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Finally, manufacturers and designers of charts and instruments should,
when necessary, consult specialists in human factors to match per-
formance characteristics of vehicle operators to future chart displays
and cockpit designs. Static medical defects that degrade operator per-
formance and safety must also be considered. Dille and Booze (22) have
examined the effects of selected physical defects on accident rates of
civilian airmen and discussed several factors that are necessary if
reliable information concerning accident reporting is to be obtained.

Sunmary and Recommendations.

The results indicate that without spectacle lenses, intermediate
visual acuity is variable, depending on the individual's ocular refrac-
tive error, accommodative power, and the viewing distance. Also, data
indicate that for individuals with accommodative power less than 2.0 D
visual acuity at the intermediate range is generally weaker without lenses
than with spectacle lenses for distance or near vision. However, indi-
viduals with myopia of less than 2.0 D generally have better intermediate
vision without rather than with bifocal or distance vision corrective
lenses. Data indicate that individuals with higher levels of accommoda-
tive power have less difficulty throughout the intermediate range of
vision.

When viewing the test numerals through the optimum lens for near
vision (40 cm, 16 in) hyperopic and myopic subjects with low accommodative
power have relatively good acuity at 51 cm (20 in) and a progressive
decline in acuity at 76 and 102 cm (30 and 40 in). Under the same test
conditions, subjects with more accommodative power have improved acuity
at the middle (76 cm) and distal (102 cm) portions of the intermediate
range.

Conversely, individuals with low accomnodative power have relatively
good visual acuity at 102 cm when using a spectacle lens for distance
vision, but their performance declines progressively at 76 and 51 cm.
Also, visual acuity at the middle and proximal portions of the inter-
mediate range (76 and 51 cm) improves with increasing levels of accommo-
dative amplitude for all refractive error subcategories.

At the 76-cm viewing distance, neither the distance nor the near lens
correction provides optimum visual acuity for individuals with low accom-
modative amplitude. Vision at’ 76 cm ranges from 20/30 to 20/50 for
individuals with advanced presbyopia.

The data from this study and the data from the referenced sources
point to the need for further research directed at intermediate vision
of people operating aircraft and other vehicles. Several suggestions
that merit further investigation are listed below.

1. A conprehensive description of intermediate vision tasks common
to various segments of the transportaion environment should be compiled.
Emphasis should be directed to the display characteristics, frequency of
viewing, and ambient operational conditions.

12


M-IIL M-IL M-I

1007
so}

4o-

20F

100 r r r r

ACUITY

100 . r r r r
sot ° b + +
A-t1 60+ L L L
4oF L L L L
20h L | == L
1 ) 1 1 1 1 1 1 1 1 1 1 1 1 1
5176 102 51 76102 51 76102 SI 76102 1 76 102

View. Dist. (CM)

Figure 3. Mean visual acuity scores at 51, 76, and 102 cm for myopic
subjects viewing test numerals without lenses (dotted line),
with lenses for distant vision (solid line), and with bifocal
lens (dashed line).



2)
ia 5.0;-
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: ~~
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= ~~~0
°
s) l i 1 | i ]
< 35-39 40-44 45-49 50-54 55-59 60-64 65-69
N=33 N=I59° N=9I N=89 N=7l N=30 N=8
AGE GROUPS
Figure 1.

Accommodative power was measured by the amount of concave lens that
blurred or convex lens that cleared a row of letters at 40 cm
combined with a base value of +2.50 D.


monitor displays located above or to either side of the operator's
central visual field, Illumination and viewing distance may also vary
considerably between work stations. Consideration must therefore be
given to the adaptability and limitations of the human visual system,
particularly when older individuals are involved.

Data indicate not only decreasing accommodative power with advancing
age, but also decrements in several other performance attributes common
to presbyopic individuals, McFarland et al. (7) found that visual sen-
sitivity to a stimulus light decreased progressively for each decade of
age for subjects ranging in age from 16 to 89 years, Guth's data (8)
indicate that individuals in their sixties required twice as much light
as 20-year-olds to read printed words, A study by Mourant and Langolf
(9) indicated that elderly individuals required 10 times more light than
did young people to obtain 95-percent-correctness scores for a symbol
readability task, Other investigators (10, 11) have shown that visual
acuity of elderly individuals may decline because of retinal changes or
clouding of the ocular media, Melton and Wicks (12) have shown that
accomplishing binocular fusion to overcome double vision is slower for
older men (45 to 60 years) than for younger men (25 to 30 years). Fur-
ther degradation in visual performance has been reported to occur under
conditions of hypoxia (13, 14), hypoglycemia (15), small doses of carbon
monoxide (16), therapeutic doses of common drugs (17), and alcohol (18).

Trifocal lenses, generally considered the most effective measure to
improve intermediate visual acuity, are not worn by many older pilots.
Harper and Kidera (19) cited natural human reluctance to wearing muilti-
focal lenses and unawareness of reduced intermediate vision as contribut-
ing factors. Their investigation revealed that more than 90 percent of
the senior pilots with minimal accommodation had less than 20/20 vision
at 76 cm (30 in) under bright ambient illumination. Furthermore, they
stated that a lens correction for near vision (35 to 40 cm) could not be
used effectively for reading aircraft instruments because of narrowed
depth of focus through bifocal lenses, Data from a recent survey (20)
indicate that only 14 percent of the private pilots with accommodative
power less than 2.0 D wear trifocal lenses.

It is incumbent on vision specialists to obtain sufficient information
about their patients' jobs that appropriate ophthalmic lenses can be pre-
scribed. Often an individual will require several pairs of glasses or
occupational multifocal lenses to perform effectively all phases of an
operational task. Also, recipients of new eyewear must be given instruc-
tions concerning the capabilities and limitations of their spectacles or
contact lenses. Backman and Smith (21) investigated the acceptance of
several occupational multifocal lenses designed for pilots and engineers
flying commercial aircraft. Their report includes data concerning flight
deck specifications of 10 commercial aircraft and recommends design
characteristics for spectacle lenses used by flight crews with incipient
and advanced presbyopia,

aa



Technical Report Documentation Page

T. Report No. 2. Government Accession No. 3. Recipient's Catolog No.
FAA-AM-77-7

‘4. Title and Subtitle 5. Report Date
INTERMEDIATE VISUAL ACULTY OF PRESBYOPIC INDIVIDUALS March 1977
WITH AND WITHOUT DISTANCE AND BIFOCAL LENS &. Performing Organization Code
CORRECTIONS

[8. Performing Orgenizotion Report No.

7, kuhor's) Kenneth W. Welsh, Paul G. Rasmussen, and
John A, Vaughan

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)
FAA Civil Aeromedical Institute
P. 0. Box 25082 TT. Contract or Grant No.

Oklahoma City, Oklahoma 73125

13. Type of Report ond Period Covered

12. Sponsoring Agency Name and Address
Office of Aviation Medicine
Federal Aviation Administration
800 Independence Avenue, S.W. 14. Sponsoring Agency Code
Washington, D.C. 20591 FAA

15. Supplementary Notes

Work was performed under Task AM-D-76-PHY-90.

16. Abstract

Visual acuity was determined at the intermediate range for older individuals with
various combinations of ocular refractive error (nine subcategories) and accommoda~
tive power (three subcategories). Subjects (N=249) read numerals ranging in size

to measure visual acuity from 20/80 to 20/15 at 51, 76, and 102 cm (20, 30, and

40 in) with 50 f£L luminance. Monocular visual acuity scores were determined both
with the subject's optimum spectacle lens correction for distance vision (6 m, 20 ft)
and near vision (0.4 m, 16 in) and without lenses.

For subjects with low accommodative power, neither the distance nor the near lens
correctioa provides normal vision throughout the intermediate range. At 102 cm
vision is better with the distance correction, while at 51 cm vision is better with
the near correction. At 76 cm neither lens offers normal vision. Uncorrected
intermediate visual acuity generally declines with the loss of accommodative power
and increasing refractive error. Individuals with myopia of less than 2.0 diopters
(D) generally have better intermediate visual acuity without correction than with
either a near or a distance correction. Younger subjects generally have better
acuity under all conditions except subjects with uncorrected myopia exceeding 2.0 D.

The factors that act to influence intermediate visual performance and recommenda-~
tions for further research are discussed,

17, Key Words 78. Distribution Statement

Visual Acuity Document is available to the public

Intermediate Vision through the National Technical Information

Presbyopia Service, Springfield, Virginia 22151

Spectacle Lenses

T9. Security Classif, (of this report) 20. Security Classif. (of this page) 21. No. of Pages | 22. Price
Unclassified Unclassified 15

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



10.

il.

12,

13.

14,

15.

References

Sorsby, A,, and G, A. Leary: Correlation Ametropia and Components
of Ametropia, VISION RES,, 2:309-313, 1962.

Hirsch, M, J.: A Longitudinal Study in Refraction, AM. J. OPTOM.,
41:137-141, 1964.

Hirsch, M. J.: Changes in Refractive State After the Age of 45,
AM, J. OPTOM,, 35:229-237, 1958.

Turner, M, J.: Observations in the Normal Subjective Amplitude
of Accommodation, BRIT, J, PHYSIOL, OPT., 15:70, 1958.

Hamasaki, D., I, Ong, and E, Marg: The Amplitude of Accommodation
in Presbyopia, AM, J, OPTOM,, 33:3-14, 1956.

Davson, H,: The Eye, Volume 4, Academic Press, New York and London,
1962, pp. 133-179.

McFarland, R, A,, R. G. Domey, A, B, Warren, and D, C, Ward: Dark
Adaptation as a Function of Age, J, GERONTOL., 15:149-154, 1960.

Guth, S, K,: Effects of Age on Visibility, AM. J. OPTOM., 34:9,
463-477, 1957.

Mourant, R, R,, and G, D, Langolf: Luminance Specifications for
Automobile Instrument Panels, HUM, FACTORS.,, 18:71-84, 1976.

Walton, W, G,: Visual Problems of the Institutional Aged, AM. J.
OPTOM., 44:319-335, 1967.

Waymouth, F, W.: Vision of the Aging Patient, Chilton Books,
Philadelphia, Pennsylvania, 1960, pp. 37-62.

Melton, C, E, Jr., and M. Wicks: Pilot Vision Considerations:
The Effect of Age on Binocular Fusion Time, FAA Office of Aviation
Medicine Report No, AM-66-35, 1966,

Whiteside, T, C. D.: The Problems of Vision in Flight at High

Altitudes, Butterworth Scientific Publications, London, 1957.

Ohlbaum, M. K.: The Effects of Hypoxia on Certain Aspects of
Visual Performance, AM, J, OPTOM., 46:4, 235-249, 1969.

McFarland, R, A., M. H., Halperin, and J, I, Niven: Visual
Thresholds as an Index of Physiological Imbalance During Insulin
Hypoglycemia, AM, J, PHYSIOL., 145:299-313, 1946.

14


2. Data should be obtained concerning minimum visual acuity levels
necessary to read various instrument displays with respect to time,
accuracy, age, and stress created,

3. The optimum size, location, and shape of the bifocal and trifocal
segments of a multifocal lens used in performing specific visual tasks
need further investigation. In addition, the effectiveness of continuous
power (Varilux, Omnifocal, or Younger Seamless) multifocal lenses should
be evaluated in the operational environment.

4. Consideration of medical standards for intermediate vision should
be made with respect to operator classification, diagnostic techniques,
and minimum performance levels.

5. Techniques should be developed to demonstrate the effectiveness
and limitations of bifocal and multifocal lenses in the operational
environment. Methods to ease the sometimes adverse psychophysiological
impact of these lenses should be explored.

13



100 -
\ ee aA-I
O---O A-III
8of-
> L
a 60
Da
oO
ft 40+
0
~—_ ee OT
20F O--—-0
1 iT 1 _1 i 1 l J

M-V M-IV M-III M-IE M-I H-I H-II H-III H-IV
REFRACTIVE ERROR

Figure 4. Mean intermediate visual acuity scores at 76 cm without
corrective lenses for subjects with lowest (0.25 to 1.00 D.)
and highest (2.25 to 5.00 D.) level of accommodative power.

10


III.

diffuser plaque located adjacent to the viewing aperture in the
occluder plate. The digits were presented monocularly, starting with
the largest (20/80) and proceeding to the smallest (20/15), for each
of the nine viewing conditions.

As the subject read the numerals, an operator scored errors by mark-
ing the digits read incorrectly on the subject's data sheet, Scoring
was based on the last line that was read with not more than two errors,
For example, if the subject incorrectly read three, four, or five of the
digits in the 20/40 row and made no more than two errors in the 20/50 row
(next largest digits), the score was 20/50. Ifa subject missed three or
more digits on the 20/80 line, the score was arbitrarily assigned a value
of 20/100. All test distances and viewing conditions were counter-
balanced to minimize fatigue and/or learning effects.

Results.

Table 2 provides data relevant to each of the 27 subcategories of
refractive error and accommodative power. Listed from top to bottom
in each block is the following information: subcategory designation,
number of eyes evaluated, mean age of the subjects, and range of ages.
The marked variation in sample size between subcategories occurred because
sufficient numbers of subjects with higher refractive errors did not
volunteer for the evaluation. Mean performance scores for 16 or 27 sub-
categories are based oa less than the desired sample size of 20 eyes;
excessive numbers of subjects are included in several other subcategories,
There were no volunteer subjects for subcategory H-IV, A-III.

The mean ages for subjects with acconmodative power in ranges A-I,
A-II, and A-III are 54, 47, and 41 years respectively. Figure 1 shows
the effect of age on accommodative power for all test subjects.

Data presented in Figure 2 show the effect of viewing distance (51,
76, and 102 cm) on visual acuity for all subjects with hyperopic refrac-
tive errors. Mean visual acuity scores are shown for subjects under the
following viewing coaditions: with optimum lens correction for distance
vision (solid line), with optimum lens correction for near vision (dashed
line), and without lens correction (dotted line).

When hyperopic subjects with the lowest accommodative power (A-I) were
evaluated with their optimum lens corrections for distance vision, mean
acuity scores decreased from 20/30 or better at 102 cm to less than 20/50
at 51 cm, Visual performance under these conditions was generally con-
sistent for all levels of hyperopia (H-I to H-IV). The same hyperopic
subjects corrected optimally for near vision (40 cm) had visual acuity
of 20/25 or better at 51 cm decreasing to 20/50 to 20/60 range at 102 cm.
Again, performance scores were similar for all hyperopic subjects with low
accomnodative power. Data for these subjects indicate that at 76 cm,
acuity better than 20/40 cannot be achieved with the spectacle lens for
correction of either distance or near vision. With one exception, data
indicate that visual acuity is consistently better at the intermediate

4


Il.

optimum spectacle lenses for distance vision and near vision and
without lens correction.

Methods.

Test subjects consisted of 249 volunteers from the FAA Aeronautical
Center in Oklahoma City, of whon 194 (78 percent) were men and 55 (22
percent) were women, The mean age of the subjects was 48 years with a
range of 35 to 69 years. Approximately 4 percent of the 498 eyes
were screened but not evaluated because they had ocular astigmatism
greater than 1.00 D or their visual acuity could not be corrected to
20/20 because of amblyopia, previous eye injury, or ocular disease.

Prior to data collection, a visual examination was made on each
subject to determine his or her ocular history, visual acuity, ocular
health, accommodative power, and optimum spectacle lens correction for
distance (6 m, 20 ft) and for near (40 cm, 16 in) vision. On the basis
of the results of the examination, each acceptable eye was categorized
with respect to one of 27 subcategories of refractive error and accom-
modative power. Accommodation was measured monocularly under bright
illumination by calculating the amount of concave lens that blurred or
convex lens that cleared a row of equivalent 20/20 letters at 40 cm com-
bined with a base value of +2.50 D.

The optimum lens to correct the subject's refractive error was
determined objectively and subjectively without cycloplegic drops. The
bifocal lens correction for near vision at 40 cm was determined under a
luminance of 50 fL by adding convex spherical lenses in +0.25-D incre-
ments to the distance prescription until the subject reported no further
acuity improvement in a row of 20/20 letters suspended 40 cm fron the
subject's eyes. Younger subjects frequently required no additional con-
vex lens power to see clearly at 40 cm.

Test targets consisted of numerals graded in size to measure visual
acuity over a range from 20/80 to 20/15. Numeral size was adjusted to
measure visual acuity over the range noted above at each test distance
(51, 76, and 102 cm). The numerals were white oa a black background with
a contrast ratio of 94 percent as measured with a Pritchard Spectra
photometer, model 1970-PR. Photographs containing the numerals were
attached to the display drum in horizontal rows with five digits of equal
size in each row. All digits including zero were used in the display bat
no digit occurred more than once in the same row. A rotatable display
drum assembly was used to present individual rows of digits through the
central aperture in the occluder plate. Each subject viewed the digits
while wearing a trial frame with the appropriate prescription lenses or
with plano lenses (0.0 D) when a prescription was not required, Subjects
having astigmatism of 1.0 D or less were corrected with an appropriate
cylindrical lens during testing.

The test numerals were illuminated by a moderately bright white light
adjusted in voltage so that 50 fL were reflected from a white oxide

3



TABLE 1.

Factors Influencing Readability at the Intermediate Range

Primary Factors Component Factors
Visual Refractive Error
Accommodation
Adaptation
Oculomotor
Psychosensory Interpretative Skills

Emotional Status
Reaction Time
Experience
Training

Biomedical General Health
Ocular Health
Fatigue

Drugs

Display Digit Size
Contrast Ratios
Lighting

View Distance
Parallax

Environmental Glare
Hypoxia
Vibration
Toxic Products



16.

17.

18,

19.

20.

21.

22.

McFarland, R. A,, F. J, Roughton, M, H, Halperin, and J, I, Niven:
The Effects of Carbon Monoxide and Altitude on Visual Thresholds,
J, AVIAT, MED,, 15:381-394, 1944,

Luria, S. M,, H. M, Paulson, J. S, Kinney, C, L. McKay, M. S,
Strauss, and A, P, Ryan: The Effect of Common Therapeutic Drugs
on Vision, U. S, Navy, Bureau of Medicine and Surgery, Work Unit
M 4305.08-3001.11, Report No. 808, 1975.

Brecher, G, A., A. P. Hartman, and D, D, Leonard: Effect of Alcohol
on Binocular Vision, AM. J. OPHTHAL,, Volume 39, Part II, 1955,
pp. 44-52.

Harper, C, R,, and G, J, Kidera: Flight Deck Vision and the Aging
Eye, AEROSPACE MED,, 39:1119-1122, 1968.

Welsh, K, W., J, A, Vaughan, and P, G, Rasmussen: Survey of
Cockpit Visual Problems of Senior Pilots, FAA Offiee of Aviation
Medicine Report No, AM-77-2, 1977.

Backman, H., and F, D, Smith: The Design and Prescription of
Multifocal Lenses for Civil Pilots, AM, J. OPTOM, 52:591-599,
1975.

Dille, J. R., and C, F. Booze: Accident Experience of Civilian

Pilots with Static Physical Defects, FAA Office of Aviation
Medicine Report No. AM-76-7, 1976.

15