Optical elements may be formed from any transparent substance and due to their unusual properties, plastics may prove superior to glass in some applications. Possible advantages of plastic optics are:

Light weight. In a portable instrument using large or numerous optics this saving of weight may be considerable.

Breakage without shattering. Many plastics do not break into sharp pieces, an often valuable feature in lenses used near the eye.

Unusual combinations in index of refraction and dispersion. Some plastics have combinations of values different from any known glass. This permits the lens designer an additional degree of freedom with in some cases, a simpler, cheaper and optically superior product.

Transparency to various radiations. Different plastics have a greater range of transparency than the glasses and may be used in scientific work where unusual transmission properties are needed.

Ease of working. While at present it is more difficult to produce an optical surface on plastics than on glass, the machinability of plastics permits the production of odd shapes and moldability allows the production of unusually large and deep lens elements that would be impracticable in glass.

Thermal insulating properties. Many plastics are such good heat insulators that they make excellent windows into apparatus operated at very low temperatures.

Ease in production of aspherical surfaces. Molding procedure produces any curve in quantity.

Corrosion resistance. Certain plastics resist agents that attack glass and can be used where hydrofluoric acid fumes, for example, are present.

On the other hand, there are several properties of plastics which may make them inferior to glass in most applications: Softness. Most plastics are so soft that an optical surface cannot be cleaned without scratching.

Variation with temperature. A small change in temperature will change the index of refraction of most plastics several times as much as glass. Also, variation in dimensions is considerably greater.

Dimensional instability. Many plastics, depending on their chemical constitution, may lose solvent and become brittle, check, crack, or develop a crazed surface discolor or crystallize and become opaque with age.

High cost. At present it is more expensive to make plastic elements than glass of the same degree of precision.

The following optical elements have been successfully made of plastic materials: lenses (simple and achromatic to 18 inches aperture and to exceedingly deep curves); prisms (right-angle, special, and infrared-transmitting, some of unusual size); mirrors (flat and concave, first-surface-aluminized); Schmidt correcting plates; plane-parallel windows; filters incorporating organic dyes.

In general, two methods of fabrication have been used-molding to shape and forming by cutting and abrasion.

The casting process has been highly developed by the Polaroid Corporation of Cambridge, Mass. and details are available from them in a paper by Edwin H. Land, director of research for that organization. The casting process is particularly adapted to mass production and consequently will be only of academic interest to the amateur who wants to make only one or a few elements. Two plastics are generally used, polycyclohexylmethacrylate with an index of refraction of 1.5064, which makes it similar to crown glass, and polystyrene with an index of 1.5916, similar to flint glass. A mold is made of Pyrex and, for casting a biconvex lens, two concave molds are made. Liquid plastic is poured into the mold and allowed to harden, taking the shape of the mold. The mold is opened and the finished lens removed. The surfaces are perfect and need no polishing. The lenses produced by this process are of good optical quality and may be used for any except the most exacting applications. They have not been suitable for astronomical telescope mirrors or objectives. This process is feasible only where a large number of identical elements are justified. Even then the cost per element is as high as that of glass elements of the same quality. The reasons for this are stated in Land's paper. This is distinctly not a cheap process.

Plastic optics may be shaped out of stock blanks by processes similar to those used in making glass elements. This procedure seems more applicable to amateur needs and small production. A process for making optics where great exactness is not required is useful for making mockups, condenser lenses, magnifiers and simple systems where only low power is to be used. Since such optics are not usually achromatic, a single material such as Lucite or Plexiglas may be used in sheets or rods. This is not made for optical use and may show striae and other obvious optical defects; hence each piece should be selected by eye.

The procedure for making a three-inch plano-convex condensing lens will be described with the assumption that the reader is familiar with common glass lensmaking methods.

Out of a plastic sheet of adequate thickness saw a disk somewhat larger than the desired lens. Attach to it a support for holding in the lathe. It is recommended that the blank be attached to a hardwood or plastic backing with pitch, using hot water to warm the surfaces and to melt the pitch. With the blank rotating on the lathe cut the curved side to shape with a sharp tool and water lubrication. Check the curve with a template. Sandpaper or steel wool may also be used to bring the curve to shape.

Now make a concave lap of opposite curvature from the lens. Metal, plastic or wood may be used. Satisfactory laps have been made by the unorthodox process of casting blocking plaster against the turned surface of the plastic lens. Coat the lap with melted pitch and cover with ordinary department-store felt. Shape the felt-covered lap to the lens. When cool, saturate the lap with rouge paste and polish the, lens as it rotates on the lathe or transferred to a spindle. Hold the lap in the hand and oscillate it over the lens surface. Comparatively deep scratches and tool marks will polish out quickly.

The process is complete when the surface is polished and all marks are removed. The surface will appear rather dull due to minute scratches produced by the felt hairs. Coat the palm of the hand with dry rouge and stroke the rotating lens a few times, using the palm in a manner similar to a lap. A dozen strokes should put a beautiful polish on the lens.

Detach the lens from the blocking piece by heating in water. Turn it over and finish the other side. A special concave backing block may be helpful in holding the curved face. If the second surface is to be flat, machine it flat on the lathe and polish with a flat lap made by covering a flat tool with felt and forming this lap against another flat surface while warm. Edge the lens, separate it from the backing, and clean it.

Any kind of polishing abrasive is suitable-ordinary, cerium or Barnesite, though cerium is preferred. For final polishing, a lap covered with rayon or silk velvet may be used instead of the palm. Ordinary rouge or precipitated chalk may be used in this step but best is precipitated

titanium oxide, the pigment of good white paint. DuPont Ti-pure R-300 is good, though available only in 50-pound quantities, but Universal Shellae and Supply Co. Brooklyn, N. Y., furnish it in one-pound cans as "White Glassite."

To make a precision surface on plastics a different procedure is followed. For a precision lens or prism optical quality plastic must be used and this is as hard or harder to obtain than optical glass. Each piece must be inspected by the methods used for glass. You can obtain the raw unpolymerized plastic from the manufacturer and mold it yourself in simple molds or buy second-hand plastic lenses and cut them up for material. At present, three-inch plastic achromats may be had from salvage companies at low prices due to scratched surfaces.

Turn the blank on the lathe to approximate shape and check by template. Make a lap similar to a brass "true-tool" (see Twyman, Prism and Lens Making, or Dévé, Optical workshop Principles) or a glass one. Both male and female are cut to templates and worked together with fine emery to get true surfaces on the laps. Now grind the surfaces to the lap. This is the hardest part because the abrasives sink into the soft plastic and it grinds the lap, instead of vice versa. Some means of anchoring the abrasive is almost a necessity. For rough grinding, sandpaper, preferably the waterproof kind, may be cut, gored, and cemented to the lap. For fine grinding the lap is cleaned, painted with a thin layer of rubber cement and sprinkled with abrasive.

For fine grinding emery is a better abrasive than Carbo, but best of all is cuttlefish powder. Cuttlefish bones in ground form make a most peculiar abrasive. These are sold in pet stores to canary owners. Cuttlefish powder and paper, which looks like sandpaper, are on the market and have long been used by cabinet makers to polish lacquer ware. These materials are, however, so hard to find that personal preparation will be described.

A cuttlebone is pounded in a mortar and sieved through screening. Several grades may be obtained and the finest washed with water and graded like emery by elutriation or levigation. Plastics are so soft that, in grinding, several grades may be skipped, and no definite recommendations may be given. Incidentally, cuttlefish powder will polish quartz and many other hard materials. Some brands of scouring powder can be used as fine abrasives on plastics, but they are so likely to contain coarse particles that they cannot be used without levigation.

Grinding of the plastic is done with a metal lap and various grades of abrasive, using rubber cement when necessary to anchor the abrasive. Lead, brass, copper and wood laps have been used without discovering any special advantage in any.

Polishing is done with a pitch or beeswax-coated pitch lap. Unfortunately, pitch sticks to many plastics to an extent that is most surprising to a worker used to glass. The only way to prevent this is to first work the pitch surface full of rouge. To do this, a glass dummy exactly like the plastic surface is made up and the pitch lap formed to it, channeled, and the glass polished, using rouge that is almost dry. This grinds rouge deep into the lap. When the lap is thoroughly impregnated and no pitch shows, it is cold-pressed to the plastic surface and used as a polisher with little rouge and plenty of water. Slow polishing speeds must be used, as the thermal expansion of plastic is high.

Using the above procedure, plane-parallel windows showing Haidinger's fringes equal to good glass samples have been made. Plastic flats have been made that have retained a quarter wave figure for one year.

Plastic optics are cemented with butyl methacrylate obtainable from the Eastman Kodak Company. The liquid is placed between the elements and set by heating in an oven to 60 degrees Centigrade (140 degrees Fahrenheit) for one half hour. During the war a great many glass lenses were cemented with this material instead of balsam. Such lenses are almost impossible to separate.

At present plastic optics leave much to be desired, especially in cost and permanency. When we have been making plastic lenses as long as we have been making optical glass, big improvements will have arrived. Today its most promising applications are those where glass cannot be used but there are more of these than is generally known. The full possibilities have not been explored, nor can they be in a short length of time. The possibilities of glass haven't been exhausted in 400 years of optic manufacture.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/

At Surplus Shed, you'll find optical components such as lenses, prisms, mirrors, beamsplitters, achromats, optical flats, lens and mirror blanks, and unique optical pieces. In addition, there are borescopes, boresights, microscopes, telescopes, aerial cameras, filters, electronic test equipment, and other optical and electronic stuff. All available at a fraction of the original cost.

SURPLUS SHED
407 U.S. Route 222
Blandon, PA 19510 USA
Phone/fax : 610-926-9226
Phone/fax toll free: 877-7SURPLUS (877-778-7758)
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