Page 270 - Modern Optical Engineering The Design of Optical Systems

250 Chapter Eleven

11.3 Reticles
A reticle is a pattern used at or near the focus of an optical system,
such as the cross hairs in a telescope. For a simple cross-hair pattern,
fine wire or spider (web) hair is occasionally used, stretched across an
open frame. However, a pattern which is supported on a glass (or other
material) substrate offers considerably more versatility, and most ret-
icles, scales, divided circles, and patterns are of this type.
The simplest type of reticle is produced by scribing, or scoring, the
glass surface with a diamond tool. A line produced this way, while not
opaque, modifies the glass sufficiently so that under the proper type of
illumination the line will appear dark. Where clear lines in an opaque
background are desired, the glass can be coated with an opaque coating,
such as evaporated aluminum, and the lines scribed through the coat-
ing with a diamond or hardened steel tool, depending on the type of line
desired. Scribing produces very fine lines.
Another old technique is to etch the substrate material. A waxy resist
is coated on the substrate and the desired pattern cut through the
resist. The exposed portion of the substrate is then etched (with hydro-
fluoric acid in the case of glass) to produce a groove in the material. The
groove can be filled with titanium dioxide (white), or lamp black in a
waterglass medium, or evaporated metal. Etched reticles are durable
and have the advantage that they can be edge-lighted if illumination is
necessary. Any substrate that is readily etched can be used. This
process is used for many military reticles and also for accurate metrol-
ogy scales on steel.
The most versatile processes for production of reticles are based on the
use of a photoresist, or photosensitive material. Photoresists are exposed
like a photographic emulsion, either by contact printing through a mas-
ter or by photography. However, when the photoresist is “developed,” the
exposed areas are left covered with the resist and the unexposed areas
are completely clear. Thus, an evaporated coating of any of a number of
metals (aluminum, chrome, inconel, nichrome, copper, germanium, etc.)
can be deposited over the resist. In the clear areas the coating adheres to
the substrate; when the resist is removed, it carries away the coating
deposited upon it, leaving a durable pattern which is an exact duplicate
of the master. The precision, versatility, ruggedness, and suitability for
mass production of this technique have earned it a prominent place in
the field of reticle manufacture.
The photoresist technique may also be combined with etching, where
the material to be etched is either a metal substrate or an evaporated
metal film.
Where the reticle pattern must be nonreflecting, the glue silver
process or the black-print process is used. The technique is similar
to that used in producing the photoresist pattern, except that the