WAR DEPARTMENT
FIELD MANUAL (1944)
FM 5-20
CAMOUFLAGE, BASIC PRINCIPLES
CHAPTER 2
5. KINDS OF ENEMY OBSERVATION. -- Of the perceptive senses available to the enemy, sight is by far the most useful; hearing is second, while smell is of only occasional importance. The comparative usefulness of the perceptive senses is primarily a matter of range. For this reason basic camouflage stresses visual concealment, while sound camouflage is covered only briefly. Camouflage must be effective against both ground and air observation. All our lives we have looked from one position on the ground to another position on the ground. Before we can conceal ourselves from aerial observation, therefore, we must become familiar with what our activities look like from the air -- both in an aerial photograph and in direct observation (fig. 3)
a. Ground observation. -- Ground observation can be maintained for long periods. Ground observers increase their range of vision with binoculars, telescopes, and radar, and their range of hearing by sound ranging.
b. Aerial observation. -- Aerial observation may be visual or photographic; it is frequently intermittent and cannot be maintained for long periods. Haze, fog, storms, smoke, antiaircraft fire, and attack by hostile aircraft impose limitations on aerial observation.
(1) Visual. -- Aircraft are used for visual aerial observation to discover quickly signs of suspicious activity within an area, to direct fire of fighter and bomber aircraft and artillery, and to locate targets for artillery. The observer's eye may be aided by binoculars.
(2) Photographic. -- Aerial photographic observation is usually more revealing than visual observation because it permits longer and more detailed study of the terrain. Panchromatic, infrared, and color film (fig. 18) are used to detect camouflaged installations. For the role of aerial photography in military intelligence, see FM 30-21; for technical data, see TM 1-220. For interpretation of aerial photographs, see FM 21-26 and TM 5-246. Photographs are frequently taken only of suspicious areas. Therefore, concealment from visual aerial observation often eliminates much of the danger of detection by the aerial camera.
FIGURE 3. (139K)
-- Aerial View, at scale of 1:3,500, of a hilltop occupied by an infantry company.
The company has organized a typical defensive position. Numbered points are
identified below. 1. Light machine-gun positions. 2. Heavy machine-gun positions.
3. Right platoon front. -d. Antitank gun positions. 5. Support platoon front.
6. 60-mm mortar positions. 7. Observation posts. 8. Left platoon front.
FIGURE 4. (131K) -- What the enemy
airman sees as he approaches the hilltop. Scale at center of photograph is approximately
1:1,500.
FIGURE 5.
(84K) -- A 360degree panoramic view, which the company commander sees from the
highest point on the hill.
FIGURE 6. (25K)
6. THE CLUES TO IDENTIFICATION FROM THE AIR.- Figure 7 shows aerial photographs of different landscapes. Many objects hidden from the ground observer are easily recognizable from the air. The main factors which enable the observer to determine the nature of objects seen from the air are form, shadow, texture, and color. Other contributing factors are movement, shine, and tone.
FIGURE 7 (71K)
1 -- Agricultural area bordering on
sparsely wooded areas.
2. (187K) -- Urban area, showing regularly shaped pattern of streets and
buildings.
3. -- Desert area, showing terrain lines and scraggly vegetation.
4. -- Jungle area, showing dense growth and deep texture.
FIGURE 8. (53K) -- Size and shape identify 1/2-ton truck and pup tent. Other
objects are half-tracks.
FIGURE 9. (49K) -- Truck in shadow
of trees is more likely to be overlooked than truck in open.
a. Form. -- Man-made objects or groups of objects tend to have straight or uniformly curved lines and are laid out in regular patterns, while nature tends to form irregular patterns. In an area of irregularity, a regular object attracts attention. If the object is of a military nature, it will be conspicuous to the enemy (fig. 8) . Its shape and its relative size are clues to its identity.
b. Shadow. -- From the air, shadows are very noticeable, particularly on aerial photographs. Often a shadow reveals the form of an object better than its top outline (fig. 11). Objects such as factory chimneys, telegraph poles, vehicles, and tents, for example, have distinctive shadows (fig. 10). Objects in the shadow of another object are more likely to be overlooked (fig. 9).
c. Texture. -- (1) Texture is the degree of roughness of a surface. A rough surface has the ability to cast shadows within itself. Perfectly smooth surfaces cast no shadow, absorb no light, and therefore have smooth texture (fig. 12). From such surfaces a large proportion of the reflected light rays enters the eye or the camera, and the surface appears light. Barren or rocky surfaces having no vegetation reflect most of the light they receive; they have little texture (fig. 13) and appear light gray when photographed. (2) Surfaces which contain large numbers of shadows, such as grass and brush or heavy woods, absorb light; they have much texture. They reflect very little light and appear dark on photographs (fig. 14). The degree of darkness is largely dependent upon the amount of texture. For instance, a field of tall grass will appear darker in a photograph than a field of short grass, since the former has a greater shadow content than the latter.
d. Color. -- Color differences at close range distinguish one object from another.
e. Movement. The area in view below an aerial observer is so large that small objects fade into the landscape and do not attract his eye. If the object moves, however, the eye is immediately attracted, and what was unnoticed before is suddenly conspicuous. The aerial camera records the fact that something has moved when two photographs of the same area are taken with a time interval between. If an object has moved, the changed position is apparent when the two photographs are compared. The same principles hold true in ground observation.
FIGURE 10. (41K) -- Without clear shadows
such as these, it is extremely difficult for an aerial photo interpreter to
identify certain objects.
FIGURE 11. (41K) -- From the air, shadows
often reveal more than the shape of the object itself. Monument is over 500
feet high.
FIGURE 12 (147K) Smooth surface --
no texture. 13 Irregular surface -- little texture. 14 Tufted surface -- much
texture.
FIGURE 15.
(160K) --Shine--one of the most revealing signs to observers on the ground and
in the air. Shine alone can betray the best camouflage.
FIGURE 16. (259K) -- Study in tone.
Tone is the relative shade of gray in which objects appear on an aerial panchromatic
photograph.
f. Shine. -- A particularly revealing signal to an observer is shine. Shine is the flashing of light reflected from a smooth surface. Wherever light strikes such smooth surfaces as windows, roofs, the windshields (fig. 15) and tops of vehicles, smooth concrete surfaces, or steel helmets, that light may be reflected directly into the observer's eye or the camera's lens with striking emphasis.
g. Tone. -- The shade of gray in which an object appears in a black and white photograph is known as tone. All objects are represented on a black and white photograph as patches ranging from black through various shades of gray to white (fig. 16). By the addition of texturing material a smooth or shiny surface may be made to produce a dark tone or value in a photograph.
7. FACTORS AFFECTING OBSERVATION. -- The kind and quality of observation obtainable by the enemy govern camouflage.
a. Visual. -- (1) The height and distance of an aerial observer from an object determine what he can see and identify. As a general rule, contrasting objects are seen from the air, under ideal conditions, at a scale of 1 foot per thousand feet of elevation. For example, an object 2 feet in size can be seen at 2,000 feet, if the observer knows where to look for it.
(2) Colors change rapidly above 10,000 feet, becoming gray.
FIGURE 18 1 (163K) -- Panchromatic
film -- the most common kind used in aerial photography. The aerial photo interpreter
gathers information from size, shape, shadow, and tone of objects.
FIGURE 18 2--
An infrared photograph of the same area shows up certain camouflage areas. Artificial
vegetation, for example, is revealed by this infrared photograph.
FIGURE 18 3-- Color photography detects faulty matching of color in camouflage.
Many clues which evade the panchromatic film may be picked up by color film.
But color film has many operational disadvantages and is not commonly used.
FIGURE 19. (55K) -- A diagram of bombing
practice by high-altitude bombers.
(3) Figure 19 illustrates the problem confronting high-altitude bombers. It is clear from this illustration that a bombardier at 20,000 feet must identify his target correctly while still 4 to 6 miles away from it.
b. Photographic. -- (1) Photographs may be taken of a suspected area at frequent intervals. By comparing these photographs, enemy interpreters find changes in the terrain (fig. 20).
FIGURE 20 (a) and (b). (73K) -- Changes
in the terrain are revealed by comparing successive photographs of the same
area. Many foxholes appear in the right-hand photograph, taken one day after
the left-hand photograph.
FIGURE 21. (92K) -- Stereopair of desert
scene. The rough ground is clearly defined in stereovision.
(2) Stereopairs or overlapping photographs taken at the same altitude provide enemy observers with photographs with which they may study the terrain in three dimensions (fig. 21). See TM 5-246 for technical information on stereopairs.
(3) Under favorable conditions, vertical night photographs can be taken (fig. 23). Concealment provided by cover of darkness may be of no value where night photography is possible.
(4) An object which measures approximately .0125 inches on a photograph can usually be identified. A foxhole with a parapet made of spoil is 7 to 8 feet in diameter. On a photograph to scale 1 :5,000 it measures 0.018 inches and can be identified (fig. 22).
c. Sound. -- The intensity of sound at the source and the enemy's means of intercepting it are major effective factors. Sound carries well at night, in mist, fine rain, and on water. Both lack of wind and wind blowing from the camouflaged object favor sound-carrying. Wind blowing from the observer to the camouflaged object detracts from sound-carrying. Wind velocities of more than 13 1/2 miles per hour make sound-carrying impossible. Heavy and thundery weather have the effect of subduing sound. These influences must be considered.
FIGURE 22. (98K) --Scale 1:5,000. Diameter
of foxhole, including spoil, is 0.018 inch on photograph. The aerial photo interpreter
can identify it easily.
FIGURE 23. (69K) --High-altitude aerial
photographs taken at night can pick up details as sharply as day photographs.
Notice unsuspecting truck convoy.