Keratometry

Question 1

What is Keratometry

Answer 1

Keratometry measures the central 3-4mm (optic cap) anterior corneal radius using Purkinje image number 1

Question 2

Why is image doubling necessary in keratometry?

Answer 2

Patients eye movements make it impossible to compare the image size with the graticule/scale

Question 3

The doubling system in a keratometer may be fixed or variable. What is altered to obtain a measurement in

i) Fixed doubling
ii) Variable doubling

Answer 3

i) Fixed doubling - Object size/seperation of mires/image size

ii) Variable doubling - Position of prism/image seperation

Question 4

If a keratometer is described as being a “ONE POSITION” instrument. What does this mean?

Answer 4

It can measure “BOTH” meridians “WITHOUT” being rotated

Question 5

What is NOT altered in a keratometer to obtain a measurement in a fixed doubling keratometer?

Answer 5

Prism power

Question 6

If a keratometer is described as a ‘one position instrument’ what does this mean?

Answer 6

It can measure both corneal meridians at once