13.3 Measuring Segment Height

Section 13.3: Measuring Segment Height

Segment Height (commonly referred to as "seg height") is the vertical distance from the lowest point of the lens bevel (in the frame's eyewire) to the top of the multifocal segment or the fitting cross of a progressive addition lens (PAL). Accurate segment height measurement ensures that the patient can access the distance, intermediate, and near zones of their lenses seamlessly.

An error in segment height leads directly to postural strain. If the segment is fit too high, the patient will experience distance blur and must tilt their head downward to see ahead. If the segment is fit too low, the patient must hyperextend their neck (tilt their head back) to access the near reading zone, leading to neck strain and asthenopia.

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The Pre-Measurement Protocol: Adjusting the Frame First

The most critical clinical rule in ophthalmic dispensing is: You must fully adjust the frame to the patient's face before taking any segment height measurements.

If an optician measures the segment height on an unadjusted frame, the measurement will be rendered completely incorrect once the frame is later adjusted. The following four adjustments must be finalized before marking the lenses:

1. Frame Alignment and Levelness: The frame must sit level on the patient's face. If one ear is higher than the other, the temples must be adjusted so the front is parallel to the pupillary line.
2. Vertex Distance: The distance from the cornea to the back of the lens should be adjusted to a standard distance of 12 mm to 14 mm. Bringing a frame closer to or further from the face changes the patient’s field of view through the near segment.
3. Pantoscopic Tilt: The frame front should be angled downward toward the cheeks at approximately 8° to 12°. This tilt ensures that the patient's visual axis is perpendicular to the lens surface when looking downward into the near segment, minimizing aberrations.
4. Temple Fit and Nosepads: The temples must be bent to fit the mastoid process behind the ears to prevent the frame from sliding forward. Nosepads must be adjusted to distribute the weight evenly on the bridge, establishing the frame’s permanent resting height.

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Measuring Standards by Lens Type

Different lens designs require different reference landmarks on the eye.

1. Flat-Top Bifocals (e.g., FT-28, FT-35)

The standard measurement landmark for a flat-top bifocal is the lower eyelid margin (also known as the lower limbus).

• Technique: The optician sits at eye level with the patient. Instruct the patient to look straight ahead at the optician’s nose. The optician uses a marking pen or PD ruler to align the top of the bifocal segment with the lower eyelid.
• Rationale: This placement allows the patient to look straight ahead for distance vision without seeing the line, but a slight downward gaze of 15° to 20° easily transitions their line of sight into the near segment.

2. Trifocal Lenses (e.g., 7x28, 8x35)

Trifocals feature an intermediate segment (often 7 or 8 mm in height) positioned directly above the near segment. The standard landmark for a trifocal is the lower edge of the pupil (lower pupillary margin).

• Technique: With the patient looking straight ahead, the optician marks the lens at the lower boundary of the pupil.
• Rationale: Placing the line at the lower pupil margin ensures that the intermediate zone is immediately available when the patient lowers their gaze slightly, while the distance portion remains completely clear for straight-ahead viewing.

3. Progressive Addition Lenses (PALs)

Progressive lenses have a continuous gradient of power with no visible lines. The standard landmark for a progressive lens is the center of the pupil (which aligns with the fitting cross of the PAL).

• Technique: The patient looks straight ahead at the optician’s opposite eye. The optician marks a dot directly over the center of the pupil on the demonstration lens of the frame.
• Rationale: The fitting cross is the starting point of the progressive power corridor. Aligning it with the center of the pupil ensures that the patient is in the pure distance zone under normal gaze and enters the corridor as soon as they look downward.

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Visual, Physical, and Postural Adjustments

While the standard rules provide a starting point, the optician must adjust these values based on the patient's physical stature, habitual posture, and occupational needs:

• Stature and Height:
  • Taller Patients: A tall patient typically looks down at a steeper angle when reading and walks with a downward head tilt. The optician should consider lowering the segment height by 1 or 2 mm to prevent the distance field from feeling restricted.
  • Shorter Patients: A short patient looks up more frequently to view their environment. The optician should consider raising the segment height by 1 mm so they do not have to tilt their head back excessively.
• Habitual Posture: Observe the patient as they stand and walk. If they have a natural slouch or a forward head posture (e.g., due to kyphosis), their "straight-ahead" gaze will be relatively higher on the lens, requiring a lower segment height.
• Occupational Demands:
  • Double-Segment Bifocals (Double-D): Designed for mechanics, pilots, or librarians who need to read materials overhead. The lens contains a near segment at the bottom and an inverted near segment at the top of the lens, requiring measurements of both the lower and upper segments.
  • Minimum Fitting Heights: For progressives, the frame must have sufficient vertical depth (the B measurement). Most progressive designs require a minimum B measurement of 28 to 30 mm, with a minimum fitting height of 14 to 18 mm from the pupil center to the lowest point of the frame, to prevent cutting off the near zone during edging.

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Exhibit: Segment Height Measurement Rules

The following table summarizes the standard measurement rules and modifications for multifocal and progressive lenses: