Visual boundary for human perception of vertical rate of descen

Assadourian, A.

Lina, L. J.

English

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TECHNICAL NOTE
D- 1591
INVESTIGATION OF THE
VISUAL BOUNDARY FOR IMMEDIATE PERCEPTION OF
VERTICAL RATE OF DESCENT
By Lindsay J. Lina and Arthur Assadourian
Langley Research Center
Langley Station, Hampton, Va.
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON February 1963
https://ntrs.nasa.gov/search.jsp?R=19630002827 2020-03-24T06:04:59+00:00Z

NATIONAL AERONAUTICS AND SPACE _NISTRATION
TECHNICAL NOTE D-1591
INVESTIGATION 0FTHE
VISUAL BOUNDARY FOR IMMEDIATE PERCEPTION OF
VERTICAL RATE OF DESCENT
By Lindsay J. Lina and Arthur Assadourian
SUMMARY
Optical projection equipment was used to determine the threshold of immediate
perception of visual angular rates. The threshold is defined as the minimum angu-
lar rate that can be detected visually with a high degree of probability in
2 seconds. A brief analytical treatment shows the relationship of visual angular
rate to closure speed or rate of vertical descent.
Application of the data was made by means of the analysis to encompass ranges
of speed and distance that might be encountered, for example, in an emergency ver-
tical landing from a straight-in lunar approach. The results indicate that a
human pilot has adequate visual ability to control a landing_ even for this case
of extreme closure velocityj if a reasonable braking thrust and adequate fuel
supply are available.
INTRODUCTION
One of the questions arising in the planning for future manned lunar landings
concerns the capabilities of a pilot to make final corrections required to land a
rocket vehicle on the moon's surface. For the lunar landing, judgment of vertical
velocity has fundamental similarity to the necessary ability displayed by a heli-
copter pilot in making a vertical landing. However, in the case of the helicopter
landing, small distances and velocities are involved, whereas the lunar landing
deceleration might_ under emergency conditions, encompass much larger distances
and velocities with commensurate fuel use. Thereforej it is desirable to deter-
mine human visual capability to perform this more difficult task.
In order to obtain a measure of visual capability (beyond the range of pres-
ent experience) an investigation was made to determine the threshold of percep-
tion of rates of change of visual angle for various initial angles. The thresh-
old is defined as the minimum angular rate that can be detected visually with a
high degree of probability in 2 seconds. The ranges covered in the study were
determined from an analysis of the conditions that might occur as surface features
appear to grow in a vertical descent.
Since surface features maybe unfamiliar, the optical projections used in
the investigation were designed so that no dependencehad to be placed on familiar
features of known size and shape.
SYMBOLS
h
hmin
s
Atmax
8
Aemin
6
Omin
distance perpendicular to reference plane, ft
velocity normal to plane_ ft/sec
threshold perception of velocity, ft/sec
distance on the reference plane between features which are equally
spaced from the perpendicular line of sight, ft
maximum image-retention time of the eye_ sec
total visual angle which is subtended by the reference base-line
distance s, radians unless otherwise indicated
angular resolution of the eye, radians
rate of change of visual angle, radians/sec
threshold perception of visual angular rate, radians/see
ANALYSIS OF THE VISUAL ASPECTS OF A VERTICAL DESCENT
In order to relate visual-angle projection tests to the real case of percep-
tion of velocity, a brief analysis was made with some effort directed toward
estimating the range of parameters of the tests. Because the investigation was
motivated by the lunar landing problem, judgment of height was not considered a
part of the problem• Height Judgment is dependent on previous experience with
familiar objects and for this reason is assumed to be unreliable in the presence
of unknown surface conditions that might occur in any planetary landing.
Human judgment of speed (or vertical velocity) relative to a two-dimensional
object is based on rate of change of the subtended visual angle. As can be seen
in the following sketch,
2
a descent from height ho to height hI will result in an increase of the visual
angle from 8o to 81 whenan object of size s on the ground is seen. The
visual reference distance s maybe the size of a single object such as a boulder
or it maybe the distance between two objects. In the case of a descent above a
reference circle or crater, the increase in visual angle would be seen as an
apparent growth in diameter.
The relation between height, size of object_ and subtended visual angle is
tan 8_= _ (i)2 2h
The threshold of perception of relative velocity is related to the threshold of
detecting visual-angle rate. The relationship_ obtained by differentiating equa-
tion (i) with respect to time, is
and can also be expressed
• i + tan2 _I
hmin _ _Smin(i_.._-
\ 2 tan [
(2)
• h 2 (3)
by substituting equation (i) in equation (2). If 8min is a constant and has no
dependence on visual angle, then for the case of a single reference object, the
most sensitive velocity cue will be at impact with the surface. When the view is
unobstructed, however, distinctive terrain features may be seen over a wide range
of visual angles. In this case, it is necessary to find the object size at a
given height that will minimize the vertical velocity at the threshold; this is
done by differentiating equation (3) with respect to s, holding emi n and h
constant, to give
• . /h 2 ¼)dhminds - -emink_ -. + : 0-" (4)
3
and thus
2
The maximum sensitivity to velocity would therefore be provided by terrain fea-
tures on a 90 ° visual cone. This angle, for example, might be put to the best
use with a photoelectric cell for detecting velocity in an automatic landing
device. However, for human control, it is the product on the right side of equa-
tion (2) that must be minimized (@rain cannot be assumed a constant with no
dependence on e) and, therefore, measurements are necessary to determine varia-
tions in the threshold of the angular-velocity perception of the eye over a range
of visual angles.
MEASUR]_4ENTS OF VELOCITYPERCEPTION
Simulation Apparatus
Since relative velocity sensation is related to rate of change of the visual
angle, a simple motion projection apparatus was necessary to provide controlled
and measured angular velocities. A schematic illustration of the test equipment
is shown in figure 1. The tests were conducted in a dark room with vertical lines
projected by a point source of light shining through two clear lines in an opaque
mask. In all tests the light projected was bright and represented velocity judg-
ment for terrain in sharp contrast. The point source of light was driven in or
out relative to the mask by a controllable electric motor so that an operator
could select different rates of horizontal motion of the two projected vertical
light lines. For increasing visual angle the lines were driven apart and, con-
versely, for decreasing visual angle the line images were driven closer together.
In addition to the mask with two vertical lines, a mask with lO equally spaced
concentric circles was used.
Te st Methods
The tests were made in two phases. Two subjects, or observers, with normal
vision took part in the tests. In the first phase, visual perception of angular
velocity was determined by the motion of the projected vertical lines at a partic-
ular visual angle, with only a small range of cone angles covered by the lines.
In the second phase of the tests, simulation of relative velocity with respect to
features covering a large range of visual angle was made by projecting the growth
and reduction of concentric circles.
The observers in the tests were seated at a distance from the screen with
the vertical lines appearing at a particular visual angle for the initial-phase
tests. The second-phase tests were made with the distance from the screen repre-
senting a scale range from the pattern of concentric circles. The visual angles
at which the tests were made and the distances of the observers' eyes from the
4
images are as follows:
Light pattern Visual angle, deg Distance from observer
to image, ft
Vertical lines
Concentric circles
ll
41
68
128
0 to 103
53
15
9
17
13
The total change in angle required for each test was small.
To start the tests_ the motor operator quickly accelerated the motor to a
constant speed and signaled the start by voice command. The subject replied, as
quickly as possible, with his decision of direction of travel. The time delay
for the reply was measured and angular rates were determined by timing the image
movement on the screen.
RESULTS AND DISCUSSION
Angular-Rate Perception
The time necessary for the observers to discriminate the direction of angular
motion and to reply is shown in figure 2, plotted as a function of angular veloc-
ity in radians per second. In the plots and in the following discussions, angular
rates for both increasing and decreasing angle are treated as positive. The
results are shown for two observers and, as indicated in the preceding table, were
made at visual angles of ll °, 41°, 68°, and 128 °.
Characteristically, the reply time quickly approaches a minimum as angular
velocity exceeds a threshold region of sensation. At rates below the threshold,
immediate perception is no longer possible and the direction of motion can only
be sensed by noting angle changes over a greatly increased period of time. Bound-
aries of reply time for each direction of motion are shown on the plots indicating
the maximum time that would rarely be exceeded. The boundaries for closure veloc-
ity (increasing angle) and departure velocity (decreasing angle) are appreciably
different and both observers noticed and commented on the difference.
The data of figure 2 are summarized in figure 3 to show the threshold of
immediate angular-velocity perception _in as a function of the visual angle B.
The threshold is chosen as the value of 0 for which the reply time indicated by
the boundary line was 2 seconds. Considering the time delay inherent in the test
procedure, this reply time is believed to represent a conservative estimate of the
boundary of immediate perception. Visual performance better than this limit can
be relied on in a large percentage of trials. The data indicate a marked increase
of the angular rate necessary for quick perception, as the subtended visual
angle e increases from ii ° to 128°. This increase maybe due to reduced sensi-
tivity as the edges of the retina are approachedbut mayalso be caused by the
increased separation of the images reducing the ability to use a feature on one
side of the center as a reference for judging motion on the other side of the
field of view. Observer A showedgreater sensitivity for immediate perception of
angular-velocity rates than did observer B over most of the visual-angle range.
Individual differences in performance result in a considerable difference in the
plot of the threshold variation with angle. Observer B apparently had about the
samesensitivity as observer A only at a visual cone angle of about 40° and lost
sensitivity more rapidly than observer A as the visual angle increased. At the
boundary for immediate perception there is no appreciable difference for
increasing or decreasing angular rates, although there is a marked difference in
the required reply time at rates less than the threshold.
Velocity Perception
The data presented in the previous figures concern only angular-rate percep-
tion at several different visual angles. These data can be applied to the rela-
tion of velocity, height, angular rate, and visual angle expressed in equation (2)
to find the boundary for immediate perception of velocity as a function of the
visual angle. The results of applying the data in this manner are shownin
figure 4.
The plots in figure 4 indicate that observer A had greater visual sensitivity
to velocity perception than observer B and that this visual ability of observer A
was present over a greater range of visual angle than for observer B. The minimum
point on the curve for observer A is at h - 0.013 and for observer B is at
_in _ 0.O16.
h
Although the treatment of the data in figure 4 is used to determine the
boundary for rapid detection of velocity, the data are individual measurements
madeat several different visual angles. Perception tests were therefore madeto
give the observer a view wlth a numberof objects covering a range of visual
angles of about 103°. A pattern of i0 equally spaced concentric circles was
projected to provide a field of view of a numberof well-lighted circles with
good contrast. The data from these tests are plotted in figure 5 and showgood
agreementwith the previous results obtained by using the data collected from
tests for each of several visual angles.
Logical application of well-established thresholds of vision capabilities
yields a value that agrees reasonably well with the experimental results. For
example, velocity sensation appears to depend upon continuous comparison of the
changing sizes of images. It is reasonable to believe that the threshold percep-
tion of velocity is dependent on the minimumangular resolution and maximumimage-
retention time of the eye. The resolution of the eye, for ideal conditions of
lighting and contrast, has been found to be about 1 minute of arc (ref. 1). An
estimate of maximumimage-retention time can be derived from the fact that movies
begin to flicker as projection speedis reduced to about 16 frames per second.
An estimate of threshold perception of angular rate is then
Omin - Aemin _ 0.0003 radian
_tma x 0.06 second
for each side of the field of view. For the total subtended visual angle this
threshold estimate must be doubled and therefore
8min _ 0.01 radian/second
This estimated value agrees reasonably well with the experimental results for
both observers near the visual angle of 40° (see fig. 3), and this cone angle is
in the retina area that seems to contribute the greatest sensitivity to velocity
perception. Thus the results obtained are consistent with known visual ability.
An additional verification of the experimental results was obtained near an
altitude of 3,000 feet in flight observations from a descending helicopter. Ver-
tical descent velocities less than 1,000 feet per minute were discernible by
observer B. The corresponding value of h/h is 0.006, indicating that the
boundary value chosen from the projection tests is a conservative estimate of
visual performance.
APPLICATION TO LUNAR LANDING
The velocity perception ability of observer B was used to determine the
point on an assumed vertical-approach path to the moon at which velocity is
recognized very rapidly. The point is shown in figure 6 as the crossing of the
visual-perceptlon boundary and the assumed approach velocity and altitude varia-
tion. The dashed line indicates that a constant deceleration of 64.4 feet per
second per second will enable a landing to be made from the point of velocity
detection. This deceleration, however, would probably be applied earlier in most
cases since the vlsual-perception boundary chosen is believed to represent a con-
servative estimate of human ability. If thrust were applied earlier or a greater
braking deceleration were available, a landing could probably be made with several
periods of alternate retrofire and free fall, with the operator using the visual-
perception boundary as a reference for control. This method of thrust control
might be useful as an emergency operation if sufficient fuel is available, but
would probably not be planned as an operational procedure because of inefficient
use of fuel.
\
7
CONCLUDING REMARKS
Optical projection equipment was used to detennine a boundary of human per-
ception of visual angular rates. The boundary established was an approximate
lower threshold of perception of angular rate with a high level of reliability
for correctly recognizing the direction of motion. A brief analysis was made to
determine the relationship of visual angular rate with velocity of closure or
departure. The tests indicate a conservative estimate of the boundary as compared
with flight observations obtained in a descending helicopter.
Application of the results was made by means of the analysis to encompass
ranges of speed and distance that could be encountered in a vertical landing from
a straight-in lunar approach. The results indicate that a human operator has
sufficient perception of closure for braking to a safe landing if reasonable
thrust is available and enough fuel is provided to allow for an inefficient
landing technique.
Langley Research Center 3
National Aeronautics and Space Administration,
Langley Station, Hampton, Va., October i, 1962.
REFERENCE
i. Institute for Applied Experimental Psychology, Tufts College: Handbook of
Human Engineering Data. Second ed. (rev.), Human Eng. Rep. SDC 199-I-2a
(Nav Exos P-643), Naval Res., Nov. l, 19_2.
8
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Figure _.- Variation of time necessary for decision and reply with change
in the ratio of velocity to distance. Curves represent boundaries of
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INVESTIGATION OF THE VISUAL BOUNDARY FOR IMMEDIATE PERCEPTION OF VERTICAL RATE OF DESCENT

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