What is Segment Height and How Do You Measure It?
Segment height refers to the vertical distance from the bottom of your lens to the center of your pupil, and it’s an important measurement for progressive lenses (or bifocal lenses) to ensure proper alignment of the different focal zones (distance, intermediate, and near vision).
Why is Segment Height Important?
For progressive lenses, the segment height helps determine where each of the lens's focal zones will be positioned. This ensures a smooth transition between different prescriptions (like for distance, computer work, and reading) without abrupt changes in focus.
Unlike single vision lenses, which only have one focal zone, progressive lenses need to be customized with accurate segment height to provide clear vision at all distances.
How to Measure Segment Height at Home
You can easily measure your segment height at home with your glasses frames, a ruler, and a mirror.
Here’s how:
- Stand about 8 inches (20 cm) away from a mirror.
- Wear your glasses (this ensures the measurement is correct according to the frame you're using).
- Hold a ruler vertically, aligning its 0-mm mark with the bottom of your lens.
- Look straight ahead, and measure the distance from the bottom of the lens to the center of your pupil.
- Repeat the process three times to ensure the measurement is consistent.
How Does Segment Height Vary by Frame Type?
Segment height will vary depending on the size and shape of your frames. For example, if you wear oversized glasses, your segment height will likely be larger compared to wearing smaller frames.
Why Segment Height Matters for Progressive Lenses
Segment height ensures that:
- The distance vision portion is positioned correctly at the top of the lens.
- The intermediate vision area (for tasks like working at a computer) is in the middle.
- The reading area is aligned with the bottom of the lens.
Getting an accurate segment height is essential for progressive lenses to offer smooth transitions between these different focal zones.