Figure 4-28. Principles of polarization of light.

TM 9-258

(3) Glare is due to light which is reflected from

smooth or wet horizontal surfaces. It can be very

annoying and detrimental to clear vision. Such light as

being reflected at a glancing angle from a smooth

horizontal surface is found to be also polarized in the

horizontal plane, the vertical waves being substantially

eliminated. A polarizing filter consisting of vertical grain

or grid will destroy the remaining horizontal component of

that portion of the light which causes glare while

permitting normal vision to continue practically

unobstructed. The principal function of polarizing filters

is to eliminate glare in the field of vision.

4-12. Lighting.

a. The reticles of fire-control instruments require

illumination for night operation and under certain

conditions for day operation in order that their markings

may be seen clearly.

b. Reticles are edge-lighted. The markings are

etched or engraved into the surface of the glass. When

light is introduced at the edge of the reticle, it travels

through the glass and is diffused at these etched or

engraved marks illuminating them. Light is introduced at

the edges of the reticles through small glass windows set

in openings in the body of the instrument. At night, tiny

shielded electric lamps are placed over the windows.

c. Lucite is a transparent plastic having the

property of transmitting light which enters one end of a

rod of this material. The light travels from end to end of

the rod with little loss through the outside surface,

despite the fact that the rod may be bent. Lucite is

employed in the lighting systems of a number of

instruments (fig 4-29).

Figure 4-29. Instrument light for panoramic telescope.

4-30