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Visual impairement > optics of low vision aids > Flashcards

optics of low vision aids Flashcards

1
Q

what is the definition of magnification of LVAs

A

Ratio of the retinal image size with a magnifier to the retinal image size without a magnifying device

M = h’ (with) / h’ (without)

h’ values not accessible but not necessary as at back of eye

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2
Q

what does magnification do to the size of the object for a presbyope

A

increases it

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3
Q

what can you not do with the object for a presbyope

A

move the object closer

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4
Q

which 2 ways do different optical devices for a larger image of the object

A
  • Larger image of a real (finite) object

- Increased angular subtense of a distant object

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5
Q

list 5 things that can provide a increasing object size

A
  • Large text books (1.5 – 2.5x)
  • Most companies will provide large print versions of their literature from bills to playing cards
  • Large TV screens
  • Adjust colour, size of text on computer screen – accessibility options
  • Magnifier software for mobiles and PCs
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6
Q

what magnification is CCTV/video magnifiers based on

A

relative size magnification

M = Linear size of image on screen/Linear size of object (what is on the page)

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7
Q

what is the available mag range of a CCTV

A

2-70x

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8
Q

what is the price range of a CCTV

A

£1300 - £3000

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9
Q

what is the price range of a portable CCTV

A

£300 - £600

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10
Q

what can tablet PCs do to an image/text

A

display it at greatly enlarged sizes

can scroll it down with your finger

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11
Q

what is an advantage of a tablet PC

A

reasonably light to hold

but can get heavy after a long time

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12
Q

what magnification is bringing an object closer based on

A

relative distance magnification

M = old distance/new distance

e.g. 40cm/10cm = 4x

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13
Q

what is required when bringing an object closer and why

A

accommodation or add to help focus

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14
Q

what does reducing the distance do for a fixed size object e.g. text

A

increases the angle subtended and hence the retinal image size

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15
Q

how is the magnification formula: M = F/4 established

M - required for text

A
  • The “old distance” is taken as –25cm to fix the baseline
  • The magnification is 1 if the object distance is -25cm

+4D of accommodation or a +4D ‘add’ needed
- The magnification formula is therefore

M = F/4

4 is always as 25cm

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16
Q

how do you modify the magnification formula: M = F/4 for a persons habitual reading distance, if it isn’t at 25cm

A

e.g. if px reads at 40cm, then

M = F/2.5

(100/40 = 2.5)

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17
Q

nominal magnification is defined in ISO _________

A

nominal magnification is defined in ISO 15253

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18
Q

what is nominal magnification the product of

A

reference seeing distance in metres multiplied with equivalent power

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19
Q

what is the agreed value for reference seeing distance when calculating nominal magnification

A

25cm (is a baseline)

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20
Q

at what power is a lens designed as a spectacles magnifier to be used

A

+10D

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21
Q

spectacles magnifiers may have ________ lens or _________ lenses placed in _________

A

spectacles magnifiers may have single lens or several lenses placed in contact

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22
Q

what is it called when a spectacle magnifier has several lenses placed in contact

A

compound magnifiers

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23
Q

why may it be better to have several lenses placed in contact in a spectacle magnifier

A

the optical quality of high powered lenses are better if there are several

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24
Q

what type of lens on a spectacle magnifier gives a better image quality

A

aspherics

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25
Q

what do spectacle magnifiers have base in prisms

A

to reduce convergence demands

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26
Q

how is the amount of convergence calculated from a patient pd, which gets incorporated into their spectacle magnifier

A

for a PD that = 60mm and a working distance of 10cm

convergence = 60^

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27
Q

list the 3 advantages of spectacles magnifiers

A
  • Good FoV
  • Patients ‘expecting’ more powerful specs (psychologically what the patient expects)
  • Use of both hands
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28
Q

what is a disadvantage of spectacle magnifiers

A

Short working distance – physical constraint for reading

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29
Q

what power high power positive lens range is plus magnifiers available in

A

5-80D

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30
Q

what are plus magnifiers used to produce

A

a magnified image of the object

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31
Q

where is the object of regard in relation to a plus magnifier

A

usually at or just inside front focal point of magnifier

this distance can be short

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32
Q

what do higher powers of plus magnifiers mean

A

shallower depth of focus

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33
Q

what is meant by shallower depth of focus with plus magnifiers

A

things either side of it goes out of focus quickly

e.g. a small movement causes a change in focus

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34
Q

what is an advantage of having the object of regard at or just inside the focal point of the magnifier, with a plus magnifier

A

this means that light coming out of the magnifier will come out parallel, so the patient doesn’t need any accommodation or add to be able to see
therefore having an object close to the front focal point if useful

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35
Q

what is visual magnification

A

Mv

Ratio of the angle subtended by the object seen through the magnifier to the angle subtended by the object at the ‘reference seeing distance’ (25cm)

36
Q

what are the three assumptions of visual magnification and which F power must be used and why

A
  • Image at infinity
  • No accommodation or ‘add’
  • Reference seeing distance is 25cm

Fe must be used since F’v will be higher for magnifiers and give a false magnification due to there being a difference between these values in a thick +ve lens

Eye to magnifier distance affects field of view but not magnification under these assumptions

Fe = the affected magnification 
F'v = the back vertex power, which affects where the image is
37
Q

what is trade magnification and it’s calculation

A

Comparison of object viewed at 250mm with same object viewed through magnifier

M trade = 1 + F/4

sometimes used by manufacturers in labelling

38
Q

if you ant to use trade magnification and hence view something at 250mm, then what must you have

A

have to have some accommodation

39
Q

what does the ISO 15253 state about trade magnification

A

This term should not be used in future

40
Q

what does field of view assume

A

that the aperture stop is pupil of eye and magnifier is field stop
y is the semi-linear image field

2y = D/Fd

D = FoV proportional to useful diameter of magnifier
d = FoV inversely proportional distance of magnifier from eye
41
Q

what happens if an add or accommodation is used with a magnifier and which F power needs to be used with this

A

There is now the effect of the increased power at the eye and the magnifier to eye separation

Equivalent power must be used

Fe = F + A – dFA

F = Equivalent power of magnifier
A = Additional power at eye (e.g. add at spec plane or accommodation of the eye)
dFA = Eye to magnifier separation
42
Q

what mag can hand held magnifiers go up to

A

10-12x

43
Q

list 5 advantages of hand held magnifiers

A
  • Ease of use
  • Wide range of magnification
  • Cheap
  • More expensive aspheric forms available
  • Illuminated versions
44
Q

list 3 disadvantages of hand help magnifiers

A
  • One hand used
  • Must be moved across text – not suitable for volume reading
  • Hand tremor or joint problems
45
Q

name 2 advantages of a stand magnifier (compared to a hand magnifier)

A
  • Helps if shaky hands

- Higher magnifications

46
Q

name 3 disadvantages of a stand magnifier (compared to a hand magnifier)

A
  • More bulky
  • Need a flat surface
  • Need to get close to increase FoV
47
Q

what does a stand magnifier require and why

A

an add for presbyopes who don’t have accommodation

as there is a fixed position from the object to the magnifier in order to get the focus correct

48
Q

where is the image with a stand magnifier

A

Usually at front focal point so image at infinity. Max. mag.

49
Q

what is the range of magnification of a flat field dome magnifier and under what condition is this

A

for n = 1.5 M varies between 1 and 3x (not any more)

50
Q

list 5 advantages of a flat field dome magnifier

A
  • Image optical quality very good
  • Gather light from surroundings making image brighter
  • Can be used with spectacle magnifiers for more mag
  • Unaffected by hand tremors
  • Usual reading distance
51
Q

list 2 disadvantages of a flat field dome magnifier

A
  • Heavy – glass to prevent scratching. (can use Acrylic as a lighter material but will scratch more easily)
  • Need a firm, flat surface
52
Q

which type of lens is a bar magnifier

A

semi cylindrical lens

53
Q

what other magnifier does the bar magnifiers have the same principle as

A

dome magnifiers

54
Q

list 4 disadvantages of a bar magnifier

A
  • Magnify 1 line at a time, hence need to move
  • Limited mag (up to 3x) – people need relatively good vision
  • Reflexes off surfaces makes it difficult to see
  • Distortion, so the image stretches more in the vertical, causes differences in magnification compared to the horizontal plane
55
Q

when will you use a telescope

A

when you can’t get close to an object or make it bigger with a magnifier

56
Q

which distances can telescopes be used for

A

distance (e.g. binoculars) and near

57
Q

what is the magnification caused by telescopes created by

A

angular magnification

angle subtended at the eye

58
Q

what is the mag up to of a telescope if its spectacle mounted

A

3x

59
Q

what is the mag up to of a near telescope and what are they called

A

8x

Galilean

60
Q

what are 2 disadvantages of telescopes

A
  • Restricted field of view

- Can’t move around – distortion (swimming sensation) of space and movement

61
Q

what lenses are the astronomical telescope made up of and what does this cause

A
  • 2 +ve lenses

- but flips the images upside down

62
Q

what lenses are the Galilean telescope made up of

A

1 +ve lens and 1 -ve lens

63
Q

what are all telescopes, and what does not change as a consequence of this

A

they are all afocal

so does not change the focus/power, therefore does not correct the patients rx

64
Q

telescopes produce a magnified _______ at _________

A

telescopes produce a magnified image at infinity

65
Q

what tend to be limited with telescopes

A

field of view

66
Q

what is the calculation of the separation (d) of both lenses in a telescope

A

Separation d = (fo’+ fe’/2)

o = objective 
e = eyepiece
67
Q

list 3 properties of a Galilean telescope

A
  • Shorter
  • Erect image
  • More aberrations = poor image quality
68
Q

list 4 properties of a astronomical telescope

A
  • Longer & heavier
  • Needs prism - to flip image upright
  • Better FoV - than Galilean
  • Better quality
69
Q

what is the magnification calculation of angular magnification which is produced by telescopes

A

M0 = -Fe/Fo

o = Subscript ‘o’ for objective
e = Subscript ‘e’ for eyepiece
70
Q

explain why does changing the spacing between the lenses in a telescope make it difficult to do in practice

A

Mag depends on the power of the eyepiece and the power of objective lens, but the problem is the spacing between the lenses, because if we want to decrease the spacing between the lenses then you have to make the lenses stronger, but then that causes a poorer image quality making it difficult in practice

71
Q

what type of affect does a Galilean telescope have on a myope px

A

a lower magnification (so they do worse)

72
Q

what type of affect does a Galilean telescope have on a hypermetrope px

A

a higher magnification (they do better)

73
Q

which type of telescope do myopes get a higher mag with

A

astronomical

74
Q

which type of telescope to hypermetropes get a lower mag (hence do worse with)

A

astronomical

75
Q

list the 4 forms/types that telescopes can come in

A
  • Monocular (handheld)
  • Binocular – better for some
  • Spectacle mounted
  • Implantable Miniature Telescope (US)
76
Q

wich type of telescope is used for nearer tasks

A

spectacle mounted

77
Q

which type of telescope is best used for AMD patients

A

Implantable Miniature Telescope (US)

78
Q

what is a bioptic telescope

A

Minature Keplerian telescopes mounted towards the top of specs

79
Q

what are bioptic telescopes used for

A

Drivers “sight” through them for short periods
therefore not for use all the time

Can be used for driving in some US states and countries
Not currently acceptable for driving in the UK

80
Q

what are Max glasses

A

Eschenbach telescopes

Galilean telescopes mounted on a spectacle frame

81
Q

what are Max glasses used for

A

TV or sporting events etc

82
Q

by who are Max glasses dispensed regularly by

A

the NHS

83
Q

what are the three types of Max glasses

A
  • Max detail
  • Max TV
  • Max events
84
Q

what is the magnification and working distance of Max detail glasses

A
  • M = 2x

- 400mm working distance

85
Q

what is the magnification and working distance of Max TV glasses

A
  • M = 2.1x

- 3m working distance

86
Q

what is the magnification and working distance of Max events glasses

A
  • M = 2.1x

- 3m - infinity working distance

87
Q

list the 3 means that magnification can be provided by

A
  • Increased object size
  • Reduced distance to object
  • Produce a larger image optically

Visual impairement (8 decks)

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