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Optometry Year 1 > OV1014 Visual Optics and Measurement techniques > Flashcards

OV1014 Visual Optics and Measurement techniques Flashcards

1
Q

What is Ametropia?

A

Abnormal refractive eye condition (myopia, hyperopia or astigmatism) which images fail to focus on the retina

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2
Q

What is vision?

A
  • Perception of difference in the external world resulting from stimulation of the retina by light
  • Smallest line of letter read with the unaided eye
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3
Q

Why assess vision?

A

Quantify what people can and cannot see

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4
Q

What were Snellen’s innovations?

A
  • Special characters (optotypes/test types)
  • Letter chart to test distance vision
  • Defined ‘the standard observer’ for comparison
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5
Q

In a snellen chart what do the number by each line represent?

A

The letter size in meters
The distance at which a letter height subtends 5’ arc
or
The gap/limb width subtends 1’ arc

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6
Q

Why is ‘serif’ font used in snellen charts?

A

Helps readability in large sections of texts

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7
Q

How can you increase reliability of tests for Snellen charts?

A
  • Include one or more difficult letters on each line

- Is ‘common confusion pairs’ (UV, FP. HN)

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8
Q

What is the progression of letter in Snellen chart?

A

No common ration so no geometric progression

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9
Q

What is the letter/line spacinging Snellen charts?

A
  • Letter width - 4 units

- 20 mm between successive lines

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10
Q

What is the standard observer?

A

Visual acuity that just allows him/her to identify a letter with a stroke width subtending 1’ arc

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11
Q

What is form vision?

A

The ability to recognise an object from its form

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12
Q

What is resolution?

A

The ability to detect fine detail

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13
Q

What requires both resolution and form vision?

A

Recognition of letters as they get progressively smaller

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14
Q

What is visual acuity?

A

The smallest line of letters that can be read with spectacles or contact lenses worn ( provided all defects of focus are corrected)

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15
Q

What is the minimum angle of resolution?

A

Angular separation of two points/lines at the resolution acuity limits

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16
Q

What is the MAR for the standard observer?

A

1’

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17
Q

What to do if VA is below 6/60?

A
  • Reduce distance to chart until Px can read top letter - 3/60
  • Counting finger - Record CF at 25 cm if Px can count
  • Hand movements - Record if Px can detect your HM
  • Perception of light - Record if Px can detect PL ad which quadrant light is in
  • No perception of light - Record NPL if patient cannot detect light
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18
Q

What are the advantages of Snellen Chart?

A
  • Quick and Reliable
  • Sensitive to ametropia
  • Narrow
  • Less affected by guessing compared to grating acuity etc
  • Enshrined in legislation
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19
Q

What are the limitations of Snellen Chart?

A
  • Unequal number of letters per line, so tasks in not equivalent at differing levels of VA
  • Non-uniform change in letter size for successive lines
  • Line by line scoring
  • Fixed row spacing
  • Unequal letter spacing, creates crowding
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20
Q

What are the features of LogMAR chart?

A
  • Same number of letters per line (5)
  • Letter of similar legibility chosen
  • Approx equivalent task at each level
  • Uses a fixed raitio between each line (0.1 Log unit)
  • Each letter represents 0.02 LogMAR unit
  • Letter spacing = letter width
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21
Q

What are Geometrical optics?

A
  • That rays can be represented by straight lines along which light propagates
  • Assumes light travels as rays in homogeneous medium
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22
Q

What are wavefronts?

A
  • Surfaces of constant phase or more easily constant optical distance from the surface
  • Perpendicular to the rays
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23
Q

What is the quantity of reduced vergence inked to ?

A

Curvature of wavefronts

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24
Q

What does vergence tell us?

A

How rapidly the wavefronts are converging or diverging

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25
Q

What do axial objects/images show?

A

the location of the object/image

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26
Q

What do off axial objects/images show?

A

The height of the object/image

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27
Q

What is the thin lens approximation?

A
  • Light does not converge/diverge significantly between the surface
  • Refraction effects can be added
  • Represent lens as one surface
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28
Q

What happens to light for a distant object?

A

Becomes less divergent as it strikes the lens

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29
Q

What is light from an infinitely distant object represented by?

A

Parallel rays

L= 0

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30
Q

Where does an image form from an infinitely distant object?

A

F (Back focal point)

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31
Q

What is F’?

A

A point which is conjugate with an object at infinity

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32
Q

Where does an image form when an object is placed at F ?

A

Infinity

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33
Q

What does surface power depend on?

A

Radius of curvature and difference in refractive index

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34
Q

What is the fovea centralis?

A

Most sensitive point on the retina

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35
Q

Where does visual axis pass?

A

Passes through pupil and hits fovea centralis

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36
Q

What is angle alpha?

A

Angle between the visual and optical axis

37
Q

Why does light fall on fovea centralis?

A

To use best visual acuity

38
Q

What do we approximate for a reduced emmetropic eye?

A

Cornea as one surface
Lens as a homogeneous n
Fe = +60D
n = 4/3

39
Q

What is optic neuritis?

A

Px is at risk to multiple sclerosis

40
Q

Why do people carry out index test?

A

Quicker / easier than diagnosis
Cheaper
Invasive

41
Q

What is the bench mark test called ?

A

Gold Standard/ Reference standard

42
Q

What does the light leaving the negative lens depend on?

A

The vergence of the incident light

43
Q

What are the two types of ametropia?

A

Spherical ametropia

Astigmatism

44
Q

Define Emmetropia?

A

Second focal point Fe’ coincides with the retina for an unaccommodated eye

45
Q

Where is the far point for an emmetropic eye?

A

Infinity

46
Q

Define Far point Mr?

A

Object point conjugate with the retina in the unaccommodated eye

47
Q

Define Myopia?

A

Second focal point Fe’ falls in front of the retina for an unaccommodated eye

48
Q

Where is the Mr in Myopic eye?

A

In front of the retina - real point

49
Q

What is the effect of myopia?

A

Blurred vision
Objects closer to the accommodated eye can be seen clearly
Anything beyond Mr would not be sharply in focused

50
Q

What is ocular refraction

A

The vergence of light striking a reduced eye from the far point.

51
Q

When a contact lens is needed to correct ametropia what is the ocular refraction

A

The thin lens power

52
Q

When are L and L’ equal to K and K’?

A

When the image is formed at the retina and the object is at the far point

53
Q

Define hypermetropia?

A

Second focal point (image of a distant object) falls behind the retina in the unaccommodated eye

54
Q

Where is the MR located in a hypermetropic eye

A

far point (MR) lies at behind the eye

55
Q

Define Accommodation

A

Increase in power of the eye with an effort to focus at near

56
Q

What happens in accommodation

A

Lens changes to increase its dioptric power

57
Q

When does eye have its weakest power

A

Unaccommodated eye

58
Q

What occurs in presbyopia

A

Accommodation declines with age

Ability to focus on near objects affected unless myopic

59
Q

Define Presbyopia

A

Condition where accommodation is insufficient for near work

60
Q

What occurs in accommodation in hypermetropia

A

Increasing the dioptric power of the eye can place the optical image on the retina

61
Q

What causes spherical ametropia

A

Failure in co-ordination during growth of the optical component eye
Ametropia larger that +_4 - eye is too long or short
Pathology

62
Q

When is a negative lens required

A

When the second focal point of the spectacle lens Fsp coincides with MR

63
Q

What happens when ray incident on the eye appear to come from the MR

A

Forms a sharp image on the retina

64
Q

When is a positive lens required

A

Second focal point of spectacle lens Fsp’ coincide with MR

65
Q

When does the vergence remain the same

A

Parallel lines travelling from infinity

66
Q

what is spectacle effectivity

A

Effective power of a spectacle lens at another plane (usually the eye). It is caused by the change in reduced vergence as the light propagates from the lens

67
Q

Whats the effectivity for myopes

A

Magnitude of Fsp is higher than K

68
Q

Whats the effectivity for hypermetropes

A

Magnitude of K is higher than Fsp

69
Q

Define transverse magnification

A

Ratio of image height to object height

70
Q

What is the magnification in the eye

A

Eye is a high positive powered optical system
Magnification is negative so image is inverted
Magnification is smaller than 1 so retinal image is much smaller than the object

71
Q

Define Nodal points N and N’

A

Conjugate points along optical axis of unit angular magnification

Coincident for a single surface

72
Q

Define principal points

A

Conjugate points along the optical axis of unit transverse magnification

73
Q

Where are the principal points for a single surface/think lens

A

Always at the surface/lens

74
Q

Define blur ratio

A

Ratio of the blur circle diameter to the basic blurred retinal image height

75
Q

Does it matter whether blur circle is +ve or -ve

A

No

76
Q

What is blur analysis based on

A

Geometrical optics where in an ideal system points are imaged as points

77
Q

What does the Pinhole do

A

Artificially reduces the pupil size significantly decreasing the blur circle

78
Q

What is the effect of Pinhole

A

Reduces light entering the eye
Improves vision if blurring is significant
Reduces light level adversely affecting vision
Increases diffraction if vision is already good (possibly decreasing vision)

79
Q

If the vision improves with pin hole what does this mean

A

Indication that the deficit is due to retinal image blur

80
Q

Define basic blurred image height

A

The height of the principal ray at the retina

81
Q

What does letter recognition depend on

A

The size of the blur circle and blur ratio

82
Q

Define projected blur

A

The retinal blur circle projected back on to the object.

83
Q

What is important about the object when referring to the projected blur super imposed

A

It will only be a scaled version of the image

Can be used for comparison with an experiment of perceived blur

84
Q

Why may the actual blur perceived be different fro our values?

A

Edges of real blurred objects are unclear

Objects often appear to have structure possibly from diffraction in the eye

85
Q

What does blur ratio and projected blur provide?

A

First approximation to understanding letter recognition with blur present

86
Q

What does it mean when the angle subtended by h1’ at eh eye (u) is less than w

A

The retinal image is smaller than in the uncorrected eye

87
Q

Define Spectacle Magnification

A

Ratio of the retinal image height in the corrected eye to the basic blurred retinal image height in the uncorrected eye

88
Q

What happens to the retinal image for spectacle corrected myopes

A

Gets smaller

89
Q

What happens to the retinal image for spectacle corrected hypermetropes

A

Gets larger

Optometry Year 1 (5 decks)

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