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1 2@ 3 4 5 6 7
SIG?) NUMBER (Size)



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TOTAL OPTICAL DENSITY

0.032

0.100

0.320

= x 3lced/m2 s 1.000
© 89 cd/m2
{ A 158 cd/m2

ee
1 2 3 4 5 6 7
SIGN NUMBER (Size)

Figure 3. Density values two standard deviations below the means
at which seven sign sizes were identified when presented
at three background luminance levels.

PERCENT LIGHT TRANSMISSION


The output of the photometric smoke measurement system was recorded
on a strip chart recorder. As the output rose from zero in dense smoke
toward the cleaz air value, the chart was indexed at the values at
which the subjects responded to each size of sign. The millivolt (mv)
values recorded from the measurement system were converted to optical
density values by the formula:

Optical Density = 1/€ log Tc/Ts

where: qd is the unit distance between the reference
light and the phototube (0.5m),

Te is the clear air (mv) value,
and

Ts is the (mv) value at which the signs were
identified.

Residual smoke deposits on the interior surfaces of the viewing
windows resulted in an apparent increase in the effective smoke density
as viewed by the subjects over and above the density measured by the
system. These deposits, with a mean combined density of 0.17, and ail
falling within a narrow range of values, were found to accumulate
during the time the chamber was being charged with, and during the
initial evacuation of, the smoke prior to identification of the first
sign. In the absence of significant increases of such deposits during
the period of subject responses, a constant correction factor of 0.17
was added to the individual density values calculated from the output
of the smoke measurement system.

Results.

The means, standard deviations, and ranges of the optical densities
ef black smoxe at which the subjects could read the seven sizes of signs
at three background luminance levels are listed in Table 2. Figure 2
plots the means for the cotal density values.

Increasing character sizes and background luminance levels made
the signs readable at higher overall density levels, but the gains were
relatively smail in comparison to the magnitude of increases in physical
size and luminance levels.

Figure 3 shows the density values two standard deviations below
the means for the three background luminance levels. These are the
values at which approximately 98 percent of the general population with
20/20 distant visual acuity anc comparable brightness and contrast
discrimination thresholds can be expected to be able to read similar
signs under similar conGitions. The ranges of density values spanned



character format was based on that used by the Grimes* exit light sign
and is described in the e_rlier report. Table 1 gives the height and
siroke width of the characters together with the corresponding visual
angles for the 196-cm (77-in) viewing distance. The smoke chamber
occupied 183 cm (72 in) of the total viewing distance. Sign No. 4
approximates the size commonly found on commercial aircraft.

TABLE 1. Character Sizes and Visual Angles of
Experimental Signs

Sign Character Height Visual Angle Stroke Width Visual Angie
No. (za) {in) * (min) (ram) {in) (min)

1 211.5 (8.33) 371.4 25.40 (1.00) 44.6

2 108.2 (4.26) 190.1 13.10 (0.52) 23.0

3 52.7 (2.07) 92.6 6.35 (0.25) 11.2

4 46.2 (2.84) 21.2 5.45 (0.21) 9.6

5 26.3 (1.04) 46.2 3.20 (0.13) 5.6

6 13.4 (9.53) 23.6 1.60 (0.06) 2.8

7 6.4 (0.25) 11.2 0.77 (9.03) 1.4

The three luminance levels were the average of measurements taken
at 24 equally spaced grid points on the observer's side of the available
viewing area of the diffusing screen before a sign was mounted for view-
ing. The 89-cd/m2 (26-fL) luminance level is approximately the exit
sign background luminance level required by FAR 25.812. The 31- and
158-cd/m2 (9- and 46-fL) vaiues were the minimum and maximum levels
obtainable with the lighting system. ‘fhe candela (cd) is the Inter-
national System of Units (SI) unit of luminous intensity. The metric
unit 1 cd/m? is approximately equivalent to 0.29 fL.

Procedure.

Prior to each test session the lenses and interior window surfaces
were cleaned, the chamber sealed, the recording system zeroed to base~-
line, the largest sign inserted in the mounting slot, the luminance
adjusted to the desired level, and the smoke introduced into the chamber
until it reached a density at which the recording system no longer
registered a measurable light output.

The subject, who had spent several minutes adapting to the mesopic
ambient lighting condition, was then seated behind the rear viewing
window with eye height adjusteé to give normal line-of-sight viewing of the
signs through the smoke-filled chamber. Smoke was then drawn out of the
chamber by an exhaust fan at a controlled rate until it reached the
@epsity at which the subject could first distinguish the light from the
sign. From this point the remaining smoke was allowed to dissipate at
its slower normal rate until the subject correctly identified the sign.
When a s‘gn had been identified the next smalier sign was immediately
presented until all seven signs had been identified.

*Grimes Mfg. Co., Urbana, Ohio, Emergency Exit Light 10-0067-3.

3



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4.5r 70.063
. 2
2°
or 40. n
> 4.0 0.010 8
= =
wo on
2 . 3
Wo35h 40.032%
7 F
< bE
oO =
= 4
ao 3.0F 40.100 5
° b
2 ig
< °
© 2.5 x 31 cd/m? 4 0.320 &
© 89 cd/m? o

& 158 cd/m?
2.0F 41.000
4
4 i i. tL. lL L i 3

I 2 3 4 5 6 7
SIGN NUMBER (Size)

Figure 2. Means of smcke density values at which seven sign
Sizes were identified when presented at three
background luminance levels.


REESE a TE nn ha Sie He epee

FAA-AM-80- 13

READABILITY OF SELF-ILLUMINATED SIGNS
OBSCURED BY BLACK FUEL-FIRE SMOK=

BP. G. Rasmussen
B. P. Chesterfield
D. i. Lowrey

Civil Aeromedical Institute
Federal Aviation Administration
Ckliahoma City, Oklahoma

AD A092529

ely 1980

Document is available to the public through the
National Technical Information Service
Sprincfield, Yirginia 22161

Prepared for
U. S. DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
Office of Aviation Medicine
Washington. B.C. 20591

COPY,

DDC ur

2n 3109 AM HOA


jithin the range studiec, there is a2 marginal over: increase in
the mean smoxe densities at which the signs can be vead es @ function
of increasing backgrouné luminance levels. However, increases past

89 cé/n? (26 £1) appear to be effective only for the signs with the
smallest character sizes. The values two standard deviations below the
means show the 89-c@/z? (26-£5) luminance level to be most effective
for ail but the two smallest charecter sizes. s change from the
relative mean values results from variability of the standard deviations
oz the means at the 89-cd/m* {26-£1) luminance level.

With the opaque letters used in the signs, an increase
background luminance should not only provide more light but also increase
the contrast between the chaiacters and the background. In the presence
of smoke, however, the back: = tends to be scattereé throughout
the chatber. ‘his causes the ambient ievel tc be ea once the smoke
Gensity falis below the valve that biccks the visibie iicht. This
scattering also causes some o he light to be reflected back on the
surface of the sign, reducing the effective contrast ratio. The less of
sign effectiveness in the presence
function cf light attenuation, bet also e degradation of coherence or
Sefinition of the transi nt and @ greater reduction contrast

nance level viewed under normal conditions.

The use ons with all seven
sign sizes may have influenced results attributed to the background
luminance level. With this ¢ the sicns the sma
characters had a larger ratio of the area
covered by the characte han the signs with the larger characters,
while the signs with +5 naracters had more total
face area.
at by an opt
more effective than
of legibility of smoke obscu:

ee.

ty, may De
variable

affect vision.
increased carbon monoxide con:
visual process but also the
required for escape (23.
nat: , smoke may aiso
sobstances which, even
when dissolved in the moist:
rymai glends,

the smoxe

therefore 5.

Gensities of

ndicates they are

o


by these data @ctively reduce the anparent brightness
to ketween 0.08 ang 1.00 percent of the clear air valtes. creasing

Sharacter sizes still compensates in part for nereasing smoke density,
hut the role of background luminance becomes less spec:

Discussion.

The density values reported in Table 2 are baseé on threst
Qeterminations of what is just recognizably visible ang do not evaluate
Rhe overall effectiveness of the signs in meeting their intended pur-
Boses the presence of smoke. The subjects' knowledge of the location
and nature of the signs, directed attention to the task, lack of situe-
tional distractions, and absence any direct contact o£ the eyes with
the smoke contributed to better vistal performance than can be expected
for aircraft passengers and crew under conditions involving smoke in
the cabin area. For these reasons the reported density values should
he considered the upper limits at which the signs can be read rather
than the values at which comparable signs can be effectively read under
&ctual emergency Conditions.

Many factors contribute to establishing tne brichtness and contrast
discrimination thresholds that determine visibility or legibi
At dim brightness levels, contrast retios become more critical than at
Righer brightness levels (10,11). If a reaction tine criterion
adéed to that c= Simple recognition, the advantage of hicher contrast
ratios and higher brightness levels hecones even more apparent (6,11).
These thresholds are also related to age (8). A number of investigators
have demonstrated a need by older age groups for more light and higher
Contrast ratios to make effective discriminations than is required by
Younger groups {4,6,10).

Common design practices for improving legibility and attention
Values of Gisplays include increasing character size, increasing the
luminance level, and increasing the contrast ratio between the legend
and the background, The character Sizes used in this study spanned,
and considerably exceeded, the rence of sizes listed by smith (9) in
his summary of recommended letter sizes used in various display applica~
tions with unobstructed viewing. The largest character height exceeded
the high end of the recommended design rance for critical data with
fixed position and a luminance level below 3.4 cé/m? (approximately
1.0 £L} by more than 10 to 1 and the low end of the des. gn range >y
ore than 19 te 1. Sign No. 4, which approximates the size format
© the exit sign Commonly founé
Craft, exceeded these design recommendations by factors of 2.2 to i and
4.3 to 1, respectively. The characte: sizes in Signs Nos. ani 4, for
example, have @ height ratio of 4.6 to 1, but the increased densities
at which the large: gn was igentified were only doubied;
teguired light transmission was only halved. creasing character
Sizes would therefore seem tc be an inefficient nethed for overcoming
increasing smcke Censities.

i the cabin area of commercial air~

2.e., th

wo



TABLE

2. Means, Standard Deviations, and Ranges of the Optical
Densities of Black Smoke at Which Signs With Seven Character

Sizes Were Identified When Presented at Three
Background Luminance Levels

Smoke Density

Smoke Density

Sign (total) (per meter)

No. Mean S.D. Range Mean s.D. Range

31 _cd/m2 (9 £1)
1 3.88 0.45 3.25 - 4.76 2.20 0.25 1.85 2.68
2 3.74 0.46 3.08 - 4.60 2.12 0.25 1.76 2.55
3 3.58 0.43 2.96 - 4.40 2,04 0.24 1.70 2.48
4 3.50 0.41 2.87 - 4.28 1.99 0.22 1.65 2.41
5 3.37 0.39 2.78 - 3.98 1.92 0.21 1.59 2.25
6 3.10 0.30 2.49 - 3.49 1.77 0.16 1.44 1.99
7 2.55 0.25 2.11 - 3.07 1.47 0.13 1.23 1.82

89 cd/m2 (26 £L)
1 4.05 0.47 2.98 ~ 5.08 2.29 0.25 1.71 2.85
2 3.91 0.42 2.92 - 4.73 2.21 0.23 1.67 2.66
3 3.75 0.43 2.83 - 4.73 2.13 0.23 1.62 2.66
4 3.63 0.38 2.82 - 4.17 2.06 0.21 1.62 2.36
5 3.48 0.32e 2.75 - 4.03 1.98 0.21 1.58 2.28
6 3.20 0.38 2-56 - 3.80 1.83 0.21 1.48 2.16
7 2.71 0.33 2.33 - 3.56 1.55 0.18 1.35 2.02

158 c@/m2 (46 £L)
a 4.00 0.46 3.01 - 4.71 2.26 0.25 1.72 2.65
2 3.88 0.48 2.95 ~ 4.80 2.20 0.26 1.69 - 2.70
2 3.75 0.49 2.87 - 4.77 2.13 0.26 1.65 2.68
4 3.67 0.47 2.83 - 4.59 2.08 0.26 1.62 2.59
5 3.55 0.45 2.70 - 4.36 2.02 0.25 1.55 2.46
6 3.32 0.38 2.56 - 3.80 1.89 0.21 1.47 2.22
7 2.84 0.26 2.20 - 3.30 1.63 0.14 1.28 1.88



READABILITY OF SELF-iLLUMINATED SIGNS
OBSCURED BY BLACK FUEL-~FIRE SMOKE

Introduction.

Minimal design requirements for emergency exit signs used in civil
air transports are specified in Federal Aviation Regulations (FAR)
25.812. These requirements insure adequate visibility and readability
under normal viewing conditions. However, some emergency conditions,
such as postcrash fires, may introduce smoke into the cabin area and
obscure the signs at a time when their prominent visibility is most
critical.

The present study, using black fuel-fire smoke, is a partial replica-

tion of an earlier study (7) that used an inert white smoke to investi-
gate the readability of smoke~obscured, self-illuminated emergency exit
signs of various sizes and background iuminance levels. The use of an
inert white smoke offers many practical advantages as a researc! tool,
put a quantitative comparison with black smoke is required to determine
its effective relative visual screening ability before it can be equated
with, or used to predict visual performance in the presence of, black
fire-generated smoke.

thods.

Subjects. Subjects consisted of 11 male and 8 female aduit volun-
teers and paid participants. Six subjects required corrective lenses to
achieve 20/20 distant visual acuity. Ages ranged from 20 to 55 years
with a mean of 35 years.

Equipmert. The equipment arrangement is schematically illustrated
in Figure 1 and is described in greater detail in the report on the
study using white smoke (7).

The National Bureau of Standards (NBS) Photometric Smoke Measure-
ment System was mounted beneath the subjects' line of sight and was
physically obscured from view. The system was modified and calibrated
for operation with a 50-cm (19.7-in} separation between the reference
source light and the phototube detector.

A firebox was constructed from steel drums in which cotton rags
soaked in JP-4 jet fuel and hydraulic cil were burned to generate the
black smoke. The smcke was drawn through a screen to filter out the
larger soot particles and collected in a cooling vessel, also con-
structed of steel drums. From there it was Gucted to the smoke chamber
as needed and evenly dispersed by a smaj.l agitating fan mounted in a
corner of the chamber.

Displays. The sicns were composed of the characters "E, X, I, T"
except that the largest sign was limited to two characters. Character
sequences were varied tc minimize anticipatory identification. The



10.

Brown, J. L., and C. G. Mueller: Brightness Discrimination and
Brightness Contrast. In C. 3. Graham (£é.}, Vision and Visual
Perception, New York, John Wiley and Sons, Inc., 1966, pp. 208-250.

Graham, L.: Research Into Post-Crash Fires, AVIAT. ENG. & MAINT.
1:52-56, 1977.

Grant, V. M.: Toxicology of the Eye, Springfield, Illinois, Charies
Cc, Thomas, 1974, p. 922.

Guth, S. K.: Effects of Age on Visibility, AM. J. OPTOM. 34:463-477,
1957.

Henahan, J. F.: Fire, SCIENCE 80(1):29-38, 1980.

Mourant, R. R., and G. Langolf: Luminance Specifications for Auto-
mobile Instrumenc Paneis, HUM. FACTORS 18:71-84, 1976.

Rasmussen, P. G., 3. B. Garner, 5. G. Blethrow, and D. u. Lowrey:
Readability of Self-Illuminated Sigms in 2 Smoke-Obscured Environment.
PAA Office of Aviation Medicine Report Ne. FAA-AM-79-22, 1979.

Richards, O. W.: Effects of Luminance and Contrast on Visual Acuity,
Ages 16 to 90 Years, AM. J. OPTOM. PHYSIOL. OPTICS 54:178-184, 1977.

Smith, S. L.: Letter Size and Legibility, BUM. FACTORS 21:661-670,
3979.

Welsh, K. W., P. G. Rasmussen, and 3. A. Vaughan: Readability of
Alphanumeric Characters Having Various Contrast Levels as a Funstion
Age and Iilumination Mode. FAA Office of Aviation TM cine
Report No. FAA-AM-77-13, 1977.

Williams, C. M.: Legibility of Members as a Punction of Contrast
ang Illumination, RUM. FACTORS 9:455-460, 1967.



Technicel Report Documentation Page

] 2. Government Accession Now

¥_ fo-A099524

—

'3,_Recipient's Cotalog No.

tie ond Subtitle

READABILITY OF SELF-JLLUMINATED SIGNS OBSCURED BY
BLACK FUEL-PIRE ‘SMOKE’. =
= = 2

ee wee

ee Fe

wh

5 _Bort:

if
i “iwaan ad i

5 Reckoning Otganizetion Code

8. Performing Organization Report No.

7. Author s)

ee

B. P sterfield

Rasmussen D. uf Lowrey

F tion Name ond Address © a 7
FAA Civil Aeromedical Institute

P. O. Box 25082

Oklahoma City, Oklahoma 73125

10. Work Unit No. (TRAIS)

; IT. Consract or Gran? No-

13. Type of Report and Period Covered

12. Sponsering Agency Name end Address
Office of Aviation Medicine
Federal Aviation Administration
800 Independence Avenue
Washington, D.C. 20591

14. Sponsoring Agency Code

15. Supplementary Noves

RDS&E Effort (FAA Form 3550-1) No. ARD-50G-76~2.

Research reported here was conducted under Task AM-B-80-PRS-38 and Request for

16. Abstrect

>This study, using black fuel-fire generated smoke, is a partial replication of an
earlier study using an inert white smoke as the obscuring agent in the study of the
readability of smoke-obscured,self-illuminated emergency exit signs.

to 3.0.

shroud the illuminated signs.

The results indicate that, within the range of sign sizes and background luminance
levels studied, and under otherwise favorable viewing conditions, most of the -igns
were read through black smoke with optical densities ranging between 3.0 and 4,0.
At two standard deviations below the means, most of the size and luminance level
combinations were identified at optical densities ranging from approximateiy 2.5

A comparison of the results with those obtained in th: earlier study using white
smoke, shows both colors of smoke to be approximatel; equal in their ability to
Black smore, however, appears somewhat more effective
in obscuring smali details at or near the normal visual acuity threshold.

17. Key Words 78.
Visual performance

Distribution Statement

Aviation safety
Posterash fire
Emergency evacuation

Document is available to the U.S. public
through the National Technical Information
Service, Springfield, Virginia 22161.

19. Security Classif. (of this report)

Unclassified

[ 2 Security Ci
Unclassified

(of this page? Bie No. of Pages

12

22 Price

Form DOT F 1700.7 @-72)

Reproduction of completed poge authorized

LEYSQO


NOTICE

This document is disseminated under the sponsorship of the
Department of Transportation in the interest of informatic. exchange.
The United States Government assumes no liability for its contents
or use thereof.



{See Pigure 4). The steeper slope of the curve for black smoke suggests

4 sign size (character height and luminovs surface area) is more
exitical for readability in the presence of black smoke thai with white
stoke.

At two standard deviations below the means, black smoke appears
considerebiy more e=fective then whites smoke in obscuring all seven
sign sizes. However, these more prorounced differences result from
the greater variability the black smoke Gata, with the standard
Geviations being approximately twice the magnitude of those for the
white smoke Gata. ‘This qreater variability is not necessarily solely
attributable to a specific optical quality of the Slack smcke itself but
j@ay also be due in part to uncontrollable variations in the character-

istics of the black smoke proGuced on successive trials.