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Beyond centration: how to create, read, and use a datum system per ISO-10110-6 (2015)

Optical designs have traditionally been designed around an axis of symmetry to which the actual components must conform within specified tolerances. Until recently and with few exceptions, it has been sufficient for the drawings of the several elements to call out local surface tilt tolerances at the physical centers of each element (or between two plane faces.) DIN 3140 and ISO 10110-6 (1996) created a formal framework for specifying surface tilt. While surface tilt may have been the specification, it wasn’t always a convenient method of measurement in the shop: Edge thickness variation, beam deviation, or axis decenter are sometimes better fits to the production and QA capabilities or optomechanical requirements. In the U.S., subsets of ASME Y14.5M1 were combined with text drawing notes allowing both greater flexibility and frequent and occasionally expensive misunderstandings. With modern optical manufacturing and design techniques allowing widespread utilization of rotationally invariant aspheres, torics and biconics, and freeform optics – many with non-circular edges – the terms “centration” and “tilt” are no longer adequate to specify the geometric relationships between one or more surfaces and their mechanical boundaries and constraints. In response, ISO 10110-6 (2015) has been greatly expanded to meet the needs of an advancing industry – and to support measurement methods more directly associated with optomechanical requirements and shop capabilities. This article is a tutorial to aid in understanding and using this essential document.