grazing land and cultivated fields. Shallow water partakes somewhat of the
color and brightness of the bed, and deep ocean water is somewhat darker
than wooded areas. Quiet stagnant pools or small lakes are sometimes
exceedingly dark; in fact, they appear like pools of ink, owing to the
fact that their brightness as viewed vertically is almost entirely due to
surface reflection. If it is due entirely to reflection at the surface,
the brightness will be about 2 per cent of the brightness of the zenith
sky. That is, when viewing such a body of water vertically one sees an
image of the zenith sky reduced in brightness to about 2 per cent.
The earth patterns were extensively studied with the result that definite
conclusions were formulated pertaining to the best patterns to be used.
Although it is out of the question to present a detailed discussion of
this important phase in this resume, attention will be called to the
manner in which the earth patterns diminish with increasing altitude. The
insert in Fig. 100 shows the actual size of an image of a 50-foot airplane
from 0 to 16,000 feet below the observer as compared with corresponding
images (to the same scale) of objects and areas on the earth's surface
10,000 feet below the observer.
For simplicity assume a camera lens to have a focal length equal to 10
inches, then the length _x_ of the image of an object 100 feet long will
be related to the altitude _h_ in this manner:
_x_ 100
----- = ----- or _xh_ = 1000
10 _h_
By substituting the values of altitude _h_ in the equation the values of
the length _x_ of the image are found. The following values illustrate the
change in size of the image with altitude:
Altitude _h_ in feet Size of image _x_ in inches
1,000 1.00
2,000 0.50
3,000 0.33
4,000 0.25
10,000 0.10
20,000 0.05
It is seen that the image diminishes less rapidly in size as the altitude
increases. For example, going from 1000 feet to 2000 feet the image is
reduced to one-half. The same reduction takes place in ascending from
10,000 to 20,000 feet. By taking a series of photographs and knowing the
reduction-factor of the lens it is a simple matter to study pattern. An
airplane of known dimensions can be placed in the imagination at any
altitude on a photograph taken at a known altitude and the futility of
certain patterns and the advantages of others are at once evident.
[Illustration: Fig. 100.--Illustrating the study of pattern for airplanes.
The photograph was taken from an altitude of 10,000 feet. The insert shows
the relative lengths (vertical scale) of an airplane of 50-foot spread at
various distances below the observer.]
It is impracticable to present colored illustrations in this resume and
values expressed in numbers are meaningless to most persons, so a few
general remarks will be made in closing the discussion of low visibility
as viewed from above in spring, summer and fall. A black craft is of much
lower visibility than a white one. White should not be used. The paints
should be very dark shades. The hues are approximately the same for the
earth areas as seen at the earth's surface. Inland waters are a dirty
blue-green or bluish-green, and deep ocean water is a greenish-blue when
viewed vertically, or nearly so. Mean hues of these were determined
approximately.
Before considering other aspects of camouflage it is well to consider such
features as haze, clouds and sky. There appear to be two kinds of haze
which the writer will arbitrarily call earth and high haze, respectively.
The former consists chiefly of dust and smoke and usually extends to an
altitude of about one mile, although it occasionally extends much higher.
Its upper limit is very distinct, as seen by the "false" horizon. This
horizon is used more by the pilot when flying at certain altitudes than
the true horizon. At the top of this haze cumulus clouds are commonly seen
to be poking out like nearly submerged icebergs. The upper haze appears
somewhat whiter in color and appears to extend sometimes to altitudes of
several or even many miles. The fact that the "earth" haze may be seen to
end usually at about 5000 to 6000 feet and the upper haze to persist even
beyond 20,000 feet has led the author to apply different names for
convenience. The upper limit of the "earth" haze is determined by the
height of diurnal atmospheric convection. Haze aids in lowering the
visibility of airplanes by providing a luminous veil, but it also operates
at some altitudes to increase the brightness of the sky, which is the
background in this case.
The sky generally decreases considerably in brightness as the observer
ascends. The brightness of the sky is due to scattered light, that is, to
light being reflected by particles of dust, smoke, thinly diffused clouds,
etc. By making a series of measurements of the brightness of the zenith
sky for various altitudes, the altitude where the earth haze ends is
usually plainly distinguishable. Many observations of this character were
accumulated. In some extreme cases the sky was found to be only one-tenth
as bright when observed at high altitudes of 15,000 to 20,000 feet as seen
from the earth's surface. This accounts partly for the decrease in the
visibility of an airplane as it ascends. At 20,000 feet the sky was found
to contribute as little as 4 per cent of the total light on a horizontal
plane and the extreme harshness of the lighting is very noticeable when
the upper sky is cloudless and clear.
Doubtless, it has been commonly noted that airplanes are generally very
dark objects as viewed from below against the sky. Even when painted white
they are usually much darker than the sky. As they ascend the sky above
them becomes darker, although to the observer on the ground the sky
remains constant in brightness. However, in ascending, the airplane is
leaving below it more and more luminous haze which acts as a veil in
aiding to screen it until, when it reaches a high altitude, the
combination of dark sky behind it and luminous haze between it and the
observer on the ground, it becomes of much lower visibility. Another
factor which contributes somewhat is its diminishing size as viewed from a
fixed position at the earth. The minimum perceptible contrast becomes
larger as the size of the contrasting patch diminishes.
Inasmuch as there is not enough light reflected upward from the earth to
illuminate the lower side of an opaque surface sufficiently to make it as
bright as the sky ordinarily, excepting at very high altitudes for very
clear skies, it is necessary, in order to attain low visibility for
airplanes as viewed from below, to supply some additional illumination to
the lower surfaces. Computations have shown that artificial lighting is
impracticable, but measurements on undoped airplane fabrics indicate that
on sunny days a sufficient brightness can be obtained from direct sunlight
diffused by the fabric to increase the brightness to the order of
magnitude of the brightness of the sky. On overcast days an airplane will
nearly always appear very much darker than the sky. That is, the
brightness of the lower sides can in no other manner be made equal to that
of the sky. However, low visibility can be obtained on sunny days which is
an advantage over high visibility at all times, as is the case with
airplanes now in use. Many observations and computations of these and
other factors have been made, so that it is possible to predict results.
Transparent media have obvious advantages, but no satisfactory ones are
available at present.
Having considered low visibility of aircraft as viewed from above and from
below, respectively, it is of interest to discuss briefly the possibility
of attaining both of these simultaneously with a given airplane. Frankly,
it is not practicable to do this. An airplane to be of low visibility
against the earth background must be painted or dyed very dark shades of
appropriate color and pattern. This renders it almost opaque and it will
be a very dark object when viewed against the sky. If the lower surfaces
of the airplane are painted as white as possible the airplane still
remains a dark object against the blue sky and a very dark object against
an overcast sky, except at high altitudes. In the latter cases the
contrast is not as great as already explained. A practicable method of
decreasing the visibility of airplanes at present as viewed from below is
to increase the brightness by the diffuse transmission of direct sun-light
on clear days. On overcast days clouds and haze must be depended upon to
screen the craft.
In considering these aspects it is well to recall that the two sources of
light are the sun and the sky. Assuming the sun to contribute 80 per cent
of the total light which reaches the upper side of an opaque horizontal
diffusing surface at midday at the earth and assuming the sky to be
cloudless and uniform in brightness, then the brightness of the horizontal
upper surface will equal 5 _RB_, where _R_ is the reflection-factor of the
surface and _B_ is the brightness (different in the two cases) of the sky.
On a uniformly overcast day the brightness of the surface would be equal
to _RB_. Now assuming _R{e}_ to be the mean reflection-factor of the
earth, then the lower side of a horizontal opaque surface suspended in the
air would receive light in proportion to _R{e}B_. If this lower surface
were a perfect mirror or a perfectly reflecting and diffusing surface its
brightness would equal 5 _R{e}B_ on the sunny day and _R{e}B_ on the
overcast day where _B_ is the value (different in the two cases) of the
brightness of the uniform sky. The surface can never be a perfect
reflector, so on an overcast day its brightness will be a fraction
(_RR{e}_) of the brightness _B_ of the uniform sky. Inasmuch as _R{e}_
is a very small value it is seen that low visibility of airplanes as
viewed from below generally cannot be attained on an overcast day. It can
be approached on a sunny day and even realized by adopting the expedient
already mentioned. Further computations are to be found elsewhere.[12]
Seasonable changes present no difficulties, for from a practical
standpoint only summer and winter need be generally considered. If the
earth is covered with snow an airplane covered completely with white or
gray paint would be fairly satisfactory as viewed from above, and if a
certain shade of a blue tint be applied to the lower surfaces, low
visibility as viewed from below would result. The white paint would
possess a reflection-factor about equal to that of snow, thus providing
low visibility from above. Inasmuch as the reflection-factor of snow is
very high, the white lower sides of an airplane would receive a great deal
more light in winter than they would in summer. Obviously, a blue tint is
necessary for low visibility against the sky, but color has not been
primarily considered in the preceding paragraphs because the chief
difficulty in achieving low visibility from below lies in obtaining
brightness of the proper order of magnitude. In winter the barren ground
would be approximately of the same color and reflection-factor as in
summer, so it would not be difficult to take this into consideration.
Seaplanes whose backgrounds generally consist of water would be painted of
the color and brightness of water with perhaps a slight mottling. The
color would generally be a very dark shade, approximating blue-green in
hue.
Aircraft for night use would be treated in the same manner as aircraft for
day use, if the moonlight is to be considered a dominant factor. This is
one of the cases where the judgment must be based on actual experience. It
appears that the great enemy of night raiders is the searchlight. If this
is true the obvious expedient is to paint the craft a dull jet black.
Experiments indicate that it is more difficult to pick up a black craft
than a gray or white one and also it is more difficult to hold it in the
beam of the searchlight. This can be readily proved by the use of black,
gray, and white cards in the beam of an automobile head-light. The white
card can be seen in the outskirts of the beam where the gray or black
cannot be seen, and the gray can be picked up where the black one is
invisible. The science of vision accounts for this as it does for many
other questions which arise in the consideration of camouflage or low
visibility.
Some attempts have been made to apply the principle of confusibility to
airplanes as finally developed for vessels to circumvent the submarine,
but the folly of this appears to be evident. Air battles are conducted at
terrific speeds and with skillful maneuvering. Triggers are pulled without
computations and the whole activity is almost lightning-like. To expect to
confuse an opponent as to the course and position of the airplane is
folly.
The camouflage of observation balloons has not been developed, though
experiments were being considered in this direction as the war closed.
Inasmuch as they are low-altitude crafts it appears that they would be
best camouflaged for the earth as a background. Their enemies pounce down
upon them from the sky so that low visibility from above seems to be the
better choice.
In the foregoing it has been aimed to give the reader the general
underlying principles of camouflage and low visibility, but at best this
is only a resume. In the following references will be found more extensive
discussions of various phases of the subject.
REFERENCES
1. A Study of Zoellner's Figures and Other Related Figures, J. Jastrow,
Amer. Jour. of Psych. 1891, 4, p. 381.
2. A Study of Geometrical Illusions, C. H. Judd, Psych. Rev. 1899, 6, p.
241.
3. Visual Illusions of Depth, H. A. Carr, Psych. Rev. 1909, 16, p. 219.
4. Irradiation of Light, F. P. Boswell, Psych. Bul. 1905, 2, p. 200.
5. Retiring and Advancing Colors, M. Luckiesh, Amer. Jour. Psych. 1918,
29, p. 182.
6. The Language of Color, 1918, M. Luckiesh.
7. Apparent Form of the Dome of the Sky, Ann. d. Physik, 1918, 55, p. 387;
Sci. Abs. 1918, No. 1147.
8. Course on Optics, 1738, Robert Smith.
9. Color and Its Applications, 1915 and 1921; Light and Shade and Their
Applications, 1916, M. Luckiesh.
10. Report of The Submarine Defense Association, L. T. Bates and L. A.
Jones.
11. Marine Camouflage Design, E. L. Warner, Trans. I. E. S. 1919, 14, p.
215.
12. The Visibility of Airplanes, M. Luckiesh, Jour. Frank. Inst. March and
April, 1919; also Aerial Photometry, Astrophys. Jour. 1919, 49, p. 108.
13. Jour. Amer. Opt. Soc., E. Karrer, 1921.
The foregoing are only a few references indicated in the text. Hundreds of
references are available and obviously it is impracticable to include such
a list. The most fruitful sources of references are the general works on
psychology. E. B. Titchener's Experimental Psychology (vol. 1) contains an
excellent list. A chapter on Space in William James' Principles of
Psychology (vol. II) will be found of interest to those who wish to delve
deeper into visual perception. Other general references are Elements of
Physiological Psychology by Ladd and Woodworth; the works of Helmholtz; a
contribution by Hering in Hermann's Handb. d. Phys. Bk. III, part 1;
Physiological Psychology by Wundt; E. B. Delabarre, Amer. Jour. Psych.
1898, 9, p. 573; W. Wundt, Taeuschungen, p. 157 and Philos. Stud. 1898, 14,
p. 1; T. Lipps, Raumaesthetik and Zeit. f. Psych. 1896, 12, 39.
INDEX
Aberration, 19
spherical, 122
chromatic, 135
Aerial perspective, 165, 183
After-images, 24, 25, 59, 128, 186
positive and negative, 129
Airplanes, visibility of, 233
camouflage for different types, 234
size of image at various altitudes, 238
camouflage for various conditions, 240
Alhazen, 8
Angles, influence of, 76
various effects of, 81
contours and, 87
apparent effect on length, 91
theories, 98
Animals, protective coloration, 211
Architecture, 195
balance in, 201
Arcs, circular, illusions due to, 86
Areas, juxtaposed, illusions of, 96
Artist, 179
Attention, fluctuation of, 65, 106, 141, 164
Aubert, 49
Auerbach's indirect vision theory, 100
Aureole, 178
Balance in architecture, 201
Bas-relief, 143
Battleships, 222
Binocular disparity, 105
Binocular vision, 29, 31
Blending of colors in camouflage, 216
Blind spot, 21
Blue light on war-vessels, 230
Boswell, varieties of irradiation, 122
Brightness,
illusions due to
variations in, 107
and color contrasts, 114
apparatus, 115
and hue, 125
sky, 241
Brucke's theory, 37
Brunot's mean distance theory, 101
Camouflage, 210
terrestrial, 210
detection of, 215
marine, 222
airplane, 234
Carr, observations on distance illusions, 108
Chromatic aberration, 19, 135
Chlorophyl, 214
Circle, 11
arcs of, illusion, 86
contracting and expanding illusion, 98
Clouds, 235
Color, 124
after-images, 128
blending in camouflage, 216
contrasts and brightness, 114
growth and decay of sensation, 131
illusions of, 156
retiring and advancing, 138
saturation, 154
sensibility of retina, 138
warm and cold, 158
Confusability, 226
Confusion theory of angular illusions, 100
Contour, illusions of, 52
and angles, illusions, 87
Contracting and expanding circles, illusion of, 98
Contrasts, illusions of, 53
simultaneous, 124, 154
apparatus for, 115, 125
color, 114, 154, 188
brightness, 114
Convergence, illusions of, 108, 191
Cord, twisted, illusion, 88
Daylight, artificial, 189
Decoration, painting and, 179
Decorator, 188
Dember, 167
Depth and distance, illusions, 102
Direction, illusions of
Zoellner's, 76
Wundt's, 79
Hering's, 80
Disk, Mason, 132
Distance and depth, illusions, 102
and size, 35, 104, 166
Distance illusions, Carr's observations, 108
Double images, 37
Dynamic theory of angular illusions, 99
Enlargement of sun and moon at horizon, apparent, 169
Equivocal figures, 64
Euclid, 8
Extent, interrupted, illusions of, 48
External image, 15, 17, 34
Eye, physiology, 13
position, 30
adjustments, 33
defects, 19
Fatigue, 128
Field, visual, effect of location in, 44
Figures, equivocal, 64
Filters, color, 214
Fluctuation of attention, 65, 106, 141, 164
Focusing, 14
Form of sky, apparent, 166
Fovea centralis, 22, 23, 139
Frames, picture, effect of wood grain, 190
Geometrical illusions, 44
Glare, 119
Grain of wood, apparent distortions due to, 190
Grecian art, 196
Growth and decay of color sensation, 131
Guttman, 175
Hallucination, 4, 72
Halo, 178
Haze, illusions, etc., 103, 166, 183
earth and high, 240
Helmholtz, 13, 74
Hering, 74
illusion of direction, 80
Hue and brightness, 125
Illusions, geometrical, 44
equivocal figures, 64
influence of angles, 76
of depth and distance, 102
irradiation and brightness contrast, 114
color, 124
light and shadows, 144
in nature, 164
in painting and decoration, 179
mirror, 205
camouflage, 210
Image
after-, 24, 25, 59, 128, 129, 186
double, 37
external, 15, 17, 34
retinal, inversion of, 16
of airplane, size at various altitudes, 238
Indirect vision theory, Auerbach's, 100
Intaglio, 143
Interrupted extent, illusions of, 48
Iris, 15
Irradiation, 120
and brightness contrast, 114
varieties of (Boswell), 122
in architecture, 199
James, 170
Jastrow, 80
Jones, L. A., 223
Judd, 86, 93
Judgment, 1
Karrer, 160
Kepler, 8
Light, effect of spectral character, 189
Lighting, illusions of depth and distance due to, 102
contrasts, 154
diffusion, effect of, 145
direction, effect of, 144, 151
ending of searchlight beam, 160
warm and cold colors, 158
Lipps, 10, 11
dynamic theory of, 99
Location in visual field, effect, 44
Mean distance theory, Brunot's, 101
Mechanical, esthetic unity, 11
Magician, 3
Magic, mirror, 205
Marine camouflage, 222
Mason disk, 132
Mirage, 3, 176
Mirror magic, 205
Miscellaneous color effects, 140
Moon, apparent size at horizon, 8, 36, 169
theories of, 173
apparent radius of crescent, 121
Mueller-Lyer illusion, 53, 93
National Research Council, Committee on Camouflage, 234
Nature, 164
Necker, 74
Oppel, 9
Painting and decoration, 179
Painter, 2, 179, 186
Parallax, 105
Parthenon of Athens, 196
Persistence of vision, 131
Perspective, 58
in architecture, 198
aerial 165, 183
theory, 98
Photographer, 152
Photography, use in detection of camouflage, 220
Photometer, 156, 217
Pigments, 184
Poggendorff illusion, 85
Protective coloration, animals, 211
Psychology, 2, 6, 157
of light, 193
Purkinje phenomenon, 26, 139
Reflection-factors, 236
Retina, 14, 20
inertia, 130
color sensibility, 138
Retinal rivalry, 140
Retiring and advancing colors, 136
Reversal of mirror image, 205
Rods and cones, 21
Schroeder's staircase, 70
Sculpture, 204
Searchlight beam, ending of, 160
Sensation, color, growth and decay, 131
Sense, 1
Shading, counter, for vessels, 224
Shadows, importance in camouflage, 215
Size and distance, 35, 36
illusions of, 104, 166
Sky
apparent form of, 166
brightness, 241
Skylight and sunlight, relative proportions of, 215, 243
Smith, Robert, 173
Smoke-screens, 230
Spectral character of light, 189
Sphere, illusions, 145, 150, 151
Spherical aberration, 19
Sphinx illusion, 206
Spiral illusions, 90
Spraying, paint, 187
Stereoscope, 39, 142
Stereoscopic vision, 38, 41, 141
Submarines, 225
camouflage for, 232
Sun, apparent enlargement at horizon, 169
Sunlight and skylight, relative proportions in nature, 215, 243
Terrestrial camouflage, 210
Theory of influence of angles, 98
perspective, 98
dynamic, 99
confusion, 100
indirect vision, 100
mean distance, 101
Thiery's figure, 71
Thiery's perspective theory, 99
Transparencies, 185
Twisted cord illusions, 88
Uibe, 167
Vertical vs. horizontal distances, 11, 36, 46
Visibility, low, for vessels, 222
of airplanes, 233
Vision, 29
persistence of, 131
stereoscopic, 38
Visual perception, 32, 33
Warm and cold colors, 158
Warner, E. L., 227
Wheatstone, 37
Wood grain, illusions caused by, 191
World War, 213
Wundt, 10, 11, 32, 74
illusion of direction, 79
Yellow spot, 139
Zoellner's illusion, 67, 76
Zoth, 175
OTHER BOOKS BY M. LUCKIESH
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