W9 Objective Refraction

Question 1

what is Retinoscopy comprised of?

Answer 1

Fenestrated sight hole and semi-silvered mirror

Question 2

How is reflex formed?

Answer 2

Myopic and hyperopic- dark annulus surrounds less bright reflex
Reflex size and brightness inversely proportional degree of ametropia, i.e. smaller and duller reflex indicates larger ametropia

Question 3

Myopia is with or against movement?

Answer 3

against movement

Question 4

Hyperopia is with or against movement?

Answer 4

with movement

Question 5

Autorefractor 5 principals

Answer 5

1. Badal optometer
2. Analysis of image quality
3. Retinoscopic scanning
4. scheiner disc
5. Photorefraction

Question 6

Badal optometer

Answer 6

Eye placed at the back focal length of Badal lens = target image vergence is proportional to target distance from the front focal point of the lens. Angular size is independent of target vergence

Target moves away or toward lens:
Change in light vergence hitting lens
No change in size
Emmetropic: Target is at the focal point of eye
Myopic: The target is within the focal point - divergent light exits lens
Hyperopic: Target away from the focal length of eye - convergent light exits the lens

Question 7

Analysis of image quality

Answer 7

Infared radiation illuminates a tses a test graticule = T
Image correctly focused it will pass through mass with the maximum radiation reaching the detector.

Question 8

Retinoscopic scanning

Answer 8

The returning beam can show with, against or neutral movement
Like retinoscopy, speed and direction of the reflex are used to determine refractive error with lenses to neutralise the refractive area.
The amount of meridional ametropia is calculated based from the time interval btw returning beam striking the upper and lower detectors

Chopper drum

Question 9

Scheiner disc

Answer 9

Based on a double pinhole being placed infront of the eye
Two infrared light sources are imaged in the plane of the pupil to stimulate the scheiner pinhole autoreceptors
The focus is adjusted by axial displacement of the illumination and detection systems.

For emetropic eye: a sharp spot image is formed

Ametropic: two blurred spots are formed

Question 10

Photorefraction

Answer 10

Directs light into eye by flash photography - examines part of light that emerges from the pupil after scattering of light from back fo eyeball.
Against movement for myopia
With movement for hyperopia
Measures size of crescent to determine degree of ametrpia
To determine degree of ametropia - camera must be defocused by a known amount
Pupils appear dark in photograph as flash returns to source

Question 11

Traditional vs wavefront autorefractor?

Question 12

Should autorefraction replace retinoscopy?

Question 13

Keratometer:

Answer 13

Assessment of corneal shape by measuring anterior corneal radius of curvature and regularity of corneal surface.
FOR:
Contact lens
Corneal disease - keratoconus

Limitation: only mesures curvature of anterior central cornea - radius 2-4mm

Optical principles:
Object of known size and distance is reflected off corneal surface to determine the size of the reflected image with a measuring telescope

The purkinje image is virtual:
The height of the image is directly proportional to mirror radius

Question 14

Keratometry readings

Answer 14

D= (n-1)/R -mm

K<41D is falt
K> 41D is steep

Question 15

Doubling principle

Answer 15

Involuntary eye movements cause image formed on cornea to always be moving
Uses an optical doubling system where observer has to align images of mires refrelcted from cornea
Prisms introduced so two images are formed
Risms moved until images touch

Keratometer: Measures only anterior corneal radius - it cannot measure posterior radius - have two diff radii

Question 16

Types of keratometry?

Answer 16

Baush and lomb type: fixed object size with variable image size

Javal Schiotz: fixed image size with variable object size