How visible are exit signs in smoke?

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Lighting Magazine, 2, 5, pp. 40-41, 44-45, 1988-05

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How visible are exit signs in smoke?

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How Visible are Exit Signs in

Smoke?

by M.J. Ouellette

Reprinted from

Lighting Magazine

Vol.

2,

No. 5,1988

pp. 40-41,44-45

(IRC Paper No. 1808)

NRCC 34023

How visible are exit signs in smoke?

Smoke density and ambient illumination strongly influence the visibility of three

types of internally lighted exit signs studied by the

NRC.

by Michael J. Ouellette

National Research Council

A

s technologies advance, so do expectations of the assured maintenance of building ser- vices and commodities.

It is assumed that in the unlikely event of a failure of lights and ventila- tion in a building, sufficient safeguards have been incorporated to ensure oc- cupant safety.

In the UK, for instance, the British Standards institution (BSI) publishes recommendations for the design, con- struction and illumination of exit signs. Building occupants generally as- sume that adherence to these recom- mendations will ensure highly visible exit signs that convey sufficient infor-

mation to guide them safely outdoors. The assumption, unfortunately, is not necessarily valid.

For years, brightly coloured signals and signs have been considered to be more conspicuous than dimmer ones. The BSI, however, restricts sign luminance to 80 cd1sq.m. or less.

In Canada there are rro specifications for sign luminance (1) although some agencies, notably the Canadian General Standards Board and the Office of the Fire Commissioner of Canada, attempt to specify sign luminance in- directly in terms of the electrical power consumption of the sign.

The National Fire Protection As-

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2- _{Figure 1. (a) Threshold sign illuminance and (b)}

1.86 threshold sign luminance for the three types of exit signs (A, Band 0. Each bar represents the average of the common logarithm of 144 meas- urements (12 subjects x 2 smoke densities x 2 states of ambient illumination x 3 repeated measures).

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sociation in the USA is quite restrictive. It recommends luminances ranging be- tween 6.9 and 10.3 cd1sq.m. (2 and 3 fL), depending on the nature of the sign. for certain signs (those having both letters and background trans-il- luminated), the Standards Association of Australia specifies a minimum luminance of 8 cd/sq.m., with no im- posed maximum limit. Other types of sign (trans-illuminated letters with opa- que background) are subject to a mini- mum of 2cdlsq.m. and a maximum of 25 cd1sq.m.

The wide range of sign design re- quirements demonstrates a lack of con- clusive research in parameters affecting exit sign visibility, especially for condi- tions in which exit signs are perhaps needed the most, when visibility i s reduced by smoke. Smoke can severe- ly alter the appearance of objects by both light attenuation and scatter.

A controlled experiment was con- ducted recently by the National Re- search Council of Canada (2) to assess the readability of three common exit sign designs under different levels of smoke density and ambient illumina- tion.

The study identified a numberof fac- tors that likely affected sign visibility in smoke. For example, ambient illumina- tion (room lighting) in smoke creates a luminous veil between the observer and the exit sign, much as high beam automobile headlights in fog may obscure the view of the road. This veil reduces sign visibility by reducing the contrast of the letters against their back- ground.

Signs with trans-illuminated letters and opaque backgrounds (arbitrarily called type A) appeared to perform bet- ter in smoke than signs with trans-il- luminated background and opaque let- ters (type B) or those with both trans-il-

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luminated backgrounds and letters (type C).

As with sign brightness, this factor was not varied parametrically. The re- searchers speculated, however, that signs with large luminous areas may create a veil in smoke which reduces their own visibility as does ambient il- lumination (though probably to a lesser degree). The objective of the study sum- marized here was to delineate more clearly the visibility of each of these three types of exit signs in smoke, both in darkness and in simulated emergen- cy lighting. Complete technical details are described elsewhere (3).

Experimentation

Human subjects viewed, through a smoke-filled cavity, three types of exit sign at low and high levels of smoke density, with and without ambient il-

lumination. At every combination of these independently manipulated vari- ables, every subject adjusted the bright- ness of the signs to the threshold criterion of 'just readable'. The ex- perimenter then measured the amount of light falling on the back of the exit sign. From these measurements threshold sign luminance was deter- mined.

Smoke density was found to be the strongest variable affecting the threshold readability of exit signs. On average, the quantity of back illumina- tion on the exit signs had to be in- creased by about a factor of 50 to com- pensate for a ten-fold increase in the amount of smoke.

A similar relation was observed for the minimum required luminance of exit signs. This is consistent with our earlier reports proposing that bright signs are more visible in smoke than dim ones.

It is therefore quite possible, and foreseeable, that signs meeting existing minimum brightness specifications (and even maximum specifications) for smoke-free conditions will not be visible in smoke.

Ambient illumination

Ambient illumination also affected threshold readability. On average, the quantity of back illumination on the exit

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sign had to be increased by a factor of approximately 1.6 to compensate for the effect of increased ambient

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lumination from the low (essentially 0) to the high (0.55 lux vertical on face of signs) experimental levels. Similar in- creases in sign luminance were also required.

These results support our previous conclusions that ambient illumination reduces sign visibility in smoke. It i s reasonable to propose, therefore, that lighting along lines of sight to exit signs should be reduced whenever smoke is detected.

This may bea relatively simple, but not necessarily inexpensive, task for build- ings operating on automated control sys- tems. If this is not practical, the task might

be

performed manually by designated personnel in the building, as should the closing of doors and windows.

Of course, separate and accessible switches would be required for control- ling the potentially problematic luminaires. However handled, it is probable that buildings adopting such strategies will have more readable exit signs in smoke. Buildings without such strategies should therefore require even brighter exit signs to compensate for the luminous veils created by uncontrolled ambient lighting.

Sign Types

In terms of threshold illuminance, the three signs differed greatly. Their rank ordering was: coloured trans-il- luminated letters on opaque back- ground (type A); opaque letters and coloured trans-illuminated back- grounds (type B); and coloured trans-il- luminated letters on white trans-il- luminated backgrounds (type C).

On average, threshold illuminance had to be approximately five times greater for the first sign than for the third sign. Thus, for signs at threshold readability, type C signs will tend to be about five times more efficient.

These distinctions are not recog- nized in some codes which specify exit -signs in terms of the wattage of the internal light source. By specifying wat- tage, signs that just meet the minimum requirements of these codes will con- sume approximately the same energy but will not necessarily be equally

readable.

In terms of threshold luminance, the signs were approximately the same. When expressed in this way, there is less need to distinguish between dif- ferent design types. Signs meeting the minimum requirements of codes ex- pressed in this manner will have ap- proximately the same threshold readability but will not necessarily have the same lamp wattage requirements.

Although there were only small dif- ferences in threshold sign luminance for the three signs studied, it was found that, on average, signs with white trans- illuminated backgrounds (type C) re- quired greater luminance than those with darker backgrounds (types A an B).

This is consistent with our supposi- tion that signs with trans-illuminated backgrounds generate more luminous veil in smoke, thus reducing their own readability. The effect i s relatively small, but other features of signs may produce larger effects, for example if downlighting apertures are designed to illuminate the exit route. lt i s

reasonable to suppose that the il- lumination produced by such signs would be sufficient to decrease readability in smoke.

Turning off the ambient illumination improves sign readability in terms of both threshold sign illuminance and threshold sign luminance. The effect was slightly stronger, however, at high smoke density.

This interaction may be explained by considering that ambient illumination reduces sign visibility by creating a luminous veil scattered from particles of smoke. The magnitude of this veil is expected to increase when either the quantity of airborne smoke particles in- creasesorthe level ofambient illumina- tion increases.

Thus, at high smoke density, the am- bient illumination more effectively produces a luminous veil, which in turn more effectively reduces the readability of the signs.

Recommendations

Based on these observations, the fol- lowing recommendations are made:

Building code guidelines should consider exit sign visibility in smoke specifically.

Limitations on maximum sign luminance should be reconsidered since the present study has found no evidence that sign readability in smoke is reduced at luminances ex- ceeding the maximum limit of British Standard 2560 (specification for internally illuminated exit signs). To improve the visibility of exit signs, room lighting along all lines of sight to exit signs should be reduced as much as possible whenever smoke is present.

4 Exit signs should be specified in terms of their luminance. Specifica- tion of the light source alone is less germane to sign readability. When specifying exit signs in terms of their luminance, there is little need to distinguish between sign types A, B and C.

References

(1) National Research Council of Canada, 1985, "National Building Code of Canada 1985", NRCC 231 74, Sections 3.4.6 and 9.9.1 0

(2) M.S. Rea, F.R.S. Clark, and M.J. Ouellette, "Photometric and Psycho- I

physical Measurements of Exit Signs Through Smoke", National Research

Council of Canada publication NRCC I

24627, Ottawa, 1985.

(3) M.J. Ouellette, "Proceedings of the National Lighting Conference", March 27-30, 1988, Cambridge, U.K., also Lighting Research and Technology, V0l.20 NO. 4, PPS 155-1 60-1 988

Michael

1.

Ouellette is senior Techni-

cal Oficer, Building Performance Section of the Institute for Research in Construction, the National Re- search Council of Canada, Ottawa. Material in this article is taken in part from his paper "Design Parameters For More Visible Exit Signs in Smoke" contained in the Proceedings of the

National Lighting Conference,

Cambridge, U.K., March 27

-

30,

1988, pp. 89

-

100.