What does a lensometer measure?

A lensometer measures the complete optical prescription of a spectacle lens: sphere power, cylinder power, cylinder axis, add power (for bifocal and progressive lenses), and prismatic power. It is used to verify new glasses against the prescription before dispensing to ensure accuracy, and to read existing glasses when a written prescription is unavailable.

How do you determine the cylinder power from a lensometer reading?

When a cylindrical lens is placed in the lensometer, the target line sets focus at two different power wheel positions 90° apart (corresponding to the two principal meridians). The algebraically lower (more minus) reading is the sphere power. The difference between the two readings is the cylinder power. The axis position of the first lines to focus is the cylinder axis.

Why must spectacle lenses be measured from the back surface?

Lensometer accuracy depends on the lens being placed at a standard distance from the instrument's optical system. The lens stop on a lensometer is designed to contact the back (ocular) surface. Measuring from the front surface changes this distance and introduces error into the power reading. Always flip the glasses so the lens back is against the stop.

Lensometer Principles and Use: CPO Exam Guide

What Is a Lensometer?

The lensometer (also called a lensmeter or focimeter) is an instrument used to measure the optical power of spectacle lenses. It determines sphere, cylinder, axis, and prism, and is used for two primary purposes: verifying new lenses against the prescription before dispensing, and reading the prescription of a patient's existing glasses when a written record is not available.

How the Lensometer Works

The lensometer projects a target (typically a set of lines or rings) through the lens being measured. The examiner adjusts the instrument's power wheel to bring the target into sharp focus. The power reading at which the target sharpens indicates the lens power in that meridian. For lenses with cylinder, the target lines have two separate focusing positions 90° apart, corresponding to the two principal meridians of the cylindrical correction.

Most manual lensometers use a bullseye or triplet-line target. When all lines are focused simultaneously, you have a spherical lens with no cylinder. When the vertical and horizontal line sets focus at different power wheel positions, the difference in those readings is the cylinder power.

Reading a Spectacle Prescription from the Lensometer

Step-by-Step Procedure

Place the lens in the lens holder with the back surface facing the examiner (for spectacle lenses, always measure from the back surface)
Adjust the eyepiece for your own vision if using a manual lensometer
Turn the power wheel until the target comes into sharpest focus
Read the sphere power on the wheel
If lines focus at different positions, note both positions: the algebraically lower (more minus) position is the sphere; the difference is the cylinder; the axis of the lines that focus first is the cylinder axis
Mark the optical center with a lens marking pen for verification against PD
Check for prism by observing whether the target center is displaced from the bullseye center

Marking the Optical Center

After determining the power, use the lensometer's marking device to mark three dots on the lens surface: one central dot marking the optical center and two flanking dots indicating the axis. These marks allow you to verify that the optical center is correctly positioned relative to the patient's pupil position.

Always measure and mark both lenses before removing the glasses from the lensometer stage. Having both optical centers marked simultaneously lets you check for vertical imbalance (one lens sitting higher than the other) before dispensing.

Measuring Add Power

For bifocal and progressive lenses, measure the near add power:

Measure the distance portion first and record the sphere and cylinder
Move the lens so the near segment (or the near zone of a progressive) is in the lensometer aperture
Note the new sphere reading; the difference from the distance reading is the add power
Add power is always plus (it adds converging power for near focus)

Prism Measurement

Prism is present when the target center is displaced from the bullseye center of the lensometer reticle. The direction of displacement indicates the base direction of the prism. The amount of displacement (in scale divisions) indicates the prism diopters.

Measuring the wrong surface of the lens. Spectacle lenses should be measured from the back (ocular) surface. Measuring from the front introduces error because lensometer readings depend on the distance of the lens from the instrument's standard plane. Placing the back surface against the lens stop ensures accurate readings.

Digital vs. Manual Lensometers

Modern practices often use digital (automated) lensometers that display power readings directly on a screen without manual focusing. The operator simply places the lens in position and the instrument measures automatically. Results can often be printed or transferred to the patient record electronically.

Key Takeaways

Lensometer measures sphere, cylinder, axis, prism, and add power of spectacle lenses
Always measure from the back (ocular) surface of the lens
Sphere = position of first line focus; cylinder = difference between two line focus positions
Add power = near reading minus distance reading (always plus)
Mark the optical center with three dots for PD verification
Prism: target displacement from bullseye center indicates base direction and amount