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What Is a Keratometry Drum/Target (Mires)?

A keratometry drum and target refer to parts of a manual keratometer used to measure corneal curvature. The target, also called the mires, is a pattern of illuminated shapes of known size that is reflected by the cornea. The drum is the control that adjusts the instrument's doubling prism system until the reflected images align. The final drum setting is used to calculate corneal curvature in diopters and radius.

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What Is a Keratometry Drum/Target (Mires)?

A keratometry drum and target refer to parts of a manual keratometer used to measure corneal curvature. The target, also called the mires, is a pattern of illuminated shapes of known size that is reflected by the cornea. The drum is the control that adjusts the instrument's doubling prism system until the reflected images align. The final drum setting is used to calculate corneal curvature in diopters and radius.

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What Are Mires in Keratometry?

Mires are the illuminated target shapes projected by a keratometer, often appearing as circles, plus signs, or segmented patterns. The cornea acts like a convex mirror, so the size of the reflected mire image changes with corneal curvature. By analyzing the reflected image size, the instrument estimates the radius of curvature of the central cornea. This is most accurate when the cornea is smooth and has regular astigmatism.

How Does the Drum Measurement Work?

A manual keratometer creates doubled images of the mires using prisms so small differences can be measured precisely. The examiner rotates the drum to move the doubled images until specific endpoints touch or overlap in the viewing telescope. Once aligned, the instrument scale or drum reading corresponds to curvature in diopters and the equivalent radius in millimeters. Readings are typically taken in two principal meridians to measure astigmatism.

What Are Keratometry Readings Used For?

Keratometry is used to estimate corneal power and astigmatism in the central cornea. These values support contact lens fitting, especially for rigid and toric lenses, and help evaluate regular astigmatism. Keratometry readings also play a role in cataract surgery planning and in monitoring corneal changes over time. When the cornea is highly irregular, topography or tomography is often needed for a fuller picture.

Common Sources of Error

Errors can occur if the tear film is unstable, the lids block part of the mires, or the patient cannot fixate steadily. Recent contact lens wear can temporarily reshape the cornea and change readings, especially with rigid lenses. Irregular corneas may produce distorted mires that are difficult to align accurately, reducing reliability. Repeating measurements and treating dryness can improve consistency.

FAQs on Keratometry Mires and the Drum

Do you need to remove contact lenses before keratometry?

Often, yes. Contact lenses can temporarily alter corneal shape, so clinics commonly recommend a lens-free period before measurement, especially for rigid lenses. Your eye doctor will tell you the appropriate break time.

What do two keratometry readings represent?

They usually represent the steep and flat principal meridians of the cornea. The difference between them reflects the amount of corneal astigmatism.

Can dry eye affect the mire pattern?

Yes. A patchy tear film can distort the reflected mires and make alignment harder, which can lead to inconsistent readings. Lubrication and normal blinking before measurement can help.

How is a keratometer different from corneal topography?

A keratometer measures curvature in a small central zone and assumes the cornea is fairly regular. Corneal topography maps curvature over a larger area and is better for detecting and characterizing irregular corneal conditions.

References

Corneal Imaging (Placido disc mires/rings and how reflections are analyzed). NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK562157/. Date Accessed February 2, 2026.

Accurate Keratometry Readings (includes examples of mires and manual keratometry technique). Cataract & Refractive Surgery Today. https://crstoday.com/wp-content/themes/crst/assets/downloads/CRST0609_06.pdf. Date Accessed February 2, 2026.

Verifying the calibration of a manual one-position keratometer. Community Eye Health Journal. https://europepmc.org/article/med/24782588. Date Accessed February 2, 2026.

Corneal Topography (Medical Policy) (describes projecting light rings and interpreting reflected patterns). UnitedHealthcare. https://www.uhcprovider.com/content/dam/provider/docs/public/policies/medadv-mp/corneal-topography.pdf. Date Accessed February 2, 2026.

Corneal Topography. EyeWiki. https://eyewiki.org/Corneal_Topography. Date Accessed February 2, 2026.