CRT versus LCD: A pilot study on visual performance and suitability oftwo display technologies for use in office work

Marino Menozzi* , Urs Napflin, Helmut Krueger

Institute of Hygiene and Applied Physiology, Swiss Federal Institute of Technology, Clausiusstrasse 25, CH - 8092 Zu¨rich, Switzerland

Received 24 July 1998; received in revised form 22 September 1998; accepted 22 September 1998

Abstract

Cathode ray tube (CRT) display and liquid crystal display (LCD) were compared for their suitability in visual tasks. For this purpose visualperformance was assessed by means of a search task carried out using both displays with different levels of ambient light. In addition,suitability was rated subjectively by users of visual display units (VDUs). Error frequency for search tasks carried out using LCD weresignificantly smaller when compared to error frequency for tasks at CRT. LCD gave rise to 34% less errors than did CRT. Reaction time insearch task was found to be significantly shorter using LCD when tasks were carried out in darkness. Subjective rated suitability of LCD wasscored twice as high as suitability of CRT. Results indicate that LCD used in this experiment may give better viewing conditions incomparison to CRT display.q 1999 Elsevier Science B.V. All rights reserved.

Keywords:Cathode ray tube (CRT); Liquid crystal display (LCD); Visual performance

1. Introduction

Liquid crystal displays (LCD) have become more andmore popular in visual display units (VDUs) (Cladis [1],Firester [2]). Size of actual LCD can cover the needs ofmost applications running on computers. For many reasonsLCD might become more important and might replace cath-ode ray tube (CRT) displays in many applications. Weightand volume of LCD are among most the important advan-tages when compared to CRT VDU. A 1700 CRT typicallyoccupies an area of 40 cm× 45 cm (width × depth). Ifaccording to suggested settings for work place (e.g. DIN4549 [3]) a desk of 120 cm× 80 cm is considered, a 1700

CRT may occupy about one-fifth of the surface of the desk.Dimensions of LCD monitors are smaller, therefore requir-ing less space and facilitating handling of the monitor. Froman ecological point of view operation of an LCD is moreadvantageous than that of a CRT. Owing to lower powerconsumption, LCDs emit less heat, therefore causing lessproblems in air conditioning at offices where many displaysrun at the same time. Low power consumption also givesLCD an advantage over CRT with regard to potential ofelectromagnetic radiation for causing possible effects onbiological matter.

Notebook PCs are very popular during travel or at any job

requiring a high frequency of changes in location of thework place as is the case with jobs like those of a represen-tative or a salesman. Minimal power consumption and lightweight is a must for displays used in notebooks. Actuallythere are only very few alternatives to LCD for displays foruse in notebooks. Data compiled by Nelson [4] and byCaladis [1] demonstrate that market volume of flat paneldisplays is predominantly controlled by LCD technologies.

As a result of technical progress, physical-optical qualityof LCD has immensly improved (Tannas [5]). Amongothers, backlight techniques, thin film super twisted LCDand new materials enable a better visibility of informationdisplayed on an LCD and make requirements for ambientlight of VDU less critical. In contrast to CRTs, LCDs havesharp edged pixels being therefore more suited to producesharp edged horizontal and vertical lines. Moreover, pixelsof LCD are not subject to spatial instabilities such as jitter.Uncontrolled external electromagnetic radiation may inducejitter at a CRT reducing legibility of characters displayed.Visibility of flicker may be less at LCD because of a morefavorable time-course of luminance of single pixels. MostCRT displays are equipped with phosphors with a shortpersistence-time. Light emitted by pixels of CRT can becompared to series of single flashes causing the perceptionof flicker phenomenon which is especially pronounced atlarge screens with a low refresh rate (Farrell [6]). Incontrast, shape of time-course of luminance of a single

Displays 20 (1999) 3–10

0141-9382/99/$ - see front matterq 1999 Elsevier Science B.V. All rights reserved.PII: S0141-9382(98)00051-1

* Corresponding author. Tel.:1 41 1 632 39 81; Fax:1 41 1 632 11 73.

pixel at LCD is square wave-like causing less or no percep-tion of flicker. Flickering light is supposed to disturb controlof eye movements (Neary [7]), possibly a cause for visualcomplaints. Drawbacks of use of LCD are, among others,reduced brightness and restricted viewing angle (Nelson[4]).

VDUs are often used in the presence of ambient light.Ambient light is reflected on the front of the screen therebyreducing contrast of information displayed. Contrast reduc-tion can be controlled by measures applied on the surface ofthe screen. With regards to material used, glass in CRT andorganic material in LCD, LCD may have an advantage overCRT displays when used in bright ambient light. To ourknowledge, reflectivity of LCD has not yet been assessedand compared to reflectivity of CRT. Activation of pixels ofLCD might change reflectivity of the display. To account forthis possibility, reflectivity should also be assessed whileinformation is presented on the display.

Insufficient optical quality of displays is a potential candi-date for causing visual complaints (Cole [8], Jackson [9],Krueger [10], Laubli [11]). The domain of visual complaintsis complex. Inter-individual variations in physiological andpsychological predispositions of VDU users are probablyamongst the most confounding factors in localization ofcauses of the complaints. An accurate assessment of opticalquality of displays may help in identification of causes forcomplaints. In accordance with the concept of strain andstress (Hacker [12]) we may deduce that complaints areclosely related to visual performance. Good optical qualityof a display constitutes a low visual strain and facilitatesreading (Grisham [13], Legge [14]). Scientific literatureoffers some papers on optical quality of VDU and visualperformance (Edwards [15], Farrell [6], Montegut [16],Roufs [17]). MacKenzie et al. [18] compared user perfor-mance in manipulative tasks carried out using CRT or LCD.In their experiments subjects had to select targets on thedisplay by using a mouse. Time between successive buttondown actions of the mouse were recorded and defined asmovement time. MacKenzie et al. [18] found that LCD gaverise to 34% longer movement times than did CRT. Saito etal. [19] recorded several visual functions during differenttasks carried out using different displays, such as LCD,CRT and plasma displays. Visual accommodation wasfound to be faster while using CRT when compared totasks where LCD or plasma display was used.

Based on actual results reported in the literature it isdifficult to draw a conclusion on the suitability of a parti-cular display technology to improve viewing conditions. Wetherefore set up an experiment by means of which visualperformance was assessed using either LCD or CRT atotherwise identical conditions.

There are many parameters contributing to an overallvisual performance. As a first attempt in investigating suit-ability of mentioned techniques we were interested in para-meters related to visual performance at a common task at aVDU. If office work is considered to be a common task at

VDU, search tasks may constitute a good starting point. Weinvestigated visual performance by assessing time forcompleting a search task and the amount of errors occurringduring a search task. Suitability of a display for use in officework is controlled by factors more than only visual perfor-mance. Field studies constitute a method accounting for alarge quantity of factors. However, a large number ofsubjects must be observed over a long period of time inorder to be able to cancel out individual interferences.Long-term observations in the field lack from constancyof environmental variables confounding possible effects.Mentioned limitations can partially be overcome, if suitabil-ity is assessed subjectively. Users of VDUs may judge suit-ability considering a multitude of factors and their impact onstress at long-term exposure. In this study we undertook anattempt to rank suitability of the two displays by interview-ing VDU users.

2. Method

2.1. Procedure

Performance was evaluated by means of a search task inwhich we recorded reaction times for detecting targets andamount of errors which occurred during the task. The para-digm used in our task was adapted from paradigmata used toassess visual performance in human factors in lighting (seeBoyce [20]) and from paradigmata used in basic visualscience (e. g. Fiorentini [21], Treisman [22]). Adaptationsaimed to consider particular conditions of office work. Atwo-alternative forced-choice task was set up using anuppercase letter ‘‘F’’ as target and uppercase letters ‘‘E’’as distractors. Target and distractors were arranged in a 20× 20 matrix with equally spaced horizontal and verticalgaps. Target was shown in 50% of the displays. The taskconsisted in scanning the display and pressing either the‘‘yes’’ or the ‘‘no’’ button of an answer box, dependingon whether target was seen or not. Subjects were informedafter each trial on the correctness of the given answer. Anacoustic feedback was used for this purpose. The subjectswere asked to accomplish the task within a minimum timeavoiding errors.

In order to account for the fact that ambient light mayvary depending on location of workplace, experiments werecarried out at two levels of ambient light. In one condition,further on referred to as darkness, horizontal and verticalillumination was about 50 lux. In the other condition,further on referred to as brightness, vertical illuminationwas 250 lux while horizontal illumination was set to550 lux. Only diffused light consisting out of indirectdaylight or artificial light reflected from the surroundingwalls was used to install described levels of ambient light.Each subject completed the task at both displays and on bothconditions ofambient light.The fourdifferentsettings, i. e. twodisplays used at two different conditions of illumination, were

M. Menozzi et al. / Displays 20 (1999) 3–104

administered in random order. The experimental sessionconsisted out of four blocks of 40 trials each. A trainingcourse comprising one block of 15 trials each was carriedout in brightness at each of the two displays before theexperimental session. Data obtained from trial sessionwere discarded. At the end of the experiment, subjectswere asked to rate subjectively how much they wouldappreciate completing office work in the four experimentalconditions.

Subjects were informed on aim and procedure of theexperiment. Before starting the experiment, subjects gavetheir written consent for participating in the experiment andfor using their data for scientific evaluation.

2.2. Instrumentation

A program was developed and run on a notebook(Toshiba Satellite 100CS). This hardware and softwareenabled to display a matrix (20× 20) consisting of targetand distractors. The program enabled control of the percen-tage of displays containing a target and to present displayswith and without targets in random order within a sequenceof 40 displays. As mentioned earlier probability for targetwas set to 50%. The location of the target was selectedrandomly to be one of the 400 places of the 20× 20 matrix.Reaction time and answer given by the subjects wererecorded using an answer box (Von Buol, submitted [23])which was connected to the notebook. The answer box wasequipped with a quartz driven timer. The timer was startedby a photodiode connected to the display on which thematrix was presented. This technique enabled synchroniza-tion of the start of the timer with the switching on and off ofpixels within a particular area of the screen, avoiding there-fore errors caused by buffering of data occurring in highlevel programming languages and high level operatingsystems. The timer was stopped by the subjects by pressingeither the ‘‘yes’’ or the ‘‘no’’ button on the answer boxdepending on whether they detected a target or not. Anew display was presented three seconds after the answerhad been recorded. During these three seconds a numberindicating the number of trials completed was displayed inthe center of the display. The number served as a target forfixation for the subjects during the intermediate periodbetween the displays.

The display of the notebook was used as LCD in onecondition, further on referred as LCD-task. The LCD wasa backlight dual scan STN 10.400 color display with a reso-lution of 640 pixels × 480 pixels (horizontal× vertical).The distance between pixels was 0.3 mm in horizontal andin vertical direction. Referring to the manual of the displaythe visible area of the display was 217.2 mm× 164.4 mm.A 1400 CRT (Dell model D1428-HS, shadow mask colordisplay, no surface treatment) was connected to the externaldisplay connector of the notebook and used in the taskwhere the matrix was displayed on a CRT. This task isfurther on referred as CRT-task. The dot pitch of the CRT

was 0.28 mm. The CRT display was run at a resolution of800 pixels × 600 pixels at 60 Hz frame rate.

Black distractors and targets were generated on a whitebackground. The luminance of the background was set at57 cd/m2 at both displays, CRT and LCD. In order to avoidchanges in light adaptation, background of the display wasalso set to 57 cd/m2 during the intermediate period. A largeblack area served to estimate luminance of the black char-acters. Luminance measured within this black field was1.4 cd/m2 when measured with no ambient light, i. e., indarkness. In brightness luminance on the screen is increasedbecause of reflections on the screen.

Reflection properties of the two displays were assessed inaccordance with ISO 9241-7 (1998) [24] procedure. Bymeans of this procedure diffuse (RD) and specular (Rs)reflectance properties of a display were evaluated. Specularreflectance is evaluated using an extended light-sourcesubtending 158 (Rs(EXT)) and a small light-source subtend-ing aproximately 0.98 (Rs(SML)). In general, reflectances ofLCD used in our experiment were much smaller than theones of CRT. For LCD all three reflectances were found tobe less than 0.01 (RD � 0.007,Rs(EXT) , 0.001,Rs(SML), 0.001). For CRT values of reflectance wereRD � 0.035,Rs(EXT) � 0.037,Rs(SML) � 0.005. We can thereforeestimate that for CRT, surplus luminance owing to reflec-tion of ambient light is about 3 cd/m2 (specular reflection).In brightness, contrast is therefore reduced by about 10% ifexpressed in term of modulation (Michelson contrast).Contrast reduction based on diffuse reflections can beneglected in any of the experimental conditions. As illumi-nation was carried out using diffuse light, we expect reflec-tions not to be a factor interfering with visual task.

Viewing distance in LCD-task was set to 50 cm. Equalityof viewing angle of characters and matrix in LCD-task andin CRT-task was achieved by adjusting settings of CRTmonitor (magnification of image) and by adjusting viewingdistance in CRT-task. As a result of limitation in settings ofCRT, viewing distance in CRT-task had to be increased by10 cm compared to viewing distance at LCD-task in order toequalize the sizes of characters and matrix displayed inLCD-task. The size of the letter E subtended 20.60 ×26.80 (horizontal × vertical), roughly corresponding to a12 point size letter E viewed at a distance of 50 cm. Hori-zontal and vertical spacing of neighboring characters wereapproximately equal (about 0.68). The size of the matrix wasabout 10.78 × 12.88 (horizontal × vertical). We did not useany rest to fix viewing distance. Subjects were asked to keeptheir viewing distance fixed during the experiment.

Subjective ratings of suitability of each of the four experi-mental settings for office work was accomplished by askingthe subjects ‘‘How much would you like to work using thisscreen on this ambient light?’’ (German: ‘‘Wie gernewurden Sie an diesem Bildschirm bei diesen Beleuchtungs-bedingungen arbeiten?’’). Subjects answered by putting across on an interval scale of six steps. They were instructedto make the position of the cross coincide with their ratings.

M. Menozzi et al. / Displays 20 (1999) 3–10 5

The scale ranged from ‘‘not at all’’ (‘‘sehr ungerne’’) to‘‘very much (‘‘sehr gerne’’). Subjects were also asked tocomment their decision by shorthand notes.

All the measurements were carried out within two days.Measurements lasted 35–80 min (median less than 50 min)including completion of formalities.

2.3. Subjects

A total of 10 subjects, 5 males and 5 females, participatedin this study. All subjects had a good visual acuity of at least1.0 for near (33 cm, Landolt rings) and 1.25 for far. Subjectsage ranged from 26 to 34 years. VDU work constituted animportant part of the daily work of our subjects. Samesubjects were used to assess suitability subjectively.

2.4. Data analysis

First, reaction time and error frequency were analyzed inorder to determine whether effects of learning were presentin the data. For this purpose an analysis of variance(ANOVA) and Pearson chi-square tests were used.

Second, we investigated whether errors were equallydistributed within the matrix displayed or whether theyoccurred more frequent in a specific direction of gaze, there-fore in a particular part of the matrix. For this purpose the 20× 20 matrix was subdivided into nine sectors (3× 3) eachdefining a sub-matrix of either 7× 7, 6 × 7 and 7 × 6(periphery) or 6 × 6 (central) characters. By means of aPearson chi-square test homogeneity of distribution of errorfrequency within this sectors was analyzed.

Third, the central question whether different technologiesfor displays and different ambient light give rise to varia-tions in visual performance was studied by applyingANOVA statistics to reaction times. On account of a depar-ture of the distribution of errors from normal distributionerror frequencies were processed by means of Wilcoxonrank test.

Fourth, subjective ratings on suitability of settings foroffice work were compared. For this purpose, position ofthe cross on the rating scale mentioned before wasconverted to scores and fed to a Wilcoxon rank test.Keywords were assigned to shorthand comments made bythe subjects.

3. Results

A total of 1600 displays were presented (four sets of 40trials, 10 subjects), 800 with and 800 without target. Anincorrect answer was given at 246 trials, i.e., at each seventhdisplay or at about 15% of the displays shown. Nearly allincorrect answers, i.e., 241, resulted from missed detectionof target. About 30% of the targets shown were missed. Intrials where target was present, a correct answer was givenafter 3.28 ^ 2.20 s (mean^ standard deviation over allsubjects and all four experimental settings) after onset of the

display. Correct detection of target occurred in 559 trials. Inconditions where displays did not contain the target, acorrect answer was given 4.14 2.74 s after onset ofthe display. A correct answer was given at nearly all (795)of the 800 displays without targets.

In order to assess learning effects, blocks were numberedaccording to the order they were administered. We will referto this number further on as block number. An ANOVA wasrun with subject and block number as factors and reactiontime as independent variable. The results show that betweensubjects variations are significant (F(9) � 15.17, p ,0.001). The same is true for the factor block number (F(3)� 3.87,p , 0.01), indicating that reaction time increasessignificantly with block number. However, the model usedin ANOVA turned out to explain only about 14% of varia-tions in reaction time. If means for reaction times over 40trials were used instead of single reaction times in ANOVA,learning effects disappear (F(3) � 0.953,p . 0.4). Errorfrequency, defined as the number of errors within one blockof 40 trials, were calculated for each subject and each blockseparately. Pearson chi-square test applied on a two waytable listing error frequency within each block and foreach subject indicates no (x 2(3, 9) � 28.03,p . 0.4)systematic effect of subject or block number on errorfrequency. Therefore error frequency are not subject tolearning effects.

In order to investigate whether errors preferably occurredwithin a specific area of the display, error frequency of thenine sectors defined earlier on in the text, one central squaresector surrounded by eight square sectors, were normalized.This procedure consisted of dividing error frequency foreach sector with the number of targets presented withinthe same sector. Owing to low frequency, errors of allsubjects were pooled. Normalization enabled to accountfor different number of targets presented within a sector.Differences were because of unequal size of sectors aswell as the random selection of position of target. Pearsonchi-square test indicates that frequency of errors did notvary (x 2(2, 2) � 2.66,p . 0.6) among sectors.

An ANOVA was used to determine the variation of reac-tion time with experimental conditions, i.e., the displaytechnology and the ambient light. Multivariate ANOVAanalysis included factors such as block number, settings(CRT-task, LCD-task, darkness, brightness), subject,presence of target (yes, no) and error of target detection.In order to account for individual preferences for settings,interaction between settings and subjects was also consid-ered in statistics. Reaction time was found to be independentof block number (F(2) � 1.15,p . 0.3) and of settings(F(1) � 1.24,p . 0.2). Reaction time depends on subject(F(9) � 14.72,p , 0.001), on presence of target (F(1) �36.6,p , 0.001), on error of detection (F(1) � 19.5,p ,0.001) as well as on the interaction between subject andsettings (F(26) � 4.39, p , 0.001). We may supposethat failure to demonstrate any effects of settings on reactiontime may be a result of the strong effect of the factors

M. Menozzi et al. / Displays 20 (1999) 3–106

presence of target, error detection of target and their inter-action. We therefore carried out post hoc ANOVA on areduced data set in which only trials with targets wereconsidered. Table 1 shows mean values and standard devia-tions for reaction time as well as error frequency assessedusing the reduced data set.

Reaction times assessed for displays without target arenot shown. First two rows list mean and standard deviationof reaction time for trials in which the target was detected(first row) and in which the target was missed (second row).None of the difference in reaction time were significantwithin each of the first two rows. If an ANOVA is runconsidering reaction time of all trials at which target waspresent, independent of whether target was detected ormissed, reaction time turns out to depend significantly onsettings (F(3) � 2.81,p , 0.05). A Bonferroni adjustedmatrix of pairwise comparison of data of the third rowreveals that in dark ambient light a significant (p ,0.05) longer reaction time results for CRT-task (3.93 s)when compared to LCD-task (3.25 s) at same ambient light.

As only few errors (5 out of 800 possible) were done atdisplays without target, trials without target were not takeninto account in the analysis on error frequency. Settingswere found to exhibit a significant influence on error

frequency. As can be seen from Table 1, median errorfrequency varied from 7 errors per block (40 trials) in theCRT-task in brightness to 4 errors per block in the LCD-taskfor same conditions of ambient light level. Significance ofdifference in error frequency between the four settings islisted in Table 2.

In general, error frequency at LCD-tasks was signifi-cantly lower when compared to error frequency at CRT-task. In brightness error frequency in CRT-task was foundto be significantly higher (p , 0.02) than error frequency atLCD-task. The same difference applies for darkness (p ,0.01). Error frequency at CRT-task on brightness are signif-icant (p , 0.01) higher than the ones assessed at LCD-taskin darkness. In accordance with Table 2 error frequencieswithin display type do not differ significantly. Errors occur-ring at the same display type can therefore be pooled. If totalerrors of CRT-task are pooled (145) and compared to pooledtotal errors of LCD-task (96), we can conclude that at LCD-task the total amount of errors is 34% less than it is at CRT-task. A pairedt-test reveals that difference in pooled data issignificant at a level ofp , 0.001 (t(9) � 4.72) for twosided probability (p , 0.005 if non-parametric Wilcoxonrank test is used).

Distribution of scores for suitability of settings for officework are plotted in Fig. 1. One subject did not answer thetwo questions on suitability of settings of LCD-task.

A Wilcoxon rank test failed (p . 0.1) to showany significant differences in scores given to the fourdifferent settings. A comparison of pooled dataassessed at CRT-tasks with pooled data assessed atLCD-tasks also fails to show significant effects(Wilcoxon rank,p . 0.1; two-tailedt-test, p . 0.1).Shorthand notes used to comment on given scoreswere mostly of negative nature. Table 3 summarizesnegative remarks which were mentioned at least twice.Interestingly two subjects found fault with sharp char-acters of LCD, a characteristic which usually denoteshigh quality. Two subjects contested inhomogeneousluminance of LCD.

M. Menozzi et al. / Displays 20 (1999) 3–10 7

Table 1Reaction time (RT) and error frequency. Experimental settings: CRT-task, LCD-task, darkness or brightness. First to third row show mean and standarddeviation for reaction time in seconds. Only differences in third row are statistically significant. Bonferroni adjusted Student’st-test shows significantdifference of mean of CRT-task and LCD-task for dark ambient light. Fourth row denotes median and quartile of errors for each setting. Total number oferrors is reported in fifth row. Median and quartiles of pooled error frequency for CRT-task and for LCD-task are shown in the sixth row (Standard deviationsappear in parantheses)

CRT-task LCD-taskBrightness Darkness Brightness Darkness

RT mean (s) for detecting target 3.02 (1.88) 3.48 (2.30) 3.52 (2.47) 2.91 (2.03)RT mean (s) for missing target 4.11 (2.69) 4.70 (2.91) 4.07 (3.00) 4.18 (2.87)RT mean (s) when target present 3.52 (2.23) 3.93 (2.60) 3.64 (2.60) 3.25 (2.34)Error frequency (median[quartiles])

7 [4;9] 6.5 [5;10] 4 [3.5;7.5] 5 [4;6]

Total errors 71 74 43 53Error frequency of pooled data(median [quartiles])

12.5 [9;19] 8.5 [6;11]

Table 2Significance of difference in error frequency. Each entry denotesp value fordifference in error frequency between two settings. Probabilities listed infirst four lines were calculated using Wilcoxon rank test whereas probabil-ity listed in the last line of the table refers to two tailedt-test statistics forpaired samples

CRT-task LCD-taskBright Dark Bright Dark

CRT-task Bright 1Dark .587 1

LCD-task Bright .015 .070 1Dark .007 .005 .147 1

CRT versus LCD task (pooled) 0.0011

4. Discussion

Reaction time was found to depend on learning effect.Apparently, two training sessions before the experimentwere not enough to eliminate the effect. Learning effectdisappeared after reducing amount of data. Therefore, learn-ing effect can be supposed to be weak and will probably notinterfere with conclusions reported at the end of this article.Although time needed to search the display is reduced bylearning, error frequency is found to be constant throughoutthe experiment. We might therefore postulate that accuracyof detection could not be improved whereas speed ofprocessing result of detection was improved by training.In our experiment speed of processing depends on motoras well as on mental skills, both of which can be improvedby training. Given these circumstances we may concludethat optimal viewing conditions are imperative for ergo-nomics if accuracy is a significant requirement in a taskbecause no training can make up for poor viewing condi-tions in order to improve accuracy of vision.

Among reasons for involuntary missing a target in ourtask are, low visibility of the target and an insufficient atten-tion. Visibility may be biased by reflections and by inhomo-geneous emission characteristics of the displays. As wasshown above, it is unlikely that reflections interfere withvisibility of target. Subjective estimation of reflections(Table 3) deviates slightly from this assumption. However,deviations are small. Probably some subjects rated

possibility of specular reflections of displays based ontheir personal experience rather than visible reflectionspresent during the experiment.

Attention is influenced by behavior, and its focus mustnot necessarily be at the location of the fixation point (Treis-man [22]). Experiments reported by Posner [27] showed aredirection of focus of attention towards locations in which apre-attentive stimulus was shown. We may postulate thatfocus of attention can be dislocated by manipulative tasksas is the case with spatial orientation. Shebilske [25] demon-strated that a task requiring downward gaze distorted spatialorientation, e.g., that of baseball batters. As there are manytasks in office work requiring downward gaze and down-wards focused attention, we could assume a drift of attentiontowards lower half of visual fields. However, errorfrequency was found to be independent of location of targeton display. If it is supposed that variations of visibility didnot cancel out effects of varying attention, we may state thatattention does not vary with direction of gaze or orientationin space in a critical manner. We may therefore concludethat position of information to be displayed on the screenmust not be determined in consideration with relevance ofthe information to be displayed. This conclusion seems to besomewhat in contrast to actual recommendations made instyle guides (e.g. DIN 66234 [26]).

Differences between error rates at CRT-task and at LCD-task are considerable. Total amount of errors assessed atLCD-task was 34% lower than amount of errors assessedat CRT-task. As mentioned in the introduction several tech-nical factors may be responsible for causing differences inaccuracy of detection of target such as sharpness of pixelsand time characteristics of luminance. Given the lowcomplexity of presented distractors and targets we suggestspatial characteristics to be of minor importance in deter-mining visual performance. Time course of luminancemight have played a decisive role in vision so as to giverise to more convenient viewing condition at LCD-task.However, LCD based technology is not necessarily requiredto produce a time course of luminance needed for goodvisual accuracy. The same beneficial effects result byincreasing refresh rate or by installing phosphors withlong persistence in CRT. Increasing refresh rate has beenshown to facilitate reading (Montegut [16]). Technicalreasons limit refresh rate in screens with high spatial resolu-tion and with high depth of color representation. There aredrawbacks in the use of phosphors with long persistence.First, effects like smearing while scrolling and transientghost images while changing mask reduce legibility ofdisplayed information at CRT equipped with long-persis-tence phosphor. Second, there are no triplets of long-persis-tence phosphors enabling to generate a color space ofsimilar size as can be done if triplets of short-persistencephosphors are combined.

Our experiment revealed a weak dependency of reactiontime on settings or on technology of display used. Failure todemonstrate strong effects might be because of low

M. Menozzi et al. / Displays 20 (1999) 3–108

Fig. 1. Box plot of scores. Scores (median, quartiles, extreme values) wereassessed by means of the question ‘‘How much would you like to workusing this screen in this ambient light?’’ using a scale of six intervals. *�outlier.

Table 3Shorthand remarks on suitability of settings for office work (summary)

CRT LCDBright Dark Bright Dark

Size too small 3 3 5 5Flicker 5 6 0 0Blur 5 4 0 0Reflections 2 1 0 0Inhomogeneous luminance 0 0 2 2

Total 14 15 7 7

sensitivity of our experimental procedure. As mentionedbefore, it is possible that mechanisms other than visionsuch as motor behavior, played a more significant role indetermining reaction time and could have cancelled out anyeffect on vision. Efficiency of target detection should beassessed without manipulatory tasks and without require-ments in time needed for highly cognitive work for decisionmaking. We propose to assess detection performance bymeans of an experiment in which target is displayed tachis-toscopically. Visual performance shall thereby be assessedin terms of percentage of correct detection of target as afunction of duration of presentation of target. By usingthis approach it has been shown elsewhere (Fiorentini[21]) that important changes in correctness of detectionare found if duration of visibility of display is changedfrom 100 to 200 ms.

No subjective preference of either display technology orsettings could be demonstrated statistically even thoughfrequency distribution of subjective ratings reported inFig. 1 suggests a possible advantage of LCD technologyfor office work. We conclude that the number of subjectsused to carry out the rating is too small to demonstratesignificant effects. Summary of shorthand notes on scoringsuitability of settings (Table 3) also indicate a possibleadvantage of LCD over CRT. Subjects reported only fewdrawbacks of LCD. Too small size of the screen as well asan inhomogeneous luminance were complained at LCD.The former complaint can be accommodated by usingdisplays with an adequate size. Screen size plays a role insuitability of a display for office work. However, screen sizewas not subject of our investigation. Findings made here areindependent of screen size. Latter factor might have exhib-ited some effect on our results, i.e., that errors might appearmore frequent in specific areas within LCD. As we did notrecord variation of luminance across display area we are notable to estimate influence of inhomogeneity of luminanceon error frequency.

5. Conclusion

The fact that the use of LCD improves accuracy in detect-ing targets and might also improve time for visual detectionof the target is an advantage of LCD over CRT. It is there-fore expected that LCD will give rise to lower visual strainand therefore cause less visual complaints as CRT will do.Given the broad variety in technology of LCD available onthe market we may expect that there may be even furtherimprovements in visual performance if other LCD are used.

Unfortunately subjective ratings did not reflect findingsmade on visual performance as was demonstrated by Roufs[17] and Edwards [15].

Acknowledgements

We thank the following people for supporting us in this

experiment. The computer program used for the search taskwas written by Ara Hagopian. Andreas Hoffmann adaptedthe program to the needs of the notebook. Andreas von Buolprovided electronics for precise assessment of reaction time.A special thank is addressed to our subjects who kindlyparticipated at the experiment without being paid. Last butnot least we thank our reviewers for fruitful comments.

References

[1] Patricia E. Cladis, World view of liquid crystal flat panel displays, in:L.E. Tannas, E. Glenn, J.W. Dorne (Eds.), Flat-panel Display Tech-nologies, Noyes Publications, Parkridge, NJ, 1995.

[2] A.H. Firester, Active matrix technology, in: L.E. Tannas, E. Glenn,J.W. Dorne (Eds.), Flat-panel Display Technologies, Noyes Publica-tions, Parkridge, NJ, 1995.

[3] DIN 4549 Buromobel, Schreibtische, Bu¨romaschinentische undBildschirmtische. Masse. Deutsches Institut fu¨r Normung e. V.,(Office furniture, desks, desks for office machines and desks forVDUs.) German Standards Institute, November, 1982.

[4] T.J. Nelson, J.R. Wullert II, Electronic Information Display Technol-ogies, World Scientific, Singapore, 1997.

[5] L.E. Tannas, E. Glenn, J.W. Dorne (Eds.), Flat-panel Display Tech-nologies Noyes Publications, Parkridge, NJ, 1995.

[6] E. Farrell-Joyce, Fitting physical screen parameters to the human eye,in: J.A.J. Roufs (Ed.), Vision and Visual Dysfunction. The Man–Machine Interface, Mocmillan Press, Hondmills, 1991.

[7] Catherine Neary, A.J. Wilkins, Effects of phosphor on perception andthe control of eye movements, Perception 18 (1995) 257–264.

[8] B.L. Cole, Jennifer D. Maddocks, Effects of VDUs on the eyes:Report of a 6-year epidemiological study, Optometry and VisionScience 73 (8) (1996) 512–528.

[9] A.J. Jackson, E.S. Barnett, A.B. Stevens, M. McClure, C. Patterson,M.J. McReynolds, Vision screening, eye examination and risk assess-ment of display screen users in a large regional teaching hospital,Ophthalmics and Physiological Optics 17 (3) (1997) 187–195.

[10] H. Krueger, Visual function and monitor use, in: J.A.J. Roufs (Ed.),Vision and Visual Dysfunction. The Man–Machine Interface,Mocmillan Press, Hondmills, 1991.

[11] T. Laubli, W. Hunting, E. Grandjean, Postural and visual loads atVDT workplaces. II: Lighting conditions and visual impairments,Ergonomics 24 (1981) 933–944.

[12] W. Hacker, Allgemeine Arbeits- und Ingenieurpsychologie (GeneralWork- and Engineering-Psychology), 2, Huber, Bern, 1978.

[13] J.D. Grisham, Melissa M. Sheppard, Wendy U. Tran, Visual symp-toms and reading performance, Optometry and Vision Science 70 (5)(1993) 384–391.

[14] G.E. Legge, G.S. Rubin, A. Luebker, Psychophysics of reading V:The role of contrast in normal vision, Vision Research 27 (1987)1165–1177.

[15] C.B. Edwards, V.J. Demczuk, Legibility of visual display terminalsand an investigation of subjective image quality ratings with respectto physical image quality measures, in: H. Luczak, A. Cakir, G. Cakir(Eds.), Work with Display Units 92, Elsevier Science Publishers,Amsterdam, 1993.

[16] M.J. Montegut, B. Bridgeman, J. Sykes, High refresh rate and oculo-motor adaptation facilitate reading from video displays, SpatialVision 10 (4) (1997) 305–322.

[17] J.A.J. Roufs, M.C. Boschman, Visual comfort and performance, in:J.A.J. Roufs (Ed.), Vision and Visual Dysfunction. The Man–Machine Interface, Mocmillan Press, Hondmills, 1991.

[18] I.S. MacKenzie, S. Riddersma, Effects of output display and control-display gain on human performance in interactive systems, Behaviourand Information Technology 13 (5) (1994) 328–337.

M. Menozzi et al. / Displays 20 (1999) 3–10 9

[19] S. Saito, S. Taptagaporn, G. Salvendy, Visual comfort using differentVDT screens, International Journal of Human–Computer Interaction5 (4) (1993) 313–323.

[20] P.R. Boyce, Human Factors in Lighting, Applied Science Publishers,London, 1981.

[21] Adriana Fiorentini, Differences between fovea and parafovea in visualsearch, Vision Research 29 (9) (1989) 1153–1164.

[22] Anne Treisman, P. Cavanagh, B. Fischer, V.S. Ramachandran, R. vonder Heydt, Form perception and attention. Striate cortex and beyond,in: L. Spillmann, J.S. Werner (Eds.), Visual Perception: The Neuro-physiological Foundations, Academic Press, San Diego, CA, 1990.

[23] A. Von Buol, The measurement of reaction time by computers. Asimple hardware solution to provide millisecond resolution time.(Submitted)

[24] ISO 9241 - 7 (1998): Ergonomic requirements for office work withvisual display terminal (VDTs) – Part 7: Requirements for displaywith reflections. Gene`ve, Switzerland, 1998.

[25] W.L. Shebilske, Baseball batters support an ecological efferencemediation theory of natural event perception, Acta Psychologica 63(1986) 117–131.

[26] DIN 66234. Bildschirmarbeitspla¨tze. Gruppierung von Informationund Daten. Hinweise und Beispiele. Beiblatt 1 zu DIN 66234, Teil3. Deutsches Institut fu¨r Normung e. V., Ma¨rz 1983. (Workplaceswith VDUs. Grouping of Information and Data. Remarks and Exam-ples. German Standards Institute, March 1983).

[27] M.I. Posner, Chronometric Explorations of Mind, Lawrence ErlbaumAssociates, Hillsdale, NJ, 1978.

M. Menozzi et al. / Displays 20 (1999) 3–1010