Optical Lenses and Prisms – How to specify material properties correctly ISO 10110 part 18 and revision of ISO 12123 have been published

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 old wisdom of 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 unique statements to product properties. It is the task of standards to provide such means.

Two standards have been published in December 2018 about the specification of optical elements such as lenses and prisms and the optical glass material they consist of, which will 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 from optical glass manufacturers have worked together on the international platform of the technical committee ISO/TC 172 of ISO, the international standardization organization, under Secretariat of DIN, the German Institute for Standardization. The important goal was to harmonize both standards with each other.

 

ISO 10110-18 replaces the former parts 2, 3 and 4, which regulated drawing indications for the material properties stress birefringence, bubbles and inclusions, homogeneity and striae. All these properties are regulated in part 18 together. While doing so special care was taken to cover the broad variety of elements in size and quality requirements. So it is now possible to specify the raw glass for the 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 formulated. 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 raw optical glass specification standard in 2010 now contains quality grade abbreviations related to the limit values and introduces narrower grades for the refractive index and the Abbe number. Now there is the possibility to indicate the required minimum aperture for the refractive index homogeneity and a second and third perpendicular inspection direction for striae can be specified. The standard defines the deviation of the relative partial dispersion from the normal line due to precision dispersion data for the standard crown and flint glasses. This allows comparing the catalog data of the deviation of the relative partial dispersion from the normal line among different glass manufacturers. ISO 12123, too, provides additional explanations and recommendations for the quality level choice in its Appendix. 

Both standards have been developed mainly with regard to optical glass. They cover the minimum requirements. Nevertheless, they can be used for other materials also. However, other materials can have additional properties, which might need additional specifications.

Peter Hartmann ehemals SCHOTT AG
Clara Engesser, DIN Pforzheim
Allen Krisiloff, Triptar Lens Company

More information about the DIN Standards Committee Optics and Precision Mechanics:

https://www.din.de/en/getting-involved/standards-committees/nafuo

For more detailed information see the articles: 

Hartmann, P., “Optical glass: standards – present state and outlook,“ Adv. Opt. Techn. 2015; 4(5-6): 377–388

Hartmann, P. “Optical Glass: Deviation of relative partial dispersion from the normal line – Need for a common definition,” Optical Engineering. Vol. 54(10), p. 105–112. 2015

P. Hartmann, Wiesbaden 16.1.2019