Previously detected deviations observed when measuring the transverse ray aberrations of polycrystalline germanium lenses have been shown to be due to localised refractive index changes at the crystal grain boundaries (striae). It is suggested that these changes are due to stress introduced into the material during manufacture rather than being due to the compositional alterations which are responsible for enhanced optical absorption in these regions. These localised variations in index have been shown to adversely affect the optical performance of lenses constructed from small grain size polycrystalline germanium.

Glasses in the system Ge-As-Se have attractive properties for use in the 8-12μm band to correct chromatic aberration in optical systems based on germanium. The selection of a glass composition is described ana included consideration of the various desirable features of a practical optical material. A manufacturing method has been developed in which the glass is melted in a sealed tube to produce 7 Kg boules of glass at 75mm diameter and current slumping techniques increase the maximum diameter of blank to 160mm. The optical properties include a transmission range of 1 to 1511m, refractive index of 2.49 and an 8-1211m reciprocal dispersive power of 113. An anti-reflection coating developed for the 6-12μm band reduces surface reflection losses to about 2(), per surface. The physical properties indicate that the material is more suited to use as internal components. The glass can be optically worked using basically conventional techniques ana an outline of the prevention of toxic hazards during working is giyen. The optical quality has been assessed and a refractive index variation of 1 x 10-4 over 90% of the area found which allows the glass to be used in high resolution systems..

A region of the Ge-As-Se-Te glass phase diagram is identified as suitable for industrial exploitation. The potential range of optical properties of these glasses is established and measured thermal and optical data on selected glasses is presented.

For routine precision measurements of refractive index, many optical glass manufacturers, such as Chance-Pilkington are presently using the Hilger-Chance Vee block refractometer which was first marketed in 1947. With a need for a higher throughput of glass samples, greater accuracy in measurements and a reduction in operator fatigue, a new automatic instrument based on the same 'vee' block principle has been commissioned by Chance-Pilkingtons. The new instrument is capable of accuracies in refractive index of ± 1 x 10-5 over a range of 1.48 to 1.95 and many of the limitations of the original Hilger-Chance design have been overcome. An interferometric technique is being included to encode the deviation angle for electronic readout. The design of this new system is described and discussed.

The quality in the production of optical systems is limited by the accuracy in the several steps of the used production technique, and by the precision of the mounting procedure. The tolerances to be achieved with the technique of the today's state of the art are discussed in comparison with those in the past and with a prospective look into the future. Important parameters are: the quality of the optical materials with regard to homogeneity and repeatibility of the refractive index, the production of spherical optical surfaces, the state of the art of the centering technique, the mechanical production technique and its accuracy, the mounting techniques for optical systems. Instruments are needed to measure the quality of optical systems. The essential parameters of lenses to be controlled are MTF, focal length, vignetting, veiling glare, distortion and modulation transfer function.

A special polishing pad is described which is very little known up to now. It was developed by CARL ZEISS some years ago it was named Polytron. General concept of Polytronpolishing is discussed and special examples of workshopapplications are given.

Technology Develops Glass and Machines often more advanced than operators experience. Management who are conscious of labour costs accept that Diamond Tooling is the logical equipment for use on modern machines. Unfortunately, worn Diamond Wheels and Discs examined by us indicate operators are often failing to get best results from machines or tools. The Diamond Wheel Manufacturers Association are actively engaged trying to promote a co-ordinated education programme which when complete will form a base for specialised technical courses to be run at selected Polytechnics and similar teaching establishments. Meanwhile this paper indicates the types of Diamond Wheels that are most commonly misused and will enable you to begin your own investigations. Improvements in the range of precision machines which are now available enable your Diamond Wheel supplier, to supply alternative bond structures (other than conventional Metal Bond matrices) which would previously have been totally uneconomic due to the vagaries of the usual hand operated equipment in use.

A limited range of materials is available for optical components of the highest quality. The quality which can be achieved depends not only on the material but also on the blank manufacturing method, the machining and finishing techniques and often, in the case of metals, on the properties of a more readily machinable or polishable overlayer. The materials and manufacturing methods employed will also determine whether the component will maintain its specification throughout its proposed lifetime during which it may be subjected to adverse mechanical and thermal stresses and corrosive conditions. For the highest quality optics, pure synthetic vitreous silica and the remelted quartzes are the preferred materials. If severe thermal changes are encountered in use, then ULE silica or the optical glass ceramics may be preferred despite their somewhat greater inhomogeneity. The best metallic optical components are made of beryllium coated with electroless nickel. This combination is inferior to glasses in its stability but has a superior stiffness to weight ratio and is superior to all but ULE silica and the glass ceramics in its response to rapid thermal fluctuations.

A new machine tool now in the final stages of development at the Pacific Northwest Laboratory uses a unique tool motion to produce diamond-turned surfaces of exceptionally high quality. The cutting tool is programmed to move in 4-nm increments along two axes: an X axis and an Omega axis. Exceptionally stiff and accurate control of the tool is possible with this "Omega-X" system. Copper surfaces of revolution have been produced with a 12.3-Å rms surface finish and a contour accuracy of 75 nm. In conjuction with a unique, thermally stabilized air bearing spindle and machine calibration equipment, the Omega-X system permits a significant advance in the fabrication of optical-quality surfaces for use with the visible spectrum.

High precision machining provides an extended fabrication capability for optical parts having geometries heretofore prohibitively expensive to achieve. The conventional optical fabrication process of "cut and try" makes complex optical parts exponentially more time consuming than simple flats and spheres and, therefore, expensive; yet a numerical control machine tool can cut complex contours easily. Combining this geometric capability of the numerical control machine tool with high precision machining technology makes possible a form accuracy and finish of optical quality. Moreover, the machining process allows added features such as mounting flanges and fittings to be designed as an integral part of the optical part usually not possible to accommodate on optical grinding and polishing machines. This means that the part can be held during the fabrication process as it will be held in the final assembly, reducing the changes of distortion that might otherwise occur during assembly.

Holographic Optical Elements (HOE) are used instead of usual glass lenses for incoherent imaging with polychromatic illumination. Such HOE can easily be manufactured either as holograms of real optical systems or as computer holograms. Single HOE suffer from strong chromatic aberrations, these chromatic aberrations, however, can greatly be corrected by combining two or more HOE. Optical transfer functions and photographs for white light and filtered light taken with imaging systems consisting of two HOE are shown. Holographic systems for incoherent imaging show very simple design and manufacture, the experimental results are of comparably good quality.

Optical components in which light is redirected by diffraction at a periodic structure may be made by recording fringe patterns generated by the interference of coherent beams of light. Such components can offer significant advantages in terms of cost, weight and ease of manufacture. This paper describes some of the components that can be made in this way and considers the mechanical and optical properties of the materials used to make them.

The design and manufacture of zoom lenses involves a range of scientific and engineering problems not normally encountered in fixed focal length lens manufacture. This is not solely related to the increased number of elements but also to the manner of use of such lenses. Two lenses are described to illustrate different approaches to the problems. In one lens having a specification of 25-250mm f/3.6 for 35mm cine the zoom action is achieved by sliding movements. In the second lens the specification is 20-100mm f/2.8 and the zoom action is obtained by rolling movements. The latter lens also achieves a shorter minimum object distance and superior optical performance. The lens elements are located in steel cells which are cemented into aluminium carriages using a flexible adhesive so as to obtain improved alignment over a wide temperature range.

A profile measuring machine has been developed for use in the production of X-ray optical mirrors, and its configuration permits the internal measurement of open-ended cylinders. It has been designed to provide for rapid measurement and evaluation of the figure of both ground and polished surfaces. Typically, the shallow curves being measured have lengths of up to 200 ram and sags of a few mm, and positions on the curve are measured with resolutions of 1 μm and 80 nm in the respective directions. The axial position is measured by a Moire fringe grating system, and the sagittal position by a He-Ne laser interferometer which monitors the separation of the stylus head from a non-load-bearing reference straight edge which is referred directly to the work being measured. A tight metrological loop confers high immunity from the effects of environmental vibration. Measurements in the two axes are simultaneously transferred into buffer stores, and then processed to provide output in both analogue and digital forms, the latter permitting further rapid data processing. The ability to measure rapidly and accurately in a workshop environment has greatly facilitated the efficient production of X-ray optical mirrors. This philosophy is equally relevant to other areas of optical and precision surface measurement.

The three-dimensional display, with peak-to-valley, rms and power error data shown in Figure 1 represents the surface of a fast convex lens element. It provides an accurate, objective, and useful evaluation of the surface contour of this element.

Whatever the intended application for an optical component an integral part of its manufacturing process involves inspection and testing of that component to ascertain its conformance with design tolerances. Very often that inspection and testing can conveniently be carried out interferometrically. Past NPL research in this area has included development of the Fizeau interferometer for the accurate measurement of flatness; common path interferometers for the examination of large optical elements including mirrors; the use of computer generated holograms to facilitate null-testing of aspheric surfaces and, more recently, oblique incidence interferometers for the examination of non-optical components. Space permits just two examples to be discussed in detail. Lenses manufactured at NPL as standards for measurement of the Optical Transfer function are assessed after assembly by interferometric measurement of their wavefront aberration. A Twyman-Green interferometer is used to generate interferograms which are measured and computer analysed to elicit the shape of the wave-front, for comparison with that predicted from the design data. A polynomial representation of that wave-front may then be used in calculation of the lens OTF, again for comparison with theoretical predictions. For these comparisons with theoretical performance to be meaningful considerable care must be taken in the design and operation of the interferometer and in the measurement of the interferograms. Certain aspects of these operations will be discussed in detail. The assessment of optical flatness has traditionally been an interferometric measurement, the high sensitivity of normal incidence interferometers, approximately 0.25µm per contouring fringe, coupled with display of the flatness error of complete surfaces in a single interferogram being important features. However such interferometers are limited to optically polished surfaces whereas flatness is also often specified for other components that are not finished to the same degree. For the inspection of these components two distinct forms of oblique incidence interferometer have been developed with desensitised fringe contour intervals of 2.5 µm. These interferometers are able to indicate departure from flatness of lightly lapped or fine-ground metal and glass surfaces covering areas of up to 75 mm square in one design and surface areas up to 750 mm long by 75 mm wide in a single interferogram in the second design. Evaluation of departure from flatness of even larger surface areas is accomplished by computer assisted assembly of overlapping interferograms.

As an alternative to using an interferometer, Modulation Transfer Function (MTF) tests may be set up for evaluating any prism or combination of prisms. Advantages can frequently be obtained both in a reduction of the time to carry out the test and in the directly quantitative nature of the result which permits pass or reject decisions to be readily made. In the case of roof prisms performance can be affected by homogeneity of the prism material, flatness of the surfaces and accuracy of the 90 degree roof angle. Testing methods for other prisms ranging from the very weak prism, which is nominally a plane parallel window, to corner-cube retro-reflectors are described. The procedures are equally applicable to single mirrors and to fabrications of mirrors which perform the same function as prisms. The apparatus used for the MTF tests was evolved at MVEE for the testing of telescopic systems and operates at a single chosen spatial frequency scanning through all azimuths. The short focal length image analyser collimator is replaced by one of longer focal length for the tests on windows, prisms and mirrors. Where comparisons with interferometric testing are made, a Twyman-Green instrument of 140 mm working aperture has been used.