Figure 1, Photographic reproduction of the
five gray background materials
used in the study. Measured con-
trast ratio values for each sector
are (1) 16.2:1, (2) 12.8:1, (3)
9.3:1, (4) 6.6:1, and (5) 3.2:1.

Photometer Alignment Aperture

Reflective Plaque

| Display Drum

Subject Hitt sas

Headrest i /

Artificial Pupil Lamp and

Lenses Filters ‘/ Experimenter's
Control Unit

Occluder Plate

Figure 2, Diagram showing relative location of
display, illumination, and recording
apparatus.



1.

9.

10.

11.

12,

REFERENCES

Ludvigh, E.: Effect of Reduced Contrast on Visual Acuity as
Measured with Snellen Test Letters, ARCH. OPHTHALMOL.
25:469-474, 1941.

Welsh, K. W., J. A. Vaughan, and P. G. Rasmussen: Readability
of Approach Charts as a Function of Visual Acuity, Luminance,
and Printing Format, AVIAT, SPACE ENVIRON. MED. 47:1027-1031,
1976.

Trendelenburg, W.: Der Gesichtssinn, 2d ed., Berlin, Springer-
Verlag, 1961,

Shlaer, S.: The Relation Between Visual Acuity and Illumination,
J. GEN. PHYSIOL, 21:165-188, 1937,

Westheimer, G.: The Maxwellian View, VISION RES. 6:669~682,
1966,

Luckiesh, M., and A, H, Taylor: A Summary of Researches in
Seeing at Low Brightness Levels, ILLUM. ENG. 38:189-207, 1943.

Graham, C. H.: Vision and Visual Perception, New York, John
Wiley and Sons, Inc., 1966, pp. 321-349.

Watkins, R. D.: The Presentation of Printed Information to
Aircraft Pilots, Aviation Medical Branch, Department of Civil
Aviation, Aviation Medicine Memorandum No. 27, Melbourne,
Australia, 1970.

Weale, R, A.: The Aging Eye, New York, Harper and Row, 1963,
Wulfeck, J. W., A. Weisz, and M, W. Raben: Vision in Military
Aviation, Wright Air Development Center Technical Report No.
58-399, Wright-Patterson Air Force Base, Ohio, 1958.

Blackwell, H, R.: Visual Basis of Desirable Standards of
Quantity and Quality of Illumination, AM. J. OPTOM. 40:581-613,
1963.

Mourant, R. R., and G, Langolf: Luminance Specifications for

Automobile Instrument Panels, HUM. FACTORS 18:71-84, 1976.

10


IIl.

Iv.

subject, These measurements, obtained with a Goldmann-Weekers
adaptometer, were compared to population norms to rule out
abnormal dark adaption processes that may affect visual per-
formance under mesopic illumination levels.

Results.

To minimize variability for different target contrast
levels and thus facilitate statistical comparisons, we trans~
lated the means of the five determinations (20 characters) made
by each subject at each contrast level into their base 10 log
values, The data points appearing in Figure 3 for the white
illumination trials and Figure 4 for the red illumination trials
are the means of these log values.

Data indicate a significant difference (P < -01) in lumi-
nance values for successive changes in contrast ratio for both
illumination modes.

To evaluate differences in lighting requirements between
the three age groups, we made tests for linear trend for each
contrast level and illumination mode. Under white illumination,
statistically significant F-values were found across age groups
for three of the five contrast levels. Significant trends
indicating more illumination with increasing age occurred at
the 0.05 level for contrast ratios 9.3:1 and 6.6:1, while the
lowest contrast ratio (3,2:1) was significant at the 0.01
level. Under red illumination, a linear trend with age was
indicated for all contrast levels, Values of F at the 0.05
level were found for contrast levels 12.8:1 and 3,2:1; the
remaining contrast levels were significant at the 0.01 level.
These trends are indicated graphically in Figures 3 and 4.

Table 1 shows the illumination required by the older and
middle-aged groups as a percentage of the younger age group
(20-25 yr) for each of the five contrast levels.

Discussion.

To observe the effects of contrast ratio variations within
each age group, we held constant the following parameters:
(i) target size, (ii) viewing distance, (iii) visual acuity,
and (iv) pupil size, The dark adaptation state of the subjects
could not be precisely controlled during the testing period
because illumination requirements varied between contrast

5



The minimum luminance level required to identify the test
characters was measured with a photometer (Pritchard Spectra,
1970 PR) located near the subject's left shoulder. The
photometer was directed toward a small white reflectance
plaque located immediately below the viewing aperture in the
occluder plate as shown in Figure 2. To correct the photometer
readings taken from an oblique angle 22° to the subject's
line-of-sight, the experimenter obtained a correction factor
and applied it to all photometer readings. The correction fac-
tor was found by taking a series of readings from both angles
(oblique and perpendicular to the surface of the plaque) under
a variety of luminance conditions, These readings were found
to be consistent at each location and amounted to a 14-percent
reduction in all luminance measurements.

The subject population was divided into three groups of 12
each as follows: (i) a younger group with a mean age of 22.9
years, range 20-25; (ii) a middle-aged group with a mean age
of 43.6 years, range 40-46; and (iii) an older group with a
mean age of 63.8 years, range 60-70. Prior to testing, each
subject was given a complete visual examination.

Each subject was tested monocularly by using the eye with
the lowest ocular refractive error. This method was used to
minimize the thickness of the lens and increase light trans-
mission into the eye, Any subject not requiring a prescrip-
tion lens wore a plano (0.0 D) lens in the trial frame.
Located behind either the plano or prescription lens was a
2.0-mm artificial pupil centered in the trial frame to match
the visual axis of the eye. The opposite eye was occluded
during the testing period,

A forehead rest located 40 cm (15.7 in) from the target
and a circular alignment aperture 2.0 cm (0.78 in) in diameter
standardized the distance and viewing angle for all subjects,
Following a brief demonstration of the experimental procedure
and a 10-min dark-adaptation period, the illumination in the
test area was raised gradually until the subject correctly
identified all four alphanumeric characters, The luminance
value was recorded and the light on the test characters was
dimmed for approximately 10's prior to the next trial.
Approximately 45 min, including a 5-min rest period, were
required to obtain threshold luminance values for all contrast
levels and both illumination modes. Eight additional minutes
were required to obtain a dark. adaptation profile curve on each

4


Several investigators (12, 13) have sought to determine
the effect of age on recognition of characters having various
contrast levels. However, none of these investigators equalized
pupil size or corrected visual acuity completely with lenses
for the viewing distance. Their data, although realistic and
predictive of the population at large, are limited when com-
parisons of neuroretinal sensitivity are made among age groups,
albeit their data under white illumination indicate a pro-
gressive decrease in performance with age at scotopic lumi-
nance levels, Our data indicate that additional illumination
is required with increasing age, although only under red light-
ing are significant increases required for most contrast levels,

Guth (15), using one contrast level (black on white, 6-point
type at 76 cm), found that illumination required for fixed read-
ability values increased gradually up to the fourth decade and
more rapidly thereafter. Ferree et al. (16), presenting Landolt
Ring targets (black on white), also found that older individuals
require additional illumination and that visibility of small
targets reaches a maximum at levels of 10 and 20 fc for younger
and older subjects respectively,

Rock (17) investigated the effect of dim illumination on
other visual attributes, including depth perception, absolute
motion threshold, the Muller-Lyer illusion, and a simple addi-
tion task.

Summary.

The minimum illumination required to identify black alpha-
numeric characters on gray background materials selected to
provide a broad range of contrast levels was determined for
three age groups (mean ages 22.9, 43.6, and 63.8).

Data indicate that significant increases in luminance were
required for decreasing contrast levels under both illumination
modes, red and white. Under red illumination, a significant
trend in brightness with age was indicated for all five contrast
levels. Under white illumination, a significant trend was noted
for three of the contrast levels.



Technical Report Documentation Page

T. Report No. 2. Government Accession No. 3, Recipient's Catalog No.
FAA-AM-77- 13

G. Title and Subtitle 5. Report Date
READABILITY OF ALPHANUMERIC CHARACTERS HAVING JuLy 19727
VARIOUS CONTRAST LEVELS AS A FUNCTION OF AGE 6. Performing Organization Code

AND ILLUMINATION MODE

|B. Performing Organization Report No.

7. Author!s)

K. W. Welsh, P. G, Rasmussen, and J. A. Vaughan

9. Performing Orgonization Nome and Address 10. Work Unit No. (TRAIS)
FAA Civil Aeromedical Institute

P. 0, Box 25082 11. Contract or Grant No.

Oklahoma City, Oklahoma 73125

T3. Type of Report and Period Covered

12. Sponsoring Agency Name and Address

Office of Aviation Medicine
Federal Aviation Administration
800 Independence Avenue, S.W. T4. Sponsoring Agency Code
Washington, D.C. 20591 FAA

15, Supplementary Notes

Work was performed under Task No. AM-D-77~-PHY-93.

T6. Abstract
Readability data of alphanumeric characters that vary in figure-to-ground contrast
ratio were obtained from 36 subjects; 12 subjects were placed in each of three age
groups (20-25 yr, 40-45 yr, and 60-65 yr). Minimum illuminance required to
identify all contrast combinations was determined at a viewing distance of 40 cm
(15.7 in) under dim white and red illumination, Subjects identified all characters
while viewing through an artificial pupil (2.0 mm) and, when required, while
wearing a spectacle lens correction.

Data indicate a significant difference in luminance values for successive changes
in contrast ratio under both illumination modes, Under red illumination,
threshold luminance values showed a significant trend with age for all five con-
trast levels. Under white illumination, significant trends were indicated for
three of the five contrast levels.

With reference to the younger group, individuals in the middle-aged and older
groups required an average luminance increase of 18 and 63 percent respectively
for equivalent readability scores under white illumination, Under red lighting,
corresponding values were 18 and 58 percent.

17. Key Words 18. Distribution Statement
Chart readability Document is available to the public
Luminance threshold through the National Technical Information
Target contrast Service, Springfield, Virginia 22161
Effect of age
19. Security Classif. (of this report) WO, Security Classif. (of this page) DI. No. of Pages | 22. Price
Unclassified Unclassified 1

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized





13,

14,

15.

16.

17.

Richards, 0. W.: Effects of Luminance and Contrast on Visual
Acuity, Ages 16 to 90 Years, AM. J. OPTOM. PHYSIOL. OPTICS
54:178-184, 1977.

Williams, C. M.: Legibility of Numbers as a Function of
Contrast and Illumination, HUM. FACTORS 9:455-460, 1967.

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

Ferree, C. E., G. Rand, and E. F, Lewis: The Effect of
Intensity of Light on the Visual Acuity of Presbyopic and
Non-presbyopic Eyes, TRANS. ILLUM. ENG, SOC. 29:296-313,
1934,

Rock, M. L,: Visual Performance as a Function of Low

Photopic Brightness Levels, J. APPL. PSYCHOL. 37:412-427,
1953.

11



4.0
4 Older Group

Or
3 © Middle-Aged Group a
2.0F O Younger Group ‘
1.55 oO
1.O-
0.8
0.6 4

\\

Threshold Brightness (fL)

co}
BS
7
\
Lie}

1 L n
(62:1 12.8:1 9.3:1 66:1 3.231
Target Contrast Ratio

Figure 3, Luminance values measured from a
reflectance plaque for threshold
readability under white illumination
for all contrast ratios and age groups.

4.0

3 ai 4 Older Group

. © Middle-Aged Group

2.0 D Younger Group A

LS} P
‘O

1.OF

0.8F

0.6 F

\\

Threshold Brightness (fL)

°
ny
T

— SE

oO. n n n 1 n
16.271 12.851 9.361 6.6: 1 3.241

Target Contrast Ratio

Figure 4, Luminance values measured from a
reflectance plaque for threshold
readability under red illumination
for all contrast ratios and age groups,


characters. For example, if the brightness of the background
and test characters are 13.8 and 2.1 fL respectively, the ratio
is shown as 6.6:1.

Test characters were 8-point futura demibold (Chartpak)
measuring 1.9 mm high, 1.0 to 1.9 mm wide, and 0.35 to 0.40 mm
stroke width, Characters selected for evaluation included
numerals 2 through 9 and letters B through H, P, R, S, Y, and Z.
A spacing of 3.5 mm was used between adjacent characters.

Four different but equivalent lists were prepared on each
of the selected background panels. Each list was composed of
five lines of four characters each. All 20 characters selected
for the study appeared only once in each list. The lists were
then mounted in columns on the display drum so that each column
contained one each of the five different contrast panels, but
in different sequences, to allow control of position effects.

A multiple-detent system was used to align each row of charac~
ters with the central viewing aperture in the occluder plate
during manual rotation of the display drum,

The illumination system consisted of a frosted white incan-
descent bulb (GE, 100 W) positioned as shown in Figure 2.
Illumination was directed toward the target area through a cir-
cular aperture 3.0 cm (1,2 in) in diameter in a metal enclosure
surrounding the bulb. To obtain threshold luminance data for
each contrast level, the experimenter operated a variable power
source to increase illumination on the test characters and sur-
rounding area, However, to operate the light at or near the
specified voltage (120 V) and color temperature, yet still
obtain dim illumination levels on the target, the experimenter
placed a neutral density filter between the light source and
the target. Using data from preliminary testing, the operator
selected a filter (1.0, 5.0, or 9.5 percent transmission) prior
to beginning each contrast level that allowed threshold read-
ability to occur within the desired voltage range (90 - 130 V).
This system operated independently from photometer (luminance)
measurements that were taken directly from the target area.

Red illumination on the target was obtained by placing a
Wratten No, 25 (tricolor) filter into the light path, All
filters could be interchanged rapidly by a manually operated
slide holder,


Il.

READABILITY OF ALPHANUMERIC CHARACTERS HAVING VARIOUS
CONTRAST LEVELS AS A FUNCTION OF AGE AND ILLUMINATION MODE

Introduction,

Alphanumeric symbols on aeronautical charts and other
graphic displays are often overprinted on backgrounds that
reduce optimal figure-to-ground contrast levels. For example,
chart backgrounds are frequently printed with shades of gray
or other colors to depict water or terrain elevation. In
addition, alphanumeric symbols that are screened (contain small
white dots) cause a further reduction in target contrast. An
early investigation by Ludvigh (1) indicated that visual acuity
remained relatively constant for contrast levels greater than
30 percent but decreased rapidly as contrast dropped below this
value, Results of a recent study (2) conducted by the authors
indicated, but did not define, the role of contrast ratio in
determining readability of characters printed on aeronautical
instrument approach charts (National Ocean Survey and Jeppesen
Company). To expand the existing data base in this area, we
conducted a study designed to assess minimum illumination
required to identify black alphanumeric characters presented
on gray background materials that alter contrast ratio in equal
increments. Additional parameters evaluated were the effects
of age on readability under dim red and white illumination.
Assessment of neuroretinal sensitivity among individuals in
younger, middle-aged, and older groups required the use of an
artificial pupil to avoid normal variations in pupil size.

These data, when combined with those of other investigators,
are necessary to specify minimum contrast ratios acceptable to
all chart users under viewing conditions that may include night
flying operations and air traffic control activities.

Methods.

To obtain approximately equal contrast increments, we
measured 15 shades of neutral gray paper (Pantone, Inc.) for
reflective brightness under 50 fL of white incandescent lighting.
Ratios between all shades of gray and the black alphanumeric
characters (2.1 fL) were plotted, from which five shades were
selected for evaluation (Figure 1). Gray background materials
selected for the display included Pantone Nos. 422, 428, 430,
432, and 441 for contrast ratios of 9.3:1, 12.8:1, 6.6:1, 3.2:1,
and 16.2:1 respectively. Contrast notations refer to the
differences in brightness between the background and black test