An optical surface is generated with a very high accuracy with regard to its macro and micro geometry so that rays falling onto this surface are deflected with image-forming quality. Materials for optical elements must have high transmission or reflectivity in a given spectral range. These requirements are met by some crystals, optical glasses and plastics and some metals. There are different methods for generating optical surfaces: - precision moulding and pressing - turning or flycutting - grinding, lapping and polishing Moulding and pressing are applied to optical plastics and to glasses. Normally elements produced in this way fulfil only low requirements with regard to surface quality and homogeneity. However precision moulding of glass lenses with remarkable high quality could be achieved recently. On soft materials optical surfaces can be generated by precision diamond turning or flycutting. These techniques require high-sophisticated design of computer controlled machines. Optical glasses cannot be machined by single point diamond turning because of the hardness and brittleness. For special applications the problems are solved by turning the glass at an elevated temperature. Under certain conditions several types of optical glasses can be turned to a transparent finish. Also in the classical methods for generating optical surfaces, i.e. gringing and polishing, changes can be observed at present. This relates to rigidity and precision in machine construction, to materials and geometry of grinding and polishing tools and to polishing agents.
The finishing of optically effective surfaces is done almost exclusively on horizontal or swivel lever machines. These machines are characterized by a rotating work spindle and the oscillating movement of the polishing lever. This oscillating movement is particularly important for obtaining the desired geometry of the workpiece.
In the production of optical lenses, particular importance is attached to centering, the final processing stage. This operation has very high demands on quality because it has the greatest influence on the standard of a lens objective. This paper describes the traditional centering method of the cementing or exchange spindle technique compared to centering by bell clamping. The laser centering devices developed in the last few years are explained with details about their principle and function. Finally, modern centering machines are presented together with laser centering devices for checking centered lenses and for precision cementing for lens assemblies. The lecture is completed by remarks on diamond tools used for centering.
For different optical glasses the high speed grinding process has been simulated by scratching glass surfaces under dry conditions with single diamond tools in the speed range from 1μm/s to 100 m/s only once and the removal mechanisms as plastic flow and brittle fracture have been analysed by direct high speed microscopic observation and subsequent scanning electron microscopy (SEM) of the scratched grooves. The generation of median cracks along the grooves is found to be restricted to low speeds in the range of some meters per second,dependent on the type of glass.A drastic change in the plastic behavior from high-viscous flow to low viscous flow has been found in the same scratching speed range. This change in the plastic behavior is assumed to be due to an increase in the generation of heat caused by internal friction and by an adiabatic pile-up of heat in front of the fast moving diamond.
This paper gives a survey of the activities in the Federal Republic of Germany - from spectacle lens optics through photographic optics to astronomical and X-ray optics. The production techniques employed - grinding with loose abrasive, machining with bonded grain, single-point diamond turning and copying techniques - are discussed. Testing techniques which play an important part in production can only be touched on in passing.
This paper will discuss the current state of optical fabrication employing precision machining. While it is a perspective based upon both our own work and that of others, it lays no claim to being a comprehensive survey.
In the most general sense, the process of image formation involves a succession of refracting or reflecting surfaces whose purpose is to alter the curvature of impingent wavefronts originating from some object scene. The character and arrangement of these surfaces determines the ultimate location, magnification, and fidelity of the image formed by the process. In designing an optical system, surface curvatures, their spacings, and the properties of the intervening media are adjusted to achieve the desired results. In practice, as design requirements become increasingly stringent, the availability of additional degrees of parameter freedom (read "surfaces") may be required; in theory, the media may be non-homogeneous, and there exist no constraints upon the geometrical form of the refracting/reflecting surfaces. In practice, we are limited in our creativity by our ability to execute all design parameters accurately, and by our ability to tailor refractive index gradients and surface shapes.
Injection molded optics, though originally limited to nonprecision applications and to components with unconventional, intricate shapes, became also important for high-quality photographic lenses during the last 15 years. This paper presents a survey of the principal optical plastics, their physical properties and some aspects concerning optical design with plastic materials. We discuss the relationship between lens quality, injection mold, and injection process. At last some examples show the imaging properties of plastic-glass systems and technical and economic limitations of the manufacturing process.
In comparison to precision optics, the demands made on a spectacle lens with respect to the macrogeometry are probably smaller, those made on material and surface quality are, however, in part even greater. The relatively large thermal expansion coefficient of plastics is only of secondary importance for a spectacle lens so, that the advantages of organic materials for ophthalmic optics such as low weight and high impact resistance can be utilized to a very great extent. The use of the thermosetting plastic CR 39 for the production. of high quality corrective lenses is at the moment indisputable thanks to its good scratch and wear resistance. Plastic lenses are either moulded directly as finished lenses or a second surface is mechanically joined to a moulded blank. The specific material properties must also be given special consideration. 'Hy the coating of plastic surfaces and this requires costly production processes in particular for the relatively thick hard, coatings for the mechanical protection of the surfaces.
Replication of optical surfaces became a commercial process around 1950 and quickly became the key to routine production of Diffraction Gratings. The procedures also lend themselves quite well to making all sorts of special mirrors and aspheric refracting elements, but it took nearly 20 years more for this to find acceptance in the optical industry. The possibilities are of increasing interest, although natural limitations must be respected.
In the production of optical elements, surface layers can be created on optical glass in various ways, particularly if we are dealing with sensitive glasses of low acid or alcali resistivity. Basically, any exposure of a glass surface to a wet or a humid environment can cause a surface layer, although different mechanisms may be determining its thickness and composition. In the production of optical elements, their surface can be endangered before coating by polishing, cleaning, general handling and even by storage. Some of these cases will be discussed in general and illustrated by examples. It is shown that all of these surface layers - independent of how they originate - alter the optical properties of subse-quently deposited thin film interference coatings, particularly of antireflection coatings, in a much disturbing manner. Besides more or less homogeneously extended surface layers, localized topographic defects on optical surfaces such as polishing or cleaning residues even much below any scratch and dig specification can degrade the performance of optical coatings deposited thereon. Such minute defects are not visible to the naked eye and perhaps also not detectable with the standard 4 - 8 x magnifiers which are still in use in many optical shops. However, these submicron defects can initiate structural defects in optical coatings which are preferably disturbing in multilayers as the size of the defects in the coating increases with its over-all thickness.
Chemical analysis of thin surface layers and analysis of the in-depth profiles of elements with an in-depth resolution of a few nm can be applied to detect production defects of polished optical surfaces and allow to select means in the production to avoid them. Especially Secondary Ion Mass Spectrometry can be applied successfully in investigating optical surfaces because it allows the detection of all elements. It is demonstrated with SIMS results, that differences in the spectral reflectance e.g. caused by stains due to the treatment of optical surface can be correlated with differences in the in-depth profiles of elements in a thin surface layer of the glass. Complementary investigations of production defects are required to support the results using several methods especially microscopy, electron microscopy and electron diffraction. Solid state reactions at the interfaces between the glass, the glass surface layer and the coating also can contribute to the differences in the reflectance in the stained areas. The chemical aspects of the composition changes induced within glass surface layers and coatings occurring during processing of the surface and coating are discussed.
Optical coatings and surfaces are prepared by a wide variety of processing methods. Since their optical properties are dependent upon both composition and structure, it is often necessary to independently characterize the chemistry of these thin films and surface layers. The ability to measure the composition-depth profile, the impurity content and distribution, and chemical bonding states, can be of great value. These data can be used for understanding the role of the various processing parameters, for quality control and for failure analysis. Of course, many optical coatings and surfaces exhibit unique optical properties which cannot be related directly to their chemical characteristics. In these instances, it may be the microstructure which must be closely examined. Nonetheless, a chemical analysis can be of value if only to verify the absence of compositional or impurity effects.
The various parameter of the polishing process of optical glasses are discussed. The attainable surface qualities are investigated by means of a special straylight measuring method and by ellipsometric measurements. Our experiments show clearly that the values of strayligth and of thin films on glass surfaces are dependent on the glass-types, on the working conditions of the polishing process and on the cleaning method of the surfaces. These experiments demonstrate how an optimization of the polishing process may be achieved.
The dip coating process is described and the properties of the coating solutions are outlined briefly. Especially the advantages and disadvantages of the dip coating process are discussed in comparison to the vacuum processes. The many products manufactured by the dip coating process are summarized.
For vacuum coating of optical surfaces a variety of new processes like ion assisted evaporation, cathode sputtering and plasma deposition have been added in the last years to the classical evaporation technique. The industrial development was concentrated mainly on automatic process control of evaporation, optical thickness control for roll coating and improvement of aging performance of optical layer systems by ion assisted coating techniques. Plasma deposition shows growing importance for IR applications, whereby microwave plasmas indicate interesting features.
This paper provides an overview of the current requirements and state of the art for laser quality optics. Particular emphasis is placed on optics for the visible and near in-frared with some discussion of the requirements applicable to higher power commercial CO2 lasers. Requirements for He-Ne and Argon ion lasers are used to exemplify this discussion for commercial applications. Optical requirements for laser fusion systems are used to il-lustrate the state of the art for large optics.
Amorphous carbon as an optical coating material offers the combination of extreme hardness, chemical inertness and transparency over a wide spectral range. Consequently, this material has found increasing interest as a protective AR coating especially for IR-optical components. We report on the rf-plasma deposition of hydrogenated amorphous carbon (a-C:H). Coating properties including density, hydrogen content, hardness, resistance against chemical at-tack, CO2 laser damage threshold, optical absorption and refractive index have been meas-ured and the dependence of most of these quantities on deposition parameters will be discussed. Of particular interest to the design of optical coatings is the possibility to adjust the refractive index of a-C:H between 1.8 and 2.2. As a result, quarter wave AR coatings on germanium for 10.6 Ã‚Âµm (with a residual reflection below 0.2%) and hard carbon terminated multilayer AR systems have been realized. In addition, a-C:H is a promising coating material for HF, DF and CO laser applications, since absorption coefficients in the mid infrared as low as 10 cm-1 can be obtained.
Over the past decade there has been a tremendous surge of interest in the use of plastic optical elements to supplement or replace glass optics. While the technology of molding and polishing plastic optics has been the chief interest, there has been increasing need for precision coatings for these elements. In some instances these coatings are as critical as the elements themselves. In this paper we will describe the difficulties incurred in coating plastic and some of the many coatings presently available today despite the difficulties encountered. We will then cover the durability aspects of these coatings and lastly, point out some areas to consider when evaluating using plastic instead of glass.
Fine form errors, or surface microroughness, are localized height deviations of the actual surface from the ideal desired surface; in contrast, surface waviness has a longer spatial wavelength range (from a few millimeters to a centimeter) on the surface, and optical figure variations are height variations extending from approximately 1 cm to distances covering the entire surface. Surface microroughness produces scattering which degrades the performance of an optical system; thus, it is desirable to be able to measure it and then minimize it as much as possible. Another approach is to measure the scattering directly, either as a function of angle or as total integrated scattering (TIS) into a hemisphere. In this paper, methods for observing and measuring surface microroughness will be described, as well as methods for measuring TIS and angular scattering. The former include Nomarski microscopy, transmission electron microscopy, Fizeau and FECO (fringes of equal chromatic order) interferometry, optical heterodyne profilometry, and stylus profilometry. TIS can be measured using a light beam normally incident on an opaque reflecting sample and collecting all the scattering into a hemisphere, while angular scattering is most easily measured in the plane of incidence. Two other useful but more specialized techniques will also be described; total internal reflection microscopy (TIRM) is particularly useful for observing transparent filmed and unfilmed surfaces, while with ellipsometry small changes in the optical properties of a surface can be detected. By having the appropriate tools available for characterizing optical surfaces, it is possible to determine the effect of various surface preparation techniques so that the surface finishing process can be varied to produce higher quality optical surfaces.
New technologies to generate optical surfaces have been introduced recently. Interferometric testing of optical surfaces is widely used in laboratories but has only recently been applied in industry. Interferometric testing of plane, spherical and aspherical surfaces will be described together with the fringe analysis. Electronic phase measurement techniques eliminate photographing the fringe pattern for the analysis of the wavefront. Aspherical surfaces can be tested successfully by using computer generated holograms. Alternatively, aspheric surfaces in production can be tested with a master surface, using a holographic technique to be described. A slightly modified fringe analysis technique can be used for the study of the microgeometry of optical surfaces. For testing surfaces in the grinding stage two wavelength holography will be described.
In the course of the last two decades holographic interferometry has developed into a branch of metrology which has become essential for production metrology and quality control in the optical industry. Two techniques are particularly important: holographic real-time interferometry using either photographic methods or computer holograms. Examples are given.