ISO standards: a means for quality assurance for optical elements and systems

A report is given on the contents and the structure of ISO DIS 101 10 "Preparation of drawings for optical elements and systems", including a comprehensive selection of important details. This future International Standard gives rules for the indication of quality characteristics of optical elements and subsystems. In our time "standardization' means "international standardization". The International Standard Organization ISO, Geneva, covers the main activities in this domain. The ISO Technical Committee TC172 "Optics and optical instruments" was founded in 1979. Since then eight subcommittees (SCs) have set up and are working in the following fields: SC 1 Fundamental standards, (SC2 Materials for optical processing, not yet active), SC3 Optical materials and components, 5C4 Telescopes, SC5 Microscopes, SC6 Geodetic instruments, SC7 Ophtalmic, endoscopic, metrological instruments and test methods, SC8 Ophtalmic optics, SC9 Electrooptical systems. SC1 and SC3 are preparing standards which are necessary for designing, manufacturing and testing components used for optical instruments for any application (general or "horizontal' standards). Standards dealing with qualities significant only for special classes of optical systems or instruments are treated in SCs 4 to 8 ("vertical" standards), whereas SC9 deals with both classes of standards. Participating members of ISO/TC172 are standardization organizations of 12 nations: Austria, Canada, China, France, Germany, India, Japan, Netherlands, Russia, Switzerland, United Kingdom, and the USA. In 1990, 383 experts have been actively working on 140 work items within ISOIFC172. One of the first decisions of TC172/SC1 'Fundamental standards was to work on an international standard for drawings of optical elements and systems. It was soon recognized that existing general standards for mechanical components are not always adequate to express tolerance information necessary for optical elements; materials, production technologies, and test methods used in optical and mechanical workshops differ considerably. Consequently a working group ISOTFC172/SC1/WG2 was given the task of drafting a new standard on drawing indications of dimensions and tolerances of opti0-8194-0960-X/93/$4.00 SPIE Vol. 1781 Specification and Measurement of Optical Systems (1992) /55 cal elements. At present WG2 "Indications in optical drawings" has about 20 members from several well known optical companies or university institutes: Cilas (France) Corning (France) Institute d'Optique (France) Leica (Germany and Switzerland) Litton (USA) Martin Marietta (USA) Olympus (Japan) ORA (USA) Rodenstock (Germany) SIRA (UK) Sopelem (France) Science University of Tokyo (Japan) University of Arizona (USA) Vavilow Optical Institute (Russia) Zeiss (Germany). The formal task of WG2 was to decide iic to indicate specific qualities in drawings, but of course the question "which qualities?" had to be answered first of all. A careful selection of the qualities to be incorporated into the new standard was essential to ensuring the quality of the standard. On the other hand the contents of the standard became rather independent of the medium used for the mdication: the rules for indicating tolerances on paper drawings can be used without modification for CAD presentations. There is an interaction between measuring technique and standards. On the one hand technical progress achieved during the work of WG2 caused the necessity of a revision of part 5 of the standard: more than five years ago the chapter "Surface form tolerances" was finished, but without taking into account the feasibility of digital interference fringe pattern evaluation, which had been established meanwhile as a commonly applied technique. To utilize the facilities of digital interferometry part 5 had to be rewritten completely. On the other hand the introduction of a new visibility method in part 7 (surface imperfection tolerances) and the rms roughness as a standard quality in part 8 (surface texture) could stimulate the development of appropriate forms of measuring instrumentation. During the group meetings the guidelines for future work have been elaborated, but often plenty of time was used for deliberations on detail problems. Extensive homework of many group members and discussions of the results led finally to a convergency of the opinions. A two-person editing committee had to unify the entire material with respect to nomenclature and style. Thus the last version of ISO DIS 101 10 can be regarded as the result of careful considerations having taken into account the opinions of experts from the most important countries with activities in optics. The Draft International Standard ISO DIS 10110 "Preparation of drawings for optical elements and systems" comprises 131 pages text and is subdived into 13 parts: Part 1 • General Part 2 Material imperfections Stress birefringence Part 3 Material imperfections Bubbles and inclusions Part 4 Material imperfections Inhomogeneity and striae Part 5 Surface form tolerances Part 6 Centring tolerances Part 7 Surface imperfection tolerances Part 8 Surface texture Part 9 Surface treatment and coating Part 10 Table representing data of a lens element Part 11 Non-toleranced data Part 12 Aspheric surfaces Part 13 : Laser irradiation damage threshold 56 / SPIE Vol. 1781 Specification and Measurement of Optical Systems (1992) Parts 2 9, 12, and 13 are independent of each other, but there is a certain interdependency of this group with parts 1, 10, and 11. The parts concerning tolerance indications have in most cases the following structure: 1 Scope 2 Normative references 3 Definitions 4 Specifications 5 Indication in drawings 6 Examples In some parts the contents are explained more detailed in one or more annexes which can have normative or only informative character. Code numbers which can be understood as a generalization of the German DIN 3 140 Standard are used for identifying certain tolerance indications: 0/ stress birefringence 1/ bubbles and inclusions 2/ inhomogeneity and stria 3/ surface form tolerances 4/ centring tolerances 5/ surface imperfection tolerances 6/ laser irradiation damage threshold Note : The definitions of the indications following these code numbers are not necessarily identical with those of DIN 3140. It is supposed that technical documents after ISO DIS 101 10 carry a reference to this standard. In the following, the contents of the 13 parts are sketched, together with a selection of especially important details. SPIE Vol. 1781 Specification and Measurement of Optical Systems (1992)157 Part 1 : General (23 pages) 1 Scope: This International Standard applies to the presentation of design and functional requirements for optical elements and systems in technical drawings used for manufacturing and inspection. 2 Normative references: inter alia ISO 128, ISO 406, ISO 1101, ISO 8015. 3 Fundamental stipulations: All indications shall apply to the final form. 4 Presentation and dimensioning : Hatching axes leaders test regions Dimensioning of radii of curvature thicknesses diameters edges bevels chamfers lengths angles Material specifications Optical sub-assemblies 5 Layout drawings (see fig. 1) : Axial separations Images pupils field stops and other apertures. Annex: Examples (figs. 2. 3, and 4) Part 2 : Material imperfections Stress birefringence (4 pages) In parts 2, 3, and 4 tolerances for material imperfections, which refer to the indivudual optical element are specified, in contrast to global material specifications to be given elsewhere. 4 Specification of permissible stress birefringence The optical path difference between orthogonal polarizations of transmitted light over the thickness of the sample is a measure of birefringence. This is given by the equation OPD = a • s • c where OPD is the optical path difference in nm, a is the sample thickness in cm, S iS the residual stress in units of N mm2 C 15 the difference in the photoelastic constants in units of 10 mm2 N'. The residual stress induced birefringence is specified in terms of OPD of retardation, in nm, per cm optical path length. 5 Indication The indication read as follows : 0/A The value A is the maximum permissible optical path difference (OPD) in nm per cm path length. 58 / SPIE Vol. 1781 Specification and Measurement of Optical Systems (1992) Part 3 : Material imperfections Bubbles and inclusions (4 pages) 4 Specification The specification for bubbles and inclusions, which are permissible in the element, is given by a term N x A (affected area principle). This term contains the allowed number N of bubbles and inclusions of maximal permitted size, and the grade number A which is the measure of their size. A is equal to the square root of the projected area of the largest permissible bubble and/or inclusion, expressed in mm. Preferred values for A are given in the first column of Table 1 (see fig. 5). 4.2 Sub-division A larger number of bubbles and other inclusions with a smaller grade number is permitted if the sum of the projected areas of all bubbles and inclusions does not exceed N . A2 = Maximum total area. When determining the number of permissible bubbles and other inclusions, those with a grade number of 0, 1 6 A or smaller shall not be counted. 5 Indication The indication reads as follows : 1/NxA Table 1 shows the preferred size designation and factors sub-division (Fig. 5). Part 4 : Material imperfections Inhomogeneity and striae (4 pages) 3 Definitions The value of the inhomogeneity of an optical element is equal to the difference between maximum and minimum values of the refractive index within the element. Measurement of the inhomogeneity within an optical element is often difficult in a non-destructive manner. Therefore the specification of an inhomogeneity class for an optical element is primarely useful for the selection of the raw material. Striae are inhomogeneities having small spatial extent. A stria usually has th