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OPTOM > OPTOM 263B > Flashcards

OPTOM 263B Flashcards

1
Q

What aberrations are rotationally and not rotationally symmetric

A

Spherical aberrations are symmetric
Monochromatic aberrations are not symmetric

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2
Q

Types of spherical aberrations ( SA )

A

Positive and Negative SA

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3
Q

How to correct Positive and Negative Spherical aberrations

A

Positive SA use Negative lens
Negative SA use Positive lens

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4
Q

How does Spherical aberration arise

A

From spherical surfaces and Larger pupils

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5
Q

What do eyedrops do to vision

A

Vision becomes blurry and should not drive

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6
Q

What type of spherical aberration is induced with accommodation and no accommodation

A

Negative spherical aberration on accommodation and Positive spherical aberration without accommodation

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7
Q

What is Coma, its effects and where on the retina it occurs

A

Off axis aberration that produces flare and non-uniform lighting on the retina, it also occurs on the fovea due to assymetry

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8
Q

What is astigmatism

A

The lack of rotational symmetry in a reference axis in at least 1 surface like the anterior cornea

Occurs when objects are more than 20 degrees off axis and is the main aberrator

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9
Q

How is central astigmatism corrected for, and what if the astigmatism isnt due to the cornea

A

Central astigmatism is corrected with lenses or LASIK. If astigmatism not from cornea then only corrected using lenses

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10
Q

Name the two loci in Astigmatism

A

T and S loci ( Tangential and Saggital )

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11
Q

Where is the T loci relative to the S loci

A

T is always left of S

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12
Q

How to quantify amount of astigmatism

A

The difference between the T and S loci power

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13
Q

Characteristics of larger glasses

A

Gathers more light but more astigmatism

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14
Q

Characteristics of smaller glasses

A

Gathers less light but reduces astigmatism

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15
Q

What size glasses are better optically

A

Smaller glasses

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16
Q

What is Field of Curvature and effects on vision

A

Off axis astigmatism where the image is formed behind the retina on an imaginary plane called the petzval surface.

It affects peripheral vision more than central vision

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17
Q

What is Distortion and effects on vision

A

Objects in the periphery and off axis known as primary monochromatic aberrations. The image lands on the Gaussian image plane so it is less likely to affect vision

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18
Q

What factor deteriorates image quality more

A

Larger pupil size deteriorates vision more than diffraction induced vision loss

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19
Q

What size pupil gives the best VA and why

A

2-3mm diameter as it is the least amount of diffraction

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20
Q

What do ocular aberrations affect

A

Image quality and pupil size

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21
Q

Which aberration affects the pupil size most significantly

A

Distortion

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22
Q

Why is Eye-Instrument alignment important

A

So accommodation and convergence does not occur when viewing binocularly and may also give more aberrations

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23
Q

What is the most common ophthalmic device and what is its purpose

A

Glasses, to have good foveal viewing at different gazes

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24
Q

What is the stop aperture in Glasses

A

The Pupil

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25
Q

What is the function of a stop aperture

A

Ensure light goes through a single point and in this case, through the pupil

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26
Q

Why is peripheral aberrations ignored

A

Due to poor peripheral resolution

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27
Q

What does Contact lenses do and compare with glasses and its optical characteristics

A

They correct vision, it differs since it rotates with the eye so off axis aberrations do not happen. Contact lens also fit well so spherical aberrations are the main aberrator

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28
Q

What replaces the crystalline lens

A

Artificial intraocular lens

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29
Q

Purpose and characteristics of an artifical intraocular lens

A

Replaces crystalline lens during cataracts, it moves with the eye so spherical aberrations is main aberrator

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30
Q

What happens if the artifical intraocular lens is decentered/tilted

A

Sagittal and Tangential power errors may occur

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31
Q

Types of chromatic aberration and how they occur

A

Longitudinal and Transverse, occurs from effects of dispersion

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32
Q

How does dispersion occur and what does it mean

A

From the varying refractive indices of different wavelengths, means that different wavelengths bend differently in the same refractive index

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33
Q

What is the relationship between refractive index and wavelength and what does it mean for the eye

A

Increasing wavelength decreases refractive index, so the eye has less power with a longer wavelength so red is focussed behind the retina in LCA

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34
Q

What is longitudinal chromatic aberration

A

Different wavelengths focus at different points along the optical axis

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35
Q

Characteristics of LCA

A

Chromatic difference of power and chromatic different in refraction

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36
Q

What is chromatic difference of power

A

The change in power with wavelength

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37
Q

What is chromatic difference in refraction

A

The difference in vergence between short and long wavelengths, this difference is greater for shorter wavelengths than longer ones

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38
Q

What is transverse chromatic aberration

A

Associated with foveal vision where different wavelengths are focussed on different parts on the retina

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39
Q

Characteristics of TCA

A

Chromatic difference in position

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40
Q

What is chromatic difference in position

A

The way to measure TCA known as the Angular measure of transverse chromatic aberration

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41
Q

How to measure Transverse chromatic aberration

A

Using chromatic magnification by comparing the image size between different wavelengths

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42
Q

Methods to measure longitudinal chromatic aberration

A

Best focus method, Vernier method, Double pass technique

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43
Q

What is the best focus method

A

A back illuminated target lit with different colours is moved back and fourth until the px says it is in focus

Can also use different power lenses instead of moving the target

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44
Q

What is the Vernier Method

A

Subjective measurement where two narrow targets of different wavelengths is imaged onto the fovea through a small aperture. There is a position where both targets seem to be aligned and a a position where the targets are aligned where the distance between the targets that seems aligned is the amount of LCA

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45
Q

What is the double pass technique

A

A narrow beam of light is formed on the fundus and it reflects some light to form an image outside the eye, a trial lens is used to minimise the width of the image formed for all colours

Very time consuming

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46
Q

What is more in focus for low accommodation

A

Longer wavelengths

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47
Q

What is more in focus with accommodation

A

Shorter wavelengths

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48
Q

Rate of LCA increase per 1D accommodation or 1D refractive error

A

Increase by 2.5% per 1D

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49
Q

How does age affect LCA

A

With age there is less accommodation so therefore less LCA

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50
Q

What CA does not affect foveal vision and what increases TCA

A

TCA

Decentered pupil and dislocated pupil causes more TCA which decreases VA

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51
Q

Why is LCA important in vision

A

Humans cant accommodate well in monochromatic light, LCA helps accommodation system, so increasing LCA has minimal effect on accommodation accuracy but lowering LCA may reduce accommodation accuracy

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52
Q

Why is chromatic aberration effects lost in vision

A

Effects of CA are lost due to the spectral sensitivies of the eye

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53
Q

What are achromatising lenses and their drawbacks

A

Removes chromatic aberrations where these lenses do not have power but has equal and opposite LCA to the eye

Does not improve VA and CS in white light so are useless for correction purposes

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54
Q

What is chromostereopsis

A

Allow objects of different colour to be seen as 3D where red appears closer than blue. This is binocularly driven where TCA is combined with BV

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55
Q

What induces more chromostereopsis

A

Larger pupils give more chromostereopsis due to more TCA

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56
Q

What colour is macular pigment

A

Yellow

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57
Q

What does macular pigment contribute towards and how it affects vision

A

The luminosity efficiency function as there is no macular pigment in the periphery so more aberrations occur there meaning less chromatic aberrations in the fovea

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58
Q

What is light

A

A transverse EM wave that transmits energy through space

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59
Q

Property of Transverse waves

A

Oscillations of magnetic and electric fields are perindicular to each other and to the direction of travel, can also be polarised to its components

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60
Q

What are harmonic waves

A

Repeating waves where its intensity is proportional to the square of the wave amplitude

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61
Q

How does a vacuum or non-vacuum environment relate to wavelength

A

Wavelength stays the same in a vacuum while in the eye the wavelength changes as it travels through it due to differing refractive indices.

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62
Q

What is superposition and what does it produce

A

The constructive and deconstructive interference of waves that produces interference patterns

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63
Q

What is the ripple tank analogy

A

2 vibrators producing circular waves in a tank, the final ripple is the sum of ripples from the vibrators

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64
Q

What is a standing wave and how is it created

A

When the ripple/wave does not seem moving at all. Produced by putting 2 vibrators half a wavelength apart and also works with light

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65
Q

How to produce coherent pattern

A

If phase difference between two monochromatic sources is constant

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66
Q

How to produce incoherent patterns

A

If phase difference between two monochromatic sources are not constant

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67
Q

Property of incoherent patterns

A

Distributes energy randomly in all directions

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68
Q

Example of coherent patterns

A

Laser pointer where the light travels in one direction and has a homogenous everage irradiance

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69
Q

What is the Youngs double slit experiment

A

light passes through a pinhole then through a double pinhole to produce coherent light. The outcome is an interference pattern of dark and bright fringes giving a standing wave

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70
Q

Youngs double slit experiment in health

A

Check visual health where two spots of coherent light passed through the pupil and fringes form on the retina where the orientation, spacing and intensity of the firnges is changed by tweaking the laser

Used on cataract px

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71
Q

What do thin films achieve

A

Produce colourful interference fringes

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72
Q

How do thin films produce colourful interference fringes

A

Due to multiple inner-thin-layer reflections

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73
Q

What does more reflections mean

A

Fringes spaces apart more where it is very dense as you move down

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74
Q

How is light lost and how do we prevent this

A

Light lost from reflections and anti-reflection coatings are used to counter this to increase transmission but a coloured pattern is produced

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75
Q

What happens to the image which more reflections

A

The image brightness is dimmer

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76
Q

How to let more wavelengths through a lens

A

Multiple anti-reflection coatings

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77
Q

Drawback of multiple anti-reflection coatings

A

More transmission of light but the lens becomes grey and becomes unfashionable and may let in UV light that damages the eye

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78
Q

What is diffraction

A

Ability of light to bend around corners where each wavefront is its own source

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79
Q

How to observe diffraction

A

Diffraction is on a small scale so to see it you need a dark envrionment since any background light will mask the diffraction

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80
Q

What is single slit diffraction

A

When monochromatic light goes through a slit of certain width and produces ripples on a screen where it is brightest in the centre

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81
Q

Slit width in relation to the diffraction pattern

A

Decreasing slit width reduced diffractioin

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82
Q

Single slit with polychromatic light

A

Centre of pattern is white and the periphery is coloured with red being the outermost

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83
Q

Why is red the outermost in the pattern

A

Longer wavelengths are diffracted the most

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84
Q

Two types of diffraction

A

Far and Near field

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85
Q

What is far field diffraction

A

When slit-screen distance is more than 20 metres

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86
Q

What is near field diffraction

A

When Slit-screen distance is closer than 20 metres

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87
Q

What does the diffraction pattern depend on in near field

A

The slit-screen distance

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88
Q

What does near field diffraction mean for vision

A

Floaters that form from debris and comes in all shapes and sizes

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89
Q

Are floaters normal

A

Yes, but too many may be an abnormality

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90
Q

What type of system is the eye

A

Diffraction limited system

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91
Q

Why is the eye a diffraction limited system

A

When all factors like refractive error, aberrations and scatter are corrected for, diffraction still exists

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92
Q

Where does the diffraction come from in the ey

A

Small pupil size

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93
Q

Pros and cons of large pupils

A

Better light and resolution but more aberrations that increases light spread so the ability to distinguish between two points diminishes

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94
Q

Pros and Cons of smaller pupils

A

Aberrations are reduced by introduced diffraction what increases PSF so blur is introduced

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95
Q

What does resolution is a diffraction system depend on, and what does mean for the resolution of the eye

A

The wavelength of the light where longer wavelengths diffraction more than shorter wavelengths.

Shorter wavelenths give better resolution due to less diffraction

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96
Q

How can pinholes be used to assess vision

A

If VA reduction is due to optical reasons like diffraction then pinholes would increase VA

If VA reduction is pathological then pinholes would not improve the VA

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97
Q

How are Halos produced

A

When white light goes through an aperture resulting in an airy type diffraction pattern

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98
Q

Halos outer and inner most colours

A

Outermost is red and inner most is blue due to diffraction

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99
Q

How are halos relevant to the eye

A

Corneal swelling from fluid deposits between stormal fibres produce halos when you look at light

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100
Q

What do halos in vision may indicate

A

High uncontrolled Intra-ocular pressure from uncontrolled glaucoma

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101
Q

How to treat swelling cornea

A

Using hypertonic saline droplets

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102
Q

What does scattering do to vision

A

Reduces image quality due to inhomogeneities in a medium

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103
Q

What does scattering depend on

A

Particle size, Particle distance and Strength of interaction between light and the particles

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104
Q

Name two types of scattering

A

Coherent and Incoherent

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105
Q

How does incoherent scattering arise

A

Due to small/medium/large particles in a medium

Large particles scatter light foward and Small particles scatter light in all dirextions

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106
Q

What happens in the wavelength is similar in size to the particle

A

Scattering in one and all directions simultaneously occur

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107
Q

Relation between particle size and wavelength

A

Smaller particles scatter smaller wavelengths more

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108
Q

Incoherent scattering examples

A

Blue sky and white smoke

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109
Q

How does multiple incoherent scattering occur

A

When particles are far from each other

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110
Q

What is single incoherent scattering

A

When there are more large particles in the liquid than small particles

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111
Q

What is Coherent scattering

A

When light has the same frequency and wavelenfth and occurs when particles are much closer than the wavelength of light

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112
Q

What happens in coherent scattering

A

Superposition occurs between the scattered waves from the particles since the medium acts as a diffraction grating

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113
Q

What does each successive scatter mean for the light

A

It adds more component to the phase shift which explains why light travels at different speeds in mediums

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114
Q

How is coherent scattering relevant

A

This is how the cornea works to maintain transparnency where the collagen fibrils are spaced apart and also wide of half the wavelength of light

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115
Q

How much of the light is scattered in the cornea

A

10%

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116
Q

What happens if the cornea swells

A

The stromal fibres space apart more than half the wavelength of light so coherent scattering is less effective so the cornea becomes less transparent

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117
Q

How is scattering relevant to the sclera

A

The collagen size and spacing is larger so incoherent scattering occures which renders the sclera to be white.

Scleral thinning makes the sclera bluer and less white due to small particle scattering

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118
Q

Scattering in the lens

A

Lens has a yellow pigment so it scatters more light. In catarcts the proteins aggregate which also scatters more light

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119
Q

Scattering in the vitreous humor

A

Has very small collagen fibers so it scatters 0.1% light. Age causes the collagen to aggregate forming fluid pockets causing more scattering

120
Q

Scattering in the retina

A

Healthy retina scatters same amount as cornea, if the retina is damaged then fluid collects in the nerve fibre layer that increases scattering. Mily gray spots on the retina would be seen

121
Q

How does the iris get its colour

A

Due to the amount of pigmemtation where less pigment gives blue eyes, more pigment means brown eyes.

122
Q

What are spatial frequencies

A

The details in an image

123
Q

What is the relation between spatial frequencies and the optical system

A

Low spatial frequencies are transmitted even when high spatial frequencies are gone where low frequencies are seen as blur.

124
Q

What is contrast and how is it affected

A

The luminance difference between an object and a background where blurry images decreases contrast

125
Q

What is MTF

A

How well an optical system can reproduce fine details and contrast with a ratioj

126
Q

What is the ratio of a perfect MTF system

A

1

127
Q

Factors that change MTF

A

Spatial frequency, photoreceptor density, refractive error, accommodation error etc

128
Q

What is the final luminance and contrast made of in a linear system

A

The sum of all components in a system
The lower frequency gives gross shape of the final wave and the higher frequencies give the smaller details on the final wave

129
Q

Why is optical system design important

A

An optical system with a high cut off frequency may not be the best to use as our visual system is not able to perceive it anyways

130
Q

What is the ideal optical system design

A

The highest frequency cut off point is similar to the human cut off point

131
Q

What type of aperture will increase MTF and why

A

Square apertures since it reduces diffraction and airy disc formation

132
Q

What effect do square apertures do to an image

A

It enhances contrast and preserves more high spatial frequencies

133
Q

Factors that affect MTF

A

Mis-focus produces blur that decreases MTF
Diffraction decreases MTF and occurs with 2mm or less pupil size

Larger pupil size induces aberrations which also decreases MTF

Longer wavelengths decline in MTF slower with larger pupils compared to shorter wavelengths

Halving pupil diamater increases VA by two times

Smaller pupil means slower MTF decline

A higher resolution of the eye also lowers the MTF and relates to the CSF

134
Q

What is an obstructed telescope

A

A telescope with a secondary mirror

135
Q

What type of telescope would you use to view high spatial frequencies

A

Obstructed telescope

136
Q

What type of telescope to use to view low spatial frequencies

A

A non-obstructed telescope

137
Q

How to calculate the MTF for a system

A

The product of the MTFs of each component in a system is the MTF for the system

138
Q

What type of filter is the eye

A

A low pass filter

139
Q

What does a low pass filter mean

A

It detects low spatial frequencies better without information loss

140
Q

Why type of neural pathway filters do we have more of

A

Low pass filters

141
Q

Function of low and high pass filters

A

Low pass filters give the shape information and High pass filters give the small details of the system

142
Q

How are refracted waves produced

A

By coherent scattering and it travels through the medium at a particular speed

143
Q

What does a larger refractive index mean

A

Light travels slower

144
Q

Which part of a refractive index graph shows opaqueness

A

The peak of the refractive index function in a medium shows which wavelength it is opaque to

145
Q

What is strabismus and what does it cause

A

Both eyes not fixing on same object and it causes diplopia

146
Q

What can treat strabismus and what are its effect on vision

A

Prisms, and they tend to have dispersive effects so you might see different colours of the object

147
Q

What is the relationship between dispersion and prism thickness

A

More dispersive with more thickness

148
Q

Property of EM waves

A

Transverse so can be polarised

149
Q

What is unpolarised light

A

Light vibrates in all planes

150
Q

Why might unpolarised light be useful

A

Makes surroundings appear to be illuminated by the same source

151
Q

What is needed to have superposition in waves

A

The wave needs to oscillate in the same plane

152
Q

How does polarisation work

A

Vertical components are absorbed so vertical light is passed through since the horizontal electrons are restricted from conduction

153
Q

What stops light from being polarised

A

Is the vertical wires are smaller than the wavelength of light

154
Q

What is an ideal polaroid

A

Lets 50% of light through, either vertical or horizontal light through, HN-50

155
Q

In reality, what occurs with the ideal polaroid

A

4% of light is reflected due to surface reflections at each surface so in total there are 8% reflections so the ideal polaroid is now known as HN-42, so only 42% of light is let through

156
Q

How to prevent all light from being transmitted

A

If the two polaroids are oriented perpendicularly

157
Q

3D glasses polarisation features

A

Has clockwise and anticlockwise circular polarisation so each eyes sees differently for a 3D view

158
Q

How is polarisation relevant to vision

A

Gets rid of reflections and improves vision quality and used in glasses

159
Q

What is birefringence and how does it happen

A

Effect of double refraction which splits beam into two paths where each beam travels at a different speed and has its own refractive index. This is due to anisotropia

160
Q

What is an anisotropic material

A

Where the medium has a different refractive index in different directions

161
Q

What is a factor that refractive index depends on

A

Absorption strength for V and H polarised light

162
Q

What is retinal birefringence scanning technique

A

Detects central fixation of the eye that is not used and can detect strabismus. This is non-invasive but needs patient cooperation.

163
Q

How is light lost in the eye

A

Reflection, Scatter and Absorption. So not all light entering the eye reaches the retina

164
Q

What is the problem with too much light

A

Eye damage including UV

165
Q

Factors changing eye light sensitivity

A

Time of day changes sensitivity like photopic and scotopic vision. Scotopic is sensitive to short wavelengths and photopic is sensitive to longer wavelengths

166
Q

What drives photopic vision

A

Cones

167
Q

What contributes to the luminousity efficiency function

A

All three cones

168
Q

Who has CVD

A

People with one or two missing types of cones

169
Q

Factors that change sensitivity to light and how

A

Sensitivity to blue light decreases with age due to absorption of blue from cataracts, less blue light reaches retina

170
Q

Which photoreceptor should have less variability in performance for scotopic vision

A

Rods, since there is only one type of it

171
Q

What is luminous flux

A

Measured in lumens and is the density of light

172
Q

What is Luminous Efficacy

A

Ratio of lumens to watts

173
Q

What is luminous intensity

A

Measured in Candelam it is the luminous flux in a direction

174
Q

What is luminance

A

Objective brightness measure of a light source in cd/m^2

175
Q

What is illuminance

A

Luminous flux density on a surface in Lumens/m^2

176
Q

What is the threshold to detect light on retina

A

Luminous flux

177
Q

How is increased sensitivity to light achieved

A

Light falls on many photoreceptors and spatial summation occurs which enhances sensitivity to dim light

178
Q

What does the threshold depend on

A

The amount of light collected by the retina

179
Q

What does stimulus size mean for sensitivity

A

Larger stimulus means spatial summation is less critical as more photoreceptors are excited. So the amount of retinal illuminance is more important than luminous flux

180
Q

Luminance vs Illuminance

A

Brightness observed is luminance, The brightness of a particular area is illuminance

181
Q

How is light lost in the eye

A

specular reflection, absorption and scatter

182
Q

Where does specular reflection mainly occur

A

In the cornea

183
Q

Why is specular reflection useful and its drawbacks

A

Can find radius of curvature of the cornea but makes it hard to assess due to glare

184
Q

Effects of light transmission on different parts of the eye

A

Transmission decreases in the cornes with age

Aqueous humor transmittance is maintained throughout life

Lens decreases transmittance of shorter wavelengths with age from cataracts

Vitreous transmittance not affected by age

185
Q

How is eye tissue damaged from light

A

Water in the eye has strong absorption to longer wavelengths causing heating of water resulting in thermal damage.

UV strongly absorbed by the lens causing damage

Lasers are absorbed by different parts of the eye

186
Q

Effects of scatter in the lens

A

Lens is made of more cells than cornea so it has more scatter with age. Cataracts cause scatter due to less transparency from protein aggregation increasing inhomogenity and anisotropy in the lens

187
Q

What is fluorescence

A

Absorption of wavelength and re-emitting at a longer wavelength

188
Q

What type of emission is fluorescence and does it affect vision

A

Isotropic where energy is emitted in all directions causing veiling glare onto the retina for people with cataracts decreases VA at low contrasts

189
Q

What fluoroscent compounds are in the lens

A

Tryptophan and two fluorogens

190
Q

What is intrinsic Birefringence

A

Difference refractive indices in different directions
Refers to properties of a medium

191
Q

What is form birefringence

A

Biological structures have less order but have more order at a structual level
Refers to structural order of the medium

192
Q

Why does the eye exhibit birefringence

A

The collagen arrangement which affects light transmission and polarisation

193
Q

Birefringence in the cornea

A

Fibrils are cylindrical and regularly spaces, but each layer is at a large and different angles so birefringence occurs

If light is incident perpendicularly then no birefringence occurs due to alot of layers and orientations so it cancels the birefringence

194
Q

Birefringence in the Lens

A

Fibres are radial so the birefringence is different for the lens and the cornea

195
Q

What are the 4 pigments in the eye

A

Macular pigment, Visual pigment, Melanin in Pigmented epithelium and Haemoglobin in the choroid

All absorbs light

196
Q

Light pathway in the retina ( Basic ) and retinal characteristics

A

Hits retina and reflected in the Inner limiting membrane. 6 layers between ILM and photoreceptors are transparent

197
Q

What is the macular pigment

A

Xanthophyll

198
Q

Recall the efficiency of light in the visual pathway

A

92% light enters the eye and macular pigment absorbs some meaning 53% of the light reaches the cones outer segment, only 67% of this light absorbed by the visual pigments go into a photochemical reaction

199
Q

What is the retinal efficiency

A

12%

200
Q

How much of incoming light goes to the brain as a signal

A

7%

201
Q

Light absorption in the layers of the eye

A

Absorption occurs in the pigmented epithelium, some light passes through and enters choroid which has Hb so absorbs short wavelength light ans scatters long wavelengths.

Rest of the light is absorbed by the sclera

202
Q

Why is retina reflex red

A

Most absorption is in the shorter wavelengths

203
Q

What colour visualises axons better

A

Blue

204
Q

Does retinal birefringence affect vision

A

No

205
Q

Where is retinal birefringence used

A

In scanning laser polarimetry

206
Q

What is scanning laser polarimetry

A

Estimates nerve fibre loss due to glaucoma

207
Q

How does scanning laser polarimetry work

A

Red lasers polarised and passed into retina and scatters deep in retinal layers, the choroid reflects and passes back out of the eye

Polarisation plane is changed twice through the nerve fiber layer, amount of polarisation gives nerve fibre layer thickness

Thicker retina means more polarisation

208
Q

What does a thinner retina indicate

A

Glaucoma

209
Q

Effect of age on corneal stroma

A

Decreased interfibrillar space and increased crossed sectional area of the collagen

210
Q

Other age effects on scatter

A

Increased endothelial degeneration and AH entering cornea increasing scatter

211
Q

What light transmission decreases with age an due to what reason

A

UV and Visible light transmission decreases due to cataracts with age

212
Q

How does the lens change with age and its relation to anterior chamber depth

A

Lens increases in volume and mass with age and due to increased axial thickness in the cortex. The anterior chamber depth decreases at the same rate as the lens axial length change

213
Q

What is the lens paradox

A

Becomes more myopic with age from lens thickening, after 30 goes more hyperopic as lens becomes less curved

214
Q

What happens to the eye after age 70

A

Shifts back to the myopic direction and introduces astigmatism from corneal shape change

215
Q

What is presbyopia

A

Decreased ability to accommodate with age, occurs around age 40

216
Q

General trend with presbyopia

A

People lose accommodation in their 50s before any physiological functions are affectd

217
Q

Two theories of presbyopia

A

Lenticular and Extra-Lenticular

218
Q

What is lenticular theory

A

Change in mechanical or geometrical change of lens and its capsule, assues lens hardens with age and maintained ciliary muscle strength

219
Q

What is Hess-Gullstrands theory

A

Zonule slackness means lens is more gravity influenced for older people

220
Q

What is Fincham theory

A

Lens capsule gets more rigid with age so more pressure is applied on the lens, ciliary muscle works harder to over come capsule resistance

221
Q

What is Geometric theory

A

Increasing lens curvature with age causes change in lens orientation so zonules apply tension less radially so zonule relaxation does not change lens shape

222
Q

What is the Extra-Lenticular theory

A

Change in ciliary muscle or elastic compoments of the zonules with age

223
Q

What is senile miosis

A

Pupil diamater decrease with age

224
Q

What also decreased with age regarding the pupil

A

Pupillary reaction speed

225
Q

What age is max pupil size

A

Teenage years

226
Q

What does senile miosis mean for MTF

A

MTF decreases with age due to senile miosis due to reduce ocular transmittance from smaller pupils

227
Q

What is Abbe value

A

The dioptric difference between blue and red convergence points in LCA, shows amount of chromatic aberration for a material in the lens

228
Q

What does a higher Abbe value mean

A

Means less chromatic aberration

229
Q

What is transverse chromatic aberration

A

Difference between image magnification between blue and red images

230
Q

What does the prismatic effect of lenses depend on

A

Distance form optical axis of the lens

231
Q

Where is there more prism on a lens

A

The periphery

232
Q

What does this mean in real life

A

We see through the centre of glx in real life so prismatic effects are less noticeable, but if lens is decentered then this effect is amplified

233
Q

Is prismatic effect more or less in the periphery of a high power lens

A

More

234
Q

What is an achromatic lens and how is it made

A

Does not have chromatic aberration where all wavelengths are focussed onto a point

Combining a plus lens of one material with a minus lens of a different material where the sum of the powers is the correction needed to focus the blue and red light

235
Q

What factors are taken into place for lens designs

A

Vertex distance, lens thickness, refractive index and back lens surface power

Aims to correct spherical aberrations, Coma, Astigmatism and Distortion

236
Q

What is spherical aberration

A

the power difference between periphery and centre of lens and it On Axis

237
Q

Function of the pupil

A

eliminate peripheral rays to reduce spherical aberration

238
Q

What is Coma

A

When object is off axis and a magnification difference between rays going through difference zones of a lens

239
Q

What images do Coma produce

A

Comet shaped images where the point of the comet points towards the optical axis. This aberration is not an issue for the human eye

240
Q

What is oblique astigmatism

A

Light focus at T and S points, difference between T and S is amount of astigmatism

241
Q

How can oblique astigmatism be induced with lenses

A

When lens is tilted so objects that were on axis are now off axis

242
Q

What are wrap around lenses and its function

A

A Prescribed protection glasses that protect from wind and sunlight, and common for sports use

243
Q

Problem with wrap around lenses

A

Tilting glasses changes its power, and changes its astigmatism correction

244
Q

Considerations when designing wrap around lenses

A

Assume lens is used tilted, so we tilt lens in direction light is coming from. Base in prisms used to retain BSV

245
Q

What are sports vision trainers

A

Glasses made of curved plastic LCD lenses where it switches between clear and blocked to train athletes to anticipate whats coming in the periphery and helps with hypersensitive motion cues

246
Q

Field of curvature aberration in lens design

A

Occurs even when lens made astigmatism free. The dioptric difference between where the image is focussed and where is should focus is the Image Shell Error

247
Q

Why should we use. the manufacturers base curves

A

To reduce oblique astigmatism and power errors

248
Q

What is Distortion

A

Difference in magnification in different areas of the lens periphery compared to the centre of the lens. Magnification increases in periphery for positive lenses which increases image size

249
Q

What are multifocals

A

Allows clear vision at many distances

250
Q

Who to prescribe multifocals and how to design the lens

A

Presbyopic patients, top portion of lens powerless and bottom portion with a plus lens for near vision

251
Q

What is Near Addition ( Near Add )

A

The additional power needed on top of distance correction for near distance viewing in the lower portion of the lens

252
Q

How does near addition work with astigmatism

A

Sum the sphero correction with the near addition, everything else stays the same

253
Q

What is a trifocal lens

A

These have an intermediate portion for normal viewing distances between the distance and near correction

254
Q

What is generally the intermediate prescription

A

Half the near add

255
Q

When to use tri-focals

A

When you cant accommodate well or if the add is more than +1.50D

256
Q

When would you not use a tri-focal lens

A

If you can accommodate then you can see intermediate objects if you see through the distance correction and then accommodating

257
Q

3 Ways Bi/Tri-focals are made

A

Fused, One-Piece and Cemented

258
Q

What is a fused Multi-focal lens

A

Only available in glass, the segment is made from a higher refractive index glass compared to the distance lens

259
Q

What is a one-piece multi-focal lens

A

Made from glass or plastic, the change in segment kens power due to the change in lens curvature. The One-piece multifocals are found by feeling the segmented border

260
Q

What is a cemented lens

A

Custom made lens with a glued small segment on the distance lens

261
Q

Types of Bi-focal designs

A

Round segments, Flat-top segments, curve top, panoptik, Franklin and Executive style

( Also available tri-focals )

262
Q

What are occupational lenses

A

Lens chosen carefully and positioned for special viewing conditions

263
Q

Common occupational lens designs

A

Double D segment, Quadrafocal and Rede-rite

264
Q

Who are Double D segment lenses for

A

Those who need intermediate and near viewing like plumbers, the lens recreates workers situations to get the working distance to get the upward power

265
Q

What is a Quadrafocal lens and who is it for

A

A double D segment with a flat top trifocal on the bottom and an upside down flat-top segment on the top, for those who need both trifocal and double segment lenses and has 4 viewing areas

Only available in glass material

266
Q

What are Rede-rite lenses and who is it for

A

An Upcurve bi-focal and has a large round segment at the top. For those who want a segmented lens and need a full, near working area. They also want to see clear in the distance wihtout taking glasses off

267
Q

Where is more prism effect seen in a lens

A

Towards the periphery away from the optical centre

268
Q

What is image jump

A

Sudden change in displacement of the image due to the sudden change in prism between the distance and the near correction

269
Q

What are progressive lenses

A

Multifocals that look the same as single vision lenses but has clear viewing at all distances without any lens lines in bi/tri-focals

Power of lens changes gradually and eliminates image jump but may be hard for some people to use

270
Q

Who needs bifocals sooner, Hyperopes or Myopes

A

Contact lens hyperopes do not need bifocals sooner than contact lens myopes as the spectacles is the reason why there is an accommodation difference

Hyperopes can accommodate to near, Myopes cannot accommodate to near

271
Q

What consideration is needed when prescribing occupation lenses

A

The working distance of the person needs to accommodate the same as they did their normal glasses

272
Q

Conditions when designing contact lenses

A

Need to sit well on the cornea with minimal lateral movement so reduce asymmetric aberrations

Diamater of optic zone needs to be more than the pupil size in all lighting conditions

273
Q

What will be visible to a patient after LASIK

A

Halos

274
Q

Pros and Cons of soft contact lenses

A

Shapes themsleves so back surface resembles shape of anterior corneal surface for a good fit but is less well at correcting astigmatism

275
Q

Why my soft contact lenses have on-eye power changes

A

Flexibility and hydration variations
The flexibilty means that the contact lens changes curvature and thickness so on-eye power changes
Changes to hydration means refractive index also changes causing on-eye power change

Eye has higher temperature so it may change the hydration and therefore refractive index

276
Q

What is checked for soft contact lenses before insertion

A

The back vertex power checked at room temperature in a fully hydrated state

277
Q

How much can hydration change in a contact lens

A

Can drop by 5-10% after 1-2 minutes of wear

278
Q

Thinner vs Thicker contact lens hydration equilibrium

A

Thinner lenses reach equilibrium in 5 minutes, thicker and high power lenses can keep dehydrating after 30 mins of insertion which the time depends on the material

279
Q

Why causes contact lenses to dehydrate more

A

High water content, it reaches equilibrium faster than low water content soft contact lenses of the same thickness

Can also be due to lots of near work as dehydration occurs due to less blinking

280
Q

Are power changes more significant for negative or positive lenses and why

A

Positive lenses and they are thicker

281
Q

Contact lens design and its effects on the eye

A

Well centred contacted lenses with steep surfaces introduce spherical aberrations. The eye and the contact lens moves together so spherical aberrations are induced.

282
Q

How are transverse chromatic aberrations induced in glasses, how is this different in contact lenses

A

The prism effects during oblique viewing due to the airgap between the glasses and the eyes, contact lenses do not have this gap so it reduced TCA

283
Q

Considerations for contact lenses to be mass produced

A

The spherical aberration for everyone should be similar, the spherical aberration is the major aberrator, spherical aberration should be the same off eye and any decentration of the contact lenses should not introduce any significant aberrations

284
Q

Spherical aberration in relation to age

A

Spherical aberration becomes more negative with age , this affects myopes more

Spherical aberration increases with age

285
Q

What is A^opt

A

The optical lens surface power adjustment needed to minimise spherical aberration

286
Q

Problems with correcting spherical aberration

A

Chromatic aberration and astigmatism still exist

287
Q

When is astigmatism generally corrected for

A

When is exceed 0.75D

288
Q

What still exists regardless of large pupils in mesopic and scotopic conditions

A

Monochromatic aberrations

289
Q

Characteristics of Rigid contact lenses

A

Back surface of the Contact lens maintains shape so on eye power is the same

The overall optical system giving the final image is based on the rigid contact lens, Tear lens and the eye itself

290
Q

What does the power of the tear lens depend on

A

The geometry of the optic zone at the back surface of the rigid contact lens and the anterior surface of the cornea

291
Q

How does the power of the tear lens change with the steepness of the back optic zone radius ( BOZR ) and how to counter this

A

Power increases by +0.25D for each 0.05mm the BOZR steepens, so the back vertex power of the rigid contact lens needs to be changed by -0.25D per 0.05mm the BOZR steepens to compensate

292
Q

How to find the BOZR

A

Trial lenses used to give BOZR which gives fit for a particular lens design. The power is the sum of the back vertex power of the trial lens and the over-refraction.

293
Q

What is the over-refraction for

A

Determines additional power needed with the trial lens to give clear vision

294
Q

Where do contact lens correct astigmatism for

A

For the cornea and not the lens

295
Q

How does VA change with contact lens power

A

VA gets worse with contact lens powers of more than +3.00D where the aberration from the cornea is much less than the aberrations introduced by the contact lens

296
Q

Contact lens power change considerations with high and low order aberrations

A

Px w high-order aberrations wearing CL will reduce aberrations
Px w low-order aberrations wearing CL might increase the aberrations

OPTOM (8 decks)

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