UPDATE ARTICLE

The Visual Display Terminal And Vision

Julie Quant*, B.Sc (Hons), M.Phil, MBCOResearch Fellow, Hong Kong PolytechnicGeorge C. Woo, O.D., PhDVisiting Professor, Hong Kong Polytechnic

Abstract

The increased use of visual display terminals(VDTs) has led to numerous reports of visualdiscomfort and reduced visual performance.

This paper summarises the visual, ergonomicand environmental factors related to VDT use anddiscusses their effect on visual function and comfort.

Key words: Visual display terminal, VDT, eyestrain,visual hygiene and performance.

Introduction

Despite numerous studies into visualdiscomfort following the use of visual displayterminals (VDTs) there are still many unresolvedquestions and differences in reported findings.

Some studies have shown no significantdifferences between the problems experienced byVDT users and non VDT users1,2 while others haveshown changes in visual performance and reducedvisual comfort with VDT use3'4.

Eyestrain or visual fatigue gives ocular andvisual symptoms of soreness, aching, dryness,burning and grittiness, photophobia, lacrimationtwitching and blur, and focussing problems. Theseare generally temporary and steps can be taken tominimise these effects. Systemic symptoms such as

* Address for correspondence: Department of Diagnostic Sciences,

headache and general fatigue may also beimplicated.

This paper gives a summary of the importantvisual, ergonomic and environmental factors relatingto VDT use in order to guide practitioners indiagnosing and advising on VDT related visualproblems.

Visual Comfort and Performance

Factors that influence visual comfort andperformance when using VDTs are consideredbelow. Potential irritants include illuminationlevels, glare, character legibility, posture and visualhygiene.

Lighting Levels

General office work is usually carried outunder an illumination of 700-800 lux. For VDT use,a decreased level of 300-700 lux with no sourcedocument or 500-700 lux with a source document isconsidered appropriate. The contrast betweenscreen, source document and surround should beminimised. Windows should have blinds foradjustment as weather conditions and time of daychange. Light sources should also have shades ordiffusers and allow for adjustment in light level.The VDT user's line of sight is closer to thehorizontal than that of the traditional office workerand therefore more affected by overheadluminaries.

Glare

Reflections from the slightly convex VDTscreen (diffuse or specular) cause discomfort and

Hong Kong Polytechnic, Hung Hom, Kowloon.

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Visual Display Terminal

may reduce the ability to read text5. The VDTshould be positioned to the side of overhead lightsand away from windows if possible.

Maintenance of a clean screen surface, use ofa non-glossy keyboard and documents andalteration in viewing angle (e.g. raising the rear legsof the terminal) will all help to minimise glare.Some individuals benefit from lightly tintedspectacles. Anti-reflection shields and screencoatings will minimise glare but care should betaken that character legibility is not affected.

Office decoration is also important. Mattebeige walls (less than 50% reflectance) and subduedcarpet as opposed to glossy floor are preferable. Areduced glare index of 16 is recommended for VDTuse.

Screen Contrast

For ease of viewing, it is important that theimage contrast is good. This can be improved byminimising glare, controlling surround luminanceand adjusting contrast control on the terminal. It isrecommended that character luminance should be3 times that of the screen background and this inturn should be 3 times that of the surroundluminance. Preferred contrast will vary betweenindividuals (depending on age, for example) andalso with the workplace.

Although Radl6 found improved readability andvisual comfort when his subjects viewed a positivepresentation (dark characters on a lightbackground), according to Campbell and Durden7,there is no obvious physiological reason why apositive presentation should be preferred tonegative presentation.

Colour

Research has indicated that green charactersare preferable for visual comfort and focussing

demand6,8,9. This may be as a result of therelationship between accommodation andchromatic aberration, the wavelength in focusdepending on the accommodative state. Haider etal10 found less temporary myopization with yellowcharacters, though in general their results showedvery little difference between green and yellowcharacter colour. For a working distance of 50cms, phosphors with a wavelength of 525-520 nmare suggested9.

Multichromatic screens are particularly usefulin visual search and are very common with theincrease in popularity of computer games.Stress on the accommodative system due to thevariation in, accommodative response to thedifferent wavelengths has been suggested.However, Campell and Durden7 show that depthof focus and small fluctuations inaccommodation can easily facilitate the need torefocus between colours. We believe that it ispossible that the variation in amplitude ofaccommodation produced by different colours onthe screen may be beneficial in preventingaccommodative spasm.

Occasionally, the viewing of green characterson the VDT screen may cause a colour after-effect11. White characters on a dark backgroundappear pink and larger white areas are seen witha pink fringe. This effect is thought to be due toadaptation of cortical neurones that are responsiveto specific colour and form12. It is usuallytransient, though it may persist for several days orweeks.

Character Quality

Poor quality VDTs are more likely to giveasthenopic symptoms. Image quality is dependenton the shape, size, spacing and resolution as wellas luminance and contrast. Legibility is often lessthan that of hard copy and there may be blurringat the screen periphery. A 7 x 9 matrix ispreferable.

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Jitter of the character is caused by insufficientvideo deflection voltage in a poorly designed orpoorly maintained VDT. Image flicker occurswhen the phosphorescent image decays before itcan be refreshed by the electron scan. Whetherit is noticed by the observer depends on severalfactors such as character colour,, size, orluminance and observer characteristics such as ageand pupil size. It is often more noticeable in theperiphery so that an operator may be distractedby the flickering of another terminal in hisperiphery. Flicker induced epileptic seizures areunlikely due to the high refresh rates on modernVDT screens.

If bifocals are preferred then the segment shouldbe positioned higher than is usual. Multifocallenses provide useful intermediate focus but arenot recommended if rapid and frequent eyemovements are required.

VDT users, especially contact lens wearers,should be encouraged to blink frequently to avoiddry, irritable eyes. Rigid lenses are generallypreferable in a dry atmosphere. They mayrequire re-wetting with an in-eye lubricant ormore frequent cleaning. The astigmatic erroroften left uncorrected with contact lens wear maybe more troublesome with VDT use.

Posture

Improper working posture can lead todiscomfort and stress. Eyestrain may be areflection of this general fatigue.

A viewing distance of 50-70 cms is usuallyrecommended. The screen viewing angle shouldbe 10-20 degrees below the horizontal eye level.Keyboard height should not be excessive so thatthe hands are not in an elevated position. Seatingshould be comfortable and the workstation shouldbe organised so that documents, telephone etc. arewithin reach and do not induce excessive head oreye movements. A document holder is useful forthis purpose.

Visual Hygiene

Small refractive errors contribute significantlyto the symptoms of eyestrain experienced by VDTusers13. Uncorrected hypermetropia or small-valueastigmatism are particularly troublesome.

It is therefore important that all VDT userswear spectacles with correction suitable for theworking distance and appropriate for the task tobe performed. Bifocal or multifocal wearers maybe more comfortable with single vision spectacles.

Any disruption in binocularity such as anuncompensated heterophoria will increasesymptoms of eye strain with VDT use. The topof the screen should be positioned below eyeheight so as to limit binocular stress duringconvergence.

A well functioning accommodative orfocussing system is considered essential forcomfortable VDT use. Accommodation is broughtabout by the contraction of the ciliary musclewhich increases the convexity of the crystallinelens and allows focussing of light from a nearobject onto the retina. Accurate and stablefocussing onto the object being viewed should beachieved without effort.

Spasm of the accommodative system isconsidered to be a possible cause of the functionalmyopia (a temporary increase in short-sightedness)which has been reported by some researchersfollowing VDT use14,15. It is proposed that withciliary spasm there is an increase in vitreouschamber pressure and increase in axial length. Inorder to alleviate this accommodative stress andimprove accommodative facility (the ability torespond quickly and accurately to accommodativechange) visual exercises have been used. Methodsinclude traditional orthoptic techniques such as fliplenses and prisms16, behavioural techniques17 andbiofeedback18.

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Hong Kong Practitioner 13 (8) August 1991

One simple piece of advice for office workers,in order to relax their accommodation, is to lookout of the window at a distant object. This will helpto prevent the frequently encountered distancevision blur due to over-focussing at near. Also, anadequate distance from the screen should bemaintained (50-70 cms).

Simple devices to exercise the eyes slowly andrhythmically over its accommodative range have notreceived much attention though positive results havebeen reported in China. A clinical trial of theOptolax II19 gave a small but statistically significantreduction in myopia after exercise and animprovement in unaided visual acuity.

Further investigation of this device withemphasis on the visual stress of VDT work is beingconducted at the Hong Kong Polytechnic,Department of Diagnostic Sciences.

Conclusion

The visual discomfort associated with VDTwork (or indeed with any prolonged close work) canbe kept to a minimum by creation of an appropriateworking environment, an accurate spectaclecorrection of refractive errors and reduction ofaccommodative stress.

Acknowledgements

This paper forms part of a project supportedby a grant from the University and Polytechnic

Grants Committee (UPGC). •

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