reflex movement in spherical ametropia

Question 1

How does the lab based model to demonstrate reversal of a spot ret?

Answer 1

set up of a positive lens, scan beam across along horizontal meridian
-You see bright flashy reflex in pupil- which is the equivalent of reversal

Question 2

How does the lab based model to demonstrate reversal of a streak ret?

Answer 2

Horizontal streak and sweep across vertical meridian

-will see bright flashy reflex - indication of a little against movement but fairly bright.

Question 3

What is reversal || explained in terms of the diagram?

See diagram on slide 5

Answer 3

• a reduced eye
• with single surface
• positive power
• in front of it - working distance lens (red)
• this is +1.50 lens- allows to work 66cm from eye- if eye is emmetropic it places the far point at the retinoscope - which is what we are trying to achieve
• Light from fundus is conjugate with fair point- which will be at the sight hole of retinosocpe
• Position indicated from the rays - is called straight ahead position- it is where the point source of light at the back of the eye which is generating an orange glow you see in pupil is on optical axis and so is conjugate with the far point on optical axis.
• In straight ahead position - all of light passes through sight hole of retinoscope and see orange glow in the pupil

Question 4

What happens when you sweep the ret upwards? - still referring to Reversal | diagram

see diagram on slide 5

Answer 4

point of illumination on fundus move upwards

• now can see the point where the 2 dashed rays come from - as tracing light back out of eye, has moved off optical axis and now upwards
• • so up the optical axis
• now we trace these rays back through the reduced eye - they form a focus off the axis but same plane -
• As soon as the focus moves outside the sight hole (tilt) - goes black
• hence why we see a flashy quick colour straight from being focus on sight hole (orange) (lines are in straight ahead position) to no focus (not on sight hole ) (black)

Question 5

Summary words for Reversal

Answer 5

-practitioner views pupil of subject- Important to know- not rays of sight hole - focusing on glow of pupil of eye that we are looking at
-reversal occurs when artificial far point with the work distance lens coincides with sighthole -
reflex “instantly” disappears

Question 6

What do you see in a retinoscope when the patient is myopic?

Answer 6

• using optical bench model of retinoscope beign swept horizontally
• now see as move to the left the orange glow moves to the right
• As move to the right the orange glow moves to the left
• reflex moves in oppsoite / against direction.
• reflex is slow and dull

Question 7

What are the optics behind it that are giving this against movement in myopia?
-Using a reduced eye diagram

SEE DIAGRAM ON SLIDE 9- the first diagram ‘ straight ahead’

Answer 7

• Straight ahead posiiton
• have reduced eye
• working distance lens in front of it
• retinoscope with its sight hole ( the 2 black lines)
• then the practiioners eye behind it - the globe behind the ret
• Without correction and with working distance lens in place, the artifical far point is between you (practisioner) and the reduced eye (subject )
• all rays of light that pass through pupil of eye - pass through the sight hole- orange glow in pupil

Question 8

What is the straight ahead positon ?

Answer 8

point of illumination the back of the eye is on the optical axis and can trace rays back from it

Question 9

Now what happens when you sweep the ret / mirror along the vertical meridian in a myopic reduced eye ?
-Using a reduced eye diagram

SEE DIAGRAM ON SLIDE 9- the 2nd digaram ‘Mirror moved UP’

Answer 9

• going to move mirror upwards
• so as a result of that mirror movement the patch illumination at the back of the eye that creates the reflex is going to move up
• Rays come from back of eye- being reflected out again from fundus - they fill the pupil, come through sight hole, cross over and form an image at the far point plane and by the time they reach the sight hole they spread out again
• Sight hole is now obstructing some of the rays and letting some through- obstructing the rays that are originated from the top half of the pupil of the eye- so they will obscure this bunch of rays and therefore a shadow will appear at the top part of the pupil
• as sweep the ret up, a shadow comes down from top part of the pupil - which looks as an against movement
• Sight hole vignettes rays from upper part of pupil.

Question 10

What are the optics behind it that are giving this with movement in hypermetropia bigger than 1.50 D?
-Using a reduced eye diagram

SEE DIAGRAM IN SLIDE 13 - ‘Straight ahead positon’

Answer 10

• As sweep beam - clear movement with streak
• Straight ahead posiiton-
• redued eye
• working distacne lens
• sight hole
• practioners eye behind sight hole
• If you take light emananting from the fundus creating the reflex that it passes through the components as follows , comes from a point behind the patients eye this is the artifical far point
• if hypermetropia is strong enough it will overcome the convergence added by working distance lens and will come from a point behind the patients head- as you expect

Question 11

Now what happens when you sweep the ret / mirror along the vertical meridian in a hypermetropic greater than 1.50D reduced eye ?
-Using a reduced eye diagram

SEE DIAGRAM ON SLIDE 13 the 2nd digaram ‘Mirror moved’

Answer 11

-When sweep up along vertical meiridan - mirror will be moved up from sight hole- this means that point of illumination that emanates from the back of the eye moves up away from the optical axis
-point of illumination has moved away from the optical axis in vertical direction - as you trace rays back through the eye - will see that some of the rays are obstructed/vignetted by the sighthole- these rays emanate from the fair point plane
-rays now have emmanated from the bottom part of the pupil that are being vignetted by the sight hole
-so as you sweep up, you see the shadow coming up from the bottom of the pupil
and it is in the same direction of the movement of the ret.
-Rays entered from lower part of pupil they get through to sighthole but not any further so practioner doesn’t see the lower part of pupil as it is in shadow
-in straight ahead position everything is lined up - circular glow in centre of pupil
-as we tilt the retinosocpe up we sweep up along the vertical meridian- see a shadow appearing form the bottom part of pupil- movement in same direction of retinoscope

Question 12

What are the optics behind it that are giving this with less than 1.50 D?
-Using a reduced eye diagram

SEE DIAGRAM IN SLIDE 15 - ‘Straight ahead positon’

Answer 12

• this hypermetropia isn’t suffcient enough to overcome the power of working distance lens
• As it is a weaker degree of ametropia , the w.d lens provides some convergence which the hyopermetropia of the eye can not overcome the far par point is behind the practioner

Question 13

Now what happens when you sweep the ret / mirror along the vertical meridian in a hypermetropic less than 1.50D reduced eye ?
-Using a reduced eye diagram

SEE DIAGRAM ON SLIDE 15 the 2nd digaram ‘Mirror moved’

Answer 13

• As sweep up along vertical meridian , the point of illumination at the back of the eye from the fundus also moves up away from the optical axis- trace rays of light back and find out some of them are cut out/ vignetted by the sight hole
• its the rays that emmanate from the lower part of the pupil

Question 14

Where is the vignetted section ?

Answer 14

shaded region is the lower part of pupil / on diagram

Question 15

What happens with a working distance lens?

Answer 15

myopia gives an “AGAINST” movement of the reflex

hypermetropia gives a “WITH” movement of the reflex

Question 16

What happens without a working distance lens?

Answer 16

strong myopia (more than  -1.50D) gives an "AGAINST" movement of the reflex
weak myopia (less than -1.50D) gives a "WITH" movement
hypermetropia gives a "WITH" movement