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newSchoolWorks.com › VoTechWorks › Health & Personal Services › Optician / Optical Technician › Ophthalmic Dispensing › Lesson 7
HLT-OPTI68-06 · Lesson 7 · 14 clock hours · theory

Progressive Addition Lens Dispensing

Objective. The student will explain the optical zones of a progressive addition lens, identify the fitting cross, and correctly calculate minimum fitting height requirements for three prescribed PAL designs given frame dimensions.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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What Is a Progressive Addition Lens?

A progressive addition lens (PAL) provides a continuous gradient of power from the distance prescription at the top of the lens through intermediate power in the middle to the full near (add) power at the bottom—all without a visible dividing line. Because there is no segment line, PALs are cosmetically preferred by many patients and are among the most commonly dispensed multifocal designs in modern optical practice.

Understanding PAL optics and fitting geometry is essential for every optician. An incorrectly fitted PAL causes blurred vision, eye strain, and postural problems, and almost always results in a costly remake. Patient safety and satisfaction depend on getting the fitting height right the first time.

The Four Optical Zones of a PAL

Every PAL has four distinct zones you must be able to identify:

1. Distance Zone — The upper portion of the lens contains the full distance prescription. The clear field of view here is relatively wide. Patients use this zone for driving, watching television, and other far tasks.

2. Intermediate Zone (The Corridor) — A narrow channel of gradually increasing plus power connects the distance and near zones. This is also called the progressive corridor. Its width and length vary by design. Patients use it for computer work and mid-range tasks. Because the corridor is narrow, patients must learn to point their nose—rather than just their eyes—toward what they want to see through this zone.

3. Near Zone — The lower portion contains the full add power. The clear field is moderate in width. Patients use it for reading and close work. They typically lower their chin slightly to access this zone.

4. Peripheral Distortion Areas — On either side of the corridor and near zone are unavoidable areas of astigmatic blur and swim distortion. These are an optical consequence of the power gradient and cannot be eliminated, only managed by design. Wider-corridor "soft" designs reduce distortion at the cost of a narrower intermediate channel; "hard" designs provide a wider corridor but push distortion further outward. Patients must be counseled about these areas before delivery to set realistic expectations and reduce remakes.

The Fitting Cross

The fitting cross (also called the fitting point or major reference point on some designs) is the specific mark on a PAL blank that must align with the patient's pupil center in primary gaze. It is pre-marked on uncut lenses with a temporary ink dot and is also indicated by small permanent engravings called alignment reference marks or hidden circles located 17 mm apart horizontally on most designs.

When the fitting cross sits exactly at the pupil center, the patient looks through the distance zone in straight-ahead gaze and the full add power is accessible in the lower reading area. If the fitting cross is placed too low, the patient looks through intermediate power in primary gaze and may never reach full distance clarity. If it is too high, the near zone may drop below the lower frame edge and the patient cannot read. Either error is a clinical and safety concern.

Minimum Fitting Height

Fitting height (also called seg height or mounting height in PAL work) is the vertical distance in millimeters from the lowest edge of the finished lens (or the bottom of the frame's eye wire) to the fitting cross. Each PAL design has a published minimum fitting height—the smallest mounting height at which the full progressive corridor and near zone are preserved within the lens boundary.

Minimum fitting heights typically range from about 14 mm to 22 mm depending on the design. Short-corridor designs made for smaller frames may have minimums as low as 14–16 mm. Standard designs commonly require 17–19 mm or more. Always consult the manufacturer's fitting guide for the specific product; never assume a value.

How to verify adequacy:

1. Measure the patient's pupillary height (fitting cross position) from the bottom of the frame.

2. Look up the PAL design's minimum fitting height from the manufacturer's guide.

3. If the measured fitting height ≥ minimum fitting height → the frame is adequate.

4. If the measured fitting height < minimum fitting height → the frame must be changed or a shorter-corridor design selected.

Measuring Fitting Height in Practice

With the patient wearing the chosen frame in the adjusted, as-worn position:

Safety and Scope-of-Practice Reminders

Summary

A PAL has four zones: distance, intermediate corridor, near, and peripheral distortion. The fitting cross must align with the pupil center. Every frame must be checked against the design's minimum fitting height before the order is placed. Accurate measurement and proper patient counseling are the twin foundations of successful PAL dispensing.

Key terms8

Progressive Addition Lens (PAL)A multifocal lens that provides a continuous gradient of power from distance at the top to near at the bottom with no visible dividing line.
Fitting CrossThe reference point on a PAL that must be aligned with the patient's pupil center in primary gaze; it marks the transition between the distance zone and the beginning of the progressive corridor.
Progressive CorridorThe narrow channel of gradually increasing plus power that connects the distance zone to the near zone in a PAL.
Minimum Fitting HeightThe smallest vertical distance from the bottom of the frame's eye wire to the fitting cross at which a specific PAL design retains its full optical performance; published by the manufacturer.
Fitting Height (Seg Height)The measured vertical distance in millimeters from the lowest edge of the finished lens to the patient's pupil center (fitting cross) as the patient wears the adjusted frame.
Peripheral Distortion ZoneThe areas of astigmatic blur and swim on either side of the corridor and near zone in a PAL, caused by the unavoidable optical power gradient.
Alignment Reference MarksPermanent micro-engravings (hidden circles) on a PAL lens, typically 17 mm apart, used to identify the fitting cross and verify correct lens orientation after surfacing.
Hard Design vs. Soft Design PALTerms describing how a manufacturer distributes the unavoidable distortion in a PAL: hard designs concentrate distortion to preserve a wider usable corridor; soft designs spread distortion more broadly for gentler peripheral blur.

Worked examples4

1. A patient's pupil center measures 19 mm from the bottom of the frame eye wire. The selected PAL design has a minimum fitting height of 17 mm. Is the fitting height adequate?
  1. Step 1 — Identify the measured fitting height: 19 mm.
  2. Step 2 — Identify the design's minimum fitting height from the manufacturer's guide: 17 mm.
  3. Step 3 — Compare: measured fitting height (19 mm) ≥ minimum fitting height (17 mm).
  4. Step 4 — Conclusion: The frame is adequate for this PAL design.
Answer: Yes, 19 mm ≥ 17 mm. The frame provides sufficient fitting height; proceed with the order.
2. A patient's pupil center measures 15 mm from the bottom of the frame. The optician wants to use a standard PAL design requiring a minimum fitting height of 18 mm. What should the optician do?
  1. Step 1 — Identify the measured fitting height: 15 mm.
  2. Step 2 — Identify the minimum fitting height for the selected design: 18 mm.
  3. Step 3 — Compare: 15 mm < 18 mm. The frame does not provide adequate fitting height for this design.
  4. Step 4 — The optician must either select a different, larger frame that yields a fitting height ≥ 18 mm, or choose a short-corridor PAL design with a lower minimum fitting height that can be satisfied by 15 mm.
  5. Step 5 — Document the finding and discuss alternatives with the patient before placing any order.
Answer: The frame is inadequate (15 mm < 18 mm required). The optician must recommend a frame with a larger B-dimension or switch to a short-corridor PAL design with a minimum fitting height of 15 mm or less.
3. Right eye fitting height = 18 mm; left eye fitting height = 16 mm. The chosen PAL requires a minimum fitting height of 17 mm. Evaluate both eyes.
  1. Step 1 — Evaluate the right eye: measured 18 mm vs. minimum 17 mm. 18 ≥ 17. Right eye is adequate.
  2. Step 2 — Evaluate the left eye: measured 16 mm vs. minimum 17 mm. 16 < 17. Left eye is NOT adequate.
  3. Step 3 — Even one eye failing the minimum height check means the frame-and-design combination cannot be used as specified.
  4. Step 4 — Options: adjust the frame fit to raise the left pupil center to ≥ 17 mm (if anatomically possible), select a different frame, or choose a PAL design with a minimum fitting height of 16 mm or less.
  5. Step 5 — Re-measure after any frame adjustment and document both values before ordering.
Answer: Right eye passes (18 mm ≥ 17 mm); left eye fails (16 mm < 17 mm). The frame or design must be changed before ordering.
4. A short-corridor PAL design lists a minimum fitting height of 14 mm. The patient's measured fitting heights are OD 15 mm and OS 14 mm. Is this design usable?
  1. Step 1 — Evaluate OD: 15 mm ≥ 14 mm. Adequate.
  2. Step 2 — Evaluate OS: 14 mm ≥ 14 mm. Exactly meets the minimum. Adequate.
  3. Step 3 — Both eyes meet or exceed the minimum fitting height for this design.
  4. Step 4 — The short-corridor PAL design may be ordered. Note: fitting at exactly the minimum leaves no safety margin; confirm the frame is adjusted to the patient's face before finalizing.
Answer: Yes, both eyes are adequate (OD 15 mm ≥ 14 mm; OS 14 mm = 14 mm). The short-corridor design can be used.
In class: Lecture with lens diagrams: anatomy of a PAL corridor—distance zone, intermediate zone, near zone, and peripheral distortion areas—using manufacturer fitting guides from at least two PAL families as handouts. · Hands-on exploration: students mark the fitting cross and near zone on demo PAL lenses using a dot marker; they then look through each zone while holding the lens at the correct distance to identify the clear field of view and distortion zones. · Calculation workshop: given five sets of frame measurements and patient seg heights, students calculate whether the frame provides adequate fitting height for each PAL design using manufacturer minimum height tables, and recommend a different frame when the height is insufficient.
Materials: PAL demo lenses (minimum four designs), dot marking pen, ruler, manufacturer fitting guides (min. two brands), minimum fitting height worksheets, ABO Study Guide PAL section
Check: Five-problem written calculation quiz; students must correctly determine fitting height adequacy for all five scenarios (100% required because an incorrect PAL placement causes patient harm and remakes).

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Ophthalmic Optics and Dispensing
1. Where on the patient's face should the fitting cross of a PAL be positioned when the patient is in primary gaze?
Ophthalmic Optics and Dispensing
2. Which zone of a progressive addition lens produces unavoidable astigmatic blur and swim?
Ophthalmic Dispensing and Patient Management
3. A patient's fitting height measures 16 mm. The PAL design selected has a published minimum fitting height of 18 mm. What is the correct action?
Ophthalmic Optics and Dispensing
4. Alignment reference marks (hidden circles) on a PAL lens are separated by approximately how many millimeters?
Ophthalmic Optics and Dispensing
5. A 'soft design' PAL differs from a 'hard design' PAL primarily in that a soft design:
Patient Safety and Communication
6. Why is pre-dispense patient counseling about peripheral distortion zones considered a patient safety issue?
Ophthalmic Dispensing and Patient Management
7. An optician measures fitting heights of OD 20 mm and OS 17 mm for a patient. The PAL design requires a minimum fitting height of 18 mm. Which statement is correct?
Ophthalmic Dispensing and Patient Management
8. Which reference document should the optician consult to confirm the minimum fitting height for a specific PAL design?
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