Page 242 - Modern Optical Engineering The Design of Optical Systems

222 Chapter Ten

many zoom lenses, projection TV lenses, and some high quality camera
lenses. The low cost of mass-produced plastic optics is one important fac-
tor in this popularity; another is the ease of production of aspheric sur-
faces. Once the aspheric mold has been fabricated, an aspheric surface is
as easy to make as is a spherical surface (in marked contrast to glass
optics). The rule of thumb that the introduction of an aspheric surface
allows the elimination of an element from the system attests to the val-
ue of optical plastic materials. This aspheric capability largely offsets the
unfortunate fact that the number of suitable optical plastics is very small
and that there are only relatively low index materials in that number.
In considering a venture into the plastic optics arena, one is well
advised to seek out a specialist in making plastic optics. Not only is the
typical injection molder incapable of making good optics, but he or she
usually has no conception of what is required to do so. The successful
fabricators have developed good, reliable sources of consistently high-
quality raw materials and material handling techniques, and they
have molding machines which have been adapted to the special
requirements of optical work. Temperature control is extremely criti-
cal, and a longer cycle time is necessary to achieve an optical level of
precision. I encountered an extreme case a few years ago. I had
designed a visual system for a client who insisted (against my advice)
not only on patronizing an inexperienced (in optics) injection molder
but also insisted on using an unusual material. The result was a sys-
tem which you literally could not see through.
In addition to the general, smooth aspheric capability, plastics are
widely used to make Fresnel lenses, where fine steps are necessary.
The condenser system in overhead projectors and the field lenses in
the viewfinders of single-lens reflex cameras are examples of plastic
Fresnel lenses. Another currently popular application is in diffractive
optics (discussed at greater length in Chaps. 12 and 16), where the dif-
fractive surface is basically a Fresnel surface the step height of which
is on the order of a half wavelength.
Another advantage in mass production is the ability to mold both
the lens element and its mounting cell in one shot. The cells of an
assembly can in fact be designed so that the lens assembly simply
snaps together, and a drop of a suitable solvent or adhesive can make
the assembly permanent.
The obvious advantages of plastic—that it is light and relatively
shatterproof—are offset by a number of disadvantages. It is soft and
scratches easily. Except by molding, it is difficult to fabricate. Styrene
plastic is frequently hazy, scatters light, and is occasionally yellowish.
Plastics tend to soften at 60 to 80°C. In some plastics the index is
unstable and will change as much as 0.0005 over a period of time. Most
plastics will absorb water and change dimensionally; almost all are