Displays 20 (1999) 3–10 CRT versus LCD: A pilot study on visual performance and suitability of two display technologies for use in office work Marino Menozzi*, Urs Näpflin, Helmut Krueger Institute of Hygiene and Applied Physiology, Swiss Federal Institute of Technology, Clausiusstrasse 25, CH - 8092 Zürich, 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 visual performance 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 were significantly smaller when compared to error frequency for tasks at CRT. LCD gave rise to 34% less errors than did CRT. Reaction time in search task was found to be significantly shorter using LCD when tasks were carried out in darkness. Subjective rated suitability of LCD was scored twice as high as suitability of CRT. Results indicate that LCD used in this experiment may give better viewing conditions in comparison to CRT display. 䉷 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 and more popular in visual display units (VDUs) (Cladis , Firester ). Size of actual LCD can cover the needs of most applications running on computers. For many reasons LCD might become more important and might replace cathode ray tube (CRT) displays in many applications. Weight and volume of LCD are among most the important advantages when compared to CRT VDU. A 17 00 CRT typically occupies an area of 40 cm × 45 cm (width × depth). If according to suggested settings for work place (e.g. DIN 4549 ) a desk of 120 cm × 80 cm is considered, a 17 00 CRT may occupy about one-fifth of the surface of the desk. Dimensions of LCD monitors are smaller, therefore requiring less space and facilitating handling of the monitor. From an ecological point of view operation of an LCD is more advantageous than that of a CRT. Owing to lower power consumption, LCDs emit less heat, therefore causing less problems in air conditioning at offices where many displays run at the same time. Low power consumption also gives LCD an advantage over CRT with regard to potential of electromagnetic radiation for causing possible effects on biological matter. Notebook PCs are very popular during travel or at any job * Corresponding author. Tel.: ⫹ 41 1 632 39 81; Fax: ⫹ 41 1 632 11 73. requiring a high frequency of changes in location of the work place as is the case with jobs like those of a representative or a salesman. Minimal power consumption and light weight is a must for displays used in notebooks. Actually there are only very few alternatives to LCD for displays for use in notebooks. Data compiled by Nelson  and by Caladis  demonstrate that market volume of flat panel displays is predominantly controlled by LCD technologies. As a result of technical progress, physical-optical quality of LCD has immensly improved (Tannas ). Among others, backlight techniques, thin film super twisted LCD and new materials enable a better visibility of information displayed on an LCD and make requirements for ambient light of VDU less critical. In contrast to CRTs, LCDs have sharp edged pixels being therefore more suited to produce sharp edged horizontal and vertical lines. Moreover, pixels of LCD are not subject to spatial instabilities such as jitter. Uncontrolled external electromagnetic radiation may induce jitter at a CRT reducing legibility of characters displayed. Visibility of flicker may be less at LCD because of a more favorable time-course of luminance of single pixels. Most CRT displays are equipped with phosphors with a short persistence-time. Light emitted by pixels of CRT can be compared to series of single flashes causing the perception of flicker phenomenon which is especially pronounced at large screens with a low refresh rate (Farrell ). In contrast, shape of time-course of luminance of a single 0141-9382/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S0141-938 2(98)00051-1 4 M. Menozzi et al. / Displays 20 (1999) 3–10 pixel at LCD is square wave-like causing less or no perception of flicker. Flickering light is supposed to disturb control of eye movements (Neary ), possibly a cause for visual complaints. Drawbacks of use of LCD are, among others, reduced brightness and restricted viewing angle (Nelson ). VDUs are often used in the presence of ambient light. Ambient light is reflected on the front of the screen thereby reducing contrast of information displayed. Contrast reduction can be controlled by measures applied on the surface of the screen. With regards to material used, glass in CRT and organic material in LCD, LCD may have an advantage over CRT displays when used in bright ambient light. To our knowledge, reflectivity of LCD has not yet been assessed and compared to reflectivity of CRT. Activation of pixels of LCD might change reflectivity of the display. To account for this possibility, reflectivity should also be assessed while information is presented on the display. Insufficient optical quality of displays is a potential candidate for causing visual complaints (Cole , Jackson , Krueger , Läubli ). The domain of visual complaints is complex. Inter-individual variations in physiological and psychological predispositions of VDU users are probably amongst the most confounding factors in localization of causes of the complaints. An accurate assessment of optical quality of displays may help in identification of causes for complaints. In accordance with the concept of strain and stress (Hacker ) we may deduce that complaints are closely related to visual performance. Good optical quality of a display constitutes a low visual strain and facilitates reading (Grisham , Legge ). Scientific literature offers some papers on optical quality of VDU and visual performance (Edwards , Farrell , Montegut , Roufs ). MacKenzie et al.  compared user performance in manipulative tasks carried out using CRT or LCD. In their experiments subjects had to select targets on the display by using a mouse. Time between successive button down actions of the mouse were recorded and defined as movement time. MacKenzie et al.  found that LCD gave rise to 34% longer movement times than did CRT. Saito et al.  recorded several visual functions during different tasks carried out using different displays, such as LCD, CRT and plasma displays. Visual accommodation was found to be faster while using CRT when compared to tasks where LCD or plasma display was used. Based on actual results reported in the literature it is difficult to draw a conclusion on the suitability of a particular display technology to improve viewing conditions. We therefore set up an experiment by means of which visual performance was assessed using either LCD or CRT at otherwise identical conditions. There are many parameters contributing to an overall visual performance. As a first attempt in investigating suitability of mentioned techniques we were interested in parameters related to visual performance at a common task at a VDU. If office work is considered to be a common task at VDU, search tasks may constitute a good starting point. We investigated visual performance by assessing time for completing a search task and the amount of errors occurring during a search task. Suitability of a display for use in office work is controlled by factors more than only visual performance. Field studies constitute a method accounting for a large quantity of factors. However, a large number of subjects must be observed over a long period of time in order to be able to cancel out individual interferences. Long-term observations in the field lack from constancy of environmental variables confounding possible effects. Mentioned limitations can partially be overcome, if suitability is assessed subjectively. Users of VDUs may judge suitability considering a multitude of factors and their impact on stress at long-term exposure. In this study we undertook an attempt to rank suitability of the two displays by interviewing VDU users. 2. Method 2.1. Procedure Performance was evaluated by means of a search task in which we recorded reaction times for detecting targets and amount of errors which occurred during the task. The paradigm used in our task was adapted from paradigmata used to assess visual performance in human factors in lighting (see Boyce ) and from paradigmata used in basic visual science (e. g. Fiorentini , Treisman ). Adaptations aimed to consider particular conditions of office work. A two-alternative forced-choice task was set up using an uppercase 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 vertical gaps. Target was shown in 50% of the displays. The task consisted in scanning the display and pressing either the ‘‘yes’’ or the ‘‘no’’ button of an answer box, depending on whether target was seen or not. Subjects were informed after each trial on the correctness of the given answer. An acoustic feedback was used for this purpose. The subjects were asked to accomplish the task within a minimum time avoiding errors. In order to account for the fact that ambient light may vary depending on location of workplace, experiments were carried out at two levels of ambient light. In one condition, further on referred to as darkness, horizontal and vertical illumination was about 50 lux. In the other condition, further on referred to as brightness, vertical illumination was 250 lux while horizontal illumination was set to 550 lux. Only diffused light consisting out of indirect daylight or artificial light reflected from the surrounding walls was used to install described levels of ambient light. Each subject completed the task at both displays and on both conditions of ambient light. The four different settings, i. e. two displays used at two different conditions of illumination, were M. Menozzi et al. / Displays 20 (1999) 3–10 administered in random order. The experimental session consisted out of four blocks of 40 trials each. A training course comprising one block of 15 trials each was carried out in brightness at each of the two displays before the experimental session. Data obtained from trial session were discarded. At the end of the experiment, subjects were asked to rate subjectively how much they would appreciate completing office work in the four experimental conditions. Subjects were informed on aim and procedure of the experiment. Before starting the experiment, subjects gave their written consent for participating in the experiment and for using their data for scientific evaluation. 2.2. Instrumentation A program was developed and run on a notebook (Toshiba Satellite 100CS). This hardware and software enabled to display a matrix (20 × 20) consisting of target and distractors. The program enabled control of the percentage of displays containing a target and to present displays with and without targets in random order within a sequence of 40 displays. As mentioned earlier probability for target was set to 50%. The location of the target was selected randomly to be one of the 400 places of the 20 × 20 matrix. Reaction time and answer given by the subjects were recorded using an answer box (Von Buol, submitted ) which was connected to the notebook. The answer box was equipped with a quartz driven timer. The timer was started by a photodiode connected to the display on which the matrix was presented. This technique enabled synchronization of the start of the timer with the switching on and off of pixels within a particular area of the screen, avoiding therefore errors caused by buffering of data occurring in high level programming languages and high level operating systems. The timer was stopped by the subjects by pressing either the ‘‘yes’’ or the ‘‘no’’ button on the answer box depending on whether they detected a target or not. A new display was presented three seconds after the answer had been recorded. During these three seconds a number indicating the number of trials completed was displayed in the center of the display. The number served as a target for fixation for the subjects during the intermediate period between the displays. The display of the notebook was used as LCD in one condition, further on referred as LCD-task. The LCD was a backlight dual scan STN 10.4 00 color display with a resolution of 640 pixels × 480 pixels (horizontal × vertical). The distance between pixels was 0.3 mm in horizontal and in vertical direction. Referring to the manual of the display the visible area of the display was 217.2 mm × 164.4 mm. A 14 00 CRT (Dell model D1428-HS, shadow mask color display, no surface treatment) was connected to the external display connector of the notebook and used in the task where the matrix was displayed on a CRT. This task is further on referred as CRT-task. The dot pitch of the CRT 5 was 0.28 mm. The CRT display was run at a resolution of 800 pixels × 600 pixels at 60 Hz frame rate. Black distractors and targets were generated on a white background. The luminance of the background was set at 57 cd/m 2 at both displays, CRT and LCD. In order to avoid changes in light adaptation, background of the display was also set to 57 cd/m 2 during the intermediate period. A large black area served to estimate luminance of the black characters. Luminance measured within this black field was 1.4 cd/m 2 when measured with no ambient light, i. e., in darkness. In brightness luminance on the screen is increased because of reflections on the screen. Reflection properties of the two displays were assessed in accordance with ISO 9241-7 (1998)  procedure. By means of this procedure diffuse (RD) and specular (Rs) reflectance properties of a display were evaluated. Specular reflectance is evaluated using an extended light-source subtending 15⬚ (Rs(EXT)) and a small light-source subtending aproximately 0.9⬚ (Rs(SML)). In general, reflectances of LCD used in our experiment were much smaller than the ones of CRT. For LCD all three reflectances were found to be less than 0.01 (RD 0.007, Rs(EXT) ⬍ 0.001, Rs(SML) ⬍ 0.001). For CRT values of reflectance were RD 0.035, Rs(EXT) 0.037, Rs(SML) 0.005. We can therefore estimate that for CRT, surplus luminance owing to reflection of ambient light is about 3 cd/m 2 (specular reflection). In brightness, contrast is therefore reduced by about 10% if expressed in term of modulation (Michelson contrast). Contrast reduction based on diffuse reflections can be neglected in any of the experimental conditions. As illumination was carried out using diffuse light, we expect reflections not to be a factor interfering with visual task. Viewing distance in LCD-task was set to 50 cm. Equality of viewing angle of characters and matrix in LCD-task and in CRT-task was achieved by adjusting settings of CRT monitor (magnification of image) and by adjusting viewing distance in CRT-task. As a result of limitation in settings of CRT, viewing distance in CRT-task had to be increased by 10 cm compared to viewing distance at LCD-task in order to equalize the sizes of characters and matrix displayed in LCD-task. The size of the letter E subtended 20.6 0 × 26.8 0 (horizontal × vertical), roughly corresponding to a 12 point size letter E viewed at a distance of 50 cm. Horizontal and vertical spacing of neighboring characters were approximately equal (about 0.6⬚). The size of the matrix was about 10.7⬚ × 12.8⬚ (horizontal × vertical). We did not use any rest to fix viewing distance. Subjects were asked to keep their viewing distance fixed during the experiment. Subjective ratings of suitability of each of the four experimental settings for office work was accomplished by asking the subjects ‘‘How much would you like to work using this screen on this ambient light?’’ (German: ‘‘Wie gerne würden Sie an diesem Bildschirm bei diesen Beleuchtungsbedingungen arbeiten?’’). Subjects answered by putting a cross on an interval scale of six steps. They were instructed to make the position of the cross coincide with their ratings. 6 M. Menozzi et al. / Displays 20 (1999) 3–10 The scale ranged from ‘‘not at all’’ (‘‘sehr ungerne’’) to ‘‘very much (‘‘sehr gerne’’). Subjects were also asked to comment 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, participated in this study. All subjects had a good visual acuity of at least 1.0 for near (33 cm, Landolt rings) and 1.25 for far. Subjects age ranged from 26 to 34 years. VDU work constituted an important part of the daily work of our subjects. Same subjects were used to assess suitability subjectively. 2.4. Data analysis First, reaction time and error frequency were analyzed in order to determine whether effects of learning were present in the data. For this purpose an analysis of variance (ANOVA) and Pearson chi-square tests were used. Second, we investigated whether errors were equally distributed within the matrix displayed or whether they occurred more frequent in a specific direction of gaze, therefore in a particular part of the matrix. For this purpose the 20 × 20 matrix was subdivided into nine sectors (3 × 3) each defining a sub-matrix of either 7 × 7, 6 × 7 and 7 × 6 (periphery) or 6 × 6 (central) characters. By means of a Pearson chi-square test homogeneity of distribution of error frequency within this sectors was analyzed. Third, the central question whether different technologies for displays and different ambient light give rise to variations in visual performance was studied by applying ANOVA statistics to reaction times. On account of a departure of the distribution of errors from normal distribution error frequencies were processed by means of Wilcoxon rank test. Fourth, subjective ratings on suitability of settings for office work were compared. For this purpose, position of the cross on the rating scale mentioned before was converted to scores and fed to a Wilcoxon rank test. Keywords were assigned to shorthand comments made by the subjects. 3. Results A total of 1600 displays were presented (four sets of 40 trials, 10 subjects), 800 with and 800 without target. An incorrect answer was given at 246 trials, i.e., at each seventh display or at about 15% of the displays shown. Nearly all incorrect answers, i.e., 241, resulted from missed detection of target. About 30% of the targets shown were missed. In trials where target was present, a correct answer was given after 3.28 ^ 2.20 s (mean ^ standard deviation over all subjects and all four experimental settings) after onset of the display. Correct detection of target occurred in 559 trials. In conditions where displays did not contain the target, a correct answer was given 4.14 ^ 2.74 s after onset of the display. A correct answer was given at nearly all (795) of the 800 displays without targets. In order to assess learning effects, blocks were numbered according to the order they were administered. We will refer to this number further on as block number. An ANOVA was run with subject and block number as factors and reaction time as independent variable. The results show that between subjects 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 increases significantly with block number. However, the model used in ANOVA turned out to explain only about 14% of variations in reaction time. If means for reaction times over 40 trials were used instead of single reaction times in ANOVA, learning effects disappear (F(3) 0.953, p ⬎ 0.4). Error frequency, defined as the number of errors within one block of 40 trials, were calculated for each subject and each block separately. Pearson chi-square test applied on a two way table listing error frequency within each block and for each subject indicates no (x 2(3, 9) 28.03, p ⬎ 0.4) systematic effect of subject or block number on error frequency. Therefore error frequency are not subject to learning effects. In order to investigate whether errors preferably occurred within a specific area of the display, error frequency of the nine sectors defined earlier on in the text, one central square sector surrounded by eight square sectors, were normalized. This procedure consisted of dividing error frequency for each sector with the number of targets presented within the same sector. Owing to low frequency, errors of all subjects were pooled. Normalization enabled to account for different number of targets presented within a sector. Differences were because of unequal size of sectors as well as the random selection of position of target. Pearson chi-square test indicates that frequency of errors did not vary (x 2(2, 2) 2.66, p ⬎ 0.6) among sectors. An ANOVA was used to determine the variation of reaction time with experimental conditions, i.e., the display technology and the ambient light. Multivariate ANOVA analysis 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 considered in statistics. Reaction time was found to be independent of 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 and settings (F(26) 4.39, p ⬍ 0.001). We may suppose that failure to demonstrate any effects of settings on reaction time may be a result of the strong effect of the factors M. Menozzi et al. / Displays 20 (1999) 3–10 7 Table 1 Reaction time (RT) and error frequency. Experimental settings: CRT-task, LCD-task, darkness or brightness. First to third row show mean and standard deviation for reaction time in seconds. Only differences in third row are statistically significant. Bonferroni adjusted Student’s t-test shows significant difference 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 of errors 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 deviations appear in parantheses) RT mean (s) for detecting target RT mean (s) for missing target RT mean (s) when target present Error frequency (median [quartiles]) Total errors Error frequency of pooled data (median [quartiles]) CRT-task Brightness Darkness LCD-task Brightness Darkness 3.02 (1.88) 4.11 (2.69) 3.52 (2.23) 7 [4;9] 3.48 (2.30) 4.70 (2.91) 3.93 (2.60) 6.5 [5;10] 3.52 (2.47) 4.07 (3.00) 3.64 (2.60) 4 [3.5;7.5] 2.91 (2.03) 4.18 (2.87) 3.25 (2.34) 5 [4;6] 71 12.5 [9;19] 74 43 8.5 [6;11] 53 presence of target, error detection of target and their interaction. We therefore carried out post hoc ANOVA on a reduced data set in which only trials with targets were considered. Table 1 shows mean values and standard deviations for reaction time as well as error frequency assessed using the reduced data set. Reaction times assessed for displays without target are not shown. First two rows list mean and standard deviation of 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 significant within each of the first two rows. If an ANOVA is run considering reaction time of all trials at which target was present, independent of whether target was detected or missed, reaction time turns out to depend significantly on settings (F(3) 2.81, p ⬍ 0.05). A Bonferroni adjusted matrix of pairwise comparison of data of the third row reveals 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 at displays without target, trials without target were not taken into account in the analysis on error frequency. Settings were found to exhibit a significant influence on error Table 2 Significance of difference in error frequency. Each entry denotes p value for difference in error frequency between two settings. Probabilities listed in first four lines were calculated using Wilcoxon rank test whereas probability listed in the last line of the table refers to two tailed t-test statistics for paired samples CRT-task LCD-task CRT versus LCD task (pooled) Bright Dark Bright Dark 0.0011 CRT-task Bright Dark LCD-task Bright Dark 1 .587 .015 .007 1 .147 1 .070 .005 1 frequency. As can be seen from Table 1, median error frequency varied from 7 errors per block (40 trials) in the CRT-task in brightness to 4 errors per block in the LCD-task for same conditions of ambient light level. Significance of difference in error frequency between the four settings is listed in Table 2. In general, error frequency at LCD-tasks was significantly lower when compared to error frequency at CRTtask. In brightness error frequency in CRT-task was found to be significantly higher (p ⬍ 0.02) than error frequency at LCD-task. The same difference applies for darkness (p ⬍ 0.01). Error frequency at CRT-task on brightness are significant (p ⬍ 0.01) higher than the ones assessed at LCD-task in darkness. In accordance with Table 2 error frequencies within display type do not differ significantly. Errors occurring at the same display type can therefore be pooled. If total errors of CRT-task are pooled (145) and compared to pooled total errors of LCD-task (96), we can conclude that at LCDtask the total amount of errors is 34% less than it is at CRTtask. A paired t-test reveals that difference in pooled data is significant at a level of p ⬍ 0.001 (t(9) 4.72) for two sided probability (p ⬍ 0.005 if non-parametric Wilcoxon rank test is used). Distribution of scores for suitability of settings for office work are plotted in Fig. 1. One subject did not answer the two questions on suitability of settings of LCD-task. A Wilcoxon rank test failed (p ⬎ 0.1) to show any significant differences in scores given to the four different settings. A comparison of pooled data assessed at CRT-tasks with pooled data assessed at LCD-tasks also fails to show significant effects (Wilcoxon rank, p ⬎ 0.1; two-tailed t-test, p ⬎ 0.1). Shorthand notes used to comment on given scores were mostly of negative nature. Table 3 summarizes negative remarks which were mentioned at least twice. Interestingly two subjects found fault with sharp characters of LCD, a characteristic which usually denotes high quality. Two subjects contested inhomogeneous luminance of LCD. 8 M. Menozzi et al. / Displays 20 (1999) 3–10 Fig. 1. Box plot of scores. Scores (median, quartiles, extreme values) were assessed by means of the question ‘‘How much would you like to work using this screen in this ambient light?’’ using a scale of six intervals. * outlier. 4. Discussion Reaction time was found to depend on learning effect. Apparently, two training sessions before the experiment were not enough to eliminate the effect. Learning effect disappeared after reducing amount of data. Therefore, learning effect can be supposed to be weak and will probably not interfere with conclusions reported at the end of this article. Although time needed to search the display is reduced by learning, error frequency is found to be constant throughout the experiment. We might therefore postulate that accuracy of detection could not be improved whereas speed of processing result of detection was improved by training. In our experiment speed of processing depends on motor as well as on mental skills, both of which can be improved by training. Given these circumstances we may conclude that optimal viewing conditions are imperative for ergonomics if accuracy is a significant requirement in a task because no training can make up for poor viewing conditions in order to improve accuracy of vision. Among reasons for involuntary missing a target in our task are, low visibility of the target and an insufficient attention. Visibility may be biased by reflections and by inhomogeneous emission characteristics of the displays. As was shown above, it is unlikely that reflections interfere with visibility of target. Subjective estimation of reflections (Table 3) deviates slightly from this assumption. However, deviations are small. Probably some subjects rated Table 3 Shorthand remarks on suitability of settings for office work (summary) CRT Bright Dark LCD Bright Dark Size too small Flicker Blur Reflections Inhomogeneous luminance 3 5 5 2 0 3 6 4 1 0 5 0 0 0 2 5 0 0 0 2 Total 14 15 7 7 possibility of specular reflections of displays based on their personal experience rather than visible reflections present during the experiment. Attention is influenced by behavior, and its focus must not necessarily be at the location of the fixation point (Treisman ). Experiments reported by Posner  showed a redirection of focus of attention towards locations in which a pre-attentive stimulus was shown. We may postulate that focus of attention can be dislocated by manipulative tasks as is the case with spatial orientation. Shebilske  demonstrated that a task requiring downward gaze distorted spatial orientation, e.g., that of baseball batters. As there are many tasks in office work requiring downward gaze and downwards focused attention, we could assume a drift of attention towards lower half of visual fields. However, error frequency was found to be independent of location of target on display. If it is supposed that variations of visibility did not cancel out effects of varying attention, we may state that attention does not vary with direction of gaze or orientation in space in a critical manner. We may therefore conclude that position of information to be displayed on the screen must not be determined in consideration with relevance of the information to be displayed. This conclusion seems to be somewhat in contrast to actual recommendations made in style guides (e.g. DIN 66234 ). Differences between error rates at CRT-task and at LCDtask are considerable. Total amount of errors assessed at LCD-task was 34% lower than amount of errors assessed at CRT-task. As mentioned in the introduction several technical factors may be responsible for causing differences in accuracy of detection of target such as sharpness of pixels and time characteristics of luminance. Given the low complexity of presented distractors and targets we suggest spatial characteristics to be of minor importance in determining visual performance. Time course of luminance might have played a decisive role in vision so as to give rise to more convenient viewing condition at LCD-task. However, LCD based technology is not necessarily required to produce a time course of luminance needed for good visual accuracy. The same beneficial effects result by increasing refresh rate or by installing phosphors with long persistence in CRT. Increasing refresh rate has been shown to facilitate reading (Montegut ). Technical reasons limit refresh rate in screens with high spatial resolution and with high depth of color representation. There are drawbacks in the use of phosphors with long persistence. First, effects like smearing while scrolling and transient ghost images while changing mask reduce legibility of displayed information at CRT equipped with long-persistence phosphor. Second, there are no triplets of long-persistence phosphors enabling to generate a color space of similar size as can be done if triplets of short-persistence phosphors are combined. Our experiment revealed a weak dependency of reaction time on settings or on technology of display used. Failure to demonstrate strong effects might be because of low M. Menozzi et al. / Displays 20 (1999) 3–10 sensitivity of our experimental procedure. As mentioned before, it is possible that mechanisms other than vision such as motor behavior, played a more significant role in determining reaction time and could have cancelled out any effect on vision. Efficiency of target detection should be assessed without manipulatory tasks and without requirements in time needed for highly cognitive work for decision making. We propose to assess detection performance by means of an experiment in which target is displayed tachistoscopically. Visual performance shall thereby be assessed in terms of percentage of correct detection of target as a function of duration of presentation of target. By using this approach it has been shown elsewhere (Fiorentini ) that important changes in correctness of detection are found if duration of visibility of display is changed from 100 to 200 ms. No subjective preference of either display technology or settings could be demonstrated statistically even though frequency distribution of subjective ratings reported in Fig. 1 suggests a possible advantage of LCD technology for office work. We conclude that the number of subjects used to carry out the rating is too small to demonstrate significant effects. Summary of shorthand notes on scoring suitability of settings (Table 3) also indicate a possible advantage of LCD over CRT. Subjects reported only few drawbacks of LCD. Too small size of the screen as well as an inhomogeneous luminance were complained at LCD. The former complaint can be accommodated by using displays with an adequate size. Screen size plays a role in suitability of a display for office work. However, screen size was not subject of our investigation. Findings made here are independent of screen size. Latter factor might have exhibited some effect on our results, i.e., that errors might appear more frequent in specific areas within LCD. As we did not record variation of luminance across display area we are not able to estimate influence of inhomogeneity of luminance on error frequency. 5. Conclusion The fact that the use of LCD improves accuracy in detecting targets and might also improve time for visual detection of the target is an advantage of LCD over CRT. It is therefore expected that LCD will give rise to lower visual strain and therefore cause less visual complaints as CRT will do. Given the broad variety in technology of LCD available on the market we may expect that there may be even further improvements in visual performance if other LCD are used. Unfortunately subjective ratings did not reflect findings made on visual performance as was demonstrated by Roufs  and Edwards . Acknowledgements We thank the following people for supporting us in this 9 experiment. The computer program used for the search task was written by Ara Hagopian. Andreas Hoffmann adapted the program to the needs of the notebook. Andreas von Buol provided electronics for precise assessment of reaction time. A special thank is addressed to our subjects who kindly participated at the experiment without being paid. Last but not least we thank our reviewers for fruitful comments. References  Patricia E. 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