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In a study involving three computer monitor-viewer distances in the range of 50 to 85 cm with a fixed screen character size (3.3 mm), the near distance was related to less:
- headache symptoms
- visual fatigue (convergence recovery)
- blurred vision
- dry/irritated eyes
There was no association between monitor-viewer distance and neck ache.
The authors, Rempel et al., recommend positioning the monitor between 52 and 73 cm from the user’s eyes if the screen character size is close to user visual acuity limits. However, among individuals with near convergence, a longer viewing distance with corresponding increased character size is suggested.
The subjects did not maintain the “ideal” sitting postural position (back against 110° backrest inclination angle) for any of the 2 hour monitor-viewer test conditions. In general, participants moved closer to the monitor through a combination of reduction of the torso flexion angle/lowering of torso height (suggestive of a flexion motion through the thoracic and lumbar spine) and forward positioning of the head – a slumped postural position.
Study Design
Subjects
Twenty-four subjects (16 males and 8 females, average age 25.4 years, 9.5 mean number of experience years working with computers) volunteered to participate in the study. All subjects used the mouse with the right hand and none used corrective lenses for computer interaction. All were free of current head, neck, back or arm injuries.
Computer Workstation Set Up
A Steelcase Leap chair was secured to the floor and set with an inclination angle of 110°. The chair seatpan vertical height and seatpan depth were adjusted to the subject’s dimensions.
The mouse was positioned such that the forearm was level with the floor and minimal shoulder flexion or abduction was required. The chair armrest was moved to comfortably support the subject’s forearm when using the mouse.
An 18-inch LCD flat panel monitor was attached to an adjustable support arm.
Tested Monitor Distances
The monitor was randomly placed at one of three horizontal distances from the volunteer’s eyes – 46, 66 or 86 cm (18, 26, or 34 inches). The monitor was located directly in front of the participant and centered 15° below the subject’s horizontal eye gaze.
Experimental Computer Tasks
For each monitor distance, two hours of computer tasks were performed that consisted a select sequence: 15 minutes of document editing, 90 minutes of Internet searching, and 15 minutes of document editing. The tasks were mouse intensive to avoid impacting head postural position that can occur among certain typists who visual gaze at the keyboard to locate keys.
A 30 minute break was given between each monitor distance test session.
Experiment Measurement – Posture
After practicing the experiment computer tasks, subjects were allowed to make minor chair adjustments (vertical height, seat pan depth) for improved comfort.
Infrared markers were placed on the side of the head at the canthus of the eye and just anterior to the ear to measure head position. Torso position was measured by placing infrared markers vertically along the sternum.
Reference posture measurements were taken at the beginning of each monitor distance testing condition. The subjects were seated with their back against the chair backrest while they looked straight ahead at a mark positioned at their sitting eye height. The average from measurements taken prior to each test condition became the reference posture measurement.
During a test condition, participants were allowed to move to any comfortable postural position but were not permitted to change the chair adjustment settings. From the two hours of data collection, summary posture calculations were made that included:
- head flexion angle relative to mean reference posture
- head flexion angle relative to torso angle
- head horizontal position relative to torso
- torso angle relative to mean reference posture
- torso height relative to mean reference posture
- mean viewing distance
Experimental Measurement – Vision Function
At the end of each test condition, evaluation of divergence break, divergence recovery, convergence break and convergence recovery was conducted.
Experimental Measurement – Symptoms
Subjects filled out a questionnaire following the vision function tests using a 100 mm visual analog scale with verbal anchors (none, mild, modest, bad, severe) to rate:
· eyestrain or eye fatigue
· blurred vision
· neck ache
· dry or irritated eyes
· headache
Other Findings
The subjects did not maintain the “ideal” sitting postural position (back against 110° backrest inclination angle) for any of the 2-hour test conditions. On average, participants moved closer to the monitor during the task by 2.3, 5.3 and 7.8 cm for near, middle and far monitor positioning, respectively. This change in monitor-viewer distance was accomplished by a relatively similar directional change in torso/head positioning:
· reduction in the torso flexion angle (1.7°, 7.5°, and 11.7° for near, middle and far monitor positioning, respectively)
· reduction in torso vertical height (2.5, 3.0, and 3.5 cm for near, middle and far monitor positioning, respectively)
· change in torso horizontal position (-0.4, 1.8, and 4.8 cm for near, middle and far monitor positioning, respectively)
· forward motion of the head relative to the torso (1.2, 2.2, and 3.4 cm for near, middle and far monitor positioning, respectively)
The head flexion angle did not significantly change relative to viewing distance.
Study Concern/Limitations
The authors identified the following potential shortcomings of this research:
1) Validity of the initial reference “ideal” sitting postural position (back against 110° backrest inclination angle) may be inaccurate.
2) A fixed monitor/subject viewing distance was not maintained during the study since participants were allowed to move their upper body while sitting.
3) Since all testing occurred on one day for each volunteer, subject fatigue may have influenced findings of the final test condition. This concern was likely minimized by experiment protocol whereby the three test conditions were applied in a random manner.
Article Title: The Effects of Visual Display Distance on Eye Accommodation, Head Posture, and Vision and Neck Symptoms
Publication: Human Factors 49:5 830-838, 2007
Authors: D Rempel, K Willms, J Anshel, W Jaschinski, and J Sheedy
This article originally appeared in The Ergonomics Report™ on 2007-10-17.
