OP - Vertometers & Keratometers - Week 8

Question 1

What is the function of a vertometer?

Answer 1

Measures the vertex power of ophthalmic lenses.

Question 2

What are spectacle lenses usually specified by (4)?

Answer 2

Back vertex power
Front surface power
Refractive index
Thickness

Question 3

Describe the initial setup for a vertometer with no test lens in position. Describe where the target is placed.

Answer 3

The target is placed at the front vertex point of the collimator. The light exiting the collimator is therefore collimated, and can be seen through an afocal telescope.

Question 4

Where is the test lens placed in the initial setup of a vertometer? What effect does this have on the setup?

Answer 4

It is placed at the back vertex point of the collimator.

The light entering the afocal telescope is no longer in focus as it is no longer collimated.

Question 5

What is the next step once a test lens is placed into a vertometer?

Answer 5

The target is moved until the light exiting the test lens is collimated and in focus through the afocal telescope.

Question 6

Describe the equation used to measure back vertex power once the target has been adjusted after a test lens is placed into a vertometer.

Answer 6

Fv = x Fc^2

Question 7

What are the limitations of vertometers for positive and negative lens?

Answer 7

Positive - limited by how far the target can move forward.

Negative - no theoretical limit on how far back a target can move, depends more on the design of the instrument.

Question 8

What happens to the movement of the target as the power of the test lens increases?
Keeping this in mind, what are the advantages and disadvantages of having a strong collimator?

Answer 8

Movement per dioptre decreases.
Strong collimators will have a larger range of powers assessible, butwill need finer movements, and is more prone to errors.

Question 9

Some lenses can be toric. How is this accounted for in a vertometer?

Answer 9

The target used is a cross or dot so that principle meridians can be measured.

Question 10

Suppose a test lens has prism dioptres, how would the target appear?

Answer 10

It would be displaced from the centre of the scale.

Question 11

How can soft contact lenses be measured using a vertometer?

Answer 11

Lens surface is blotted dry using lint-free tissue.

Must be measured quickly before it dries out too much.

Question 12

What is a disadvantage of using a wet cell for measuring soft contact lense using a vertometer?

Answer 12

A wet cell is is when a soft contact lens is immersed in saline.
Must now account for the refractive index of the contact lens and saline solution, which is less accurate than in air.

Question 13

What is the dioptre accuracy of vertometers?

Answer 13

Limited to 0.12D.

Question 14

What is the accuracy of a vertometer limited by?

Answer 14

The quality of the collimator lens

Question 15

Describe how to set up the eyepiece of a vertometer.

Answer 15

The eyepiece must be focused first.
Ther vertometer reading is first set to zero.
The eyepiece is screwed in until the target is sharply focused.

Question 16

What maginitude of error can be expected if a vertometer has a poorly focused eyepiece?

Answer 16

Excess of 1.00D

Question 17

What assumption is the calibration of a vertometer based on?

Answer 17

That the back surface of the test lens is coincident with the lens stop.

Question 18

What is a projection vertometer, and name 2 advantages it has over a telescope vertometer.

Answer 18

The telescope is replaced with a projection system.

• It overcomes errors due to failure to focus the eyepiece or due to accommodation.
• Large number of people can view the screen at once, less subjective.

Question 19

What is an automatic vertometer, and name 2 advantages it has over a telescope vertometer.

Answer 19

The target focusing is done automatically.

• Removes the subjectivity entirely
• Many measurements can eb done rapidly

Question 20

How do automatic vertometers work, what problems does it face, and how can this be accounted for?

Answer 20

A beam is shone onto the lens, and its deviation is depended on the lens power.
Can be problematic if the lens isnt centred correctly.
Can be overcome by using two beams of light.
The difference between the two light beams is used, and therefore independent of correct centration.

Question 21

In an automatic vertometer, what gives an indication of the prismatic component of the lens?

Answer 21

The midpoint between the two beams of light.

Question 22

In an automatic vertometer, how many beams of light are used to measure cylindrical lenses?

Answer 22

4.

Question 23

What is the purpose of a keratometer?

Answer 23

To measure the radius of curvature of the anterior surface of the cornea.

Question 24

What is the normal range and mean radius of curvature for human corneas?

Answer 24

Range - 7.0mm to 8.5mm

Mean - 7.8mm

Question 25

What are the uses of knowing the radius of corneal curvature?

Answer 25

Contact lens fitting, and assessing corneal abnormalities like keratoconus.

Question 26

What does the tear film on the anterior surface of the cornea act like, and what is this called?

Answer 26

The tear film acts like a polished convex mirror.

The imaged formed by reflection is called purkinje I.

Question 27

Define purkinje I, and 3 factors that affect it.

Answer 27

It is the image formed by the reflection on the cornea.
It is affected by:
-the size of the object
-the distance from the cornea to the target
-the radius of corneal curvature

Question 28

What kind of image does the keratometer mire produce?

Answer 28

Virtual

Question 29

What magnification does the keratometer mire have, and how can it be measured? What is a problem with this?

Answer 29

When it is 15cm away, there is a magnification of 0.03 (minification)
Microscope needed to measure it.
A graticule within the microscope can be used to measure it, but the eye moves too much, and measuring is difficult.

Question 30

What is a solution to the problem of excessive eye movement when making measurements using a keratometer? Explain how this works.

Answer 30

A doubling system using a biprism.
The amount of doubling required is proportional to the mire size.
Mire size is proportional to the radius of corneal curvature.
Therefore the radius of corneal curvature is proportional to the amount of doubling.

Question 31

In what two ways can alignment of images be achieved in keratometry?

Answer 31

Variable doubling - the biprism position is adjusted

Fixed doubling - the mires are separated

Question 32

Describe one position and two position keratometers.

Answer 32

One position - only need to find one principle meridian in a toric eye by aligning mires
Two position - once one meridian is found, the mire must be rotated 90* to find the second one

Question 33

Why will different keratometers give slightly different curvature radii?

Answer 33

Corneas arent 100% spherical.

Size of the areas of the mires depends on the keratometer’s objective lens size.

Question 34

Keratometers sometimes give a reading out in corneal power. What is the test reliability for keratometry in dioptres?

Answer 34

0.12 - 0.25 D

Question 35

How are the following determined:
Corneal astigmatism
Total ocular astigmatism
Residual astigmatism

Answer 35

Corneal - keratometry
Total ocular - ocular refraction
Residual - other components

Question 36

How is total ocular astigmatism defined?

Answer 36

Total ocular astigmatism = corneal astigmatism + residual astigmatism

Question 37

What kind of astigmatism is particularly important to measure to ensure hard contact lenses fit correctly?
What kind of astigmatism does a hard contact lens neutralise, and which one doesnt it?

Answer 37

Residual astigmatism

Hard contact lenses neutralise corneal astigmatism, but not residual astigmatism.

Question 38

How must the eypiece of a keratometer be focused before beginning keratometry?

Answer 38

Patient must close their eyes, while the practitioner then focuses the eyepiece onto the graticule.