This document discusses keratometry, which is the process of measuring the curvature of the anterior surface of the cornea. It defines keratometry and describes the optical principle, which is that the size of the image formed on the cornea depends on its radius of curvature. It then discusses the doubling principle used to avoid small eye movements, and describes types of keratometers including one-positional and two-positional models. Specific keratometers like the Bausch and Lomb are outlined, including their components and measurement procedure.

1. KERATOMETRY
By
Shoaib khattak
Kabir shah
Wahab khan

2.  Contents
Definition
Principle
Doubling principle
Types of keratometer
Advantages
Limitations of keratometer

3.  Definition
Kerato means cornea and metry means
measurment .
Keratometry is the process of measuring
anterior 2-3 mm corneal curvature .
The instrument used in this procedure is
called keratometer .
It is also called opthalmometer

5.  Optical priciple
Anterior surface of the cornea acts as a
convex mirror
Size of the image formed on convex surface
depends on the radius of curvature of the
surface .
Greater radius of curvature , smaller will be
the size of image formed .

6. Therefor from the size of the image formed
on anterior surface of cornea the radius of
curvature of cornea can be calculated by
using the formula
 r= 2u (i/o)
 r= radius of curvature of cornea
 U= distance b/t image and convex surface
 i= iamge size
 O= object size

7.  Doubling priciple
The principle of doubling the size of image
formed on the convex surface is called
doubling principle .
It is neccesary to avoid small eye
movements
 to reduce the miniature movements of
image formed on the anterior surface of
cornea .

8. Types of doubling
 Variable doubling
In this type of doubling the image size is
doubled while the size of object and
distance of doubling device is kept
constant .
Variable doubling is used in topcon and
bausch and lomb keratometers

9. Fixed doubling
 in this type the image size is kept constant
while object size and doubling device
distance is changed .
Fixed doubling is used in javal schiotz
keratometer .

10.  types
1. One positional keratomter
The keratometer which can measure
curvature of both the principle meridians
without being rotated is called one
positional keratometer .
e.g bausch and lomb ,topcon

11. 2. Two positional keratometer
 this keratometer can’t measure both the
meridians at a time .
 To measure the 2nd meridian we have to
rotate it through 90 degree .
 E.g javal schiotz keratometer .

12.  Bausch and Lomb
It is one positional keratometer i-e we can
measure both the corneal meridians
without rotating it .
This works on the principle of variable
doubling .
Its also called Reichert keratometer .

13. Bausch and lomb keratometer Images formed

14. Parts
Chin rest
Forehead rest
Light source
Focusing knob
> focusing knobs are used to overlap the
plus and minus signs during measurment of
corneal curvature in horizontal and verticle
maredian.

15.  optical system
Object
 is circuler mires with two plus and two
minus signs .
 A lamp illuminate the mires by means of
diagonally placed mirrors .
Objective lens
 Focuses the light from the image of the
object along central axis

16. Diaphrams apertures and doubling
prisms
 Four apertures diaphram is placed near
the objective lens .
 The apertures function is to double the
central image when instrument is not
focused .
 Prisms are used to double the image of
mire .
Eye piece lens
Enable the examiner to examine the
magnified view of the double images .

18.  procedure to use
Instrument ajdustment
 Caliborate the instrument before use .
Patient adjustment
 Set the patient in front of kertometer with
chin on chin rest and head on head rest.
Focusing of mires
 Focus the mires so that the central image is no
longer doubled .

19. Measurment of corneal curvature
 To measure the curvature in horizental
maredian the plus signs are aligned
through horizontal knob while the
vertical meridian is measured by aligning
the minus signs through vertical knob .

20.  javal schiotz keratometer