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Effects of pictorial size and circle-slash thickness on glance legibility for prohibitive symbols
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International Journal of Industrial Ergonomics 33 (2004) 73–83
Effects of pictorial size and circle-slash thickness on glance legibility for prohibitive symbols Kong-King Shieha,*, Shih-Miao Huangb a
Department of Industrial Management, National Taiwan University of Science and Technology, 43, Kee-Lung Road, Section 4, Taipei 106, Taiwan, ROC b Department of Industrial and Commercial Design, Oriental Institute of Technology, 58, Sze-Chuan Road. Section 2, Pan-Chiao, Taiwan, ROC Received 21 February 2002; received in revised form 28 January 2003; accepted 1 September 2003
Abstract The purpose of this study was to investigate the effects of pictorial size and thickness of the red-circle slash on glance legibility for prohibitive symbols under different figure/ground luminance contrasts and limited exposure times. Analyses showed that the two-factor interactions of luminance contrast and exposure time; luminance contrast and pictorial size; luminance contrast and slash thickness; exposure time and pictorial size; and pictorial size and slash thickness were significant. Under higher luminance contrast (1:20) or longer exposure time (50 ms), the effects of pictorial size and slash thickness were not significant. Under degraded situations resulting from the reduction of luminance contrast of the symbols or limited exposure time, pictorial size and thickness of circle slashes influenced glance legibility. Glance legibility for the 100% and 75% pictorial sizes did not significantly differ, but glance legibility for the 50% size was significantly lower. Moreover, glance legibility for the 25% and 35% thickness of the red-circle slash did not significantly differ, but glance legibility for the 45% thickness was significantly lower. Relevance to industry A red circle with a slash is used extensively to convey that some activity or thing is forbidden. The design of prohibitive symbols may profoundly affect their efficiency and effectiveness in communicating the desired message. This article provides recommendations to improve the design of prohibitive symbols. r 2003 Elsevier B.V. All rights reserved. Keywords: Prohibitive symbol design; Legibility; Pictorial size; Thickness of circle slash
1. Introduction There are two kinds of warning symbols: permissive and prohibitive. Permissive symbols *Corresponding author. Tel.: +1-886-02-2737-6332; fax: +1-886-02-2737-6344. E-mail addresses: [email protected] (K. Shieh), [email protected] (S. Huang).
refer to the symbols with positive messages providing information about permitted practices or encouraged behavior, and prohibitive symbols refer to the symbols with negative messages frequently conveying information about actions that should not be taken in specific situations, or about conditions to be prevented or avoided (Glover et al., 1996). Gough (1965) believed that
0169-8141/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2003.09.001
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positively stated concepts are more easily understood than negative one. However, Dewar (1976) thought that some concepts are difficult to express in a direct permissive way although permissive symbols may be better than prohibitive symbols with regard to glance legibility. For example, due to its ease in expressing the prohibitive situation, or the meaning, ‘‘no,’’ the prohibitive sign whose pictorial is surrounded by a red ring with a red slash through the symbol is used extensively. Besides, Ringseis and Caird (1995) believed that people might have difficulties in comprehension of negation when a warning slash was absent across some of the pictorials. Therefore, prohibitive symbol is valuable to convey prohibitive meanings to human. Because the pictorial of a prohibitive symbol is superimposed under the slash, pictorial legibility difficulties may increase when the aggregate symbol forms a ‘‘bad feature’’ (Garner, 1974), or when a distinctive feature of the pictorial is obscured (Dewar, 1976; Biederman, 1987; Martindale, 1991; Murray et al., 1998). In the views of Gestalt, the completeness of the pictorial (figure of goodness) may be reduced because a slash overlaid with a pictorial separates the pictorial into parts. Pictorial legibility difficulties may increase when the degree of completeness is reduced. Besides, a prohibitive symbol can be confusing when the distinctive feature of the pictorial is obscured (Dewar, 1976; Biederman, 1987; Martindale, 1991; Murray et al., 1998). For example, Murray et al. (1998) found that ambiguity arises when the over slash of a prohibitive symbol obscures some detailed aspect of the pictorial such that it has multiple meanings. However, one can still recognize objects with limited information, if that information does not disrupt the critical parts (Biederman, 1987). Therefore, both pictorial size and slash thickness may be important factors influencing pictorial legibility. Many studies investigated the effect of slash on the legibility of prohibitive symbols. Dewar (1976) assessed the glance legibility of prohibitive symbols by examining four prohibitive symbol variations: a red ring with a slash over the symbol, a red ring with a slash under the symbol, a red ring with a partial slash, and a slash only (not a prohibitive sign). He found that performance was best with no
slash. He reasoned that the conventional circle slash declined people’s performance because slash could increase pictorial complexity and obscure portions of the pictorial. He also found that a red ring with a slash under the symbol had worst performance and that there was interaction between symbols and slash condition. The outcomes indicated that the extent of legibility of symbols was differentially affected by the different kinds of slashes. Murray et al. (1998) studied whether people’s judgments of four types of the circle slash (a slash over the symbol, a slash under the symbol, a partial slash, and translucent slash) would differ in perceived effectiveness by preference rankings. They found that the over and under slashes were preferred to translucent or partial slashes. This finding is different from Dewar’s (1976) finding that ‘‘symbol over slash’’ is worst. They attributed this finding to familiarities of the situation of the slash. The above two researches agreed that the interaction of slashes and pictorials affected the recognition of prohibitive symbols. The above two researches agreed that the interaction of slashes and pictorials affected the recognition of prohibitive symbols. Shieh and Huang (2002) investigated, further, how properties of both the slash and the pictorial form influenced the legibility ratings for prohibitive symbols. The results showed that solid pictorials were rated more legible than pictorials in outline form. Pictorials with a size equal to or greater than 75% of the length of the inner diameter of the circle slash were rated more legible than pictorials 50% in size. The effect of pictorial extensity was not significant: Pictorials with a greater vertical extensity (i.e., tall/thin pictorials) did not differ from pictorials with a greater horizontal extensity (i.e., short/wide pictorials), in terms of their legibility ratings. However, pictorial extensity interacted with slash orientation. Diagonal slashes were rated more legible than vertical or horizontal ones. Further, symbols were rated more legible when the thickness of the red-circle slash was such that its resulting area comprised 25% of the total area inside its outer circle. Slash thickness is defined per ISO 3864 (1984), (area of the red-circle slash/ total area inside the outer circle of the prohibitive symbol) 100%. However, the experiment
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was conducted in a laboratory where pictorial/ background luminance contrast for the prohibitive symbols was high and no time-constraint was imposed on the subjects. Further, the dependent variable in their study was subjective legibility rating. There is a need to use objective performance measures to evaluate the effect of the properties of the slash and pictorial form. In real world situations, prohibitive symbols are often viewed in less favorable conditions. For instance, the contrast between the various elements of a traffic sign, the black pictorial, red-circle slash and white background, may be reduced during rain or by dirt and dust accumulation over time. Also, because drivers normally have only limited time to look at signs, real world legibility is affected by limited exposure time. Symbol sign visibility and comprehension has been great concern for highway safety (TRB, 1988). Schieber and Kline (1994) found reduction in symbol legibility when luminance was reduced from daytime to nighttime luminance, especially for older observers. In addition to luminance level, luminance contrast is another important factor affecting visual acuity and symbol legibility (Sanders and McCormick, 1992). Grether and Baker (1972) believed that with less contrast, there is lower acuity. Cobb and Moss (1928) showed minimum separable acuity as a function of background luminance by measuring the visibility of the Landolt ring. Low luminance contrast between a pictorial and its background has been found to be detrimental to visual work (Sturgis and Osgood, 1982; Synder, 1988; Bullimore and Fulton, 1991; Shieh and Chen, 1997). Zhu and Wu (1990) reported that visual performance was reduced when the luminance contrast was too low, or too high. They suggested that visual performance was relatively good and visual fatigue was not a serious concern within the luminance contrast range of 9:1–11:1. However, Synder (1988) suggested that luminous contrast be at least 0.667 (a contrast ratio of 3:1) and, for improved performance, it should be at least 0.857 (a contrast ratio of 7:1). ANSI/HFS 100-1988, 1988 (Human Factors Society, 1988) recommended that text and its background should differ by a minimum value of 100 in DE (CIE Y, u0 , v0 ) distance. DE (CIE Y,
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u0 , v0 ) is proposed by Lippert (1986) to describe the color distance between two colors. These researchers all agreed that luminance contrast between figure and its ground influenced visual performance, although their suggestions regarding the optimal contrast ratio differed. Exposure time may be another factor influencing the legibility of prohibitive symbols. Sanders and McComick (1992) found that under high illumination conditions, visual acuity improved with increased exposure time up to 100 or 200 ms and then generally leveled off. Bullimore and Fulton (1991) believed that visual performance is better the longer an observer gets to look at, or look for something. However, users, such as drivers, can have only a brief time to look at a sign, especially while driving in heavy traffic on urban streets. Hence, a measure of glance legibility by presenting a stimulus for a fraction of a second would seem appropriate (Dewar and Ells, 1984). Thus, the design of prohibitive symbols should consider the effects of exposure time on the legibility of symbols. Increasing the size of pictorials generally improves legibility. However, beyond a certain size, legibility attains asymptotic levels and may even deteriorate (Matlin, 1989; Bullimore and Fulton, 1991). Shieh and Huang (2002) found that legibility rating increased as the pictorial size increased from 50% to 100%, however, the effect of pictorial size leveled off as it reached 75%. They argued that a pictorial that covers too much area of the available symbol space might create clutter and hinder recognition. They found that a size about 75% of the diameter of the red circle of the prohibitive symbol may be enough, and 100% may not be necessary. However, as mentioned previously, their suggestion was based on a study where physical conditions were favorable and no time-constraint was imposed on the subjects. Whether the 75% size is sufficient under less favorable conditions remains to be determined. Sanders and McComick (1992) believed that when the contrast between a visual target and its background was low, the target must be larger for it to be as equally discriminable as a target with a greater contrast. Thus, the present study further investigates the effects of pictorial size under degraded conditions.
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The thickness of the red-circle slash affects the degree to which important features are concealed by the slash. The ISO 3864 (1984) recommended that the area of the red-circle slash included in the warning symbol be at least 35% of the total area inside the outer circle, leaving 65% of the area for the pictorial. Generally, a thicker slash covers more details of the pictorial, thus, increasing the difficulty in understanding the pictorial although the thicker slashes which can be seen easily may express the meaning of ‘‘no’’ prominently. Further, the slash separates the pictorial into two parts. Thicker slashes produce more separation between the two pictorial parts and further degrades the completeness of the pictorial. When the completeness of the pictorial is degraded, the prohibitive symbol may not be perceived as a ‘‘good shape,’’ and the difficulty in recognizing the pictorial would increase. Shieh and Huang (2002) found that subjects rated the 25% thickness as the best, 35% next, and 45% as the worst in pictorial recognition. They reasoned that the greater the thickness, the lesser the available space for the pictorial and the more the pictorial would be covered by the slash, hence resulting in lower legibility rating. As described previously, their experiment was conducted in a laboratory where physical conditions were favorable and no timeconstraint was imposed on the subjects. The present study further investigated the effects of thickness of slash under degraded conditions. In summary, prohibitive symbols are typically viewed under various situations, which may degrade performance. Any investigation of the effects of pictorial size and thickness of slash on the legibility of prohibitive symbols should consider the possible effects of such degraded situations. Two factors contributing to degraded situations were also evaluated: luminance contrast and exposure time of the prohibitive symbols.
2. Method 2.1. Experimental design The experiment was a 2 (exposure time) 2(luminance contrast) 3(pictorial size) 3(thick-
ness of the red-circle slash) mixed design. Exposure time was a between-subjects variable; the other variables were within-subjects. Luminance contrast was defined as (low luminance of the black pictorial/high luminance of the white background). The luminance contrast (figure: ground) had two levels: 1:3 (a contrast ratio of 3:1) and 1:20 (a contrast ratio of 20:1). The luminance contrasts of 1:3 and 1:20 simulated the prohibitive symbols’ contrast during rainy days, i.e., during degraded lighting conditions and sunny days, respectively (ISO 3864, 1984). Exposure time of the prohibitive symbol had two levels: 33 and 50 ms. Pictorials size was defined as (length of the pictorial/diameter of inner circle of prohibitive symbol) 100%. Size was comprised of three levels: 50%, 75%, and 100%. The thickness of the red-circle slash had three levels: 25%, 35%, and 45%. Thickness was defined per ISO 3864 (1984), (area of the red-circle slash/total area inside the outer circle of the prohibitive symbol) 100%. Fig. 1 shows the prohibitive symbols used in the present study with their design descriptions. 2.2. Subjects The subjects were 15 male and 15 female undergraduate students from the Oriental Institute of Technology. They were compensated US $ 5 for their time and inconvenience in participating in the experiment. Their mean age was 21.83 years with a standard deviation of 3.30 years. All subjects had normal ocular health and no clinically significant anomalies of accommodation. 2.3. Materials and equipment Ten prohibitive symbols were used in the experiment (see Fig. 1). Each was composed of a pictorial and a red-circle slash superimposed on the pictorial (ISO 3864, 1984). The pictorials in five symbols had a greater vertical extensity; whereas, in the other five symbols the pictorials had a greater horizontal extensity. For each symbol, 18 designs were constructed according to the combination of pictorial size (3 levels), the thickness (3 levels) of the red-circle slash and
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2.4. Task and procedure Subjects were seated while performing the task. The 60 cm viewing distance was controlled with a headrest. The stimulus subtended a visual angle of 4.13 of arc. Subjects performed a symbol identification task in the experiment. At the beginning of each trial, a warning tone was initiated to instruct the subjects to visually fixate on the ‘‘X’’ at the center of the screen. Two seconds later, a prohibitive symbol was presented in the central area of the screen for the designated exposure time (33 or 50 ms). The subjects were then shown a paper sheet with 15 symbols (the ten symbols shown in Fig. 1, plus five symbols used in the prior training period). Each subject received 3(pictorial size) 3(slash thickness) 2(luminance contrast) 10(symbol)=180 trials. To familiarize the subjects with the task, they performed 20 training trials with five training symbols. 2.5. Dependent measures and data analysis
Fig. 1. The ten prohibitive symbols used in the experiment. Design characteristics of the symbols are described below. Design characteristics are expressed in the order of meaning, luminance contrast, pictorial of size and thickness of slash. ‘‘A’’ is ‘‘No golfing’’, high luminance contrast, 100%, 25%; ‘‘B’’ is ‘‘No riding’’, high luminance contrast, 50%, 25%; ‘‘C’’ is ‘‘No locking’’, high luminance contrast, 50%, 35%; ‘‘D’’ is ‘‘No swimming’’, high luminance contrast, 100%, 45%; ‘‘E’’ is ‘‘No car entrance’’, high luminance contrast, 50%, 45%; ‘‘F’’ is ‘‘No cat entrance’’, low luminance contrast, 75%, 25%; ‘‘G’’ is ‘‘No touching’’, low luminance contrast, 50%, 25%; ‘‘H’’ is ‘‘No drinking’’, low luminance contrast, 75%, 35%; ‘‘I’’ is ‘‘No passing’’, low luminance contrast, 75%, 35%; ‘‘J’’ is ‘‘No bus entrance’’, low luminance contrast, 100%, 45%.
luminance contrast between the pictorial and its background. The prohibitive symbols were presented via a 17-in Philips Brilliance-107 CRT. The ambient illuminance on the screen surface was 550 lx. The luminance of the black pictorials, redcircle slashes and white backgrounds of the prohibitive symbols was recorded with Minolta CRT color analyzer CA-100.
Glance legibility score, the performance measure, was defined as the proportion of the correctly recognized symbols. Analysis of variance was employed to analyze the data. All calculations were made using the Statistical Analysis System.
3. Results Table 1 shows the means and standard deviations of the glance legibility scores for each level of the independent variables. Results of analysis of variance indicated that the main effect of luminance contrast was significant (F(1, 28)=27.11, po0:01). Symbols with a contrast ratio of 1:20 attained a higher glance legibility score (0.97) than those with contrast ratio of 1:3 (0.86). There was also a significant main effect (F(1, 28)=16.19, po0:01) for exposure time of prohibitive symbols. Symbols with the 50-ms exposure time had a higher glance legibility score (0.96) than symbols with the 33-ms exposure time (0.87). The main effect of pictorial size was significant (F(2, 56)=12.07, po0:01). Multiple comparisons using
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the Duncan’s method showed that glance legibility scores for the 100% pictorial size (0.94) and 75% size (0.93) did not differ significantly, but the glance legibility score for the 50% size (0.88) was significantly lower. With respect to the effects of thickness of the red-circle slash, the results showed a significant main effect (F(2, 56)=7.65, po0:01). Multiple comparisons using the Duncan method showed that the glance legibility scores for the 25% slash thickness (0.93) and 35% thickness (0.92) did not significantly differ, but the glance
Table 1 Means and standard deviations of glance legibility scores for each level of the independent variables Variables
Level
Mean
SD
Duncan
Luminance contrast
Low (1:3) High (1:20)
0.86 0.97
0.19 0.06
A B
Exposure time
33 ms 50 ms
0.87 0.96
0.18 0.10
A B
Pictorial size
100% 75% 50%
0.94 0.93 0.88
0.11 0.15 0.19
A A B
Thickness of the circle slash
25%
0.93
0.13
A
35% 45%
0.92 0.90
0.14 0.18
A B
legibility score for the 45% thickness (0.90) was significantly lower. Table 2 shows glance legibility scores under combinations of luminance contrast and the other factors. It was found that there were significant differences of glance legibility between levels of factors when the luminance contrast was 1:3, but no significant differences when the luminance contrast was 1:20. Firstly, the Luminance Contrast Exposure Time interaction was significant (F(1, 28)=12.33, po0:01). Table 2 and Fig. 2 show a significant glance legibility difference between 33 and 50 ms exposure times when the luminance contrast was 1:3, but no significant difference when the luminance contrast was 1:20. The results indicated that the combination of low luminance contrast with shorter exposure time degraded visual performance much more than the other conditions. Next, the Luminance Contrast Pictorial Size interaction was also significant (F(2, 56)=8.01, po0:01). Table 2 and Fig. 2 show no significant differences between levels of the pictorial size when the luminance contrast was 1:20. However, the differences among the three pictorial sizes were significant when the luminance contrast was 1:3. Analysis of simple main effect showed that the glance legibility scores for the 100% pictorial size (0.90) and 75% size (0.88) did not significantly differ, but the glance legibility score for the 50% pictorial size (0.80) was
Table 2 Glance legibility scores under combinations of luminance contrast and other factors Variables
Level
Luminance contrast Low (1:3)
High (1:20)
Mean (SD)
Duncan
Mean (SD)
Duncan
Exposure time
33 ms 50 ms
0.78 (0.21) 0.95 (0.14)
A B
0.96 (0.07) 0.98 (0.05)
A A
Pictorial size
100% 75% 50%
0.90 (0.14) 0.88 (0.19) 0.80 (0.23)
A A B
0.98 (0.15) 0.98 (0.05) 0.97 (0.06)
A A A
Slash thickness
25% 35% 45%
0.88 (0.16) 0.87 (0.18) 0.82 (0.23)
A A B
0.98 (0.06) 0.98 (0.05) 0.97 (0.08)
A A A
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significantly lower. Lastly, the interaction of luminance contrast with slash thickness was significant (F(2, 56)=6.15, po0:01). Table 2 and Fig. 2 show no significant differences between levels of slash thickness when the luminance contrast was 1:20. However, the differences between the three slash thicknesses were significant when the luminance contrast was 1:3. Analysis of simple main effect showed that the glance legibility scores for the 25% slash thickness (0.88) and 35% thickness (0.87) did not significantly differ, but the glance legibility score for the 45% slash thickness (0.82) was significantly lower. There was significant interaction (F(2, 56)= 6.99, po0:01) between exposure time and pictorial size. Table 3 and Fig. 3 show no significant differences between levels of pictorial size when the exposure time was 50 ms. However, there were significant differences between levels of pictorial size when the exposure time was 33 ms. Analysis of simple main effect showed that the glance legibility scores for the 100% pictorial size (0.91) and 75%
size (0.90) did not significantly differ, but the glance legibility score for the 50% pictorial size (0.81) was significantly lower. The results also showed a significant interaction (F(4, 112)=3.79, po0:01) between pictorial size and slash thickness. Table 4 and Fig. 3 also shows that the scores for levels of pictorial size when slash thickness was 25% had no significant differences. However, there were significant differences between levels for pictorial size when the slash thickness was 35% or 45%. Analysis of simple main effect showed that the glance legibility scores for 100% pictorial size (0.94) and 75% size (0.95) were not significantly different, but the scores for 50% pictorial size (0.89) were lower when the slash thickness was 35%; glance legibility score for 100% pictorial size (0.95) were significantly higher than 75% size (0.89), and the scores for 50% pictorial size (0.85) was the lowest when the slash thickness was 45%. Finally, there was no significant interaction between slash thickness and exposure time.
Fig. 2. Interaction between luminance contrast and other factors.
Fig. 3. Interaction between pictorial size and other factors.
Table 3 Glance legibility scores under combinations of exposure time and pictorial size Variables
Level (%)
Exposure time 50 ms
Pictorial size
100 75 50
33 ms
Mean (SD)
Duncan
Mean (SD)
Duncan
0.98 (0.05) 0.96 (0.14) 0.96 (0.11)
A A A
0.91 (0.14) 0.90 (0.15) 0.81 (0.23)
A A B
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Table 4 Glance legibility scores under combinations of pictorial size and slash thickness Variables
Level (%)
Slash thickness 25%
Pictorial size
100 75 50
35%
45%
Mean (SD)
Duncan
Mean (SD)
Duncan
Mean (SD)
Duncan
0.94 (0.12) 0.95 (0.09) 0.91 (0.17)
A A A
0.94 (0.11) 0.95 (0.11) 0.89 (0.19)
A A B
0.95 (0.10) 0.89 (0.21) 0.85 (0.21)
A B C
Summarily, the results of two-factor interactions show that there is a substantial and significant decrement in performance when two degrading levels of two factors are combined.
4. Discussion The major purpose of this study was to investigate the effects of pictorial size and thickness of the red-circle slash on glance legibility for prohibitive symbols under visually degraded situations resulting from reduced luminance contrast and limited exposure time. Data analysis showed that luminance contrast affected the glance legibility of prohibitive symbols as expected. Prohibitive symbols with higher luminance contrast produced higher glance legibility scores than symbols with lower luminance contrast. This result is consistent with many previous research findings that low luminance contrast between figure and background was detrimental to visual work (Synder, 1988; Bullimore and Fulton, 1991; Shieh and Chen, 1997; Cobb and Moss, 1928). Exposure time also affected the glance legibility of prohibitive symbols. Prohibitive symbols with the longer exposure time attained higher glance legibility scores than those with the shorter exposure time. This result also agrees with previous research findings where visual performance was better when the observer received longer viewing times (Bullimore and Fulton, 1991). Considering the interaction of luminance contrast with exposure time, the results indicated that the glance legibility scores might degrade even more when situations of both low luminance contrast and shorter exposure
existed at the same time. As expected, under both degraded conditions, the legibility performance was reduced more severely. For the effect of pictorial size, as size increased from 50% to 100%, glance legibility scores increased. However, no significant improvement in glance legibility scores resulted from the 75% to the 100% pictorial size. Generally speaking, larger pictorials are easier to recognize, and hence provide for better legibility. Caird et al. (1997) also found that small pictorials and pictorials with slashes result in difficulties in comprehension. With respect to the effect of slash thickness, as thickness decreased from 45% to 25%, glance legibility scores increased. But, the increase leveled off when the thickness was below 35%. The greater the slash’s thickness, there was less space for the pictorial, hence resulting in lower glance legibility. Both the effects of pictorial size and slash thickness agreed with Shieh and Huang’s findings (2002) that the 75% pictorial size might be large enough and the 35% slash thickness might be the upper limit for prohibitive symbol design. Further, the significant Pictorial Size Slash Thickness interaction indicated that the effect of slash thickness was small when pictorial size was large. However, when pictorial size was small, the greater the slash thickness, the worse the glance legibility. A smaller pictorial could be covered more by a thicker slash than a greater one, thus, resulting in poor glance legibility. A major purpose of the present study was to determine how the preferred levels of pictorial size and slash thickness would be affected by unfavorable viewing conditions, such as reduced luminance contrast of the pictorial and its background
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and limited viewing time. The results of data analysis showed that under higher luminance contrast (1:20) or longer exposure time (50 ms), the effects of pictorial size and slash thickness were not significant. It appeared that subjects could recognize symbols correctly when the contrast between symbols and background is high enough or when viewing time is long enough. Thus, the detrimental effects of small pictorial size (50%) might not emerge under favorable viewing conditions. However, under degraded situations resulting from the reduction of luminance contrast of the symbols or/and limited exposure time, pictorial size might be significant factors influencing glance legibility. Under degraded situations, larger pictorials could compensate for the effects of the degraded situations because the larger pictorials are easier to recognize. As shown in the results, under low luminance contrasts (1:3) and short exposure time (33 ms), as pictorial size increased from 50% to 100%, glance legibility scores increased. Besides, in this situation, legibility of smallest pictorials (50%) was seriously worse than that of 75% pictorial size and 100% pictorial size. Based on the results of this study, a pictorial size that is at least 75% of the inner diameter of the red-circle slash is appropriate when luminance contrast is low and exposure time short. A smaller pictorial size may be acceptable under more favorable viewing conditions. Like pictorial size, the detrimental effects of large slash thickness (45%) might not emerge under favorable viewing conditions. However, the thickness of circle slashes might be significant factors influencing glance legibility under degraded situations resulting from the reduction of luminance contrast of the symbols. Hence, as slash thickness increased from 25% to 45%, glance legibility scores decreased under degraded situation. A thin slash that covers less of the pictorial would have an advantage in recognizing the symbols than would a thick one. Under this condition, the data analysis showed that the 35% slash thickness might be the upper limit for prohibitive symbol design. However, the results also showed that the interactions between slash thickness and exposure time were not significant. Generally, red slash is
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not easily seen in limited time, especially in dark environment because both cons and rods are not sensitive to long wavelength (Hendee and Wells, 1997). It is possible that both levels of exposure time were so short that the red slashes were overlooked. Besides, the three-factor interactions between slash thickness, exposure time and luminance contrast were not significant, either. Finally, the appropriate thickness of the redcircle slash may be as small as 25% or 35%, instead of at least 35% as suggested by ISO 3864 under degraded situations. Under degraded situations, the greater the slash thickness, the lesser the available space for the pictorial and the more the pictorial is covered by the slash, hence resulting in lower legibility. The present study found that legibility declined significantly as the thickness of the slash was greater than 35%. Further, the best legibility was with the 25% thickness when luminance contrast was low and exposure time short. Based on these results, the recommendation of ISO 3864 (1984) that the red-circle slash should be at least 35% of the area of the sign is questionable, especially under unfavorable viewing conditions. However, a large slash thickness may be beneficial to noting prohibitive meanings. Subjects may overlook the red slash shape in prohibitive symbol when the slash thickness is not thick enough. In order to keep clear legibility of both pictorials and slashes in prohibitive symbols, the slash size should not be too small. There are at least two implications in the present study. Firstly, The luminance contrast of prohibitive symbols may be low due to environmental conditions, such as rain, snow and smog. To the extent that the environmental factors reduce the luminance contrast of prohibitive symbols, the legibility of prohibitive symbols could be reduced significantly. Second, short exposure time may degrade legibility of prohibitive symbols. Longer exposure time may be comparable to driving at a slower speed; and shorter exposure time, to a faster speed. Hence, the results of this study might indicate that as driving speed increases, the legibility of prohibitive symbols may be reduced significantly. However, it still needs more evidences to support this point because the
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subjects did not perform a driving task in the experiment.
5. Conclusion The pictorial size, thickness of the red-circle slashes, luminance contrast, and exposure time interactively affected the legibility of prohibitive symbols. When luminance contrast is high and exposure time long, the subjects have better pictorial discriminability and enough time to process the symbols, and thus the effects of pictorial size and slash thickness may not be significant. On the other hand, when luminance contrast is low and exposure time short, a large pictorial size lessens the degree to which critical features of the pictorial are covered and prevent the forming of bad features, and thus benefits glance legibility. Further, when luminance contrast is low, a small slash thickness prevents the forming of bad features, and thus benefits glance legibility. Finally, the desirable thickness of the red-circle slash would be as small as 25% or 35%; the recommendation of ISO 3864 (1984) that the redcircle slash should be at least 35% of the area of the sign seems to be questionable, especially under unfavorable viewing conditions.
Acknowledgements This study was supported by a Research Grant from the National Sciences Council of the Republic of China, Grant No. NSC-92-2213-E-161-002.
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Caird, J.F., Wheat, B., McIntosh, K.R., Dewar, R.E., 1997. The comprehensibility of airline safety card pictorials. In: Proceedings of the Human Factors and Ergonomics Society 41st Annual Meeting, Santa Monica, pp. 801–805. Cobb, P.W., Moss, F.K., 1928. The four variables in the visual threshold. Franklin Institute Journal 205, 831. Dewar, R.E., 1976. The slash obscures the symbol on prohibitive traffic signs. Human Factors 18 (3), 253–258. Dewar, R.E., Ells, J.G., 1984. Methods of evaluation of traffic signs. In: Easterby, R., Zwaga, H. (Eds.), Information Design. Wiley, NY, pp. 77–90. Garner, W.R., 1974. The Processing of Information and Structure. Potomac, MD, Erlbaum. Glover, B.L., Magurno, A.B., Murray, L.A., Wogalter, M.S., 1996. Pictorial Negations: preferences for different circleslash variations. In: Proceedings of the Human Factors and Ergonomics Society 40th Annual Meeting, Santa Monica, pp. 910–914. Gough, P.B., 1965. Grammatical transformation and speed of understanding. Journal of Verbal Learning and Verbal Behavior 4, 107–111. Grether, W.F., Baker, C.A., 1972. Visual presentation of information. In: Van Cott, H.A., Kinkade, R.G. (Eds.), Human Engineering Guide to Equipment Design. Government Printing Office, Washington DC, pp. 42–121. Hendee, W.R., Wells, P.N.T., 1997. The Perception of Visual Information. Springer, WI, p. 7. ISO 3864, 1984. International Standard for Safety Colours and Safety Signs. ISO, Switzerland. Lippert, T.M., 1986. Color-difference prediction of legibility performance for CRT raster imagery. SID Digest of Technical Paper 16, 86–89. Martindale, C., 1991. Cognitive Psychology: A Neural-network Approach. Cole, CA. Matlin, M.W., 1989. Cognition. Holt, Rinehart, Winston, NY. Murray, L.A., Magurno, A.B., Glover, B.L., Wogalter, M.S., 1998. Prohibitive pictorials: Evaluations of different circleslash negation symbols. International Journal of Industrial Ergonomics 22, 473–482. Ringseis, E.L., Caird, J.K., 1995. The comprehensibility and Legibility of twenty pharmaceutical warning pictogram. In: Proceedings of the Human Factors and Ergonomics Society 39th Annual Meeting, Santa Monica, pp. 974–978. Sanders, M.S., McCormick, E.J., 1992. Human Factors in Engineering Design. McGraw-Hill, NY, pp. 121–125. Schieber, F., Kline, D.W., 1994. Age differences in the legibility of symbol highway signs as a function of luminance and glare level: A preliminary report. In: Proceedings of the Human Factors and Ergonomics Society 38th Annual Meeting, Santa Monica, pp. 133–136. Shieh, K., Chen, M.T., 1997. Effects of screen color combination, work-break schedule, and workplace on VDT viewing distance. International Journal of Industrial Ergonomics 20 (1), 11–18. Shieh, K., Huang, S., 2002. A study of the relations between pictorials and circle-slashes in prohibitive symbol. Journal of Chinese Design Institute 18 (2), 41–60 (In Chinese).
ARTICLE IN PRESS K. Shieh, S. Huang / International Journal of Industrial Ergonomics 33 (2004) 73–83 Sturgis, S.P., Osgood, D.J., 1982. Effects of glare and background luminance on visual acuity and contrast sensitivity: Implication for driver night vision testing. Human Factors 24 (3), 347–360. Synder, H.L., 1988. Image quality. In: Helander, M. (Ed.), Handbook of Human-computer Interaction. Elsevier science publishers, Amsterdam, pp. 437–474.
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Transportation Research Board (TRB), 1988. Transportation in an Aging Society: Improving Mobility and Safety for Older Persons, Vols. 1 and 2. National Research Council, Washington, DC. Zhu, Z., Wu, J., 1990. On the standardization of VDT’s proper and optimal contrast range. Ergonomics 33 (7), 925–932.
International Journal of Industrial Ergonomics 33 (2004) 73–83
Effects of pictorial size and circle-slash thickness on glance legibility for prohibitive symbols Kong-King Shieha,*, Shih-Miao Huangb a
Department of Industrial Management, National Taiwan University of Science and Technology, 43, Kee-Lung Road, Section 4, Taipei 106, Taiwan, ROC b Department of Industrial and Commercial Design, Oriental Institute of Technology, 58, Sze-Chuan Road. Section 2, Pan-Chiao, Taiwan, ROC Received 21 February 2002; received in revised form 28 January 2003; accepted 1 September 2003
Abstract The purpose of this study was to investigate the effects of pictorial size and thickness of the red-circle slash on glance legibility for prohibitive symbols under different figure/ground luminance contrasts and limited exposure times. Analyses showed that the two-factor interactions of luminance contrast and exposure time; luminance contrast and pictorial size; luminance contrast and slash thickness; exposure time and pictorial size; and pictorial size and slash thickness were significant. Under higher luminance contrast (1:20) or longer exposure time (50 ms), the effects of pictorial size and slash thickness were not significant. Under degraded situations resulting from the reduction of luminance contrast of the symbols or limited exposure time, pictorial size and thickness of circle slashes influenced glance legibility. Glance legibility for the 100% and 75% pictorial sizes did not significantly differ, but glance legibility for the 50% size was significantly lower. Moreover, glance legibility for the 25% and 35% thickness of the red-circle slash did not significantly differ, but glance legibility for the 45% thickness was significantly lower. Relevance to industry A red circle with a slash is used extensively to convey that some activity or thing is forbidden. The design of prohibitive symbols may profoundly affect their efficiency and effectiveness in communicating the desired message. This article provides recommendations to improve the design of prohibitive symbols. r 2003 Elsevier B.V. All rights reserved. Keywords: Prohibitive symbol design; Legibility; Pictorial size; Thickness of circle slash
1. Introduction There are two kinds of warning symbols: permissive and prohibitive. Permissive symbols *Corresponding author. Tel.: +1-886-02-2737-6332; fax: +1-886-02-2737-6344. E-mail addresses: [email protected] (K. Shieh), [email protected] (S. Huang).
refer to the symbols with positive messages providing information about permitted practices or encouraged behavior, and prohibitive symbols refer to the symbols with negative messages frequently conveying information about actions that should not be taken in specific situations, or about conditions to be prevented or avoided (Glover et al., 1996). Gough (1965) believed that
0169-8141/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ergon.2003.09.001
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positively stated concepts are more easily understood than negative one. However, Dewar (1976) thought that some concepts are difficult to express in a direct permissive way although permissive symbols may be better than prohibitive symbols with regard to glance legibility. For example, due to its ease in expressing the prohibitive situation, or the meaning, ‘‘no,’’ the prohibitive sign whose pictorial is surrounded by a red ring with a red slash through the symbol is used extensively. Besides, Ringseis and Caird (1995) believed that people might have difficulties in comprehension of negation when a warning slash was absent across some of the pictorials. Therefore, prohibitive symbol is valuable to convey prohibitive meanings to human. Because the pictorial of a prohibitive symbol is superimposed under the slash, pictorial legibility difficulties may increase when the aggregate symbol forms a ‘‘bad feature’’ (Garner, 1974), or when a distinctive feature of the pictorial is obscured (Dewar, 1976; Biederman, 1987; Martindale, 1991; Murray et al., 1998). In the views of Gestalt, the completeness of the pictorial (figure of goodness) may be reduced because a slash overlaid with a pictorial separates the pictorial into parts. Pictorial legibility difficulties may increase when the degree of completeness is reduced. Besides, a prohibitive symbol can be confusing when the distinctive feature of the pictorial is obscured (Dewar, 1976; Biederman, 1987; Martindale, 1991; Murray et al., 1998). For example, Murray et al. (1998) found that ambiguity arises when the over slash of a prohibitive symbol obscures some detailed aspect of the pictorial such that it has multiple meanings. However, one can still recognize objects with limited information, if that information does not disrupt the critical parts (Biederman, 1987). Therefore, both pictorial size and slash thickness may be important factors influencing pictorial legibility. Many studies investigated the effect of slash on the legibility of prohibitive symbols. Dewar (1976) assessed the glance legibility of prohibitive symbols by examining four prohibitive symbol variations: a red ring with a slash over the symbol, a red ring with a slash under the symbol, a red ring with a partial slash, and a slash only (not a prohibitive sign). He found that performance was best with no
slash. He reasoned that the conventional circle slash declined people’s performance because slash could increase pictorial complexity and obscure portions of the pictorial. He also found that a red ring with a slash under the symbol had worst performance and that there was interaction between symbols and slash condition. The outcomes indicated that the extent of legibility of symbols was differentially affected by the different kinds of slashes. Murray et al. (1998) studied whether people’s judgments of four types of the circle slash (a slash over the symbol, a slash under the symbol, a partial slash, and translucent slash) would differ in perceived effectiveness by preference rankings. They found that the over and under slashes were preferred to translucent or partial slashes. This finding is different from Dewar’s (1976) finding that ‘‘symbol over slash’’ is worst. They attributed this finding to familiarities of the situation of the slash. The above two researches agreed that the interaction of slashes and pictorials affected the recognition of prohibitive symbols. The above two researches agreed that the interaction of slashes and pictorials affected the recognition of prohibitive symbols. Shieh and Huang (2002) investigated, further, how properties of both the slash and the pictorial form influenced the legibility ratings for prohibitive symbols. The results showed that solid pictorials were rated more legible than pictorials in outline form. Pictorials with a size equal to or greater than 75% of the length of the inner diameter of the circle slash were rated more legible than pictorials 50% in size. The effect of pictorial extensity was not significant: Pictorials with a greater vertical extensity (i.e., tall/thin pictorials) did not differ from pictorials with a greater horizontal extensity (i.e., short/wide pictorials), in terms of their legibility ratings. However, pictorial extensity interacted with slash orientation. Diagonal slashes were rated more legible than vertical or horizontal ones. Further, symbols were rated more legible when the thickness of the red-circle slash was such that its resulting area comprised 25% of the total area inside its outer circle. Slash thickness is defined per ISO 3864 (1984), (area of the red-circle slash/ total area inside the outer circle of the prohibitive symbol) 100%. However, the experiment
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was conducted in a laboratory where pictorial/ background luminance contrast for the prohibitive symbols was high and no time-constraint was imposed on the subjects. Further, the dependent variable in their study was subjective legibility rating. There is a need to use objective performance measures to evaluate the effect of the properties of the slash and pictorial form. In real world situations, prohibitive symbols are often viewed in less favorable conditions. For instance, the contrast between the various elements of a traffic sign, the black pictorial, red-circle slash and white background, may be reduced during rain or by dirt and dust accumulation over time. Also, because drivers normally have only limited time to look at signs, real world legibility is affected by limited exposure time. Symbol sign visibility and comprehension has been great concern for highway safety (TRB, 1988). Schieber and Kline (1994) found reduction in symbol legibility when luminance was reduced from daytime to nighttime luminance, especially for older observers. In addition to luminance level, luminance contrast is another important factor affecting visual acuity and symbol legibility (Sanders and McCormick, 1992). Grether and Baker (1972) believed that with less contrast, there is lower acuity. Cobb and Moss (1928) showed minimum separable acuity as a function of background luminance by measuring the visibility of the Landolt ring. Low luminance contrast between a pictorial and its background has been found to be detrimental to visual work (Sturgis and Osgood, 1982; Synder, 1988; Bullimore and Fulton, 1991; Shieh and Chen, 1997). Zhu and Wu (1990) reported that visual performance was reduced when the luminance contrast was too low, or too high. They suggested that visual performance was relatively good and visual fatigue was not a serious concern within the luminance contrast range of 9:1–11:1. However, Synder (1988) suggested that luminous contrast be at least 0.667 (a contrast ratio of 3:1) and, for improved performance, it should be at least 0.857 (a contrast ratio of 7:1). ANSI/HFS 100-1988, 1988 (Human Factors Society, 1988) recommended that text and its background should differ by a minimum value of 100 in DE (CIE Y, u0 , v0 ) distance. DE (CIE Y,
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u0 , v0 ) is proposed by Lippert (1986) to describe the color distance between two colors. These researchers all agreed that luminance contrast between figure and its ground influenced visual performance, although their suggestions regarding the optimal contrast ratio differed. Exposure time may be another factor influencing the legibility of prohibitive symbols. Sanders and McComick (1992) found that under high illumination conditions, visual acuity improved with increased exposure time up to 100 or 200 ms and then generally leveled off. Bullimore and Fulton (1991) believed that visual performance is better the longer an observer gets to look at, or look for something. However, users, such as drivers, can have only a brief time to look at a sign, especially while driving in heavy traffic on urban streets. Hence, a measure of glance legibility by presenting a stimulus for a fraction of a second would seem appropriate (Dewar and Ells, 1984). Thus, the design of prohibitive symbols should consider the effects of exposure time on the legibility of symbols. Increasing the size of pictorials generally improves legibility. However, beyond a certain size, legibility attains asymptotic levels and may even deteriorate (Matlin, 1989; Bullimore and Fulton, 1991). Shieh and Huang (2002) found that legibility rating increased as the pictorial size increased from 50% to 100%, however, the effect of pictorial size leveled off as it reached 75%. They argued that a pictorial that covers too much area of the available symbol space might create clutter and hinder recognition. They found that a size about 75% of the diameter of the red circle of the prohibitive symbol may be enough, and 100% may not be necessary. However, as mentioned previously, their suggestion was based on a study where physical conditions were favorable and no time-constraint was imposed on the subjects. Whether the 75% size is sufficient under less favorable conditions remains to be determined. Sanders and McComick (1992) believed that when the contrast between a visual target and its background was low, the target must be larger for it to be as equally discriminable as a target with a greater contrast. Thus, the present study further investigates the effects of pictorial size under degraded conditions.
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The thickness of the red-circle slash affects the degree to which important features are concealed by the slash. The ISO 3864 (1984) recommended that the area of the red-circle slash included in the warning symbol be at least 35% of the total area inside the outer circle, leaving 65% of the area for the pictorial. Generally, a thicker slash covers more details of the pictorial, thus, increasing the difficulty in understanding the pictorial although the thicker slashes which can be seen easily may express the meaning of ‘‘no’’ prominently. Further, the slash separates the pictorial into two parts. Thicker slashes produce more separation between the two pictorial parts and further degrades the completeness of the pictorial. When the completeness of the pictorial is degraded, the prohibitive symbol may not be perceived as a ‘‘good shape,’’ and the difficulty in recognizing the pictorial would increase. Shieh and Huang (2002) found that subjects rated the 25% thickness as the best, 35% next, and 45% as the worst in pictorial recognition. They reasoned that the greater the thickness, the lesser the available space for the pictorial and the more the pictorial would be covered by the slash, hence resulting in lower legibility rating. As described previously, their experiment was conducted in a laboratory where physical conditions were favorable and no timeconstraint was imposed on the subjects. The present study further investigated the effects of thickness of slash under degraded conditions. In summary, prohibitive symbols are typically viewed under various situations, which may degrade performance. Any investigation of the effects of pictorial size and thickness of slash on the legibility of prohibitive symbols should consider the possible effects of such degraded situations. Two factors contributing to degraded situations were also evaluated: luminance contrast and exposure time of the prohibitive symbols.
2. Method 2.1. Experimental design The experiment was a 2 (exposure time) 2(luminance contrast) 3(pictorial size) 3(thick-
ness of the red-circle slash) mixed design. Exposure time was a between-subjects variable; the other variables were within-subjects. Luminance contrast was defined as (low luminance of the black pictorial/high luminance of the white background). The luminance contrast (figure: ground) had two levels: 1:3 (a contrast ratio of 3:1) and 1:20 (a contrast ratio of 20:1). The luminance contrasts of 1:3 and 1:20 simulated the prohibitive symbols’ contrast during rainy days, i.e., during degraded lighting conditions and sunny days, respectively (ISO 3864, 1984). Exposure time of the prohibitive symbol had two levels: 33 and 50 ms. Pictorials size was defined as (length of the pictorial/diameter of inner circle of prohibitive symbol) 100%. Size was comprised of three levels: 50%, 75%, and 100%. The thickness of the red-circle slash had three levels: 25%, 35%, and 45%. Thickness was defined per ISO 3864 (1984), (area of the red-circle slash/total area inside the outer circle of the prohibitive symbol) 100%. Fig. 1 shows the prohibitive symbols used in the present study with their design descriptions. 2.2. Subjects The subjects were 15 male and 15 female undergraduate students from the Oriental Institute of Technology. They were compensated US $ 5 for their time and inconvenience in participating in the experiment. Their mean age was 21.83 years with a standard deviation of 3.30 years. All subjects had normal ocular health and no clinically significant anomalies of accommodation. 2.3. Materials and equipment Ten prohibitive symbols were used in the experiment (see Fig. 1). Each was composed of a pictorial and a red-circle slash superimposed on the pictorial (ISO 3864, 1984). The pictorials in five symbols had a greater vertical extensity; whereas, in the other five symbols the pictorials had a greater horizontal extensity. For each symbol, 18 designs were constructed according to the combination of pictorial size (3 levels), the thickness (3 levels) of the red-circle slash and
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2.4. Task and procedure Subjects were seated while performing the task. The 60 cm viewing distance was controlled with a headrest. The stimulus subtended a visual angle of 4.13 of arc. Subjects performed a symbol identification task in the experiment. At the beginning of each trial, a warning tone was initiated to instruct the subjects to visually fixate on the ‘‘X’’ at the center of the screen. Two seconds later, a prohibitive symbol was presented in the central area of the screen for the designated exposure time (33 or 50 ms). The subjects were then shown a paper sheet with 15 symbols (the ten symbols shown in Fig. 1, plus five symbols used in the prior training period). Each subject received 3(pictorial size) 3(slash thickness) 2(luminance contrast) 10(symbol)=180 trials. To familiarize the subjects with the task, they performed 20 training trials with five training symbols. 2.5. Dependent measures and data analysis
Fig. 1. The ten prohibitive symbols used in the experiment. Design characteristics of the symbols are described below. Design characteristics are expressed in the order of meaning, luminance contrast, pictorial of size and thickness of slash. ‘‘A’’ is ‘‘No golfing’’, high luminance contrast, 100%, 25%; ‘‘B’’ is ‘‘No riding’’, high luminance contrast, 50%, 25%; ‘‘C’’ is ‘‘No locking’’, high luminance contrast, 50%, 35%; ‘‘D’’ is ‘‘No swimming’’, high luminance contrast, 100%, 45%; ‘‘E’’ is ‘‘No car entrance’’, high luminance contrast, 50%, 45%; ‘‘F’’ is ‘‘No cat entrance’’, low luminance contrast, 75%, 25%; ‘‘G’’ is ‘‘No touching’’, low luminance contrast, 50%, 25%; ‘‘H’’ is ‘‘No drinking’’, low luminance contrast, 75%, 35%; ‘‘I’’ is ‘‘No passing’’, low luminance contrast, 75%, 35%; ‘‘J’’ is ‘‘No bus entrance’’, low luminance contrast, 100%, 45%.
luminance contrast between the pictorial and its background. The prohibitive symbols were presented via a 17-in Philips Brilliance-107 CRT. The ambient illuminance on the screen surface was 550 lx. The luminance of the black pictorials, redcircle slashes and white backgrounds of the prohibitive symbols was recorded with Minolta CRT color analyzer CA-100.
Glance legibility score, the performance measure, was defined as the proportion of the correctly recognized symbols. Analysis of variance was employed to analyze the data. All calculations were made using the Statistical Analysis System.
3. Results Table 1 shows the means and standard deviations of the glance legibility scores for each level of the independent variables. Results of analysis of variance indicated that the main effect of luminance contrast was significant (F(1, 28)=27.11, po0:01). Symbols with a contrast ratio of 1:20 attained a higher glance legibility score (0.97) than those with contrast ratio of 1:3 (0.86). There was also a significant main effect (F(1, 28)=16.19, po0:01) for exposure time of prohibitive symbols. Symbols with the 50-ms exposure time had a higher glance legibility score (0.96) than symbols with the 33-ms exposure time (0.87). The main effect of pictorial size was significant (F(2, 56)=12.07, po0:01). Multiple comparisons using
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the Duncan’s method showed that glance legibility scores for the 100% pictorial size (0.94) and 75% size (0.93) did not differ significantly, but the glance legibility score for the 50% size (0.88) was significantly lower. With respect to the effects of thickness of the red-circle slash, the results showed a significant main effect (F(2, 56)=7.65, po0:01). Multiple comparisons using the Duncan method showed that the glance legibility scores for the 25% slash thickness (0.93) and 35% thickness (0.92) did not significantly differ, but the glance
Table 1 Means and standard deviations of glance legibility scores for each level of the independent variables Variables
Level
Mean
SD
Duncan
Luminance contrast
Low (1:3) High (1:20)
0.86 0.97
0.19 0.06
A B
Exposure time
33 ms 50 ms
0.87 0.96
0.18 0.10
A B
Pictorial size
100% 75% 50%
0.94 0.93 0.88
0.11 0.15 0.19
A A B
Thickness of the circle slash
25%
0.93
0.13
A
35% 45%
0.92 0.90
0.14 0.18
A B
legibility score for the 45% thickness (0.90) was significantly lower. Table 2 shows glance legibility scores under combinations of luminance contrast and the other factors. It was found that there were significant differences of glance legibility between levels of factors when the luminance contrast was 1:3, but no significant differences when the luminance contrast was 1:20. Firstly, the Luminance Contrast Exposure Time interaction was significant (F(1, 28)=12.33, po0:01). Table 2 and Fig. 2 show a significant glance legibility difference between 33 and 50 ms exposure times when the luminance contrast was 1:3, but no significant difference when the luminance contrast was 1:20. The results indicated that the combination of low luminance contrast with shorter exposure time degraded visual performance much more than the other conditions. Next, the Luminance Contrast Pictorial Size interaction was also significant (F(2, 56)=8.01, po0:01). Table 2 and Fig. 2 show no significant differences between levels of the pictorial size when the luminance contrast was 1:20. However, the differences among the three pictorial sizes were significant when the luminance contrast was 1:3. Analysis of simple main effect showed that the glance legibility scores for the 100% pictorial size (0.90) and 75% size (0.88) did not significantly differ, but the glance legibility score for the 50% pictorial size (0.80) was
Table 2 Glance legibility scores under combinations of luminance contrast and other factors Variables
Level
Luminance contrast Low (1:3)
High (1:20)
Mean (SD)
Duncan
Mean (SD)
Duncan
Exposure time
33 ms 50 ms
0.78 (0.21) 0.95 (0.14)
A B
0.96 (0.07) 0.98 (0.05)
A A
Pictorial size
100% 75% 50%
0.90 (0.14) 0.88 (0.19) 0.80 (0.23)
A A B
0.98 (0.15) 0.98 (0.05) 0.97 (0.06)
A A A
Slash thickness
25% 35% 45%
0.88 (0.16) 0.87 (0.18) 0.82 (0.23)
A A B
0.98 (0.06) 0.98 (0.05) 0.97 (0.08)
A A A
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significantly lower. Lastly, the interaction of luminance contrast with slash thickness was significant (F(2, 56)=6.15, po0:01). Table 2 and Fig. 2 show no significant differences between levels of slash thickness when the luminance contrast was 1:20. However, the differences between the three slash thicknesses were significant when the luminance contrast was 1:3. Analysis of simple main effect showed that the glance legibility scores for the 25% slash thickness (0.88) and 35% thickness (0.87) did not significantly differ, but the glance legibility score for the 45% slash thickness (0.82) was significantly lower. There was significant interaction (F(2, 56)= 6.99, po0:01) between exposure time and pictorial size. Table 3 and Fig. 3 show no significant differences between levels of pictorial size when the exposure time was 50 ms. However, there were significant differences between levels of pictorial size when the exposure time was 33 ms. Analysis of simple main effect showed that the glance legibility scores for the 100% pictorial size (0.91) and 75%
size (0.90) did not significantly differ, but the glance legibility score for the 50% pictorial size (0.81) was significantly lower. The results also showed a significant interaction (F(4, 112)=3.79, po0:01) between pictorial size and slash thickness. Table 4 and Fig. 3 also shows that the scores for levels of pictorial size when slash thickness was 25% had no significant differences. However, there were significant differences between levels for pictorial size when the slash thickness was 35% or 45%. Analysis of simple main effect showed that the glance legibility scores for 100% pictorial size (0.94) and 75% size (0.95) were not significantly different, but the scores for 50% pictorial size (0.89) were lower when the slash thickness was 35%; glance legibility score for 100% pictorial size (0.95) were significantly higher than 75% size (0.89), and the scores for 50% pictorial size (0.85) was the lowest when the slash thickness was 45%. Finally, there was no significant interaction between slash thickness and exposure time.
Fig. 2. Interaction between luminance contrast and other factors.
Fig. 3. Interaction between pictorial size and other factors.
Table 3 Glance legibility scores under combinations of exposure time and pictorial size Variables
Level (%)
Exposure time 50 ms
Pictorial size
100 75 50
33 ms
Mean (SD)
Duncan
Mean (SD)
Duncan
0.98 (0.05) 0.96 (0.14) 0.96 (0.11)
A A A
0.91 (0.14) 0.90 (0.15) 0.81 (0.23)
A A B
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Table 4 Glance legibility scores under combinations of pictorial size and slash thickness Variables
Level (%)
Slash thickness 25%
Pictorial size
100 75 50
35%
45%
Mean (SD)
Duncan
Mean (SD)
Duncan
Mean (SD)
Duncan
0.94 (0.12) 0.95 (0.09) 0.91 (0.17)
A A A
0.94 (0.11) 0.95 (0.11) 0.89 (0.19)
A A B
0.95 (0.10) 0.89 (0.21) 0.85 (0.21)
A B C
Summarily, the results of two-factor interactions show that there is a substantial and significant decrement in performance when two degrading levels of two factors are combined.
4. Discussion The major purpose of this study was to investigate the effects of pictorial size and thickness of the red-circle slash on glance legibility for prohibitive symbols under visually degraded situations resulting from reduced luminance contrast and limited exposure time. Data analysis showed that luminance contrast affected the glance legibility of prohibitive symbols as expected. Prohibitive symbols with higher luminance contrast produced higher glance legibility scores than symbols with lower luminance contrast. This result is consistent with many previous research findings that low luminance contrast between figure and background was detrimental to visual work (Synder, 1988; Bullimore and Fulton, 1991; Shieh and Chen, 1997; Cobb and Moss, 1928). Exposure time also affected the glance legibility of prohibitive symbols. Prohibitive symbols with the longer exposure time attained higher glance legibility scores than those with the shorter exposure time. This result also agrees with previous research findings where visual performance was better when the observer received longer viewing times (Bullimore and Fulton, 1991). Considering the interaction of luminance contrast with exposure time, the results indicated that the glance legibility scores might degrade even more when situations of both low luminance contrast and shorter exposure
existed at the same time. As expected, under both degraded conditions, the legibility performance was reduced more severely. For the effect of pictorial size, as size increased from 50% to 100%, glance legibility scores increased. However, no significant improvement in glance legibility scores resulted from the 75% to the 100% pictorial size. Generally speaking, larger pictorials are easier to recognize, and hence provide for better legibility. Caird et al. (1997) also found that small pictorials and pictorials with slashes result in difficulties in comprehension. With respect to the effect of slash thickness, as thickness decreased from 45% to 25%, glance legibility scores increased. But, the increase leveled off when the thickness was below 35%. The greater the slash’s thickness, there was less space for the pictorial, hence resulting in lower glance legibility. Both the effects of pictorial size and slash thickness agreed with Shieh and Huang’s findings (2002) that the 75% pictorial size might be large enough and the 35% slash thickness might be the upper limit for prohibitive symbol design. Further, the significant Pictorial Size Slash Thickness interaction indicated that the effect of slash thickness was small when pictorial size was large. However, when pictorial size was small, the greater the slash thickness, the worse the glance legibility. A smaller pictorial could be covered more by a thicker slash than a greater one, thus, resulting in poor glance legibility. A major purpose of the present study was to determine how the preferred levels of pictorial size and slash thickness would be affected by unfavorable viewing conditions, such as reduced luminance contrast of the pictorial and its background
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and limited viewing time. The results of data analysis showed that under higher luminance contrast (1:20) or longer exposure time (50 ms), the effects of pictorial size and slash thickness were not significant. It appeared that subjects could recognize symbols correctly when the contrast between symbols and background is high enough or when viewing time is long enough. Thus, the detrimental effects of small pictorial size (50%) might not emerge under favorable viewing conditions. However, under degraded situations resulting from the reduction of luminance contrast of the symbols or/and limited exposure time, pictorial size might be significant factors influencing glance legibility. Under degraded situations, larger pictorials could compensate for the effects of the degraded situations because the larger pictorials are easier to recognize. As shown in the results, under low luminance contrasts (1:3) and short exposure time (33 ms), as pictorial size increased from 50% to 100%, glance legibility scores increased. Besides, in this situation, legibility of smallest pictorials (50%) was seriously worse than that of 75% pictorial size and 100% pictorial size. Based on the results of this study, a pictorial size that is at least 75% of the inner diameter of the red-circle slash is appropriate when luminance contrast is low and exposure time short. A smaller pictorial size may be acceptable under more favorable viewing conditions. Like pictorial size, the detrimental effects of large slash thickness (45%) might not emerge under favorable viewing conditions. However, the thickness of circle slashes might be significant factors influencing glance legibility under degraded situations resulting from the reduction of luminance contrast of the symbols. Hence, as slash thickness increased from 25% to 45%, glance legibility scores decreased under degraded situation. A thin slash that covers less of the pictorial would have an advantage in recognizing the symbols than would a thick one. Under this condition, the data analysis showed that the 35% slash thickness might be the upper limit for prohibitive symbol design. However, the results also showed that the interactions between slash thickness and exposure time were not significant. Generally, red slash is
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not easily seen in limited time, especially in dark environment because both cons and rods are not sensitive to long wavelength (Hendee and Wells, 1997). It is possible that both levels of exposure time were so short that the red slashes were overlooked. Besides, the three-factor interactions between slash thickness, exposure time and luminance contrast were not significant, either. Finally, the appropriate thickness of the redcircle slash may be as small as 25% or 35%, instead of at least 35% as suggested by ISO 3864 under degraded situations. Under degraded situations, the greater the slash thickness, the lesser the available space for the pictorial and the more the pictorial is covered by the slash, hence resulting in lower legibility. The present study found that legibility declined significantly as the thickness of the slash was greater than 35%. Further, the best legibility was with the 25% thickness when luminance contrast was low and exposure time short. Based on these results, the recommendation of ISO 3864 (1984) that the red-circle slash should be at least 35% of the area of the sign is questionable, especially under unfavorable viewing conditions. However, a large slash thickness may be beneficial to noting prohibitive meanings. Subjects may overlook the red slash shape in prohibitive symbol when the slash thickness is not thick enough. In order to keep clear legibility of both pictorials and slashes in prohibitive symbols, the slash size should not be too small. There are at least two implications in the present study. Firstly, The luminance contrast of prohibitive symbols may be low due to environmental conditions, such as rain, snow and smog. To the extent that the environmental factors reduce the luminance contrast of prohibitive symbols, the legibility of prohibitive symbols could be reduced significantly. Second, short exposure time may degrade legibility of prohibitive symbols. Longer exposure time may be comparable to driving at a slower speed; and shorter exposure time, to a faster speed. Hence, the results of this study might indicate that as driving speed increases, the legibility of prohibitive symbols may be reduced significantly. However, it still needs more evidences to support this point because the
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subjects did not perform a driving task in the experiment.
5. Conclusion The pictorial size, thickness of the red-circle slashes, luminance contrast, and exposure time interactively affected the legibility of prohibitive symbols. When luminance contrast is high and exposure time long, the subjects have better pictorial discriminability and enough time to process the symbols, and thus the effects of pictorial size and slash thickness may not be significant. On the other hand, when luminance contrast is low and exposure time short, a large pictorial size lessens the degree to which critical features of the pictorial are covered and prevent the forming of bad features, and thus benefits glance legibility. Further, when luminance contrast is low, a small slash thickness prevents the forming of bad features, and thus benefits glance legibility. Finally, the desirable thickness of the red-circle slash would be as small as 25% or 35%; the recommendation of ISO 3864 (1984) that the redcircle slash should be at least 35% of the area of the sign seems to be questionable, especially under unfavorable viewing conditions.
Acknowledgements This study was supported by a Research Grant from the National Sciences Council of the Republic of China, Grant No. NSC-92-2213-E-161-002.
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