What is the seg height typically referred to in optical dispensing?

The concept of seg height is specifically relevant in the context of bifocal and multifocal lenses. It refers to the measurement that indicates where the bifocal segment begins on the lens. This is crucial for ensuring that the segment aligns correctly with the user's line of sight, allowing for optimal viewing through both the distance and near vision zones. Correct positioning enhances the overall effectiveness of the lens by ensuring that the wearer can seamlessly transition between different visual tasks without experiencing discomfort or visual strain.

In the choices provided, the other options do not accurately define seg height. A point above the lens center does not specifically relate to where the bifocal segment starts, and the full height of the lens does not take into account the functional aspect of the segment's placement. Similarly, the distance from the lens top to the bottom encompasses a broader measurement that is not specific to the starting point of the bifocal area, which is what seg height addresses directly.