with the sides coming closer to the observer [9]. The vertical horopter, however, starts somewhere between the viewer's waist and feet and projects outward, intersecting the point of fixation and continuing in a straight line. If an observer fixates on the center of a straight vertical wire in the median plane, both ends of the wire will be seen as double until the wire is tilted backward, with its top farther away from the observer [10].

Figure 1. The vertical horopter for fixation at point A.

The development of the human visual system is conditioned by its environment during infancy and early childhood [11].

When looking at intermediate points on the ground, objects below the point of fixation tend to be progressively closer to the viewer while those that are above are generally farther away. As a result, the lower visual hemifield has developed to be better equipped to see objects nearer than the point of fixation, while the upper visual hemifield is better equipped to see objects that are further away [12].

A vertical VDT screen orientation results in an angle of view which is inconsistent with the developed abilities of the visual system. The characteristics of the vertical horopter predict that computer users with a monitor whose top is closer to the eyes than its bottom (negative angle of view) will experience greater discomfort and reduced performance. A study conducted to evaluate the effects of monitor height and angle on comfort and performance showed significant increases in four measures of discomfort, both postural and visual when subjects viewed a monitor tilted at a negative angle of view [13].

During a pilot study, it was observed that subjects tended to alter their viewing distance as a function of the tilt of the monitor. It was decided to record viewing distance as an additional dependent variable. This paper reports on the relationship between viewing distance and comfort and preference at positive, negative and horopter monitor tilts, and at gaze angles corresponding to both eye level and a low position. Based on geometric foreshortening, user selected viewing distances should vary about the same at equal, but opposite angles of view. Based upon the predictions of the vertical horopter, viewing distances should vary differently as a function of positive and negative monitor tilt.

Method

The subjects were six emmetropic students (refractive correction, if needed) with an average age of 21.5 years (range: 20-24) and an average of 6.1 years (range: 3-10) of computer experience.

The experimental task involved comparing an accurate list of names, addresses and phone numbers on hard copy to a list on the screen that contained errors. When they found mistakes, subjects corrected the screen image.

There were six experimental conditions involving all combinations of three screen angles and two gaze angles. The three screen angles (measured from the perpendicular to the line of sight) were: "horopter," tilted back 15 degrees at the high condition and 25 degrees at the low condition, more or less coincident with the horopter [14]; "positive," tilted back 40 degrees, and "negative," tilted forward 40 degrees. The gaze angles were: "high," top of screen at eye level; and "low," the top of screen 20 degrees below eye level. The center of the three lines of text on the screen was another 8 degrees lower.

The viewing distance was initiallly set at 66 cm, but subjects were free to alter their postures. The characters were 4 mm high, corresponding to a visual angle of 21 minutes at 66 cm. That is within the preferred range of 20 to 22 minutes of arc recommended by ANSI HFS-100 (1988) [2] for tasks where legibility is important. In the positive and negative monitor tilt conditions, the characters subtended a visual angle of 16 minutes, corresponding to the minimum character height required by ANSI HFS-100 (1988) (2) for tasks where legibility is important.

Each condition consisted of two 20-minute segments, separated by a 10-minute break. Each subject participated in all six conditions on separate days. The order of conditions was determined by a Latin Square. Eye to screen measurements were taken at three different times during each condition: two minutes after the beginning of the first segment; at the end of the first segment; and at the end of the second segment.

Results

An earlier article [13] reported that four measures - neck discomfort, upper back discomfort, "tired eyes," and "tired looking at the screen"- were found to be significantly greater in the negative monitor tilt conditions.

The three-way ANOVA was performed to compare the viewing distances assumed in each of the six conditions. Follow up tests were performed with a Turkey HSD procedure. Reported results are significant at the .05 level. Mean viewing distances were presented in Figure 2.

Figure 2. Mean viewing distance across the session.

A significant main effect of monitor tilt, (p =.0006), demonstrated that subjects maintained shorter viewing distances for the negative angle, and there were no significant differences between the horopter and positive angles. A significant main effect of gaze angle (p =.0015), showed that subjects chose closer viewing distances in the high gaze angle conditions. There was no significant main effect of time which demonstrated a lack of a significant change in viewing distance over the course of the session.

Table 1

The interactions between time and monitor angle, gaze angle and monitor angle, and time and gaze angle were not significant.

The three-way interaction between time, gaze angle and monitor angle was significant (p = .0260). Follow-up analysis found that subjects changed their viewing distances over the course of the session in the high gaze angle, negative monitor tilt condition. Those subjects moved closer to the screen as the session progressed.

In order to determine the relationship between viewing distance and the subjective measures of comfort and preference, Pearson Correlations were performed. As no main effect of viewing distance across time was found, the distance recorded at the end of the season was used in these comparisons. Table 1 lists the results of this analysis.

Significant relationships were found between viewing distance, and neck and lower back discomfort (p < .05). Shorter viewing distances were related to increases in reported discomfort. The conditions in which subjects assumed shorter viewing distances were rated less favorable than those conditions in which they assumed longer viewing distance.

Discussion

The results of this study suggest that monitor tilt may play a role in user-selected viewing distances at computer workstations. A vertical horopter which tilts away from the observer at the top was developed to adapt to a commonly experienced feature of the visual environment. In that environment, objects below a point of visual fixation are usually closer to the observer, while higher objects are usually farther away.

Because the results of this study appear to concur with the physiological mechanism of the vertical horopter, it suggests that monitor tilts opposite to the horopter may result in shorter viewing distances and greater increases in discomfort.

It is inappropriate to consider the angle of view adjustments commonly observed in office environments as reflecting preferred settings due to the constraints of the lighting systems and equipment. In many offices tilting the monitor back would result in glare from ceiling luminaires. If glare and reflections are not satisfactorily addressed, the potential benefits of a positive tilting monitor will be lost.

Acknowledgement

The authors wish to thank Dr. Walter Makous for introducing them to the concept of the vertical horopter.

References

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