OV1014 Visual Optics and Measurement techniques Flashcards
(89 cards)
1
Q
What is Ametropia?
A
Abnormal refractive eye condition (myopia, hyperopia or astigmatism) which images fail to focus on the retina
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
3
Q
Why assess vision?
A
Quantify what people can and cannot see
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
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
6
Q
Why is ‘serif’ font used in snellen charts?
A
Helps readability in large sections of texts
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)
8
Q
What is the progression of letter in Snellen chart?
A
No common ration so no geometric progression
9
Q
What is the letter/line spacinging Snellen charts?
A
- Letter width - 4 units
- 20 mm between successive lines
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
11
Q
What is form vision?
A
The ability to recognise an object from its form
12
Q
What is resolution?
A
The ability to detect fine detail
13
Q
What requires both resolution and form vision?
A
Recognition of letters as they get progressively smaller
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)
15
Q
What is the minimum angle of resolution?
A
Angular separation of two points/lines at the resolution acuity limits
16
Q
What is the MAR for the standard observer?
A
1’
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
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
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
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
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
22
Q
What are wavefronts?
A
- Surfaces of constant phase or more easily constant optical distance from the surface
- Perpendicular to the rays
23
Q
What is the quantity of reduced vergence inked to ?
A
Curvature of wavefronts
24
Q
What does vergence tell us?
A
How rapidly the wavefronts are converging or diverging
25
What do axial objects/images show?
the location of the object/image
26
What do off axial objects/images show?
The height of the object/image
27
What is the thin lens approximation?
- Light does not converge/diverge significantly between the surface
- Refraction effects can be added
- Represent lens as one surface
28
What happens to light for a distant object?
Becomes less divergent as it strikes the lens
29
What is light from an infinitely distant object represented by?
Parallel rays
| L= 0
30
Where does an image form from an infinitely distant object?
F (Back focal point)
31
What is F'?
A point which is conjugate with an object at infinity
32
Where does an image form when an object is placed at F ?
Infinity
33
What does surface power depend on?
Radius of curvature and difference in refractive index
34
What is the fovea centralis?
Most sensitive point on the retina
35
Where does visual axis pass?
Passes through pupil and hits fovea centralis
36
What is angle alpha?
Angle between the visual and optical axis
37
Why does light fall on fovea centralis?
To use best visual acuity
38
What do we approximate for a reduced emmetropic eye?
Cornea as one surface
Lens as a homogeneous n
Fe = +60D
n = 4/3
39
What is optic neuritis?
Px is at risk to multiple sclerosis
40
Why do people carry out index test?
Quicker / easier than diagnosis
Cheaper
Invasive
41
What is the bench mark test called ?
Gold Standard/ Reference standard
42
What does the light leaving the negative lens depend on?
The vergence of the incident light
43
What are the two types of ametropia?
Spherical ametropia
| Astigmatism
44
Define Emmetropia?
Second focal point Fe' coincides with the retina for an unaccommodated eye
45
Where is the far point for an emmetropic eye?
Infinity
46
Define Far point Mr?
Object point conjugate with the retina in the unaccommodated eye
47
Define Myopia?
Second focal point Fe' falls in front of the retina for an unaccommodated eye
48
Where is the Mr in Myopic eye?
In front of the retina - real point
49
What is the effect of myopia?
Blurred vision
Objects closer to the accommodated eye can be seen clearly
Anything beyond Mr would not be sharply in focused
50
What is ocular refraction
The vergence of light striking a reduced eye from the far point.
51
When a contact lens is needed to correct ametropia what is the ocular refraction
The thin lens power
52
When are L and L' equal to K and K'?
When the image is formed at the retina and the object is at the far point
53
Define hypermetropia?
Second focal point (image of a distant object) falls behind the retina in the unaccommodated eye
54
Where is the MR located in a hypermetropic eye
far point (MR) lies at behind the eye
55
Define Accommodation
Increase in power of the eye with an effort to focus at near
56
What happens in accommodation
Lens changes to increase its dioptric power
57
When does eye have its weakest power
Unaccommodated eye
58
What occurs in presbyopia
Accommodation declines with age
| Ability to focus on near objects affected unless myopic
59
Define Presbyopia
Condition where accommodation is insufficient for near work
60
What occurs in accommodation in hypermetropia
Increasing the dioptric power of the eye can place the optical image on the retina
61
What causes spherical ametropia
Failure in co-ordination during growth of the optical component eye
Ametropia larger that +_4 - eye is too long or short
Pathology
62
When is a negative lens required
When the second focal point of the spectacle lens Fsp coincides with MR
63
What happens when ray incident on the eye appear to come from the MR
Forms a sharp image on the retina
64
When is a positive lens required
Second focal point of spectacle lens Fsp' coincide with MR
65
When does the vergence remain the same
Parallel lines travelling from infinity
66
what is spectacle effectivity
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
Whats the effectivity for myopes
Magnitude of Fsp is higher than K
68
Whats the effectivity for hypermetropes
Magnitude of K is higher than Fsp
69
Define transverse magnification
Ratio of image height to object height
70
What is the magnification in the eye
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
Define Nodal points N and N'
Conjugate points along optical axis of unit angular magnification
Coincident for a single surface
72
Define principal points
Conjugate points along the optical axis of unit transverse magnification
73
Where are the principal points for a single surface/think lens
Always at the surface/lens
74
Define blur ratio
Ratio of the blur circle diameter to the basic blurred retinal image height
75
Does it matter whether blur circle is +ve or -ve
No
76
What is blur analysis based on
Geometrical optics where in an ideal system points are imaged as points
77
What does the Pinhole do
Artificially reduces the pupil size significantly decreasing the blur circle
78
What is the effect of Pinhole
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
If the vision improves with pin hole what does this mean
Indication that the deficit is due to retinal image blur
80
Define basic blurred image height
The height of the principal ray at the retina
81
What does letter recognition depend on
The size of the blur circle and blur ratio
82
Define projected blur
The retinal blur circle projected back on to the object.
83
What is important about the object when referring to the projected blur super imposed
It will only be a scaled version of the image
| Can be used for comparison with an experiment of perceived blur
84
Why may the actual blur perceived be different fro our values?
Edges of real blurred objects are unclear
| Objects often appear to have structure possibly from diffraction in the eye
85
What does blur ratio and projected blur provide?
First approximation to understanding letter recognition with blur present
86
What does it mean when the angle subtended by h1' at eh eye (u) is less than w
The retinal image is smaller than in the uncorrected eye
87
Define Spectacle Magnification
Ratio of the retinal image height in the corrected eye to the basic blurred retinal image height in the uncorrected eye
88
What happens to the retinal image for spectacle corrected myopes
Gets smaller
89
What happens to the retinal image for spectacle corrected hypermetropes
Gets larger
