Epoxy replication of front surface mirrors has been practiced for more than forty years. Properly applied, the technique can offer the advantages of lower cost, low mass, monolithic construction and high producibility. This paper examines most aspects of the process and discusses its virtues and weaknesses.
This paper gives a brief overview of the replication process for optical elements with epoxy resin based on a practical example, the so-called "aspherical end plate". The main viewpoints are the preparation of the master moulds with a siloxane solution and the attempt to explain the influence of the carbon acid during the epoxy cure in the epoxy system "resin-hardener-accelerator".
Delicate diamond turned Aluminum optics (DTO's) were made much more durable by Hard Carbon Coating (HCC). Submasters, steel masters and glass master surfaces were also made more rugged by the application of Hard Carbon. Multiple replicas were successfully made from these Hard Carbon coated masters and submasters. The merging of these three technologies (DTO's, HCC, and replicated optics) could produce lower cost precision aspheric mirrors and lenses.
Precision aspheric surfaces can be produced by Thin Film Epoxy Replication for operation in the ultra-violet, visible and infrared spectral regions. Accuracies to 1/10 wave at 633 nm are obtained by proper control of the master, replica substrate and replication process. Aspherics with small deviations from the best fit sphere can use spherical substrates; otherwise aspheric substrates must be used. Aspheric mirrors up to 16 inch diameter have been produced to 1/4 wave accuracy. Off axis ellipsoids with deviations from the best fit sphere measured in fractions of an inch have been produced with surface accuracies of 1/4 wave per cm. in production quantities.
Replication techniques have long been used to fabricate optical components. As early as the 1950's, serious studies of replication techniques were performed. During the late 60's and throughout the 70's replicated reflective optical components were used extensively as a cost effective alternative to conventionally fabricated components of unusual geometry. This paper addresses the problem of fabricating refractive replicated components. Until recently, these were manufactured in relatively small volumes or for prototype quantities. The obvious difference between reflective and refractive replication processes is that a metallic layer is present in the reflective replication process. The requirements of a transmissive replicated layer have significant impact of the choice of adhesives, coatings, mixing processes, and actual fabrication techniques. First, I will describe a process for fabricating a replicated refractive surface. Figure 1 depicts the geometry. A master surface is treated or coated with a release layer. This layer may then be overcoated with another coating, perhaps an anti-reflection or anti-abrasion coating. An adhesive is applied on top of this coating. The lens substrate onto which the replicated surface is to reside is then placed onto the adhesive and the sandwich is glued together. Fixtures are frequently used to keep the substrate in position while the adhesive cures. If they are properly designed, they fixtures can also assist if applying the forces required to release the substrate from the master. Specific details of a replication process depend on the particular choice of master substrate, release layers or surface treatment, adhesive and coatings.
Replicated optics make possible the design and manufacture of mirror structures of exceptionally low weight. In practice, mirrors replicated over light-weight substrates are found to exhibit problems of dimensional stability induced by the operating environment. This paper discusses issues associated with the design and fabrication of light-weight replicated mirrors and describes the results of dimensional stability tests performed on replica structures exposed to thermal and dynamic stresses.
The first section of this presentation is a review of mirror requirements specific to Low Inertia Scanners in high performance applications. The advantages of integral metal mirrors are outlined. The last section describes the design and performances of a laminated mirror construction.
Ultraviolet light absorbing monomers have been developed that can be copolymerized with acrylates. The composition of the resultant stable copolymers can be adjusted to totally block the transmission of light below about 430 nm. Fabrication of lenses from the materials is accomplished by lathe cutting and injection molding procedures. These ultraviolet light absorbing materials are non-mutagenic and non-toxic and are currently being used in intraocular lenses.
Properties of an extremely clean high-flow acrylic resin are discussed in the context of the optical disc market. Benefits include reduced birefringence and light-scattering in parts molded from this resin. Comparisons to standard acrylics and to polycarbonate resins are made. Data from a light-scattering particle counter illustrates improvements in resin purity.
A novel acrylic resin has been developed for injection molded precision lenses. This resin is a copolymer of tricyclodecyl methacrylate (TCDMA) with other acrylic monomers. The novel resin, TCDMA-Acrylic copolymer has rigid, bulky cyclic hydrocarbons in the side chain so that it possesses characteristics superior to conventional acrylic resin (PMMA) in terms of low moisture absorption properties and high temperature resistance. However, like PMMA, it also provides high transparency, low birefringence, and low dispersion of refraction. Because of these characteristics, TCDMA-Acrylic copolymer is being used for precision aspheric lenses that cannot be realized by conventional optical plastics.
The optical anisotropy which is frozen into transparent plastic parts has often been utilized to assess the level of molecular orientation produced during the injection molding process . The advent of optics based digital information encoding and retrieval technologies (optical discs) have required the development of transparent polymeric materials which can be injection molded into optical components with controlled (low) levels of optical anisotropy. The light beam typically passes through the transparent substrate, and excessive levels of optical anisotropy degrade the performance of the data retrieval system. This optical anisotropy has been shown to be caused by recoverable molecular orientations, which are produced in the filling and holding portions of the injection molding cycle, and are frozen into the part during the molding process. Processing effects on molded in birefringence have been approached from empirical , experimental , and theoretical [4,5,6] viewpoints. The latter efforts have focused on predicting the optical anisotropy in transparent moldings. This paper will present experimental results obtained for several polycarbonates of bisphenol-A. Concepts from flow birefringence in molten polymers  will be used to identify the significant process and melt rheological variables which influence molded in birefringence. Control strategies to minimize orientations produced during both mold filling and the pack-and-hold portions of the molding cycle will be developed. Results will be presented for two mold geometries, an end gated plaque and a center gated disc, to demonstrate the interrelationships between processing and melt theology which govern the optical properties of the moldings. In addition, some preliminary results on the utility of variable-cavity-volume mold designs to further reduce optical anisotropy will be discussed as well as development of a polariscope to measure in greater detail optical properties of compact disc substrates.
Extensive use of plastics has become commonplace in the optics industry over the last decade. Plastics offer many advantages in the design and manufacturing of novel optical components. A combination of advanced techniques for machining and measuring surfaces as well as process control during molding enable optics manufacturers to make tightly toleranced multielement plastic aspheric lenses. In this paper we will attempt to describe a general approach we might follow to develop and evaluate a prototype optical component. In the first part of the paper we will discuss the equipment and resources that might be used for this task, while in the second part we will present a procedure we might follow in the design and fabrication of precision prototype plastic optical components.
A plastic corner-cube reflector was designed for low cost and low mass; its total weight is less than 0.06 gram. The problems of making the die, forming, coating and testing are described, along with some applications. A mathematical model of the part, based on typical figure and angle errors, estimates beam divergence and return angle.
Injection molding of optical components on a production basis began in 1980. Development took place from the mid to late seventies with the potentially high volume defense contracts candidates for injection molding. Plastic optics are lower cost and lighter weight than optical glass. Also, injection molding offers the capability to produce complex shapes which are functionally integrated with other metal and/or plastic components. Lenses and domes with aspheric or spheric surfaces and internal or external threads have been successfully injection molded. These parts would have required costly manufacturing methods or even been impossible to produce. Although initial tooling cost and process development may be high, parts can be consistently produced in large quantities economically. Consistency from run to run is maintained by rigorous process control to ensure the high standards required in optical parts. Polycarbonate is the plastic material used most often to injection mold optical hardware. It is a thermopolastic consisting of linear polymer chains like an acrylic or a styrene but has higher lipact strength and temperature resistance. Polysulfone is also used and has even higher temperature resistance and strength although impact strength is lower. Mold design, process parameters, and part inspection (dimensional, visual, optical) as they relate to the plastic optical components injection molded at Martin Marietta are discussed. Also presented are problems encountered during pre-production and production and the corrective measures, e.g., cosmetic appearance, dimensional control, molded part handling, and enhancement of optical characteristics.
New molding processes have been successfully developed for precise aspherical lenses of cam-corder or projection TV. They are founded on our idea that precise aspherical lens can be molded by securing the molding deformation axial and constant in every shot. This paper presents a) computer simulation of thermal process and molding deformation, b) factors and process to secure the molding deformation constant i.e. repeatability of molded aspherical surface figure, c) molding dies and injection machine to realize temperature homogeneity of molding lens and stability of injected weight, and d) pre-correction of surface figure on optically polished insert to compensate surface deformation in the molding process. Additionally applications of the aspherical plastic lens manufactured industrially with new process are also provided.
This paper addresses the peculiar conditions one encounters in coating of plastic substrates with thin films. The emphasis lies on optical coatings, with decorative metallization being omitted from the discussion. Differences in the nature and preparation of the substrates, the coating processes, and resulting film properties compared with 'regular' optical coatings on glass elements are highlighted. The paper concludes with a survey of coating processes suitable for the deposition of thin films on plastics.
Coatings for plastic optics fall into two major categories in today's marketplace. One is in the opthalmic or eyeglass market, the other is precision optical instruments for industrial or military use. It is in the latter area we will concentrate this paper. The most common plastics used for these purposes are polycarbonates, polymethyl methacrylate and allyl diglycol carbonate (CR-39). They are normally made by casting or injection molding processes. Each of these materials have distinctive properties from one another but share other properties in common. We will discuss problems associated with organic substrates in general, some of the pros and cons of the three materials mentioned in relation to thin film coatings, solutions in dealing with these problems, and finally, examples of coated optics.
The tumble abrasion test proposed by J.M. Young and J.D.Masso has been modified in order to produce uniform scratching of the lens surface under test and in order to simulate closely damage which occurs during normal wear in the field. The uniformity of the abrasion enables a quantitative measurement of the abrasion resistance by a simple determination of the scattered light. The test has been applied to coated and uncoated plastic ophthalmic lenses. The results show that the scratch resistance of the coated lenses depends strongly on the composition and on the thickness of the coating.
Optical plastics can be molded or cast to replicate traditional spherical and aspheric lenses. It is possible to obtain good optical quality, but often it is necessary or desirable to enhance the surface characteristics in a variety of ways. These include improving the abrasion resistance, chemical resistance, the addition of anti-fog, or anti-static characteristics, applying electrically conductive coatings, and applying coatings or selective absorbers for light and color control. Coatings may be entirely organic or organo-silanes applied by dipping or spinning. All dielectric coatings such as quartz abrasion resistant coatings or multilayer dielectric coatings for reflection reduction or enhancement may be applied by vacuum vapor deposition. This paper discusses a number of these coatings and surface treatments. The paper describes their characteristics and includes discussions of their durability and environmental stability. The adhesion of coatings to plastic substrate depends on the specific substrate and coating materials. Pretreatments or primers are used to promote good coating adhesion. A coating used for one purpose will generally affect other properties of the plastic and trade-offs are sometimes required. A description is given of several test methods which have been found useful in evaluating the quality of the various coatings.
This survey will provide an overview of glass molding technologies, with a concentration in the newest of the technologies - Precision Glass Molding (PGM). A brief description of various forms of glass molding, including an historical review of patents associated with precision molding, is given. A worldwide survey of known commercial availability and recent innovations in PGM at Kodak are presented as examples of the potential of the precision molding technology.
Ophthalmic lens design and manufacture is changing because of the use of aspherics to correct presbyopia and because of the use of thermoplastic and thermosetting materials. These combinations are examined and compared to the aspheric component prospects in the mainstream of optics. Thermal replication of aspheric surfaces is described in relationship to injection molding and casting. Limitations on performance and constraints of cost of manufacture are examined and compared for all of these methods. Product applications considered range in scale from fiber optic connector lenses effective over millimeter apertures to elements for flight simulators approaching 1 meter in diameter.
For many years, plastic lenses were used in applications where low cost optics were required. With improvements in materials, molding equipment, and aspheric technology, more recently they are being used in optical systems where high performance is required. Plastic offers advantages of low cost, reduced weight, high volume capacity and high lens to lens repeatability. There are, however, inherent limitations to these materials which can degrade performance. These can be particularly harmful if the system is required to function at other than ambient environmental conditions. With the commercialization of glass lens molding, low cost, high repeatability optics can be produced in glass. Here material limitations are not a concern. The glass lens molding process at Kodak accurately replicates the geometry, figure, and finish of the tool. With these options, the design engineer has increased flexability when selecting materials for optical components.
The method of producing microlenses in glass by a photo-thermal technique is applied to the fabrication of a linear lens array to be accurately positioned over a CCD detector array. The process is described with particular emphasis on the control of the optical and thermal steps so as to maintain the accuracy of alignment required.