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Visual impairement > optics of low vision aids > Flashcards

optics of low vision aids Flashcards

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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
what do spectacle magnifiers have base in prisms
to reduce convergence demands
26
how is the amount of convergence calculated from a patient pd, which gets incorporated into their spectacle magnifier
for a PD that = 60mm and a working distance of 10cm | convergence = 60^
27
list the 3 advantages of spectacles magnifiers
- Good FoV - Patients ‘expecting’ more powerful specs (psychologically what the patient expects) - Use of both hands
28
what is a disadvantage of spectacle magnifiers
Short working distance – physical constraint for reading
29
what power high power positive lens range is plus magnifiers available in
5-80D
30
what are plus magnifiers used to produce
a magnified image of the object
31
where is the object of regard in relation to a plus magnifier
usually at or just inside front focal point of magnifier this distance can be short
32
what do higher powers of plus magnifiers mean
shallower depth of focus
33
what is meant by shallower depth of focus with plus magnifiers
things either side of it goes out of focus quickly | e.g. a small movement causes a change in focus
34
what is an advantage of having the object of regard at or just inside the focal point of the magnifier, with a plus magnifier
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
35
what is visual magnification
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
what are the three assumptions of visual magnification and which F power must be used and why
- 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
what is trade magnification and it's calculation
Comparison of object viewed at 250mm with same object viewed through magnifier M trade = 1 + F/4 sometimes used by manufacturers in labelling
38
if you ant to use trade magnification and hence view something at 250mm, then what must you have
have to have some accommodation
39
what does the ISO 15253 state about trade magnification
This term should not be used in future
40
what does field of view assume
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
what happens if an add or accommodation is used with a magnifier and which F power needs to be used with this
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
what mag can hand held magnifiers go up to
10-12x
43
list 5 advantages of hand held magnifiers
- Ease of use - Wide range of magnification - Cheap - More expensive aspheric forms available - Illuminated versions
44
list 3 disadvantages of hand help magnifiers
- One hand used - Must be moved across text – not suitable for volume reading - Hand tremor or joint problems
45
name 2 advantages of a stand magnifier (compared to a hand magnifier)
- Helps if shaky hands | - Higher magnifications
46
name 3 disadvantages of a stand magnifier (compared to a hand magnifier)
- More bulky - Need a flat surface - Need to get close to increase FoV
47
what does a stand magnifier require and why
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
where is the image with a stand magnifier
Usually at front focal point so image at infinity. Max. mag.
49
what is the range of magnification of a flat field dome magnifier and under what condition is this
for n = 1.5 M varies between 1 and 3x (not any more)
50
list 5 advantages of a flat field dome magnifier
- 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
list 2 disadvantages of a flat field dome magnifier
- Heavy – glass to prevent scratching. (can use Acrylic as a lighter material but will scratch more easily) - Need a firm, flat surface
52
which type of lens is a bar magnifier
semi cylindrical lens
53
what other magnifier does the bar magnifiers have the same principle as
dome magnifiers
54
list 4 disadvantages of a bar magnifier
- 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
when will you use a telescope
when you can't get close to an object or make it bigger with a magnifier
56
which distances can telescopes be used for
distance (e.g. binoculars) and near
57
what is the magnification caused by telescopes created by
angular magnification | angle subtended at the eye
58
what is the mag up to of a telescope if its spectacle mounted
3x
59
what is the mag up to of a near telescope and what are they called
8x | Galilean
60
what are 2 disadvantages of telescopes
- Restricted field of view | - Can’t move around – distortion (swimming sensation) of space and movement
61
what lenses are the astronomical telescope made up of and what does this cause
- 2 +ve lenses | - but flips the images upside down
62
what lenses are the Galilean telescope made up of
1 +ve lens and 1 -ve lens
63
what are all telescopes, and what does not change as a consequence of this
they are all afocal | so does not change the focus/power, therefore does not correct the patients rx
64
telescopes produce a magnified _______ at _________
telescopes produce a magnified image at infinity
65
what tend to be limited with telescopes
field of view
66
what is the calculation of the separation (d) of both lenses in a telescope
Separation d = (fo’+ fe’/2) ``` o = objective e = eyepiece ```
67
list 3 properties of a Galilean telescope
- Shorter - Erect image - More aberrations = poor image quality
68
list 4 properties of a astronomical telescope
- Longer & heavier - Needs prism - to flip image upright - Better FoV - than Galilean - Better quality
69
what is the magnification calculation of angular magnification which is produced by telescopes
M0 = -Fe/Fo ``` o = Subscript ‘o’ for objective e = Subscript ‘e’ for eyepiece ```
70
explain why does changing the spacing between the lenses in a telescope make it difficult to do in practice
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
what type of affect does a Galilean telescope have on a myope px
a lower magnification (so they do worse)
72
what type of affect does a Galilean telescope have on a hypermetrope px
a higher magnification (they do better)
73
which type of telescope do myopes get a higher mag with
astronomical
74
which type of telescope to hypermetropes get a lower mag (hence do worse with)
astronomical
75
list the 4 forms/types that telescopes can come in
- Monocular (handheld) - Binocular – better for some - Spectacle mounted - Implantable Miniature Telescope (US)
76
wich type of telescope is used for nearer tasks
spectacle mounted
77
which type of telescope is best used for AMD patients
Implantable Miniature Telescope (US)
78
what is a bioptic telescope
Minature Keplerian telescopes mounted towards the top of specs
79
what are bioptic telescopes used for
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
what are Max glasses
Eschenbach telescopes | Galilean telescopes mounted on a spectacle frame
81
what are Max glasses used for
TV or sporting events etc
82
by who are Max glasses dispensed regularly by
the NHS
83
what are the three types of Max glasses
- Max detail - Max TV - Max events
84
what is the magnification and working distance of Max detail glasses
- M = 2x | - 400mm working distance
85
what is the magnification and working distance of Max TV glasses
- M = 2.1x | - 3m working distance
86
what is the magnification and working distance of Max events glasses
- M = 2.1x | - 3m - infinity working distance
87
list the 3 means that magnification can be provided by
- Increased object size - Reduced distance to object - Produce a larger image optically

Visual impairement (8 decks)

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