Large diameter crystals grown by a variety of techniques have been manufactured for many years. The perameters for optimising growth conditions are well established. This paper will attempt to identify these conditions and illustrate the problems likely to be encountered. Having selected a method of growth suitable for generating crystals for optical applications, furnace design should be considered. The size and type of the optical component will influence furnace design in several areas, e.g. heating, control and atmosphere. The areas in need of careful control will be discussed, namely power input, temperature control, cooling water, pressure. Crucible material is an important feature, influencing the purity of the melt. With many materials to select from the choice is often limited by the application the crystal is put to, the compatibility of raw material and crucible material and, finally, cost. The purity of raw material needs to be considered carefully, identifying and limiting those impurities likely to have an adverse effect on the finished component. The temperature gradient in the furnace and crucible are critical to the formation of crystals that exhibit good lattice structure, establishing the usefulness of the end product. Internal stresses set up in the growing crystal must be kept to a minimum. This requires a knowledge of heat flow patterns inside the furnace and crucible. Equally important in producing stress free uniform structured crystals are the growth rates used, care must be taken in cooling the grown crystal down to ambient temperature and annealing and cooling cycles need to be produced for each type of material grown. Processing of crystaline materials pose their own problems. In general most are both mechanically and thermally weak, requiring specially developed techniques for cutting and shaping. Grinding and polishing crystaline materials follows generally recognised optical practice but special attention must be given to overcome the frailty and delequesence of some crystals.
Among the I.R.-transmittive materials, germanium takes an important place owing to its excellent optical and mechanical properties which compensate for the fact that it is rather expensive. Germanium can be easily ground and polished into appropriate shapes. Its high refractive index, varying only slowly with wavelength, makes it extremely valuable for lenses if treated with an antireflective coating. Its absorption coefficient between 2 and 12 micrometer wavelength is very low. It is not hygroscopic and shows in general a high chemical resistance. For all these reasons the use of Ge as an I.R. optical material has grown considerably. Most of the germanium used in infra-red optics up to now is polycristalline. A number of measurements, however, proves that monocrystalline Ge is optically more uniform and also has higher mechanical resistance. The difference in homogeneity can be explained by the production techniques described in this paper. The characteristics of mono- and polycrystalline Ge are also compared in detail.
Loose emery abrasive smoothing will probably be needed indefinitely for high quality small quantity glass surfacing as there is no economic alternative. The use of diamonds impregnated in a copper or bronze bond, for milling and lapping, is a well established process and can be justified where quantities are large enough. The technique tends to waste diamonds because they are pulled out of the bond before being worn out. Electro-plated nickel-on-diamond grinding tools have been used for milling plastics lenses for many years but recent research has shown that they can be efficient and economical for surfacing optical glass. A high concentration and good cooling with a soluble oil is achieved with economy in use of diamonds. The close-bonded crystals continue cutting until they are worn down to the nickel surface. Replating with new diamonds is practicable as an 'in house' process by the user of electro-plated tools. Because the nickel bond is not worn away the shape of the tool is retained throughout its life and so electro-plated tools have a special use where tool form must be retained. This is a desirable feature for consistent production.
Proc. SPIE 0109, Discussion Of A Model Of Lens Surface Geometry Through High-Speed Generating, Smoothing And Polishing And Of Aspects In Its Practical Achievement, 0000 (14 October 1977); doi: 10.1117/12.955501
Generally speaking, there are three basic steps in processing a lens surface - rough to shape, refine the surface, polish - whether loose - or bonded-abrasive methods are employed. It is, though, an occasion for disappointment when bonded-abrasive - "high-speed" - methods are substituted into a loose-abrasive system and without a fundamental change in operational philosophy. Loose-abrasive methodology tends to zero-in on the figure and surface at the end of the process. Bonded-abrasive systems begin with figure control and operate further only on surface. Unlike loose abrasive smoothing bonded-diamond laps have extremely limited capability for figure correction. Conversely they produce a continuum of work virtually identical - piece-to-piece. And in such circumstance plastic flow in the polishing lap is unnecessary. In these circumstances two significant changes in optical shop operations result. The character of the curve generating usually must be considerably enhanced, and batch processing is replaced by a continuous, sequential flow of material. Establishing a practical example and setting the dimensional and operating parameters, we may consider the unique problems and the methods of achieving a successful system of "hi-speed" production. In particular the problems in curve generating and the character of that equipment and the contribution of a continuous work flow through operational series are detailed.
There is no doubt that the industry has benefited greatly from the introduction of diamonds for grinding and smoothing processes. In many cases, as a result of this, the use of abrasives has almost disappeared. The use of diamond impregnated crowns and impregnated pellets as a means of both fast stock removal and final smoothing before polishing, are common throughout the world. This paper sets out to describe new developments that depart from the normally accepted practice - the use of a spherical generator as a smoothing machine. A number of terms have been used to describe this method of operation: Fine milling, ring tool smoothing, one shot milling, ring tool diamond fining, and one shot generation, have all been used in various parts of the world to distinguish this type of working. This operation has been referred to as "ring tool smoothing" throughout this paper.
DAMA Optical Machines of Darmstadt, Federal Republic of Germany, has had a 30-year tradition in the construction of optical machines. An experienced team of employees is always at the service of customers all over the world. All new technologies are thoroughly tested in our own test shops. As soon as a perfected type of construction is achieved, a new optical machine is produced in series after having been modified and tested under hard and continuously severe conditions. This turns out to be a great advantage for all customers who have right to demand reliable machines and who do not want to be used as guinea-pig. During the last years many a revolutionary idea has been postponed in order to be put on the market as a perfect machine at a later date. You will remember that more than 10 years ago when diamond lapping was being introduced by the leading manufacturers there were no suitable diamond-pellets available. It is only within the last five years that this modern method of glass machining has become prevalent. At the present time there is no cheaper method and I dare say there will be none in the future.
With simple changes, some standard polishing machines can be used to make possible a quantitative analysis of polishing efficiencies. By weighing stock removal of glass under controlled polishing conditions, it is possible to determine the instantaneous efficiency, and its variations as a function of time which is dependent on the nature of the polisher, the concentration and temperature of the slurry, the pressure and speed of the spindle. This test has been very useful for the development of a range of new polishing products differentiated according to the principal type of polishers. It is also employed to control the quality of the manufacture and to guide users in their choice of the best product for a specific application. Some practical examples are described.
The article describes modern production methods in manufacturing optical components. For the several operations there are specific requirements to be considered in machines and tooling. The author lays stress on centering and edging of optical lenses. However, when dealing with advances and efficiency in production technologies also the other operations for manufacturing optical components should be reviewed and therefore, these processes are described too. During the last few years special progress was made by the employment of diamond pellets in fining. Now one can assume that it became more or less a standard technology throughout the optical industry. It is similar with the development of suitable plastic materials for high speed polishing which have contributed to improve the processing of optical elements considerably. The latest advances offer further possibilities for substantial cost reduction in processing optical components. Generating and fining are done in a single step, called fine generating - ready for polishing. For centering the latest development of laser centering systems has made a great advance to improve efficiency of this process. The laser centering systems make it possible for the first time that even lenses with small centering angles can be worked cementless. It saves the costly operation of sticking on lenses for manual alignment to their optical axis.
The general requirements for optical cements are outlined. A classification of cements is given together with typical values of properties of interest. In a guide to selection, emphasis is placed on strain induction. This is time dependent and affects long term properties. After a discussion of the important practical points to consider in the cementing operation, a brief guide to failure analysis is given.
Inhomogeneity in an optical material can generally be observed at any wavelength for which the material will transmit radiation. For infra-red materials with no visual transmission, useful tests can be performed using a vidicon sensitive to radiation of up to 2 microns wavelength. The CO2 laser though difficult to use for interferometry is ideal for simple transmission measurements. The MFT which is generally accepted as a standard of imaging performance can be determined from spread functions which can be measured with simple although necessarily well engineered equipment. Standards for homogeneity of materials however need to be established to allow the system manufacturer to select suitable material. It is desirable to refine the methods described to provide adequate tests for systems of diffraction limited performance.
The problems associated with the procurement of aspheric surfaces of high quality in reasonable quantities and at low cost can be overcome by the use of replication techniques. This paper describes the process developed at MEL-Watson to produce transmitting replicated components and gives examples of the cost and performance potentialities.
Production facilities for the high-precision shaping of aspherics by mechanical processing are described. The production and test equipment are closely linked with each other. This creates optimum conditions for an economical series production.