Page 252 - Modern Optical Engineering The Design of Optical Systems

232 Chapter Ten

10.9 Cements and Liquids
Optical cements are used to fasten optical elements together. Two
main purposes are served by cementing: the elements are held in accu-
rate alignment with each other independent of their mechanical
mount, and the reflections from the surfaces (especially those from
TIR) are largely eliminated by cementing. Ordinarily the layer of cement
used is extremely thin and its effect on the optical characteristics of
the system can be totally neglected; some of the newer plastic cements,
designed to withstand extremes of temperature, are used in thicknesses
of a few thousandths of an inch (which could affect the performance of
an optical system under critical conditions where the light rays have
large slopes).
Canada balsam is made from the sap of the balsam fir. It is available
in a liquid form (dissolved in xylol) and in stick or solid form. Elements
to be cemented are cleaned and placed together on a hot plate. When the
elements are warm enough to melt the balsam, the stick is rubbed on
the lower element. The upper element is replaced and the excess cement
and any entrapped air bubbles are worked out by oscillating or rocking
the upper element. The elements are then placed in an alignment fix-
ture to cool. Balsam cement has an index of refraction of about 1.54 and
a V-value of about 42. These are conveniently midway between the
refractive characteristics of crown and flint glasses. Unfortunately,
Canada balsam will not withstand high or low temperatures. It softens
when heated and splits at low temperatures and is thus unsuited for
rigorous thermal environments. Balsam is very rarely used today.
A great number of plastic cements have been developed to withstand
extremes of both temperature and shock. For the most part, these are
thermosetting (heat-curing) or ultraviolet light-curing plastics,
although a few thermoplastic (heat-softening) materials are used.
Cements are available which will withstand temperatures from 82°C
down to 65°C without failure when properly used. In general the
thermosetting cements are supplied in two containers (sometimes
refrigerated), one of which contains a catalyst which is mixed into the
cement prior to use. A drop of cement is placed between the elements
to be cemented, the excess cement and air bubbles are worked out, and
the elements are placed in a fixture or jig for a heating cycle which
cures the cement. Once the cement has set, it is exceedingly difficult
to separate the components; the customary technique is to shock them
apart by immersion in hot (150 to 200°C) castor oil. The index of
refraction of plastic cements ranges from 1.47 to 1.61, depending on the
type, with most cements falling between 1.53 and 1.58 with a V-value
between 35 and 45. Epoxies and methacrylates are widely used.
Because of the variety of types and characteristics which are available,