Lensometer: Multifocal and Progressive Lenses for the CPOA Exam

Verifying Multifocal Lenses

Multifocal lenses -- bifocals, trifocals, and progressives -- require additional steps beyond single-vision verification. The key additional measurement is the near add power, which is the additional plus power added to the distance prescription to bring near objects into focus for presbyopic patients.

The Near Addition (Add)

The reading add (or add power) is always a positive value, typically ranging from +0.75 D to +3.50 D. It is the same for both eyes in the vast majority of patients, though occasionally the doctor may prescribe slightly different adds for each eye.

The add power is written at the bottom of the prescription: Add +2.00 or Add OD +2.25 OS +2.00.

Measuring the Add Power on a Bifocal Lens

Step 1: Measure the Distance Power

Measure the distance portion (top of the lens) as you would a single vision lens -- clear the sphere and cylinder lines and record the distance Rx.

Step 2: Measure the Near Segment Power

Shift the lens table so the lensometer reads through the near segment (the bifocal segment at the bottom of the lens). Clear the target lines at this new position and note the reading.

Step 3: Calculate the Add

The add power = near segment power - distance power.

Example: Distance power = -2.00 DS. Near segment power = 0.00 DS. Add = 0.00 - (-2.00) = +2.00 D.

💡 Clinical Tip: It is easier to remember: the near reading is always more plus (or less minus) than the distance reading. The difference between them equals the add power.

Verifying Trifocal Lenses

Trifocal lenses have three segments. Measure:

1. Distance power (top segment)
2. Intermediate power (middle segment)
3. Near power (bottom segment)

The intermediate add is typically half the near add. If the near add is +2.00, the intermediate segment should read approximately +1.00 more than the distance power.

Verifying Progressive Addition Lenses

Progressive lenses are more challenging to verify because there is no visible segment line and the power changes gradually from top to bottom.

Locating the Distance and Near Zones

Progressive lenses have hidden reference markings on the lens surface, called permanent engravings. These tiny laser-etched circles (or other marks) are placed by the manufacturer and can be seen by holding the lens at an angle to a bright light source. They mark the fitting point, the add power, and the optical corridor.

Many progressive lenses also have a temporary marker supplied by the lab -- a set of three dots (or crosses) that identify the key zones. These can be removed with a wipe after verification.

Measuring a Progressive Lens

1. Locate the distance reference point (usually marked by the lab or identified by the fitting cross above the engravings). Measure the distance power at this point.
2. Move the lens downward to the near reference zone (below and usually slightly nasal from the fitting cross). Measure the near power.
3. Calculate the add as: near power - distance power.

⚠️ Common Mistake: Measuring a progressive lens in the intermediate corridor instead of the true distance or near zones. The corridor contains intermediate power that neither matches the distance nor the full add. Ensure you are measuring in the correct zone by referencing the manufacturer's engravings or lab markings.

Progressive Lens Tolerances

Due to the nature of progressive lens design, the ANSI/ISO tolerances for progressives are slightly more generous than for single vision or bifocals. The add power should be within 0.12 D of the prescribed value; distance and near sphere/cylinder/axis should be within standard tolerances.

Checking Optical Center Position and Prism

For multifocal lenses, verify that the distance optical center (or the fitting point for progressives) aligns with the patient's distance PD. Vertical imbalance (prismatic difference between the two eyes' distance OCs) can cause significant eye strain in binocular vision.

Documenting Lensometer Verification

After verifying a pair of glasses, document in the patient record:

• Measured sphere, cylinder, axis for each eye
• Measured add power for each eye
• Whether the measurements match the prescription within tolerance
• Date and initials of the staff member who performed verification

🔑 Key Point: For multifocal lenses, the add power is the difference between the near reading and the distance reading. This simple calculation is critical for verifying that the near addition has been correctly manufactured.

Key Takeaways

• The near add power = near segment power minus distance power.
• Bifocals: measure the top (distance) and the near segment separately, then subtract.
• Trifocals: measure all three zones; intermediate add is typically half the near add.
• Progressives: use manufacturer engravings or lab markings to locate distance and near reference zones before measuring.
• Measuring in the corridor of a progressive gives intermediate power, not distance or full add.
• All lensometer readings should be documented in the patient record with staff initials.