Section II. COATED OPTICS

TM 9-258

d. Instrument lights for the majority of fire-control

instruments are detachable. They consist of a container

for one or more dry-cell batteries, a light switch which is

sometimes combined with a rheostat, a clamping

arrangement, and a miniature electric bulb with facilities

for directing the illumination to one or more points on the

instrument (fig 4-29). Some systems have an additional

"finger light" (fig 4-30) to permit directing light to any part

of the instrument.

Figure 4-30. Instrument light for BC periscope.

e. Some instrument lights are not supplied with dry-

cell batteries, but use the vehicle power source.

Section II. COATED OPTICS

light which enters the telescope is transmitted to the eye;

4-13. Introduction.

the remaining 40 percent is lost by reflection and

a. Light loss by absorption and scattered dispersive

absorption at the surfaces of the various elements. The

reflection on a silvered surface is about 7 percent.

result is that the image becomes dimmer, the stray

Whenever light strikes an ordinary glass surface, most of

reflections produce glare which reduces contrast, and

the light is transmitted through the glass and undergoes

"ghost images" are produced when viewing bright objects

refraction. However, even under the most favorable

at night due to reflection from the curved lens surfaces.

conditions generally about 4 to 6 percent of the light is

c. A process has been developed for coating the

lost by reflection from the surface. This loss takes place

air-to-glass surfaces of optical elements with a very thin

when light passes from glass to air as well as when light

film of transparent substance. The coating reduces the

passes from air to glass.

loss of light by reflection from each optical interface

b. This condition occurs at each optical surface of

surface to less than 1 percent (about 1/2 percent),

the elements in a fire-control instrument. For example,

resulting in a corresponding increase in the amount of

in a straight telescope with 6 optical elements there are

4ight transmitted through the element. Optical elements

12 surfaces at which there is reflection and loss of light.

so treated are termed coated optics.

Consequently, if each surface transmits 96 percent of the

light which it receives, only about 60 percent of the

4-31