Keratometry

Presented by, Loknath Goswami

Long- im Kro B.Sc Optometry 2nd year

Introduction Keratometry (Gk. Kerato= cornea, metry =

measurement of) It is also known as ophthalmometers Keratometers are instruments used to measure the

radius of curvature of the anterior corneal surface This measurement is utilize to fit contact lenses and

to monitor corneal changes produced through the wear of contact lenses

A Schematic Diagram

Principle

• Keratometers utilize the reflective properties of the cornea in order to measure its radius of curvature

• By measuring the size of an image, formed by reflection from the cornea, of an object of known size and position , a measurement of the radius can be calculated

Principle

• The theory of keratometry is depicted in the figure, where it can be seen that the magnification of the image is equal to h’/h where h’ equals the image the size of the image and h the size of the object. By similar triangles, it can be seen that

h’/h = f’/x = -r/2x• From the equation we find that radius of

curvature of the cornea is • r = 2mx

– Where m = magnification of the image

• In theory, the size of the mire image could be measured by simply placing a measuring graticule within the microscope.

• However a problem arises in keratometry due to the continual movement of the patient’s eye.

• Everytime the eye moves, the mire image moves, which makes it exceedingly difficult to measure with any degree of accuracy

Figure of doubling prism

One and two position Keratometers

• Keratometers that require rotation through 90˚ in order to measure the second principal meridian are known as two position keratometers

• Keratometers that do not require rotation in order to measure the second principal meridian are known as one position keratometers

Area of cornea used during keratometry

• It can be seen from the figure that the light reflected from the cornea comes not from its centre, but from two small areas on either side of the instrument axis.

• The size of this area is dependent upon the effective aperture of the keratometer’s objective.

• The principles upon which keratometry is based assume that a spherical surface exists between these two areas

• This need not be the case. It is, in fact, well known that the normal cornea is not spherical, but flattens off towards its periphery. Because of this and because different keratometers reflect their mires from different regions of the cornea, two readings of the same cornea with two different keratometers may not give the same radius.

COMMONLY FOUND MIRES

Eyepiece focusing errors and their elimination

• The first is due to the mires being positioned closer to the patient. When this occurs, the size of the mire image formed by the corneal mirror increases – Can be eliminated by optically imaging the mires at infinity

Eyepiece focusing errors and their elimination

• The second is produced because the degree of doubling in an instruments which uses a prism placed on the eyepiece side of the objective, alters with the distance from the mire image to the objective lens. As the mire image is brought closer to the objective lens, the magnification of the mire image increases and the original coincidence setting of the keratometer is upset– Can be eliminated by using two parallel glass plates in front

of the objective as a variable doubling device, rather than a moveable prism behind the objective

Calibration

• Set the eyepiece• Mount the test sphere• Measure the test sphere

Types (according to the operation)• Manual keratometer Bausch and Lomb keratometer Javal-Schiotz keratometer Zeiss (Oberkochen) ophthalmometer

• Automated keratometer IOL Master Pentacam Orbscan Corneal Topography The Humphrey Auto Keratometer

PentacamIOL Master

Orbscan

Corneal Topography The Humphrey Auto Keratometer

Types of optical system incorporated in keratometers

Most of the currently produced keratometers fall into one of the following three catogories:1. Fixed doubling, variable mires 2. Variable doubling, fixed mires 3. Telecentric

Javal-Schiotz type keratometer

• It is fixed doubling, variable mire, two position keratometer

Ziess (Oberkochen) keratometer

• It is a variable doubling, two position keratometer

Bausch and Lomb keratometer

• It is one position, variable doubling keratometer

• Two independently adjustable prisms, situated behind a special aperture stop, double the mire image along two mutually perpendicular meridians

Bausch and Lomb keratometer

Optical system of Bausch and Lomb keratometer

Optics of Bausch and Lomb keratometer

• When the instrument is correctly aligned, the operator sees three images of the instrument’s mires

• The first is produced by light passing through aperture C and the vertically displacing prism

• The second is produced by light passing through aperture D and the horizontally displacing prism, and the third by light passing through aperture A and B

• Back and forth movement of the vertically doubling prism results in movement of the vertically displaced image, while movement of the horizontally doubling prism results in movement of the horizontally displaced image

• The central image formed by the light passing through A and B is unaffected by movement of either prism

• The aperture A and B act like a Schiener disc and double the central image of the mire when the intermediate image, produced by the objective lens does not coincide with the focal point of the eyepiece lens

• This system is designed to assist the operator in judging when the microscope is out of focus

Mires in Bausch and Lomb keratometer

• The mire of the Bausch and Lomb keratometer is shown in figure. The images of the mire as seen through the doubling system of the keratometer are shown in the figure for the conditions where1. The vertical doubling is correct and the horizontal

doubling is insufficient2. The vertical doubling is too great and the horizontal

doubling is correct3. The vertical and horizontal degrees of doubling are

correct4. The mires are viewed after reflection by an astigmatic

cornea, the axes of which do not coincide with that of the keratometer

Reference

• David B. Henson, Optometric Instrumentation, Page 91-114

• https://www.ophthalmictechnician.org • https://en.m.wikipedia.org/wiki/keratometry

THANK YOU