The cost of tolerancing

The design and development of an optical system includes completing detailed drawings that specify allowable error limits, commonly referred to as tolerances. The process of deriving tolerances is iterative, requires attention in the nominal design process, must take into account adjustments in production (compensators), and is highly dependent on designer skill. The performance of the resulting as-built systems will clearly be dependent upon the specified tolerances. Additionally, while frequently overlooked, the cost of the lenses is also strongly dependent on the difference between the specified tolerances and the limits of the optics manufacturer, the coater, and the metrologist. In spite of this relationship, many drawing tolerances are not reviewed at all, and default values are frequently used. In this paper, methodologies for assessing design robustness and tolerancing optical systems are covered. Typical "default" tolerances are evaluated for effectiveness and cost. Finally, the paper has a case study that explicitly shows a design with different sets of tolerances and relative costs, along with associated expected performance.