Ch 13. Keratometry and topography

Question 1

What is the formula for transverse magnification from the geometric optics point of view applied to the keratometer.

Answer 1

Hi/Ho=F/X

Where

Hi:heith of the virtual image

Ho: Known heith of the mire

F: Known Focal point of the mirror

X: distance from the mires to the principal focus

F is calculated according to F=r/2, formula for mirror’s focal point.

Question 2

What is the formula for the focal lens of a convex mirror?

Answer 2

F=r/2

Question 3

what does keratometers messure, Corneal curvature or corneal power?

Answer 3

corneal curvature. (radious of curvature)

Corneal power (diopters) is calculated from those messurements.

Question 4

Which formulae is used to calculate power from corneal curvature?

Answer 4

P= (n¹-n⁰)/r

where n¹ is the corneal index of refraccion, n⁰ is that of the air.

(keratometers use a “keratometric” index of refraction of 1.3375 to account for posterior corneal surface. But real corneal index would be 1.376

Abrreviated calculation the is:

P=(1.3375-1)/r =

P=0.3375/r

Question 5

Name both the real corneal index of refration and the keratometric “corneal” index of refration

Answer 5

Real: 1.376

Keratometric: 1.3375

Question 6

What type of instrument is the Pentacam

Answer 6

Is a Scheimpflug, slit beam, corneal tomographer.

Question 7

In a corneal placido topographer, what means it the mires are crowded together?

Answer 7

Mirer look closer in areas of higher corneal power, thus, increased curvature.

Question 8

Define corneal enantiomorphism

Answer 8

When corneal topographic representacions are a mirror image of each other.

Question 9

Which topografic map is best suited for delimitation of specific corneal alterations p.e. the evaluation of a post lasik optical zone?

Answer 9

The Tangential map.