What is the correct measurement for the lower edge of a top segment in a double segment lens?

The correct measurement for the lower edge of a top segment in a double segment lens is typically set at 13-14 mm above the lower segment line. This specific measurement is important because it ensures proper alignment of the visual fields provided by both segments of the lens. The top segment is intended for viewing objects at a distance while the lower segment caters to near vision, such as reading.

By positioning the top segment within the 13-14 mm range above the lower segment, it allows for an optimal transition between the two segments, providing comfort and ease of use for the wearer. This measurement takes into account the typical positioning of the eye and the distance from the eye to the segments when the wearer is looking straight ahead or down.

Other potential measurements provided would either be too low or too high, which could disrupt the visual efficiency of the lenses and lead to discomfort or improper viewing angles. Thus, maintaining the 13-14 mm standard is crucial for the effective performance of double segment lenses.