【正文】 sign can be found in the literature [3–6]. For example, a 57mmwide chevron in gray fitted onto a 300mmwide white sign containing the word EXIT of 150mm height would be identified correctly at a mean distance of 30m [3]. In Hong Kong, an exit sign designed with the word EXIT in English and Chinese and an exit symbol of a running person is specified for local buildings [5,6], and a directional indicator shall be used in a directional sign with an exit symbol or the English and Chinese words. Exit symbols (E1, E2), words (W1, W2) and directional indicators (D1 to D5) used in this study are shown in Table 1. The following are their sources. The graphics of a person running through an open door (E1) [7–10] and a person running away from fire (E2) [5] from current codes were used as the exit symbols. E2 is believed to be a good exit indication and prescribed in the local code [6]. The English word EXIT (W1) contributed to accurate identification of exit messages [7–10] and has been widely adopted in design guides and standards [3–6]. The Chinese word for exit (W2) has also been specified for local applications [56]. Visibility of directional indicators and their effectiveness in indicating direction were studied extensively [7–10]. A chevron (D1) was identified as the most visible and effective directional indicator and remended for building environment [3]. Alternatively, an ‘a(chǎn)ngelfish’ (D2) was found to be eye catching and a triangle (D3) (of increased area related to D1) would be easily visible [11, 12]. An arrow containing a chevron with a ‘tail’ (D4) prescribed in British Standard [4] has been used in local buildings [5, 6]. A triangle with a ‘tail’ (D5) was hypothesized to have increased visibility because of its increased area relating to the arrow (D4) [11,12]. Visibility of the binations of E1, E2, W1, W2, D1 to D5 in six colourpairs, namely green and white (GW), red and white (RW), black and white (BW), white and green (WG), white and red (WR) and white and black (WB) as shown in Table 3, was studied and reported in this paper. . Experimental study Visibility is defined as the ability of an observer to both see and identify the sign content correctly at a distance in terms of the height of the content of an exit sign. Smaller sign content for correct identification indicates a higher visibility. Visibility of an exit sign was determined for observers at a fixed distance in smoke clear conditions. The visual distance L2 (m) of a sign of height h2 (mm) would be calculated from the measured h1 (mm) at L1 (m) in the experiment with the same visual angle assumed [22, 23]: L2=L1h1h2The visibility is alternatively defined in terms of distance between the observer and the sign with Eq. (1). Longer distance for correct identification indicates higher visibility associated with the sign. To access the visibility, a corridor of (width)_18m (length) _ (height) with an inspection window of size (height) (width) was used. As shown in Fig. 1, an exit sign installed at the end of the corridor was presented to an observer behind the window. The participants were 22 male and 8 female volunteers aged from 18 to 55 with a mean age of 36. They were invited through local advertising. All of them reported to have normal or correctedtonormal vision and no colour deficiency. A total of 10 participants wore glasses. The experiments were divided into two parts. Part 1 studied the visibility of the designations in Tables 1 and 2. For Table 2, the interference between the two ponents of a sign was also studied. The symbols, words and directional indicators of good visibility were identified and used to design the exit signs for the second part. Part 2 studied the visibility of the exit signs in different colour pairs of sign content and background under normal and emergency lighting conditions. Three exit sign designs in six colourpairs as shown in Table 3 were studied. In both experiments, the observer looked at a sign of content height of 20mm and reported whether the sign content was detected, identified or identified with confidence. The same sign in larger graphics, with a height increment of 5 mm, would be presented until it was identified with confidence. Another sign was then presented in the same way till all the signs were done. The secon