What is the name of the horizontal line that spans across a lens?

The correct term for the horizontal line that spans across a lens is known as the datum line. The datum line is an important reference point in lens manufacturing and fitting, as it defines the baseline for measuring the optical characteristics of the lens, including the placement of optical centers and the fitting heights for various types of lenses.

The datum line is usually positioned at the same height as the optical centers for balanced and accurate vision correction. It ensures that the lens is aligned properly in relation to the eye, which is critical for achieving the desired vision correction and comfort for the wearer.

Understanding the importance of the datum line helps opticians ensure that lenses are correctly fitted, thus enhancing the overall effectiveness of the eyewear. Other terms like the 0-180 line, Prentice line, and optical center line refer to different aspects of lens design and measurement but do not specifically denote the horizontal line across the lens.