Keratometry (incomplete)

Question 1

what is the purpose of keratometers/ophthalmometers, and what can it be used for?

Answer 1

measure anterior radius of curvature (optical non-contact method)

used for:
- fitting contact lenses
- monitoring corneal changes
- assist in recognition of optical abnormalities
- radii of curvatures of contact lenses

Question 2

what are the equations relating to radii of curvature?

Answer 2

r = 2dM1/(1-M1^2)
r ~ 2dM1 when l»r, M1–> 0

d is distance between object and image

Question 3

what is M1?

Answer 3

n’/n
height of image/height of object.

Question 4

briefly draw the main components of a keratometer (page 5)

Question 5

the refractive index of cornea is estimated. give an example value.

Answer 5

1.3375, 1.336

Question 6

what to do when there is a toric cornea?

Answer 6

rotate target. example on page 6. recording 7.80 along 15/7.65 along 105. these are power meridians do not use the term axis.

Question 7

what is image doubling and what is it used for?

Answer 7

used to overcome problem of eye movement. image is doubled with a transverse separation, and separation adjusted until extremities of images are in contact.

Question 8

how is image doubling achieved and what are the types of doubling.

Answer 8

prisms often used for doubling the image in one of two ways: fix and variable doubling.

Question 9

define fixed doubling

Answer 9

amount of doubling is fixed (e.g. using Wollaston prism). separation adjusted by varying size of image (object mire size n is varied)

Question 10

draw the types of fixed doubling

Answer 10

page 8

Question 11

define variable doubling. how is this often achieved?

Answer 11

object size fixed. separation adjusted by varying amount of doubling. often through moving prism back and forth.

Question 12

list an advantage of variable doubling

Answer 12

instrument can be easily calibrated to give direct reading of corneal radius of curvature as doubling is equal to image size n’’

Question 13

draw mechanism of variable doubling

Answer 13

page 9

Question 14

effect of distortion on keratometers?

Answer 14

apparent reduction in measured radius of curvature. errors reduced as target size reduced.

Question 15

if target lies on a ….., the image lies on a ….. surface due to ….. the effect is …..

Answer 15

if target lies on a plane, the image lies on a curved surface due to field curvature aberration. effect is small. relating to aberrations and calibration of keratometers.

Question 16

how do keratometers address aberration and calibration errors?

Answer 16

instruments are calibrated on spheres of known radius of curvature, reducing errors induced by use of paraxial theory.

Question 17

corneal area involved in generating corneal mire image is an …. region. hence, keratometer indicates radius of curvature in this area only.

Answer 17

annular (ring)

Question 18

eyepiece focussing errors. how is this often resolved? give an example.

Answer 18

graticule in intermediate image plane helps. e.g. if eyepiece focused too far out, instrument must be positioned closer for target (mires) to be in clear focus.

Question 19

two consequences of eyepiece focussing errors? what is the result on ROC? (as of lecture)

Answer 19

1. magnification of mires by anterior cornea increased (n’/n) due to mires moving closer to cornea.
2. as objective is moved closer to image, the image of mire image (n’’) size increases. ie. n’‘/n’. (seems to be objective (lens) moved closer to patient)

both errors combine to increase size of final image, giving a greater recording of radius of curvature.

Question 20

two consequences of eyepiece focussing error (summary)

Answer 20

1. movement of mire closer to patient (increase n’)
2. movement of objective closer to patient. (increase n’’)

results in increased size of final image, resulting in greater recording of ROC.

Question 21

solutions for eyepiece focussing errors?

Answer 21

error 1: use collimated mires –> mire image position and size do not alter

error 2: tilted glass planes for doubling (angular doubling unaffected) OR placing doubling system at focal point of objective.

it is possible to correct them together in one instrument in which the mires’ images, as seen by objective, are behind the patient’s head. error due to mires being too close to eye is cancelled by error due to objective being closer to the eye.

Question 22

describe Bauch and Lomb keratometer.

Answer 22

one position, variable doubling keratometer. two independently adjustable prisms double the mire images along two mutually perpendicular meridians

lamp illuminates the circular mire. a small mirror between the components of the objective is a fixation target. masks reduces reflected beam into four circular areas.

consists of: horizontal prism independent variable doubling by moving prisms

vertical prism

parallel glass plates act as a scheiner disc to ensure focus.

Question 23

what are the four apertures of Bausch and lomb keratometer?

Answer 23

1. horizontal prism independent variable doubling by moving prisms
2. vertical prism
   3, 4. parallel glass plates act as a scheiner disc to ensure focus.

Question 24

describe Bausch and Lomb keratometer in terms of position and doubling.

Answer 24

one position, variable doubling.

two independently adjustable prisms double the mire images along two mutually perpendicular meridians.

Question 25

draw the mire image appearances for Bausch and Lomb keratometer.

Answer 25

page 15. circle. + horizontal, - vertical.

Question 26

javal -schiotz keratometer. describe in terms of doubling and position

Answer 26

fixed doubling, 2 position instrument.

Question 27

describe Javal-schiotz keratometer

Answer 27

mires attached to the front of the small light boxes which can be moved equally in opposite directions along a circular arc (centre of curvature at subject’s eye)

doubling by Wollaston prism between components of objective.
rotate instrument about optical axis.

Question 28

draw a diagram of mire separation of javal-schiotz keratometer.

Answer 28

page 17

Question 29

describe zeiss ophthalmometer in terms of doubling and position

Answer 29

variable doubling, two position

Question 30

what is an advantage of zeiss ophthalmometers? (how is it achieved?)

Answer 30

free of focussing errors: achieved by:
1. placing mires behind positive lenses, which image them at infinity –> size and separation of images unaffected by working distance errors.

2. doubling system placed at focal point of objective lens (telecentric principle)

objective of observation system consists of two achromatic lenses. first acts as collimator. followed by beam-splitting prism to produce II intermediate optical axes. low powered minus lens on each axis. variable doubling produced by lateral displacement of lenses in opposite directions. beams are recombined by another beam splitting prism and pass through second component of objective.

Question 31

draw a diagram of mire images of zeiss ophthalmometers

Answer 31

see page 20