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

The correct answer is the datum line. The datum line is a horizontal line that runs across a lens, serving as a reference point for lens measurements and positioning. This line is essential for determining various parameters, such as the placement of the optical center, which is crucial for ensuring proper visual performance and alignment of the lenses within a frame.

Understanding the function of the datum line is important in optical practices as it helps in the accurate fitting of lenses to glasses, ensuring that the optical centers of the lenses align with the wearer's pupils. This alignment is vital for reducing lens distortions and optimizing visual acuity.

The other terms listed refer to different concepts within optics. The 0-180 line typically refers to the meridian orientation on a lens but does not serve as a general reference line across the lens itself. The Prentice line is associated with measuring prismatic effect in lenses, typically utilized for identifying the amount of prism when the lens is tilted or when the wearer's gaze is directed at different angles. The optical center line, while it might seem relevant, pertains to the specific point on a lens where light passes through without causing any prismatic effect, but it is not a horizontal line that spans the whole lens like the datum line does.