Using the Lensometer for Multifocal and Progressive Lenses

Measuring Multifocal Lenses

Verifying multifocal lenses on a lensometer requires more steps than single vision lenses because you must measure power at multiple locations. The key additional measurement is the add power, which is the extra plus power provided for near vision. The technique differs slightly between lined multifocals (bifocals and trifocals) and progressive addition lenses.

Bifocal Lenses

Measuring the Distance Portion

1. Place the lens on the lensometer stage with the back surface toward you
2. Position the lens so you are measuring through the upper (distance) portion, above the segment line
3. Neutralize the sphere and cylinder as you would for any single vision lens
4. Record the distance prescription (sphere, cylinder, axis)

Measuring the Near Segment

1. Slide the lens upward on the stage so the near segment is now centered over the lensometer aperture
2. Neutralize the mires again. You will find more plus power in the near segment.
3. Record the near sphere reading

Calculating Add Power

The add power is the algebraic difference between the near sphere reading and the distance sphere reading:

Add = Near sphere - Distance sphere

For example, if the distance sphere reads -2.00 and the near sphere reads -0.50, the add is -0.50 - (-2.00) = +1.50 D. The add power is always a positive value.

Trifocal Lenses

Trifocal verification follows the same process as bifocals, with one additional step:

1. Measure the distance portion (upper zone)
2. Measure the intermediate segment (middle zone)
3. Measure the near segment (lower zone)

The intermediate add is typically half the near add. If the near add is +2.00 D, the intermediate add should be approximately +1.00 D. This is a useful verification check.

Progressive Addition Lenses (PALs)

Progressives are more challenging because there are no visible lines to guide you. Instead, you must locate small laser engravings on the lens surface that mark specific reference points.

Finding Reference Points

Every progressive lens has manufacturer-specific engravings, usually visible as tiny markings when you hold the lens at an angle and reflect light off its surface. Common markings include:

• Distance reference point (DRP): Located in the upper portion, often marked by a small circle or cross. This is where you verify the distance prescription.
• Near reference point (NRP): Located approximately 14-18 mm below and slightly nasal to the distance point. This is where you verify the near power.
• Prism reference point (PRP): The geometric center of the lens, marked by a cross or circle. This is where you verify prescribed prism.
• Add power markings: Small numbers near the nasal engravings indicate the add power (e.g., "20" for a +2.00 add)

Verification Steps

1. Locate the engravings by holding the lens at an angle under a light source or using the lensometer's built-in illumination
2. Dot the reference points with a marking pen to make them visible during measurement
3. Measure the distance Rx at the distance reference point
4. Measure the near power at the near reference point
5. Calculate the add as the difference between near and distance sphere readings
6. Check prism at the prism reference point if prescribed

Verification Checklist

Key Takeaways

• Add power equals the near sphere reading minus the distance sphere reading and is always a positive value
• For bifocals and trifocals, the visible segment line guides where to measure distance and near portions
• The intermediate add in a trifocal is typically half the near add
• Progressive lenses require locating laser-engraved reference points before measuring
• Cylinder power and axis should remain consistent between distance and near measurements