Recent updates to standards help engineers specify material properties correctly
Optical engineers routinely ask a series of questions in order to choose appropriate materials: How many bubbles are permissible in a lens? Which striae quality is needed for a large precision prism? How do I specify the requirements in the right way to obtain just the quality that I need without paying unnecessary surcharges? How can I avoid errors leading to wrong deliveries, considerably longer delivery time, or even total loss of delivery?
The experience of most quality engineers is that more than 80% of mistakes and errors come from communication issues. How can communication be improved? This can be done using well-defined terms and descriptions of product properties. It is the task of standards to provide such means.
Two standards were published in December 2018 regarding the specification of optical elements such as lenses and prisms and the optical glass material they're made of, which should improve communication significantly: Part 18 of the already well-established standards series ISO 10110 for drawing indications of optical elements, and the revision of the raw optical glass specification standard ISO 12123. For both projects, experts from optical systems companies and optical glass manufacturers worked together on the international platform of the technical committee ISO/TC 172 of ISO, the international standardization organization, under the Secretariat of DIN, the German Institute for Standardization. The main goal was to harmonize both standards with each other.
ISO 10110-18 replaces sections 2, 3, and 4, which regulated preparation of drawings for the material properties stress birefringence; bubbles and inclusions; homogeneity; and striae. All these properties are now regulated in Part 18. While doing so, special care was taken to cover the wide variety of sizes and quality requirements of various elements. So it is now possible to specify the raw glass required for elements, the elements themselves, or groups of elements together. For small lenses it is usually sufficient to require standard optical glass quality in order to obtain application-suitable quality. The larger the elements are, and especially, the longer the light path in glass is, the more detailed the specifications should be. The standard provides the formal system for the drawing indications, contains quality grade tables, and recommendations for choosing the requirements.
The revision of the ISO 12123, first published as the specification standard for raw optical glass in 2010, now contains quality grade abbreviations related to the tolerances' limit values and introduces narrower grades for the refractive index and the Abbe number. Now it is possible to indicate the required minimum aperture for the refractive index homogeneity and specify a second and third perpendicular inspection direction for striae. The standard defines the deviation of the relative partial dispersion from the normal line due to precision dispersion data for standard crown and flint glasses. This allows a comparison of the catalog data on the deviation of the relative partial dispersion from the normal line among different glass manufacturers. ISO 12123, too, provides additional explanations and recommendations for quality level choice in its Appendix.
Both standards have been developed mainly with regard to optical glass and cover the minimum requirements. While these standards can be used for other materials, those materials can have additional properties, which might require additional specifications.
More information about the DIN Standards Committee Optics and Precision Mechanics can be found online.
For more detailed information see the articles:
Hartmann, P. "Optical glass: standards - present state and outlook," Adv. Opt. Techn. 4(5-6), 377-388 (2015)
Hartmann, P. "Optical glass: deviation of relative partial dispersion from the normal line - need for a common definition," Optical Engineering 54(10), 105-112 (2015)
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