The vigilance test was administered in four
consecutive one-hour sessions with a ten-minute
rest period between each session. Four different
and random schedules were prepared for signal
presentation through time, one schedule for each
hourly session. The schedules were based on
giving one signal within each five-minute in-
terval for the hour (12 signals for each one-hour
session). ‘The inter-signal intervals were ir-
regular and varied between 20 and 564 seconds.

When the subject detected a signal he acknowl-
edged it by depressing a button before him.
Both the occurrence of the signal and the re-
sponse to it were recorded on a moving strip

Remote At
FLASHING “(NS
LIGHTS

chart. If a signal was not acknowledged within
ten seconds after its presentation it was scored
as “undetected” or “missed.” Responses when
no signal was presented were also scored as
errors (“additions”). Performance on the test
was scored according to the number of signals
during each session that were not detected, and
the number of responses that were made without
an appropriate signal.

C. Physiological Measures. Because the vig-
ilance situation characteristically involves a
minimum of muscular activity, it occurred to us
that measures of internal body temperature and
heart rate might be useful in reflecting the gen-

sw

XK | Remore

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TEST
FLASHING
A, LIGHTS

GR
BL

SCR

2N2323 nase

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TEST yACKS

FOR
REMOTE
OPERATION

7 KRPLIDG

MICRO
SWITCHES.
sw3

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1sv
_ BATTERIES

ca)

ev

FIL TRANS 24 V

Ficure 2. Schematic diagram of flashing light circuit. Power switch SW-1 permits DC motor to run and operate

microswitches alternately. DC voltage applied alternately to test lamps A and B and to remote lamps A; and
B, Relays #1, #2 and #3 are not contacting, Contacting switch SW-2 applies 1.5 volts on the gate of SCR
2N 2323. When conducting, the SCR provides a ground for relay #1, Activating relay #1 removes voltage
from remote lamps and applies voltage to relay #2. Activating relay #2 provides a holding for relay #1 and
also provides voltage to relay #3. Activating relay #8 opens the circuit to SCR. On the next cycle the
microswitch opens to remove voltage from relay #2. Deactivating relay #2 removes holding cireult for
relay #1 and contacts are returned to original position and the lamp not deactivated will glow again. Thus
contacting SW-2 interrupts the flash sequence causing one lamp to skip a flash. Contacting SW-3 removes
voltage from both lamps so that neither flashes.



Tastz [Relative vigilance performance during the
four hour test period

Number of Subjects Scoring More
Misses in the Second Half of the Test
Relative to the Total Number of

Condition Subjects Tested
Ground level; O2 7/10
Ground level; Room Air 6/10
Altitude; O2 7/10

Altitude; Room Air 10/10

by well-oxygenated subjects. The evidence that
subjects at altitude without supplemental oxygen
were mildly hypoxic rests principally on the
oxygen saturation data. It is realized that both
the level of hypoxia (oxygen saturation) and
the response to hypoxia of the heart and respira-
tory rates are closely related to the oxygen re-
quirements of the subjects at altitude. Since
the task studied here was (by design) a sedentary
one, demanding a minimum of muscular activity,
it is apparent that “adequate” tissue oxygenation
was provided with little, if any, cardiopulmonary
adjustment. The sedentary nature of the task
is emphasized by the gradual decrease in both
internal body temperature and heart rate to
stabilized levels below initial values. The ab-
sence of a condition effect in those data further
suggest that the level of “arousal,” as might be
evident through increased muscular activity or
muscular tension, was not different in hypoxic
subjects as compared with oxygenated subjects.

Changes in the plasma glucose levels among
subjects in the different conditions were approxi-
mately the same. All groups showed a slight
increase from the starting level$ ‘within the first
two hours of the test with no further change
beyond that time. This increase in the plasma
glucose levels is believed to be a postprandial
response resulting from the standard meal given
1.5 hours before the beginning of the test. These
data provide no evidence that the plasma glucose
level responds to an altitude exposure of 11,500
feet.

Since the plasma lactate represents the net re-
sult of lactate production (muscular activity)
and lactate utilization (liver uptake, pyruvate
oxidation) the general decrease observed in the
well-oxygenated subjects may reflect more a

muscular relaxation effect rather than increased
utilization. In these terms it would appear that
the altitude subjects without supplemental oxy-
gen took longer to relax or adjust to the task.
Another possibility, though without the appro-
priate data it remains only a speculation, depends
on a decrease in pyruvate oxidation under oxygen
limiting conditions. If the subjects at altitude
without extra oxygen also experienced a decrease
in lactate production (muscular relaxation), but
at the same time incurred decreased pyruvate
oxidation, the lowering of the plasma lactate
might not be as marked as that observed in
normoxic subjects. Consistent with this sugges-
tion, there is a slight increase in the oxygen
saturation of the hypoxic group after two hours
on the test (from — 93% to — 94%; Fig. 1).
It is at about this time in the test that plasma
lactate begins to decrease in this group.

From a performance standpoint, the level of
hypoxia incurred by an altitude exposure of
11,500 feet seems to be of little consequence when
vigilance is the primary consideration. It should
be realized that the “test” of vigilance used in
this study represents only a single instance of
an extremely wide spectrum of possible “vigi-
lance situations.” While mild hypoxia may have
little additional effect in the simplest instances
of vigilance performance, it is conceivable that
more sophisticated vigilance situations might be
susceptible to performance decrements under
similar conditions of oxygen deficit. This again
brings up the question of varying sensitivities
of performance tests to hypoxia. Although the
group data would suggest the conclusion of no
hypoxia effect, viewing the data based on indi-
vidual responses might suggest that the test used
in these experiments was just at or near the
threshold of sensitivity for the degree of hypoxia
achieved. The results of the present study indi-
cate that the FAA oxygen rule (FAR 91.32,
Supplemental Oxygen) is well-conceived and that,
insofar as vigilance is concerned, no ill-effects
need be anticipated, at least to 11,500 feet. Be-
cause the study concerned itself with only young
males, however, it may not be possible to ex-
trapolate the significance of these findings to
the general aviation community. Further, the
fact that the study did not evaluate the influence
of age, obesity, smoking, fatigue, etc., may limit
the conclusiveness of our interpretations since
these factors tend to decrease altitude tolerance.


standard demand-type oxygen mask that was
worn at all times except during part of each
ten-minute rest period. The subject was per-
mitted to remove his mask at these times for
purposes of general comfort. The mask was off
the subject for approximately the first five min-
utes of the rest period.

Neither the test altitude nor the composition
of the breathing mixture was revealed to the
subjects. When an altitude condition was under
study, chamber depressurization to the test pres-
sure (498 mm Hg) occurred over a five-minute
period. Subjects studied under ground level
conditions (725 mm Hg) were made to experi-
ence an ascent to 3,000 feet and a slow return to
ground level to simulate chamber operation.
_ This procedure also required five minutes. Only
one subject was studied at a time.

Venous blood samples (5 ml) were taken from
the subjects just before the start of the test,

during each of the three rest periods, and im-
mediately following the last vigilance session.
The blood was chilled and centrifuged and the
plasma analyzed as previously indicated.

TIL. Results.

The exposure of subjects to an altitude equiv-
alent of 11,500 feet for four hours without sup-
plemental oxygen resulted in a decrease in blood
oxygen saturation to approximately 93-94%
from initial levels near 97% (Fig. 3, upper
panel). By contrast, subjects breathing 100%
oxygen, whether at ground level or at altitude,
maintained blood oxygen saturation between 96
and 98% over the four-hour test. Subjects
breathing room air at ground level showed blood
oxygen saturations near 96% over the entire
period. According to these data, the degree of
hypoxia experienced by the altitude/room air
group may be considered as no more than mild.

6 T T TTT T T T T T T T T T T T T T T T T T
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0 10 20 30 40 $0 60 70 8 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270

MINUTES.

Fraure 4, Changes in heart rate (upper panel) and in respiratory rate (lower panel) during the four-hour vigi-

lance test.

Shaded areas represent ten-minute rest periods; ea

ch point is the mean change of ten subjects.

Mean values for the measures made at the start of the test were represented by zero.

5



100-—>.—. 711 —Tt 11

98

96

94

(ear oximeter)

92

% BLOOD OXYGEN SATN

90
+010

°

-0.10

-0.20F © Ground Level - Oxygen
© Ground Level Room Air
@ Altitude - Oxygen

~0.30F © Altitude-Room Air

-o4olL_ 11 1 1 a or or

A RECTAL TEMPERATURE °C

0 10 20 30 40 50 60 70

80 90 100 HO 120 130 140 180 160 170 180 190 200 210 220 230 240 250 260 270
MINUTES

Fiaure 8. Blood oxygen saturation (upper panel) and change in rectal temperature (lower panel) during four
consecutive hours of vigilance testing. The shaded areas represent the ten-minute rest periods permitted

between one-hour testing sessions.

Each point represents the mean value (or change) of ten subjects. The

mean value for the temperature measure made at the beginning of the test was represented by zero.

eral level of “arousal” during the test. Heart
rate was counted from recordings of the elec-
trical activity of the heart (non-standard EKG)
and internal body temperature was’ monitored
with a calibrated thermistor probe inserted 10 cm
into the rectum and was read on a YSI Tele-
thermometer. Measures associated with the con-
dition of hypoxia were respiratory rate, plasma
glucose and plasma lactate concentrations and
blood oxygen saturation. Respiratory rate was
counted from recordings of thoracic displacement
sensed with a mercury-in-rubber strain gauge.
Plasma glucose was measured by an Auto-
Analyzer technique adapted from the fluoro-
metric method of Guilbault.® Plasma lactate
was measured according to the method of Schén.1*
Blood oxygen saturation was monitored at ten-
minute intervals using a Waters earpiece oxim-
eter. Because of the recognized limitations in
the measurement of arterial blood oxygen satura-

tion by indirect oximetry, percentage saturation
data given in this report should be considered
in terms of a relative change rather than in
terms of an absolute estimate of arterial oxygen
saturation. The oximetry values in this report
are consistent with other published values for
oxygen saturation obtained by ear oximetry. 1? 3°

D. Procedure. Test subjects were randomly
divided into four groups of ten men each, All
of the tests were conducted in a low-pressure
chamber that was temperature controlled at
75° F. Two of the subject groups were studied
at 725 mm Hg, which is the equivalent of 1,284
feet and is ground level in Oklahoma City. The
other two groups were tested at 493 mm Hg, the
pressure equivalent of 11,500 feet. Both ground
level and altitude groups were examined with
and without supplemental oxygen. Those sub-
jects not breathing oxygen breathed room (am-
bient) air. Each subject was provided with a


same way, condition effects were also unable to
be detected, but a significant overall effect was
found (P<0.05). This suggests that, consider-
ing all subjects, vigilance performance decreased
progressively during the course of each hourly
session with this specific task.

The second analysis considered the number of
individuals making more errors during the last
two hours of the test as compared to the first
two hours. Similarly, the Chi Square contin-
gency table revealed no significant condition or
overall effects with respect to additions. A sig-
nificant overall effect was detected in missed
signals (P<0.01), but again no significant condi-
tion effect was observed. In general then, with
all subjects considered, vigilance performance
also decreased progressively over the entire four-
hour test (Fig. 6). The observation that ald of
the hypoxic subjects responded with more misses
in the latter half of the test suggests that there
may be a subtle effect on the performance of

those subjects (Table 1). The effect, however,
was not great enough to be detected statistically
in this small sample because of the high vari-
ability in subject responses.

A third analysis, the Kruskal-Wallis analysis
of variance, was made to examine possible con-
dition effects through the use of rank data. No
statistically significant differences were found
among conditions with respect to either addi-
tions or misses.

IV. Discussion.

Considering first the physiological data, it
appears that exposure of subjects to an altitude
equivalent of 11,500 feet without supplemental
oxygen resulted in a level of hypoxia too subtle
to be reflected in heart rate and respiratory rate
measurements. In both of those measures the
changes experienced by hypoxic subjects were
not distinctly different from the changes shown

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TEST PERIOD

Ficure 6. Vigilance performance by period of testing. Test periods 1-4 represent the first through the fourth con-

secutive testing sessions,

Each point is the mean value of ten subjects.

A positive slope indicates poorer

performance (number of signals missed) as the test progresses.

7



TECHNICAL REPORT STANDARD TITLE PAGE

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
FAA-AM-71-11

4, Title and Subtitle 5. Report Date
EFFECTS OF LOW-GRADE HYPOXIA ON PERFORMANCE March 1971
IN A VIGILANCE SITUATION G. Performing Organization Code

7. Author’s) 8. Performing Organization Report No.

Vincent Fiorica, Ph.D., Mary Jo Burr, M.S., and
Russell Moses, B.A.

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

Oklahoma City, Oklahoma 73125

13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address
Office of Aviation Medicine

Federal Aviation Administration OAM Report
800 Independence Avenue, S.W. 14. Sponsoring Agency Code
Washington, D. C. 20590

15. Supplementary Notes

The work was performed under task AM-C-68-PHYS-23.

16, Abstract
Forty male subjects participated in a study to examine the relationship between

low-grade hypoxia and vigilance performance. At an altitude equivalent of 11,500
feet in a low-pressure chamber, subjects without supplemental oxygen did not
respond differently from well-oxygenated subjects at the same altitude with respect
to such physiologic measures as heart rate, respiratory frequency, internal body
temperature, or plasma concentrations of glucose or lactate. Nor were these
measures significantly different between groups studied at altitude and ground
level controls. The only evidence of hypoxia observed in the altitude/room air
group was in terms of a decrease in blood oxygen saturation measured with an
earpiece oximeter. Vigilance performance deteriorated with time in all groups.

No significant differences, however, could be detected between the hypoxic group
and the well-oxygenated groups. It is concluded that a four-hour exposure to
11,500 feet of altitude has no demonstrable effect on the performance of a simple
vigilance task under the conditions examined here. Because of the limitations
concerning test subjects and the test used in this study, its results do not affect
compliance with FAR 91.32, supplemental oxygen rule.

17, Key Words 18, Distribution Statement
Hypoxia, Vigilance, Altitude and Availability is unlimited. Document may
Performance be released to the National Technical

Information Service, Springfield,
Virginia 22151, for sale to the public.

19. Security Classif. (of this report) 20. Security Classif. (of this page) Bi. No. of Pages | 22. Price

Unclassified Unclassified 9 $3.00

Form DOT F 1700.7 (e-69)





If it is necessary to examine the problem
further, it seems clear that several tests of vigi-
lance should be studied at several altitudes above
12,000 feet in order to establish critical altitudes

and whether each of the situations is equally
sensitive to hypoxia (amount of performance
decrement per unit increase in altitude, or per
unit decrement in blood oxygen saturation).

REFERENCES

1. Arees, E. A.: The Effects of Environmental Tem-
perature and Alerting Stimuli on Prolonged Search.
Technical Note No. 2, Institute of Environmental
Psycho-Physiology, University of Massachusetts,
Amherst, Massachusetts, June 1963.

Colquhon, W. P.: Effects of Raised Ambient Tem-
perature and Event Rate on Vigilance Performance.
AEROSPACE MED., 40:413-417, 1969.

Denison, D. M., F. Ledwith, and BE. C. Poulton:
Complex Reaction Times at Simulated Cabin Alti-
tudes of 5,000 Feet and 8,000 Feet. AEROSPACE
MED., 37 :1010-1013, 1966.

+ Department of ‘Transportation, Federal Aviation
Administration, 14 CFR Parts 23, 91, Installation
and Operating Requirements for Oxygen Equipment
and Supply, Notice of Proposed Rule Making, Docket
No. 8281; Notice No. 67-30, July 11, 1967.

. Finan, J. L, S. ©. Finan, and L. D. Hartson: A
Review of Representative Tests Used for the Quan-
titative Measurements of Behavior-Decrement under
Conditions Related to Aircraft Flight. United States
Air Force, Air Materiel Command, Wright-Patter-
son Air Force Base, Dayton, Ohio, Technical Report
No. 5830, July 1949.

6. Franklin, M. E., A. W. Schumacher, and J. G.
Tiedemann: A Bibliography and Classification of
the Literature on Vigilance. Research Memorandum
64-8, U.S. Army Personnel Research Office, July
1964.

N

@

»

a

7. Green, D. M.: Variations in the Effect of Anoxia on
Performance. AM. J. PHYSIOL, 150 :588-592, 1947.

8. Guilbault, G. G., P. Brignac, and M. Zimmer: Homo-
vanillic Acid as a Fluorometric Substrate for Oxida-
tive Enzymes. Analytical Applications of the Per-
oxidase, Glucose Oxidase and Xanthine Oxidase
Systems. ANALYTICAL CHEM., 40:190-196, 1968.

9. Higgins, E. A. J. A. Vaughan, and G. B. Funk-
houser: Blood Alcohol Concentrations as Affected
by Combinations of Alcoholic Beverage Dosages and
Altitudes. FAA Office of Aviation Medicine Report
No. AM-70-5, 1970.

10.Human Factors Research, Inc. Studies of Human
Vigilance, Goleta, California, 1968.

11. Malmo, R. B., and J. L. Finan: A Comparative Study
of Eight Tests in the Decompression Chamber. AM.
J. PSYCHOL., 57 :389-405, 1944.

12.McFadden, E. B.: Evaluation of the Physiological
Protective Efficiency of a New Prototype Disposable
Passenger Oxygen Mask. FAA Office of Aviation
Medicine Report No. AM-66-7, 1966.

13. Mohler, S. R.: Physiologically Tolerable Decompres-
sion Profiles for Supersonic Transport Type Certifica-
tion. FAA Office of Aviation Medicine Report No.
AM-70-12, 1970.

14, Rahn, H, and A. B. Otis: Alveolar Air During Simu-
lated Flights to High Altitudes. AM. J. PHYSIOL.
150 :202-221, 1947.

15. Schén, R.: A Simple and Sensitive Enzymic Method
for Determination of L(+)-Lactic Acid, ANALYT-
CAL BIOCHEM., 12:413-420, 1965.



Ground
B Altitude — Oxygen
D Altitude = Room air

A PLASMA
GLUCOSE
mg/!00 ml

+0.20 rs rn a TMT1—T—T rs es rs rr ne
=
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Pad
5-020
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a

-0.40 a oo oo 1

© 10 20 30 40 80 60 70 80 90 100 110 120 130 140 160 160 170 180 190 200 210 220 230 240 250 260 270

MINUTES

Ficure 5. Changes in the plasma concentrations of glucose (upper panel) and lactate (lower panel) during four
hours of vigilance testing. Shaded areas represent the rest periods between test sessions. Blood samples were

taken at the beginning of the test (zero values) and during each of the rest periods.

the mean change of ten subjects.

The changes in internal body temperature
during the vigilance test are shown in the lower
panel of Figure 3. During the first two hours
of the test, internal body temperature declined
steadily in all groups. During the last two hours
of the test, internal body temperature stabilized
at levels 0.2-0.4° C below initial temperatures
with no marked difference appearing in any
particular group.

Heart rate changes are shown in Figure 4
(upper panel). In general, the heart rate data
are similar to the changes in internal body tem-
perature, declining during the first half of the
test and stabilizing at some lower level during
the latter half of the test. Despite the fact that
heart rate changes were variable from group to
group, the general tendencies of this measure
were clear. No condition effect was observed.

Respiratory rate changes (Fig. 4, lower panel)
were also variable among groups and fluctuated
£3 breaths per minute from the initial group
measure. No clear difference was detected in
this measure as a function of altitude or breath-
ing mixture condition.

The plasma measures of glucose and lactate
are presented in Figure 5. The general trend
in plasma glucose was to increase slightly during
the first hour of the task. Thereafter plasma
glucose levels remained elevated at about 8-10

Each point represents

mg/100 ml above the initial levels measured.
The increase detected in the group breathing
room air at ground level was somewhat less than
that observed for the other three groups. In
each of the three well-oxygenated groups plasma
lactate levels decreased during the first hour by
0,.2-0.3 mM and remained at approximately this
level for the remainder of the four-hour session.
In the group breathing room air at altitude the
plasma lactate levels were unchanged from initial
levels during the first and second hour of the
test. The first noticeable decrease in plasma
lactate levels in this group occurred after three
hours, and by the end of the four-hour session
lactate levels were similar to those of the other
three groups.

Performance data were examined in several
ways to evaluate condition effects. In the first
comparison the errors made by each subject dur-
ing the first 30 minutes of each hour testing
session were compared to his errors made during
the last 30 minutes of each session. Misses and
additions were considered independently. The
number of individuals making more errors in
the last half of each hourly session was evaluated
in a Chi Square contingency table. No signifi-
cant differences in additions were found between
conditions, nor was an overall effect detected.
With respect to missed signals examined in the


Chamber
Wall

Occurrence of Signal
Subject Response

Blood Oxygen
Saturation

Internal Body Temperature
Respiratory Rate

Heart Rate

é Z

- Remote
Tele Waters Physiological Light [4 2 Channel
Therm. Oximet Recorder Control Strip Chart
ximerer eco Recorder

Ficure 1. Diagram of experimental arrangement. The subject sits inside a low pressure chamber; the record-
ing equipment is monitored from outside the chamber. Heavy lines represent the electrical leads between the
sensors and the recording equipment. The subject's only activity is to acknowledge the occurrence of a signal
by depressing the button before him.


EFFECTS OF LOW-GRADE HYPOXIA ON PERFORMANCE IN A
VIGILANCE SITUATION

I. Introduction.

Many studies evaluating various aspects of
psychophysiological performance have indicated
that effects of hypoxia are not usually observed
below altitude equivalents of 12,000 feet*, De-
spite such a generality, however, it is recognized
that some tests of psychomotor performance
seem more sensitive to the effects of low-grade
hypoxia than do others.’ As examples, Rahn
and Otis found that hand steadiness was sig-
nificantly affected at 12,000 feet, and more re-
cently Denison e¢ al.? demonstrated that: learning
a new orientation task was affected at as low
an altitude as 5,000-8,000 feet. In the latter
study the subjects were exercised to a level of
energy expenditure comparable to that of a pilot
in active flight.

In view of the considerable amount of work
done on the effects of hypoxia on other types of
performance, it is surprising that the area of
vigilance has been ignored. Recent treatments
of the problems of vigilance have covered the
effects of such environmental factors as heat and
cold!*, but data on the effects of altitude are
conspicuously lacking.*?° It was the intent of
this study to examine whether simple vigilance
performance could be affected by low-grade
hypoxia. For the purposes of this paper, low-
grade hypoxia is defined as the degree of hypoxia
generated by altitude exposure up to 12,000 feet.
This altitude has been considered to be the maxi-
mum flight altitude that can be safely maintained
in unpressurized general aviation aircraft with-
out supplemental oxygen.t Studies examining
the relationship between hypoxia and vigilance
may have a direct application to the monitoring
functions required of the general aviation pilot
(aircraft watch, instrument scanning, etc.).

II. Methods.

A. Subjects. The subjects used in this study
were 40 male volunteers between the ages of 19

and 30 years. Most of the subjects had no prior
experience with chamber depressurization. After
arriving at the laboratory in a post-absorptive
state (12 hours), each subject was medically
examined for his fitness to participate in a de-
pressurization experiment, even though half the
tests conducted were at ground level conditions.
No attempt was made to differentiate smokers
from non-smokers. Smoking is the most com-
monly encountered condition associated with a
lowering of altitude tolerance. Because subjects
were assigned to the four groups at random, it
was assumed that the distribution of smokers in
each of the groups was uniform.

Approximately 90 minutes before the test, each
subject was given a standard meal consisting of
a commercial preparation of “instant breakfast.”
About one hour before the test run, each subject
read a standard set of test instructions and was
permitted to practice the vigilance task for a
30-minute period. Subjects were paid for their
participation.

B. Vigilance Testing. The vigilance situation
was created by requiring the subject to monitor
a simple light display. Two small blue lights
were mounted 30 cm apart on a panel. The
panel was placed approximately 65 cm in front
of the seated subject at eye level (Fig. 1). Il-
lumination of the panel lights was automatically
controlled by a relay device (Fig. 2) located out-
side the altitude chamber. The control unit per-
mitted the panel lights to be energized
alternately at the rate of one flash per second.
The duration of the flash was approximately 75
milliseconds. At any time in the cycle, an inter-
rupter switch could be activated which would
prevent the next regular flash from occurring.
The failure of a flash to occur when it should
have constituted the signal to be detected by the
subject.



ACKNOWLEDGMENTS

The authors acknowledge the technical assistance provided by Floyd
Passmore, Miss Elinore M. Galerston and Charles P. Valdez. We also thank
Dr. Earl D. Folk for help with data analysis, Miss Betty Gatliff and William
V. Flores for the research data drawings and Mrs. Dickie Price and Mrs.
La Donna Cook for manuscript preparation. The help of Jack C. Morgan
and John M. Nelson for the esign, fabrication and circuit analysis of the
light test is gratefully acknowledged.