Figure 4-31. Light ray striking uncoated surface.

TM 9-258

d. A number of types of coating have been tested

the surfaces of optical components in an instrument

and applied to optical elements. Some have proved too

tends to throw a veil of stray light over the field of view.

soft to stand up under handling in normal use and

Coating will reduce these internal reflections. Better

service. The present transparent coating consists of

contrast and sharper definition of the image will be the

magnesium fluoride applied by an evaporation method of

result even under intense illumination.

(3) Reduction of ghost images.

Internal

sublimation under heat in vacuum. The coating is

durable enough to permit handling during assembly and

reflections from the curved lens surfaces sometimes

disassembly and to withstand careful cleaning.

form an image or series of images which are slightly out

e. The main advantages of coating are as follows:

of focus and not quite as bright as the image of focus.

(1) Increased light transmission. When all elements

These ghost images are very distractive for proper

of a straight telescope are coated, except the reticle and

aiming. Coating will greatly reduce the formation of

filters, there is approximately 90 percent transmission of

ghost images.

(4) Reduction of front surface reflections.

light instead of 60 percent, or half again as much light as

would pass through the instrument were its optics

Reflections from the front surface of observation

uncoated. This improvement will result in a brighter

instruments are very noticeable at night and might give

image at times when light is scarce. It will be most

the observer's position away. By reducing reflections by

obvious at dawn, dusk, or at night; in other words, at

means of coating, this danger is reduced.

times of poor visibility. Targets are visible from 1/4 to 1/2

4-14. Theory.

hour longer at dusk and at dawn. The range of vision at

a. Consider a tiny portion of a light ray so highly

night is increased approximately 20 percent for standard

magnified as to show its wave structure. A ray of light

binoculars. Therefore, coating is essential in night

striking an uncoated glass surface could then be pictured

glasses or in instruments to be used under adverse

as a wavy line as shown in figure 4-31. The reflected

conditions and now is used in all military optical systems.

ray, which can represent 4 percent of the total light, is

(2) Reduction of haze. Light reflected from

represented by the wavy dotted line and will shift in

phase 180 (reverse direction of vibration).

Figure 4-31. Light ray striking uncoated surface.

b. If a thin film of transparent substance were

so, they would reinforce each other resulting in the

placed on the glass, there would be two surfaces from

double amount of light being reflected and the minimum

which the wave would be reflected and similarly shifted in

amount of light being transmitted through the surfaces.

phase 180ƒ. If this film were one-half as thick as a

This condition would be desirable in a reflector but

undesirable in a lens.

wavelength of light, the reflected waves from the two

surfaces would follow the same path (fig 4-32). In doing

4-32