Page 244 - Modern Optical Engineering The Design of Optical Systems

224 Chapter Ten

and a reasonably precise (negative) replica is left on the substrate.
This process has several advantages. For example, any surface (including
aspherics) for which a master can be made can be replicated relatively
inexpensively, since the master can be used over and over. Other
advantages are that a mirror can be made an integral part of its
mount, the bottom of a blind hole can have an optical polish and fig-
ure, and extremely thin and lightweight parts can be produced. In
many cases these things are effectively impossible with standard opti-
cal fabrication techniques. The limitations to replicated parts are the
inherent softness of the epoxy and the change in the surface figure
from that of the mold.

10.6 Absorption Filters
Absorption filters are composed of materials which transmit light
selectively; that is, they transmit certain wavelengths more than others.
A small percentage of the incident light is reflected, but the major por-
tion of the energy which is not transmitted through the filter is
absorbed by the filter material. Obviously, every material discussed in
the preceding sections of this chapter is, in the broadest sense, an
absorption filter, and occasionally these materials are introduced into
optical systems as filters. However, most optical glass filters are made
by the addition of metallic salts to clear glass or by dyeing a thin
gelatin film to produce a more selective absorption than is available in
“natural” materials.
The prime source of dyed gelatin filters is the Eastman Kodak
Company, the line of Wratten filters of which is widely used for appli-
cations where the versatility of dyed gelatin is required and the envi-
ronmental requirements are not too severe. Gelatin filters are usually
mounted between glass to protect the soft gelatin from damage.
The number of coloring materials which are suitable for use in
optical filter glass is limited, and the types of filter glass available are
thus not as extensive as one might desire. In the visible region, there
are several main types. The red, orange, and yellow glasses all trans-
mit the red and near-infrared and have a fairly sharp cutoff, as indi-
cated in Fig. 10.10. The position of this cutoff determines the apparent
color of the filter. Green filters tend to absorb both the red and blue
portions of the spectrum. Their transmission curves often resemble the
spectral sensitivity curve of the eye. Blue optical glass filters can be a
disappointment, since they occasionally transmit not only blue light,
but some green, yellow, orange, and frequently a sizable amount of red
light as well. The purple filters transmit both the red and blue ends of
the spectrum, with fair suppression of the yellow and green spectral
regions. Filter glass is manufactured by most optical glass companies