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 résumé 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 Re 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 ReB. If this lower surface were a perfect mirror or a perfectly reflecting and diffusing surface its brightness would equal 5 ReB on the sunny day and ReB 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 (RRe) of the brightness B of the uniform sky. Inasmuch as Re 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 résumé. In the following references will be found more extensive discussions of various phases of the subject.
REFERENCES
1. A Study of Zöllner’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, Täuschungen, 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
Zöllner’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
Müller-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
Schröder’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
Thiéry’s figure, 71
Thiéry’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
Zöllner’s illusion, 67, 76
Zoth, 175