Part 14

The convoy system is well known to the reader. This saved many vessels from destruction. Vessels of the same speed were grouped together and steamed in flocks across the Atlantic. Anyone who has had the extreme pleasure of looking down from an airplane upon these convoys led by destroyers and attended by chasers is strongly impressed with the old adage, "In unity there is strength."

Before the war began, a Brazilian battleship launched in this country was provided with a system of blue lights for use when near the enemy at night. Blue was adopted doubtless for its low range compared with light of other colors. We know that the setting sun is red because the atmospheric dust, smoke, and moisture have scattered and absorbed the blue and green rays more than the red and yellow rays. In other words the penetrating power of the red and yellow is greater than that of the blue rays. This country made use of this expedient to some extent. Of course, all other lights were extinguished and portholes were closed in ocean travel during the submarine menace.

Naturally smoke-screens were adopted as a defensive measure on sea as well as on land. Destroyers belch dense smoke from their stacks in order to screen battleships. Many types of smoke-boxes have been devised or suggested. The smoke from these is produced chemically and the apparatus must be simple and safe. If a merchantman were attacked by a submarine immediately smoke-boxes would be dumped overboard or some which were installed on deck would be put into operation and the ship would be steered in a zigzag course. These expedients were likely to render shell-fire and observations inaccurate. This mode of defense is obviously best suited to unarmed vessels. In the use of smoke-boxes the direction and velocity of the wind must be considered. The writer is unacquainted with any attempts made to camouflage submarines under water, but that this can be done is evident from aerial observations. When looking over the water from a point not far above it, as on a pier, we are unable to see into the water except at points near us where our direction of vision is not very oblique to the surface of the water. The brightness of the surface of water is due to mirrored sky and clouds ordinarily. For a perfectly smooth surface of water, the reflection factor is 2 per cent for perpendicular incidence. This increases only slightly as the obliquity increases to an angle of about 60 degrees. From this point the reflection-factor of the surface rapidly increases, becoming 100 per cent at 90 degrees incidence. This accounts for the ease with which we can see into the water from a position directly overhead and hence the airplane has been an effective hunter of submerged submarines. The depth at which an object can be seen in water depends, of course, upon its clarity. It may be surprising to many to learn that the brightness of water, such as rivers, bays, and oceans, as viewed perpendicularly to its surface, is largely due to light diffused within it. This point became strikingly evident during the progress of work in aerial photometry.

A submerged submarine may be invisible for two reasons: (1) It may be deep enough to be effectively veiled by the luminosity of the mass of water above it (including the surface brightness) or, (2) It may be of the proper brightness and color to simulate the brightness and color of the water. It is obvious that if it were white it would have to attain concealment by submerging deeply. If it were a fairly dark greenish-blue it would be invisible at very small depths. In fact, it would be of very low visibility just below the surface of the water. By the use of the writer's data on hues and reflection-factors of earth and water areas it would be easy to camouflage submarines effectively from enemies overhead. The visibility of submarines is well exemplified by viewing large fish such as sharks from airships at low altitudes. They appear as miniature submarines dark gray or almost black amid greenish-blue surroundings. Incidentally, the color of water varies considerably from the dirty yellowish-green of shallow inland waters containing much suspended matter to the greenish-blue of deep clear ocean waters. The latter as viewed vertically are about one-half the brightness of the former under the same conditions and are decidedly bluer.

_The Visibility of Airplanes._--In the Great War the airplane made its debut in warfare and in a short time made a wonderful record, yet when hostilities ceased aerial camouflage had not been put on a scientific basis. No nation had developed this general aspect of camouflage systematically or to an extent comparable with the developments on land and sea. One of the chief difficulties was that scientific data which were applicable were lacking. During the author's activities as Chairman of the Committee on Camouflage of the National Research Council he completed an extensive investigation[12] of the fundamentals upon which the attainment of low visibility for airplanes must be based. Solutions of the problems encountered in rendering airplanes of low visibility resulted and various recommendations were made, but the experiences and data will be drawn upon here only in a general way. In this general review details would consume too much space, for the intention has been to present a broad view of the subject of camouflage.

The visibility of airplanes presents some of the most interesting problems to be found in the development of the scientific basis for camouflage. The general problem may be subdivided according to the type of airplane, its field of operation, and its activity. For example, patrol craft which fly low over our own lines would primarily be camouflaged for low visibility as viewed by enemies above. (See Fig. 99.) High-flying craft would be rendered of low visibility as viewed primarily by the enemy below. Airplanes for night use present other problems and the visibility of seaplanes is a distinct problem, owing to the fact that the important background is the water, because seaplanes are not ordinarily high-flying craft. In all these considerations it will be noted that the activity of the airplanes is of primary importance, because it determines the lines of procedure in rendering the craft of low visibility. This aspect is too complicated to discuss thoroughly in a brief resume.

The same fundamentals of light, color, and vision apply in this field as in other fields of camouflage, but different data are required. When viewing aircraft from above, the earth is the background of most importance. Cumulus clouds on sunny days are generally at altitudes of 4000 to 7000 feet. Clouds are not always present and besides they are of such a different order of brightness from that of the earth that they cannot be considered in camouflage designed for low visibility from above. In other words, the compromise in this case is to accept the earth as a background and to work on this basis. We are confronted with seasonal changes of landscape, but inasmuch as the summer landscape was of greatest importance generally, it was the dominating factor in considering low visibility from above.

On looking down upon the earth one is impressed with the definite types of areas such as cultivated fields, woods, barren ground and water. Different landscapes contain these areas in various proportions, which fact must be considered. Many thousand determinations of reflection-factor and of approximate hue were made for these types of areas, and upon the mean values camouflage for low visibility as viewed from above was developed. A few values are given in the accompanying table, but a more comprehensive presentation will be found elsewhere.[12]

_Mean Reflection-Factors_

(From thousands of measurements made by viewing vertically downward during summer and fall from various altitudes.)

Per Cent

Woods 4.3 Barren ground 13.0 Fields (grazing land and growing crops) 6.8 Inland water (rivers and bays) 6.8 Deep ocean water 3.5 Dense clouds 78.0

Wooded areas are the darkest general areas in a landscape and possess a very low reflection-factor. From above one sees the deep shadows interspersed among the highlights. These shadows and the trapping of light are largely responsible for the low brightness or apparent reflection-factor. This is best illustrated by means of black velvet. If a piece of cardboard is dyed with the same black dye as that used to dye the velvet, it will diffusely reflect 2 or 3 per cent of the incident light, but the black velvet will reflect no more than 0.5 per cent. The velvet fibers provide many light traps and cast many shadows which reduce the relative brightness or reflection-factor far below that of the flat cardboard. Cultivated fields on which there are growing crops are nearly twice as bright as wooded areas, depending, of course, upon the denseness of the vegetation. Barren sunbaked lands are generally the brightest large areas in a landscape, the brightness depending upon the character of the soil. Wet soil is darker than dry soil, owing to the fact that the pores are filled with water, thus reducing the reflection-factor of the small particles of soil. A dry white blotting paper which reflects 75 per cent of the incident light will reflect only about 55 per cent when wet.

Inland waters which contain much suspended matter are about as bright as 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.

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.