7 Facts On Keratometer:What,Uses,Types,How To Use,Principle

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What is a keratometer?

A keratometer (sometimes referred to as an ophthalmometer) is a device used for measuring the curvature of the cornea’s frontal surface. In 1851, German physiologist Hermann von Helmholtz built the first keratometer, although a similar instrument was built by Jesse Ramsden and Everard Home in 1796. This diagnosing instrument helps in evaluating the degree and axis of astigmatism.

  • A keratometer provides the following information:
  • The radius of curvature of the anterior surface of the cornea.
  • The direction of the eye’s principal meridian. (With-the-rule or against-the rule astigmatism)
  • The axis and degree of corneal astigmatism.
  • The occurrence of any form of corneal distortion.

keratometer uses

  • Evaluating degree of astigmatism .
  • Fitting of eyeglasses and contact lenses.
  • Analyzing patients having keratoconus.
  • Determining intraocular lens power for patients suffering from cataracts.

Principle of keratometry?

The main purpose of a keratometer is to find the optical refracting power of the cornea (that is a convex refracting surface). When an object of known size is placed at a known distance from the corneal surface in such a way that the size of the reflected image of the object is measured by a measuring telescope.

keratometer
Image source: Mike Blyth, Eye doctor examining Nigerian patient with keratometer, CC BY-SA 3.0

The radius of curvature of the cornea is given by

R = 2 d (I/O)

R: Radius of curvature of the cornea in meters.
d: Distance of the object from the cornea.
I: Size of the image.
O: Size of the object.

Now, the refractive power of the cornea can be given by the formula

D = (n-1)/R

D: Dioptric refracting power of the corneal surface.
n: refractive index of the instrument (n= 1.3375 generally)
A keratometer displays the curvature of the cornea in terms of diopters of power or in terms of millimeters and diopters. If the readings are shown in terms of millimeters then the dioptric power can be derived by the equation shown above.

Types of keratometer:

Javal-Schiotz Keratometer

The Javal-Schiotz keratometer is based on the Javal-Schiotz principle that works with a fixed image, a doubling size, and an adjustable object size for obtaining the curvature of the corneal surface. It is a two-position instrument and uses two self-illuminated objects. One of the objects is a red square and the other is green staircase-like design.

These objects are designed to be kept on the circumferential track such that a fixed distance from the eyes is maintained. It is vital for the instrument to remain focused on obtaining accurate results. This instrument, like many autofocus devices, works on the Scheiner principle. In this, the incoming reflected light rays (converging in nature) are observed through two or more discrete symmetrical apertures.

Bausch and Lomb Keratometer

The Bausch and Lomb Keratometer is based on the Bausch and Lomb principle that works with a fixed object instead of a fixed image. This is a one-position keratometer and the image size can be varied. In this, the incoming reflected light rays (converging in nature) are passed through a Scheiner disc having four discrete apertures. The two prisms are aligned perpendicularly with each other. The powers of the major and minor axis can be independent without adjusting the orientation of the instrument.

How to use a keratometer:

kerato
Parts of a Keratometer Image source: indiamart

Steps to use a Bausch and Lomb Keratometer

  • Eyepiece Adjustment: First, the eyepiece needs to be turned counterclockwise entirely. A white sheet of paper can be placed in front of the telescope part. The eyepiece should be turned clockwise until the instrument reaches a sharp focus for a given target object
  • The height of the device should be aligned with the level of the patient’s sight sign.
  • The non-testing eye of the patient should be blocked.
  • The patient should then be asked to look directly at the center of the device. The patient should be able to see the picture of the circles in their eyes.
  • The patient should adjust the focusing knob in such a way that they are able to see a single image of the eye’s central circle.
  • The side and upper circles should be aligned with the central circle. This can be done by rotating the body of the device.
  • The horizontal alignment knob present in the instrument can be used for bringing the crosses on each other.
  • The vertical alignment knob present in the instrument can be used for bringing the crosses on each other.
  • The values will now be displayed on the keratometer scale.

Steps to use a Javal-Schiotz Keratometer

  • Eyepiece Adjustment: First, the eyepiece needs to be turned counterclockwise entirely. A white sheet of paper can be placed in front of the telescope part. The eyepiece should be turned clockwise until the instrument reaches a sharp focus for a given target object.
  • The height of the device should be aligned with the level of the patient’s sight sign.
  • The non-testing eye of the patient should be blocked.
  • The patient should then be asked to look directly at the center of the device. The patient should be able to see the picture of the circles in their eyes.
  • The instrument should be adjusted in order to see a sharp image of the staircase and rectangle targets.
  • The instrument can be rotated for aligning the line in the staircase target with the line in the rectangle target.
  • The measuring knob can be turned until both the staircase and rectangle targets overlap.
  • The values will now be displayed on the keratometer scale.

Manual Keratometer vs Automated Keratometer

Manual keratometers provides

  • A clear reading of the pre-corneal tear film.
  • A dynamic image of the corneal surface.
  • Clear visualization of the reflections produced by the corneal tear film.
  • Accurate measurements produced from manual fine adjusting.

Automated keratometer provides

  • A static image of the corneal surface.
  • Computer generated adjustments for readings.

Keratometry requires precise measurements as eye operations are critical and often irreversible. So, errors in measurements can cause huge issues. While automated keratometers are useful and comparatively easy to use for beginners, manual keratometers provide a more accurate reading. An experienced professional generally prefer using the manual instrument for fine adjustments. However, nowadays updated automated instruments are comparable to manual design. It is very important to be completely sure about the readings before one orders an eye lens. Both the patient and the doctor should be satisfied with the reading values.

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