complete topic of keratometry

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