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A Chart for Determining Minimum Blank Size of Spectacle Lens

Jie Chen, John Y. Wang, Clifford Scott, George Y. Wang, Xiaoman Li, Guang-Ji Wang

Significance: A method is needed to quickly determine the Minimum Blank Size (MBS) of spectacle lens for the chosen frame. Selecting the MBS is important in making high quality eyeglasses with appropriately thin centers and edge thicknesses, while maintaining light weight, cost effectiveness, and time efficiency. Using MBS can help make the decision whether to use an uncut lens blank from commercially available stock or to have the surfaced lens specially ordered from a lab.

Purpose: This article introduces a new chart for instantaneously, easily and accurately determining the MBS for a specific frame and will help select a suitable blank lens and therefore control the quality, cost and turnaround time of a pair of eyeglasses.

Methods: The new chart was designed with serial concentric circles matching the commonly available stock lens sizes and the surfaced lens sizes, with a series of vertical lines representing the range of inter Pupillary Distances (PD). Inside the circles, a group of horizontal lines mark vertical decentration in millimeters, while two groups of vertical lines, located above and below the horizontal lines, mark horizontal decentration. By placing the selected frame on the chart correctly, the MBS is determined immediately.

Results: This chart has been used without any special training, for determining the MBS. It can be used in any situation with or without decentration of the optical center from the patient’s visual axis. The chart has proven to be faster and more accurate than using current formula calculations.

Conclusion: Since the unfinished blank size controls the weight, thickness, and edging time of a finished pair of lenses, it is essential to determine MBS before making a pair of eyeglasses. The current method to determine the MBS, calculated by formulas using the frame measurements is tedious, time-consuming and subject to inaccuracy. This new chart is a graphically instinctive and intuitive tool that can easily be applied in clinical dispensing to meet the general and special optical requirements of the lenses in a finished pair of eyeglasses.