Visibility in Smoke With Different Extinction Coefficients

Abstract

Visibility in smoke has since the 1970s gained some attention, mostly after Tadahisa Jin ran the experiments which helped him develop todays widely used visibility equation. By showing that visibility can be assumed to be inversely proportional to the extinction coefficient by a visibility constant (equal to 8 for light emitting sign and 3 for light reflecting), Jin opened up new doors for researchers who were testing out his theory by going around experimenting in different ways and also by showing that there is way for more research to be done. The visibility constant also allowed for easier modeling of visibility which is used in egress models. However, in recent years, his suggested values for the visibility constant have been thought to be lower, as claimed by the SFPE in their more recent Handbooks. Nonetheless, many factors come into play when studying the visibility of exit signs in smoke such as brightness, light color, signage type (reflecting/ emitting), smoke type (white/ black), and others. In this thesis, a literature review as well as an overview of the current standards of exit signs is presented in the first half. In addition, a small scale experiment was run for the purpose of this thesis. It included 20 participants going through 16 different scenarios where the previously mentioned factors are studied, and their effects on visibility are looked into. The results under black smoke conditions have exhibited an unexpected behavior, not following what the currently used equation dictates, nonetheless a clear performance superiority of red signs has been seen which agrees with the previous literature. It is speculated that this unexpected behavior suggests that for higher values of the extinction coefficient, Jin’s equation does not stand. On the other hand, the results under white smoke conditions were more consistent, and showing that indeed as recently thought, the visibility constant is higher than what Jin had suggested. Finally, also in line with previous research, light emitting signs were shown to have a better performance than light reflecting ones.

Popular Abstract

Exit signs are a crucial factor in performing an efficient evacuation of a building in case of fire. While they are perfectly visible on a normal day, in a smoke filled building, locating them becomes a hard task, and especially for those not familiar with the building, being able to see those signs is crucial for their survival. However when it comes to standards and regulations, the design of these exit signs differs from country to country. In the United States for example, it is required for exit signs to spell out the word EXIT while its color could be green or red depending on the state. On the other hand, the International Organization for Standardization (ISO), widely referred back to across the world, requires the use of pictograms instead of wordings on exit signs (ISO 7010), they also require the exit signs to be of green color. With all these different design standards and requirements, a question that is fair to ask would be: which of these designs is more effective when it comes to visibility in smoke?
In an attempt to investigate visibility in smoke, many researchers have led experimental research to assess the effect of factors such as brightness, color light, signage type, and others on the visibility in smoke. One main researcher, Tadahisa Jin, carried on experiments in the 1970s which allowed him to suggest the now commonly used visibility equation: V.Cs=C, where V is the visibility [m], Cs the extinction coefficient [m-1], and C the visibility constant equal to 3 for light reflecting signs and 8 for light emitting signs (Jin, 1978). This formula along with the suggested constants has been used by engineers to calculate visibility and egress times through modeling where in some software such as Fire Dynamics Simulator (FDS) the default settings are based on the constants 3 and 8. However as more research was established, the values for that constant have been deemed to be higher than previously reported. In addition, several researchers have reported that red signs are more visible in black smoke than green ((Jin, 1977), (Clark et al., 1985, & (Collins et al., 1992)). However, green signs are still the most required by standards and regulations worldwide as people associate the color green with safety, and red with danger (Or & Wang, 2014).
As part of the thesis on the visibility of exit signage in smoke, a small scale experiment was run to investigate the effect of: color, signage type, and brightness on visibility under both black and white smoke conditions. A total of 20 participants, 8 of which being female, an average age of 26, and half of them needing collective lenses, participated in 14 scenarios each. The scenarios changed one variable at a time: high/low brightness, red/ green color, white/black smoke, emitting/ reflective sign. Participants were looking through a smoke filled duct and asked to move the sign closer or further in the duct until they could no longer see the sign, at that point two measurements were taken. The first measurement was the distance (visibility), and the second was the smoke density (from which the extinction coefficient is calculated). Using these measurements, the average visibility constant value of each scenario was calculated, data points of each scenario were plotted and compared to Jin’s suggested values and conclusions were made.
When it came to signage type, emitting signs outperformed reflective signs under all conditions. Whereas for signage color, under black smoke, red had a higher visibility constant, while green performed better under white smoke conditions. Furthermore, as previous research showed, visibility constants were higher than the ones reported by Jin. However, no conclusion could be done for the results under black smoke conditions as the results showed a great deal of scatter, which was not observed in the graphs that were presented by (Jin, 1978). The difference in scatter, especially under black smoke conditions, between the results obtained in the thesis work and those of Jin raised some concern about the reliability of his results. On the other hand, the results obtained under white smoke were of acceptable scatter and data points followed the behavior of the equation suggested by Jin which was not the case under black smoke conditions. Finally, an observation was made in regard to the visibility of females vs males, where females reported a higher visibility.
The experimental work presented gave insightful initial conclusions which should further be investigated. Some improvements could be made to the experiments, and I invite future researchers to develop on it.

• author: Elhokayem, Lea
• supervisor: Håkan Frantzich
• organization: Division of Fire Safety Engineering
• course: VBRM05 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Visibility, smoke, exit sign, extinction coefficient, visibility constant, evacuation
• publication/series: LUTVDG/TVBB
• report number: 5665
• other publication id: LUTVDG/TVBB—5665--SE
• language: English
• id: 9084340
• date added to LUP: 2022-06-07 10:02:49
• date last changed: 2022-06-16 15:51:44