Page 236 - Modern Optical Engineering The Design of Optical Systems

216 Chapter Ten

glass is also available in “water white” quality, without the greenish
tint. For elements with one or two plane surfaces and with modest pre-
cision requirements, window glass can often be used without further
processing; the accuracy of the plane surfaces is surprisingly good. By
special selection, plane parallels can be obtained which meet fairly rig-
orous requirements. The secret here is to avoid pieces cut from the
edge of the large sheets in which this type of glass is made; the center
sections are usually far more uniform in surface and thickness. Note
that the surface of “float glass” is significantly less smooth by a factor
of 3 or 4, although recent process improvements have brought the sur-
faces up to that of window and plate glass.

10.3 Special Glasses

Several glasses are available which differ sufficiently from the stan-
dard optical glasses to deserve special mention.

Low-expansion glasses. In applications where the elements of an opti-
cal system are subject to strong thermal shocks (as in projection
condensers) or where extreme stability in the presence of temperature
variations is necessary (such as astronomical telescope reflectors or
laboratory instruments), it is desirable to use a material with a low
thermal coefficient of expansion.
A number of borosilicate glasses are made with expansion coeffi-
cients which are less than half that of ordinary glass. Corning’s Pyrex
#7740 and #7760 have expansion coefficients between 30 and 40
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10 /°C. The index of refraction of these glasses is about 1.474 with a
V-value of about 60, and their density is about 2.2. Unfortunately they
are often afflicted with veins and striations so that they are suitable
only for applications such as condensing systems when used as refracting
elements. They are widely used for test plates and for mirrors. Some
of these materials are yellowish or brownish, but others are available
in a clear white grade.
Another low-expansion glass is fused quartz, which is also called
fused silica glass. This material is essentially pure (more or less,
depending on the grade and manufacturer) silicon dioxide (SiO 2 ) and
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has an extremely low expansion coefficient of 5.5 10 /°C. It was
originally made by fusing powdered crystalline quartz. Fused quartz
can be obtained in grades with homogeneity equal to that of optical
glass. Fused quartz is a completely different material than crystalline
quartz. Its index is 1.46 versus 1.55; it is amorphous (glassy) without
crystalline structure; and it is not birefringent, as is quartz.
Fused quartz has excellent spectral transmission characteristics,
extending further into both the ultraviolet and infrared than ordinary