Filters

TM 9-258

intensities. They are provided as separate elements

4-11. Filters.

which may be attached and detached (A, fig 4-27) or are

a. General. Filters or ray filters are colored glass

mounted so they may be placed in or out of position as

disks with plane parallel surfaces. They are placed in the

desired (B and C, fig 4-27).

path of light through the optical system of a fire-control

instrument

reduce

glare

and

light

Figure 4-27. Types of filter mountings.

b. Colored Filters. Filters of various colors are

c. Polarizing filters.

(1) Polarizing filters do not change the color of

used to improve visibility under different atmospheric and

objects but merely decrease light intensity and are used

light conditions. Among the colors of filters used are:

to eliminate glare. When mounted in pairs, they can be

smoke, yellow, amber, blue, red and greenish-yellow.

used to provide continuous control of light intensity.

(1) The smoke (neutral) filter reduces the

(2) Light polarizing substances can be

intensity of light and is effective when observing against

considered as being made up of very minute parallel

or in close vicinity of sun or a searchlight; usually it is too

bars or grain (A and C, fig 4-28). Light vibrates in waves

dark for other purposes.

with the crests of the waves in all directions. A polarizing

(2) The yellow and amber are used to protect

substance, placed in the path of the light, permits waves

the eyes from the reflection of sunlight on water and

vibrating parallel to the direction of the grain to pass

other general conditions of glare.

through; waves vibrating at right angles to the bars or

(3) The amber and red filters are usually

grain are stopped (B, fig 4-28). By eliminating a portion

employed under various conditions of fog and ground

of the light, the intensity of the light is decreased. When

haze. Red filters are also used in observing tracer fire.

two polarizing substances are held with the grain of one

(4) The blue filter is helpful in detecting the

at right angles to that of the other, no light passes

outlines of camouflaged objects.

through. By shifting the substance to intermediate

(5) The greenish-yellow filter is intended to

angles, light at different degrees of intensity is permitted

serve the purpose of both smoke and amber.

to pass through.

4-29