What must be measured for high index materials and polycarbonate lenses?

The measurement that is essential for high index materials and polycarbonate lenses is the MRP height, which stands for Minimum Reference Point height. This measurement refers to the vertical position on the lens where the optical center is located, and it is especially important when fitting lenses that have specific thickness and curvature characteristics.

High index materials are designed to be thinner and lighter than traditional lenses, providing improved aesthetics and comfort. However, the thinner profile can also lead to a higher sensitivity to misalignment of the optical center. Polycarbonate lenses, while known for their impact resistance, also require careful placement of the optical center to ensure optimal vision and minimize optical distortion.

By accurately measuring the MRP height, an optician can better align the lenses with the wearer's line of sight, ensuring that they receive the correct prescription at the intended point of gaze. This is of heightened importance in high index and polycarbonate lenses where deviations can significantly impact visual quality.

Other measurements like vertex distance, segment heights, and frame size are important for various reasons in eyewear fitting but are not as critical specifically for high index and polycarbonate lens considerations. MRP height directly influences the lens' performance in relation to the wearer's vision, making it the key measurement in this context.