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by Dennis R. Ankrum (ankrum@aol.com)
In the years since the introduction of the personal computer and the realization that it was the cause of workplace health problems, many guidelines have been published concerning the best viewing angles and distances. With few exceptions these guidelines recommend distances and angles that are contrary to the known qualities of the visual system. The distances allowed are too close and the angles too high. The proven relationship between viewing angle and viewing distance is mostly ignored. Near WorkComputer work takes place at near distances. When we view a monitor or any other close object, two things happen to our eyes. They accommodate and they converge. Both these reflexes contribute to the eye strain.1AccommodationWhen we look at close objects the ciliary muscle changes the shape of the lens. The lens bends the light rays so they strike the retina at a single point, allowing a sharp image to be interpreted by the brain.2 If this point is too far in front of or behind the retina, excessive blur results. The brain's reaction to this blur stimulates the ciliary muscle to alter the shape of the lens which brings the object into sharper focus.Accommodation and DistanceThe eyes have a resting point of accommodation. This is the distance at which the eye focuses when there is nothing to look at. At one time this resting point was thought to be infinity. Recent studies have shown this not to be the case.3 This resting point differs among individuals, but the average is about 31 ½ inches.4 Prolonged viewing of a monitor closer than your resting point of accommodation increases eye strain.5 The ciliary muscle must work two and a half times harder to focus on a monitor 12 inches away than it does to focus at 30 inches.The closest distance at which an object can be brought into sharp focus is called the near point of accommodation. As we get older, the lens undergoes changes which cause the near point to get farther away. A 15-year-old can focus as close as 3 inches. At age 32 the near point averages 4.5 inches; at 40 it is 10 inches; at 50 it is 20 inches; at 60 it reaches about 39 inches.6 At around the age of 40, many people with otherwise adequate vision begin to hold reading material farther and farther from their eyes until their "arms get too short". They then require reading glasses. When determining viewing distance, optometrists recommend not requiring more than half of the eye's ability to accommodate.7 Accommodation and Viewing AngleViewing angle affects the ability to accommodate. Ripple found that subjects over 42 increased their ability to accommodate by an average of 25.5 percent by directing their eyes down in the "usual reading position". The improvement for younger subjects was even greater.8 Hill and Kroemer found people preferred a more downward gaze angle when looking at a target 20 inches away than they did for a 40 inch distant target.9VergenceThe closer the object the more the extraocular muscles converge the eyes inward toward the nose. Convergence allows both eyes to focus the object at the same place on the retina.Vergence and DistanceThe eyes also have a resting point of vergence. This varies among individuals, but the average is around 40 inches.10 Looking at objects closer than one's resting point causes strain on the muscles controlling vergence. The closer the distance, the greater the strain.11The resting point of vergence has an even greater impact on eyestrain than the resting point of accommodation. Jaschinski-Kruza12 divided subjects into two groups. The first (near) group had resting points of accommodation of around 20 inches. The second (far) group's resting points of accommodation average 40 inches. Both groups performed computer work at viewing distances of 20 inches and 40 inches. As expected, the near group had less eyestrain working at 20 inches than the far group. But both the near and far groups had less eyestrain at the 40 inch distance. Both groups judged the 20 inch monitor distance as "too near", and accepted the 40 inch distance. When Jaschinski-Kruza measured productivity he found that both groups had greater productivity at the 40 inch distance. He concluded that increased convergence seems to be the crucial stressing factor in visual strain. Vergence and Viewing AngleThe resting point of vergence changes with viewing angle. The lower the gaze angle, the more the resting point of vergence tends to move closer. Averaging the results of 24 subjects, Heuer13 found that, with a 30 degree upward gaze angle, the unstimulated eyes would converge on a target about 53 inches away. Looking horizontally, the viewing distance would be about 44 inches.When looking down 30 degrees, the resting point of vergence equals about 35 inches of viewing distance. Not everyone exhibits a change in their resting point of vergence when looking up and down. For those who do, a downward gaze angle significantly reduces headaches and eye strain and, to a lesser extent, blur and mental fatigue.14 Neck AnglesChaffin15 discovered that the time it took workers to achieve pain and fatigue increased at 30 to 45 degrees forward neck tilt. He noted that a forward neck posture of 15 degrees caused no subjective feeling of fatigue or pain.In guidelines for workplace and machine control layout, Chaffin considered a head inclination of 20 to 30 degrees as normal.16 Head MovementsSeveral studies cited by Von Noorden show more benefits of a downward gaze angle. The studies found that, when looking back and forth between targets placed at eye level, head movements occurred with even very small movements of the eyes. However, when the targets were at a downward gaze angle "head movements hardly participated in the movements".17
User Preferences for Distance and AngleThe preferences of computer users correspond with the physiological findings.DistanceEvery one of the subjects studied by Jaschinski-Kruza18 judged the eye to screen distance of 20 inches to be too close. All accepted a 40 inch distance. Grandjean reported an average preferred viewing distance of 30 inches.19We do not know of a single study in which users preferred an eye to screen distance of anywhere near the viewing distance of 12 inches allowed by ANSI/HFS 100-1988. Viewing AngleIn a study which included four different head positions and two target distances, Hill and Kroemer found people preferred to look downward at an average angle of -34 degrees below the Frankfurt Plane.20In what would be a normal computer-work position, the average preferred angle was -29 degrees. They found that the closer the object, the more downward people wanted to look. When the target was 40 inches away, the preferred viewing angle was -30 degrees. When the target was moved in to 20 inches, the downward preference increased to -38 degrees. The head was not permitted to move during these tests, so the downward gaze angle was caused solely by movement of the eyes. SummaryAs Hill and Kroemer wrote in Preferred Declination of the Line of Sight, "....it is unclear what research, if any, has contributed to the current human factors recommendations on the preferred line of sight in the midsagittal plane". The same holds true for the current recommendations for viewing distance.None of the current guidelines address the critical interrelationship between viewing angle and distance. If you would like to read a more technical article on this subject (by the same author), please click here. 1 Fischer 1977, Collins 1975 2 For the latest explanation of how this happens, see the Sept. 1992 issue of Scientific American. 3 Owens 1984 4 Krueger 1984 5 Jaschinski-Kruza 1988 6 Grandjean 1987 7 Boorish 1970 8 Ripple 1952 9 Hill and Kroemer 1986 10 Jaschinski-Kruza 1991 11 Collins 1975 12 Jaschinski-Kruza 1988 13 Heuer 1989 14 Tyrrell, R. 1990 15 Chaffin 1973 16 Chaffin 1991 17 Von Noorden 1985 18 Jaschinski-Kruza 1988 19 Grandjen, E. 1983 20 The Frankfurt Plane is an imaginary reference line passing from the center of the ear hole through the bottom of the eyeball socket. When the head tilts forward, the Frankfurt Plane tilts with it. |
