Lensometer for Contact Lens Verification: RGP and Wet Cell Techniques

Purpose of Lens Power Verification

The lensometer (also called a vertometer or focimeter) is used to verify that a contact lens has the correct optical power before dispensing it to a patient. This verification step catches manufacturing errors and confirms that the patient will receive the prescribed correction. While the same basic instrument is used for both spectacle lenses and contact lenses, the technique differs for rigid and soft contact lens types.

Verifying RGP Lens Power

Measuring the power of a rigid gas permeable (RGP) lens is relatively straightforward because the lens maintains its shape when handled.

Procedure

1. Clean the lens to remove any debris or surface films
2. Place the lens on the lens stop of the lensometer with the concave (back) surface against the stop
3. The lens should rest securely with its concave surface cupping over the stop aperture
4. Focus the lensometer until the target mires are sharp
5. Read the sphere power, and if applicable, the cylinder power and axis

The concave surface contacts the lens stop because this is the surface that will face the cornea during wear. Placing the lens correctly ensures an accurate reading of the lens's back vertex power, which corresponds to the prescribed contact lens power.

Verifying Soft Lens Power: The Wet Cell

Soft contact lenses present a unique challenge: they are flexible and collapse or deform when placed directly on the lensometer stop. A deformed lens will give inaccurate power readings. To solve this, a wet cell is used.

What Is a Wet Cell?

A wet cell is a small device that holds the soft contact lens suspended in saline solution, maintaining the lens in its natural, hydrated shape. The wet cell has flat optical windows on both sides that allow the lensometer's light to pass through without altering the power reading.

Procedure

1. Fill the wet cell with non-preserved saline (preservative-free to avoid affecting the lens)
2. Place the soft contact lens inside the wet cell, ensuring it is properly oriented and free of air bubbles
3. Close the wet cell and place it on the lensometer stop
4. Focus the lensometer and read the power
5. The reading will include the lens power

Temperature Considerations

Soft lens materials are sensitive to temperature changes. A cold lens will have different hydration properties than one at room temperature, which can slightly affect the power reading. For the most accurate results, allow the lens and saline to reach room temperature before measuring.

Reading the Lensometer

The lensometer displays its reading through a target system. For contact lens measurement:

• Sphere power: Read when the target lines in one meridian are in sharp focus
• Cylinder power: The difference between the two meridian readings (relevant for toric contact lenses)
• Axis: The orientation of the cylinder power, read from the axis wheel

For spherical contact lenses, both meridians should read the same power. Any difference suggests lens warpage or manufacturing error.

When to Verify

Contact lens power should be verified in these situations:

• New lens orders: Every newly received RGP lens should be verified before dispensing
• Patient complaints: When a patient reports unexpected vision changes, verify the lens power to rule out a manufacturing or dispensing error
• Trial lenses: Verify trial lens power to ensure the correct lens is being used during fitting
• Soft lenses (selective): While routine verification of every soft lens is not standard practice due to high-volume manufacturing quality control, spot-checking and checking lenses when vision complaints arise is prudent

Auto-Lensometers

Modern practices may use automated lensometers that measure lens power electronically and display the results digitally. These instruments can measure both RGP and soft lenses (some have built-in wet cell options or specialized contact lens holders). They reduce operator variability and can measure multiple parameters simultaneously.

Key Takeaways

• The lensometer verifies contact lens power by measuring back vertex power
• RGP lenses are placed concave side against the lens stop for direct measurement
• Soft lenses require a wet cell filled with saline to maintain their shape during measurement
• Air bubbles in the wet cell must be removed to avoid distorted readings
• All RGP lenses should be verified before dispensing; soft lenses are verified selectively
• ANSI tolerance for RGP power is typically +/-0.12 D for powers up to +/-5.00 D