What is segment height for progressive lenses?
Segment height for progressive lenses refers to the vertical distance from the bottom of your lens to the centre of your pupil.
How do you measure it?
To measure your segment height, you need your glasses frames, a ruler and a mirror.
Unlike pupillary distance (PD), segment height depends on the glasses frame you're wearing because different frames can have different lens heights. For example, your segment height would be larger if you were wearing oversized glasses and smaller if you were wearing small ones.
Here's how to measure your segment height at home:
- Stand about eight inches (20 cm) away from a mirror.
- Wear your glasses.
- Hold your ruler vertically and align its 0-mm mark with the bottom of your lens.
- The mm line that aligns with the centre of your pupil is your segment height.
- Repeat the process three times to ensure a consistent measurement.
Why is segment height important?
Your segment height determines the different focal zones (e.g. distance, computer, reading) for progressive or bifocal lenses.
Segment height is not necessary for single vision prescription lenses because they have only one focal zone, hence the term single vision.