Wolfgang Jaschinski

Preferred position of visual displays relative to the eyes: a field study of visual strain and individual differences

At office workplaces equipped with visual display units (VDU) that were adjustable to various positions relative to the eyes short and long viewing distances from the eyes to the screen were imposed (mean value of about 63 and 92 cm) at two levels of screen height so that the visual target was either at eye level or 18 cm below, on the average. The change from far to near viewing distance produced a larger increase in eyestrain when the VDUs were at eye level. High screens resulted in greater eyestrain than low screens, as shown by correlations over subjects. When operators were free to adjust the most comfortable screen position, the group of 22 participants preferred viewing distances between 60 and 100 cm and vertical inclination of gaze direction between horizontal and -16 degrees downwards. However, within most subjects the range of preferred screen positions was much smaller. Between 3 days during a 1-month period the test-retest correlations of the preferred screen positions were highly significant, both for viewing distance and vertical gaze inclination. When operators were forced to work at a shorter distance than their preferred viewing distance they reported more visual strain. Thus, operators appear to prefer an individual adjustment of the screen relative to the eyes in order to avoid visual strain and discomfort at VDU work.

A procedure to determine the individually comfortable position of visual displays relative to the eyes

In an intervention phase, 38 operators used four different imposed screen positions (near versus distant, high versus low) for a full working day to experience the advantages and disadvantages. Screens at about 66 cm induced more reported strain than screens at about 98 cm. When operators later freely selected their individually most comfortable screen position, individually different changes due to the intervention were observed: some subjects changed to shorter, others to longer viewing distances, some operators adjusted the screen lower, others higher. These effects were confirmed in repeated tests. Thus, trying out different screen positions appears useful for arranging the VDU workstation to the individually most comfortable screen location relative to the eyes.

Fixation disparity and accommodation as a function of viewing distance and prism load

Fixation disparity was measured with dichoptically presented nonius lines at viewing distances of 20, 30, 40, 60, and 100cm, so that both vergence and accommodation were stimulated adequately as in normal vision. As the viewing distance was shortened, mean fixation disparity changed monotonically from 1 min arc eso (i.e., the eyes converged in front of the target) to 3 min arc exo. The average standard deviation of the psychometric function of fixation disparity, which is a measure of the temporal variability of vergence, was smallest at 100cm (when fixation disparity was eso) and increased as viewing distance decreased. Fixation disparity itself and the change of fixation disparity with distance differed reliably among subjects with normal binocular vision, but neither was related to the momentary degree of accommodation. Fixation disparity was also measured at a constant distance of 40 cm, but with prisms in front of the eyes that induced the same vergence angles as viewing distances between 20 and 100cm. The slope of these conventional fixation disparity curves as a function of prism load was generally larger than the slope of fixation disparity as a function of viewing distance (which can be explained by accommodative vergence), but the slopes of the two types of curves were correlated (r= 0.39, P= 0.02, n= 25).

Accommodation, convergence, pupil diameter and eye blinks at a CRT display flickering near fusion limit.

This study investigates possible effects of temporally modulated light stimulation near critical fusion frequency (CFF) when subjects observe a cathode ray tube (CRT) operated at different refresh rates. Various visual functions were measured in a series of tests of 2.5 min duration. In experiment 1, at a repetition rate of 50 Hz mean pupil size was 0.055 mm smaller than at 300 Hz. The precision of convergence and accommodation in binocular vision was not affected. In experiment 2, 300 Hz was compared with the lowest frequency that did not produce visible flicker for each subject. At the lower rate (55 to 90 Hz), mean accommodation in monocular vision was 0.06 D weaker, median eye blink duration was 6% shorter, and mean eye blink interval was 15% longer. Individual differences and possible fatigue effects of intermittent light at visual display units are discussed.

Fixation disparity at different viewing distances and the preferred viewing distance in a laboratory near-vision task

Previous research has shown that subjects with normal binocular vision differ reliably in the extent to which their fixation disparity changes in the exo direction when the viewing distance is shortened from 100 to 20 cm. Since fixation disparity can lead to asthenopic complaints, the present study investigates whether an exo fixation disparity induced by proximity may cause subjects to move to a longer viewing distance during a near‐vision task in order to reduce exo fixation disparity. In two optometric sessions, fixation disparity and accommodation were tested at 60, 40, and 30 cm viewing distance. In a further session, subjects were required to begin a one‐hour near‐vision task at about 40 cm viewing distance, at which the text characters subtended a comfortable visual angle of 21 min arc. Later, the subjects were free to adopt any viewing distance. In the initial phase of the task, subjects moved back from the screen to a greater or lesser extent that was correlated with the amount of proximal exo fixation disparity: the more a subjects fixation disparity changed to exo when the viewing distance had been shortened from 60 to 30 cm the more he or she moved to longer viewing distances in the course of the near‐vision task. Further, the more distant the resting position of vergence (dark vergence), the more visual complaints the subjects indicated after the task relative to before.

Fixation disparity, accommodation, dark vergence and dark focus during inclined gaze.

We investigated several oculomotor functions at different angles of vertical inclination of the gaze direction from 15 deg upwards to 45 deg downwards. The mean accommodative resting state (measured in a dark visual field) increased when the eyes or the head were declined from 0 to 45 deg. Fixation disparity (the vergence error in minutes of arc relative to the principle visual directions) became more eso when a fusion target at a viewing distance of 40 u2003cm was lowered: declining the gaze by 45 deg changed mean fixation disparity by 1.8 u2003min arc with eye inclination (keeping the head upright), and by 0.9 u2003min arc with head inclination (with eye position unchanged relative to the head). When the eyes were lowered, the individual rate of eso change in fixation disparity was correlated with the amount of the subjects’ near shifts in the resting position of vergence, measured in darkness. Significant test‐retest correlations between repeated measurements showed that the effects of eye inclination on vergence varied in a reproducible way among individuals with good binocular vision.

Contrast sensitivity after wearing prisms to correct for heterophoria

Contrast sensitivity to sinusoidal gratings was measured at spatial frequencies of 1.5–16 c/deg in 8 subjects before and after wearing prisms (for at least two weeks) to correct heterophoria, prescribed by the full correction method of H.‐J. Haase (Binocular testing and distance correction with the Berlin Polatest [trans. W. Baldwin]. J. Am. Optom. Assoc. 34, 115–125, 1962). At higher spatial frequencies of 12–16 c/deg, we found an average improvement in contrast sensitivity of 0.15 log unit. This effect is estimated to correspond to an increase in visual acuity of 15%.

Precise placement of nonius lines on a personal computer screen for measuring fixation disparity

A method is described for measuring fixation disparity psychophysically using the conventional monitor of a personal computer. Both the nonius test stimuli and the fusion stimulus were presented on the same screen. To be able to adjust the offset of the nonius lines in very small steps, special electronic circuitry was developed that allows up to 16 steps within the pixel separation of the monitor. The method is flexible in that various geometrical dimensions of the visual stimuli and different psychometric procedures can be used.

Verschiedene Kenngrößen für die physiologisch günstige Kopfneigung

ZusammenfassungDas Sehen am Bildschirm erfordert die abgestimmte Bewegungskoordination der Augen und des Kopfes. Dabei kann die eingenommene Kopfneigung von Bedeutung sein. Diese Studie untersucht horizontale Kopfb ewegungen mithilfe des Trackingverhaltens und weiterer Indikatoren für eine physiologisch günstige Kopfneigung. Die komfortable Kopfneigung, die Probanden spontan in einem Schnelltest selbst einstellen, war korreliert mit zwei weiteren Indikatoren: erstens mit der Kopfneigung, bei der die subjektiv beurteilte Nackenanstrengung minimal war, und zweitens mit der Ruhe-Kopfneigung, auf die sich der Kopf vertikal hinbewegt, wenn der Proband horizontale Kopfb ewegungen mit geschlossenen Augen ausführt. Einige Indikatoren, darunter das Elektromyogramm des Musculus Sternocleidomastoideus und die subjektive Beurteilung von Nackenbeschwerden, zeigten einen ähnlichen Beanspruchungsverlauf als Funktion der vorgegebenen Kopfneigung. Die Bestimmung der komfortablen Kopfneigung und der dazugehörigen komfortablen Blickneigung erscheint als ein einfaches und praxisgerechtes Verfahren, das als Anhaltspunkt für die individuelle Optimierung der vertikalen Bildschirmposition nützlich sein kann.AbstractA physiologically favourable head tilt should be adopted at computer work by an ergonomic optimization of the vertical monitor position, in order to reduce musculoskeletal complaints. The aim of this study was to investigate head movements during a horizontal head-tracking task and further indicators for a physiologically favourable head inclination.First, to find the comfortable head tilt, subjects performed small horizontal head movements with closed eyes in different vertical head positions until they found their individual favorable head tilt. Then, a head-mouse system — designed for computer interaction — was used in a way that subjects performed horizontal head movements to keep a cursor (vertical line) on a target (rectangle) which moved with variable velocity on a 44° wide screen. This was repeated with head tilts of -20°, -10°, 0°, +10°, +20° (Frankfurt line relative to horizontal) and the comfortable head tilt, while the activity of the musculus sternocleidomastoideus was measured by electromyography. For each head tilt, subjects judged the magnitude of perceived neck strain during tracking. The horizontal precision and the vertical deviation of the target position during tracking were measured as a function of head tilt. Further, we measured the resting position of head tilt, defined as the head tilt towards which the head tends to move vertically when performing horizontal head movements with closed eyes.The results in 16 participants showed a linear increase in tracking precision the more the head was inclined upwards, but this effect was negligible. Other indicators suggested a favorable condition more or less near a 0° Frankfurt line, specifically a U-shaped function was found for the EMG activity of the musculus sternocleidomastoideus and the subjective judgment of strain in the neck.Three indicators had significant test-retest- correlations (r > 0.5, p < 0.05) and showed the following inter-correlations: the comfortable head tilt (+5.1 ± 4.2°) was significantly correlated to the resting head tilt with horizontal head movements (+4.5 ± 8.9°, r = 0.58, p = 0.0093; boThmeasures used closed eyes to eliminate external stimuli) and to the head tilt of subjectively minimal strain when observing the visual head tracking targets (-1.6 ± 9.4°, r = 0.50, p = 0.0243); these means and standard deviations refer to Session 2 and the Frankfurt line relative to horizontal; the eye-ear-line is about 11° higher. For practical purposes, the comfortable head tilt and the comfortable gaze inclination can easily be tested at the workplace as starting value to determine the individually favorable vertical monitor position to improve subjective neck comfort and to avoid musculoskeletal strain in the neck.

Vergence Dynamics and Variability of Fixation Disparity in Children and Adults

With dichoptic nonius lines presented repetitively on a computer monitor for only short moments in time (e.g., 80 ms), temporal aspects of vergence can be assessed: the variability of fixation disparity with a stationary central fusion stimulus and the velocity of convergence or divergence changes. Results and test-retest correlations of these psychophysical vergence measures are shown for children and adults. In most children older than about 7 years, the estimation of vergence velocity turned out as well as in adults in terms of amount and reliability.