Ophthalmic Optics

Question 1

describe swim, and its causes

Answer 1

sensation where environment appears to “swim” due to rapidly changing prismatic effects caused by spectacle magnification, and distortion on rapid head and/or eye movement.

Question 2

in swim, moving head only without moving eyes causes a with movement in what sort of lenses?

Answer 2

negative

Question 3

solutions to swim

Answer 3

rotate head, eyes together
reduce prismatic effect
positive lens: reduce magnification, reduce distortion
negative lens: increase magnification, reduce distortion

Question 4

visual field size: effect and phenomenon? (positive lens)

Answer 4

reduce visual field, peripheral ring scotoma

“starting” phenomenon, “roving ring” scotoma. moving against direction of eye movement

Question 5

visual field size: effect and phenomenon? (negative lens)

Answer 5

enlarged visual field, peripheral ring diplopia

Question 6

visual field size: positive lenses. solutions to problems?

Answer 6

reduce prismatic effects. increased head movements. ^lens diameter increases visual field size, but also ring scotoma.

Question 7

visual field size: negative lenses. solutions to problems?

Answer 7

reduce prismatic effect. increased head movements.

Question 8

cosmetic factors: moderate to high positive power lenses. problems and solutions?

Answer 8

appear bulbous, eye appear magnified. full-field lenses more attractive than lenticular lenses.
solution: reduce spectacle magnification

Question 9

cosmetic factors: negative power lenses. problems and solutions?

Answer 9

thick edges. eye appear minified.
Myopic rings: multiple reflections, at back surface of lens, of light scattered into the lens.
solution: polishing lens edges, coating with varnish, AR coatings.

Question 10

cosmetic factors: more problems and solutions.

Answer 10

thickness reduced by: reducing lens size, aspherising, high index glass and plastic. however, weight up for glass, TCA up.

reflections at front surfaces produce glare, making eye less visible. bigger problem for flat surfaces. AR coatings.

Question 11

ways to reduce weight of lens?

Answer 11

high index glass and plastic. thickness reduced, but density up for glass.
reduce lens size
asphericity

Question 12

define simply catoptric power

Answer 12

power based on reflection (e.g. mirror). a flat mirror has power of 0.

Question 13

in aberrations, is the tangential part usually anterior or posterior to sagittal? (corneal is anterior to retina)

Answer 13

anterior. (at least in diagrams)

Question 14

what is Tscherning ellipse based on? (what principle?)

Answer 14

zero oblique astigmatism. the power of front and back surface respectively to produce this.

Question 15

in ghost images, what does z represent?

Answer 15

distance between eye and back surface of lens. (I think Z is something different)

Question 16

intensity of ghost image 1, description? Who would have problem with this reflection? what is another way that lenses can form similar type of ghost image?

Answer 16

intensity: 0.15%. n = 1.523

double internal reflection, generally well out of focus
problem in low myopes. focused if eye accommodates 3D.

similar type of ghost image formed by lenses incorporating prism.

Question 17

intensity of ghost image 2, description? Who would have problem with this reflection? how to solve problem?

Answer 17

intensity 0.08%
reflection of cornea and back surface of lens. cornea forms virtual image of real source 4mm behind apex.

problem for high to moderate negative lenses, susceptible to lens bending changes

solution: change vertex distance, lens bending, AR coating.

Question 18

intensity of ghost image 3, description? Who would have problem with this reflection? how to solve problem?

Answer 18

intensity: 0.08%
formed by reflection at cornea and front lens
High hypermetropes.
susceptible to changes in lens bending.
solution: change VD, lens bending, AR coating, change thickness.

Question 19

intensity of ghost image 4, description? Who would have problem with this reflection? how to solve problem?

Answer 19

intensity: 4%
reflection of object at back surface of lens.
high hypermetropes (distance obj), myopes (face)
AR coating, bending, (+vertex distance for face reflection).

Question 20

intensity of ghost image 5, description? Who would have problem with this reflection? how to solve problem?

Answer 20

3.7%
front surface, object behind lens
moderate to high myopes (distance), moderate hypermetropes (face).
AR coating, bending, (+vertex distance for face)

Question 21

anti-reflection coatings. what is a reason for doubling layers on plastic? what is it?

Answer 21

no low index materials for coatings with plastic lenses, due to adhesion problems, stress differences, cracking of coating on flexible plastic lenses.

high index material applied to plastic, then AR. materials such as titanium oxide used for high index. AR out of quartz.

Question 22

in terms of wavelengths, how do AR coatings work? this is single-layer coating. what is p in the equation?

Answer 22

reflection between material behind coating and on coating. cancelling each other out reduces reflection. this occurs if wavelengths travel 1/2 wavelength from each other –> coating 1/4 wavelength thick. in equation, nc is r. index of coating. p is odd integer.

Question 23

in anti-reflection coatings. what does nc and ng stand for?

Answer 23

coating r. index and lens r. index.

Question 24

list types of anti-reflection coatings

Answer 24

single-layer
double layers on plastics
multi-layer coatings

Question 25

purpose of multi-layer coatings? (AR coating)

Answer 25

to reduce reflectance over whole visible spectrum requires series of layers. stack of pairs of alternate high and low r. index layers used. can be manipulated to produce different coloured coatings.

Question 26

list lens surface treatments

Answer 26

hard coatings
mirror coatings
hydrophobic coatings
tinting plastic lenses
UV absorption
glass tints

Question 27

what is the equation for reflection intensity?

Answer 27

Ir = Io(u’-u)^2/(u’+u)^2
Ir is intensity of reflected light. it’s a percentage.