CHAPTER 4 OPTICAL COMPONENTS, COATED OPTICS, AND CONSTRUCTION FEATURES

TM 9-258

CHAPTER 4

OPTICAL COMPONENTS, COATED OPTICS, AND

CONSTRUCTION FEATURES

Section I. OPTICAL COMPONENTS

(6) Chemical stability and mechanical hardness

4-1.

Optical Glass.

determine the resistance of the finished optical element

a. Types. Two of the most important types of

to handling and contact with atmospheric moisture.

optical glass are known as crown glass and flint glass.

(7) Freedom from internal strain depends upon

Crown is an alkali-lime glass. Boro-silicate crown glass

annealing or very slow cooling of the glass in

contains salts derived from boric and silicic acids. Flint

manufacture. Any large amount of internal strain may

glass contains lead. The refractive indices of these

cause the glass to break during grinding or if subjected

types of glass are listed in paragraphs 2-12c and 5-31a.

to shock at any time after grinding.

b. Characteristics.

The

characteristics

c. Manufacture. Although an understanding of

affecting the values of all types of optical glass may be

the manufacture of optical glass is not essential to a

divided into the purely optical properties, which directly

knowledge of elementary optics, some appreciation of

influence the light in passing throught the glass, and the

the many processes involved will show why fine optical

general physical qualities. The purely optical properties

glass is considered a critical material. The finely divided

are constant density (homogeneity), transparency

materials are very thoroughly mixed, usually by hand.

refraction, dispersion, and freedom from color and

This mixture is melted in small charges, small quantities

defects. The general features of optical glass are

being fed to the melting pot at intervals regulated by the

chemical stability, mechanical hardness, and freedom

melting time of the charge previously introduced. Each

from internal strain.

charge is brought to a liquid state before the next charge

(1) Constant density or homogeneity is the most

is placed in the pot. The next operation, the fining

important property. The uniformity of the index of

process, consists of holding the molten glass mixture at

refraction of a given sample of glass is dependent upon

a high temperature, sometimes as long as 30 hours, to

its constant density.

drive off the bubbles contained in the liquid. To secure

(2) The greater the transparency, the less light

homogeneity and prevent striae, the molten glass is then

is absorbed and the more light will pass through.

stirred during the gradual cooling of the mass until it is so

(3) The refractive and dispersive qualities are

stiff that the stirrer cannot be moved. The mass is stirred

dependent upon the type of glass. Crown glass has

from 4 to 20 hours, depending on the glass and the size

approximately half of the dispersive power (ability to

of the pot. As soon as the stirring ceases, the pot is

spread the colors of so-called white light) of flint glass.

withdrawn from the furnace and allowed to cool to about

(4) Freedom from color tint, while preferable, is

the annealing temperature. It is then placed in an

not as essential as the other requirements. It is obtained

annealing kiln at a temperature of from 4000 to 5000 C.

by careful selection of raw materials which must be of

and slowly cooled to ordinary temperature. Annealing

the greatest possible purity.

takes from 1 to 2 weeks, according to the size of the

(5) Types of defects are known as stones,

batch. When cooled, the pot is withdrawn from the

bubbles, seeds, and striae (layers). Stones are due to

annealing kiln and broken away from the glass.

solid material that accidentally may get into the molten

glass at the time of manufacture. Bubbles are air

4-2.

Lenses.

pockets in the glass. Seeds are very minute bubbles.

a. Single Lenses. Lenses are optical elements

Striae are wavy bands which appear to be of different

with polished faces, one plane and one spherical or

density and color than the surrounding medium.

4-1