Page 157 - Chalcogenide Glasses for Infrared Optics

Unconventional Lens Fabrication, Aspheric Surfaces, and Kinos 133

The initial cost of the diamond point machine was great at first.
Since the early 1990s the machines have become much more common-
place, resulting in lower initial cost and more general use. Designs
previously impossible to consider are routinely produced but are
costly. Costs are estimated in terms of labor hours to set up the machine
and labor hours for a technician to tend to the machine as the part is
generated, machine time. The diamond turning is carried out at high
rotation speeds and under the cover of a cooling fluid continuously
sprayed on the rotating part. The process is confined in an enclosure in
part to protect the operator.

6.3 Slump Molding
Glass has been molded for centuries using simple methods. Gener-
ally, the required amount of glass is placed in an open mold and then
heated sufficiently for the glass to soften and move under the force of
gravity to fill the mold. No pressure is used. Slump molding may be
used as a cost-saving measure. Slump molding has been used at TI
and AMI to preshape a lens blank to minimize the amount of material
removed and wasted during the grinding stage of forming the lens.
At AMI, routinely a blank for a deep 7.5-in meniscus Amtir 1 lens has
been formed by slumping a flat plate of glass into a concave Pyrex
mold. The mold is generated from a Pyrex mirror blank with the con-
cave radius equal to the convex radius of the lens. The plate is placed
over the mold and heated in the furnace together until it softens and
slumps into the mold under its own weight. After slumping, it is
annealed. The thickness of the Amtir 1 plate before slumping is
slightly greater than the finished center thickness of the lens. Mostly
slump molding is used for shaping glass to dimensions that are not
precise.

6.4 Precision Molding
The desire to begin precise molding of lenses was a continuation of
the goal to lower the cost of infrared optics. The ability to diamond
point turn lenses was a real advancement but too expensive. The
expense occurred for each lens ordered. The thinking went that if the
expense produced a mold instead of one lens, the mold could be used
over and over to make many lenses with only the cost of the infrared
glass lens blank. The cost of the diamond turning would thus be aver-
aged over many lenses. In 2000, AMI joined with Lockheed Martin in
Orlando (LMCO) in a program to develop the technology required to
mold infrared lenses from chalcogenide glasses. At this time, the most
experienced and knowledgeable person in the United States regarding
molding lenses from glass was Harvey Pollicove. His efforts at
Eastman Kodak resulted in the development of a production facility