optics of low vision aids Flashcards
what is the definition of magnification of LVAs
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
what does magnification do to the size of the object for a presbyope
increases it
what can you not do with the object for a presbyope
move the object closer
which 2 ways do different optical devices for a larger image of the object
- Larger image of a real (finite) object
- Increased angular subtense of a distant object
list 5 things that can provide a increasing object size
- 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
what magnification is CCTV/video magnifiers based on
relative size magnification
M = Linear size of image on screen/Linear size of object (what is on the page)
what is the available mag range of a CCTV
2-70x
what is the price range of a CCTV
£1300 - £3000
what is the price range of a portable CCTV
£300 - £600
what can tablet PCs do to an image/text
display it at greatly enlarged sizes
can scroll it down with your finger
what is an advantage of a tablet PC
reasonably light to hold
but can get heavy after a long time
what magnification is bringing an object closer based on
relative distance magnification
M = old distance/new distance
e.g. 40cm/10cm = 4x
what is required when bringing an object closer and why
accommodation or add to help focus
what does reducing the distance do for a fixed size object e.g. text
increases the angle subtended and hence the retinal image size
how is the magnification formula: M = F/4 established
M - required for text
- 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
how do you modify the magnification formula: M = F/4 for a persons habitual reading distance, if it isn’t at 25cm
e.g. if px reads at 40cm, then
M = F/2.5
(100/40 = 2.5)
nominal magnification is defined in ISO _________
nominal magnification is defined in ISO 15253
what is nominal magnification the product of
reference seeing distance in metres multiplied with equivalent power
what is the agreed value for reference seeing distance when calculating nominal magnification
25cm (is a baseline)
at what power is a lens designed as a spectacles magnifier to be used
+10D
spectacles magnifiers may have ________ lens or _________ lenses placed in _________
spectacles magnifiers may have single lens or several lenses placed in contact
what is it called when a spectacle magnifier has several lenses placed in contact
compound magnifiers
why may it be better to have several lenses placed in contact in a spectacle magnifier
the optical quality of high powered lenses are better if there are several
what type of lens on a spectacle magnifier gives a better image quality
aspherics
what do spectacle magnifiers have base in prisms
to reduce convergence demands
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^
list the 3 advantages of spectacles magnifiers
- Good FoV
- Patients ‘expecting’ more powerful specs (psychologically what the patient expects)
- Use of both hands
what is a disadvantage of spectacle magnifiers
Short working distance – physical constraint for reading
what power high power positive lens range is plus magnifiers available in
5-80D
what are plus magnifiers used to produce
a magnified image of the object
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
what do higher powers of plus magnifiers mean
shallower depth of focus
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
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
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)
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
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
if you ant to use trade magnification and hence view something at 250mm, then what must you have
have to have some accommodation
what does the ISO 15253 state about trade magnification
This term should not be used in future
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
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
what mag can hand held magnifiers go up to
10-12x
list 5 advantages of hand held magnifiers
- Ease of use
- Wide range of magnification
- Cheap
- More expensive aspheric forms available
- Illuminated versions
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
name 2 advantages of a stand magnifier (compared to a hand magnifier)
- Helps if shaky hands
- Higher magnifications
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
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
where is the image with a stand magnifier
Usually at front focal point so image at infinity. Max. mag.
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)
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
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
which type of lens is a bar magnifier
semi cylindrical lens
what other magnifier does the bar magnifiers have the same principle as
dome magnifiers
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
when will you use a telescope
when you can’t get close to an object or make it bigger with a magnifier
which distances can telescopes be used for
distance (e.g. binoculars) and near
what is the magnification caused by telescopes created by
angular magnification
angle subtended at the eye
what is the mag up to of a telescope if its spectacle mounted
3x
what is the mag up to of a near telescope and what are they called
8x
Galilean
what are 2 disadvantages of telescopes
- Restricted field of view
- Can’t move around – distortion (swimming sensation) of space and movement
what lenses are the astronomical telescope made up of and what does this cause
- 2 +ve lenses
- but flips the images upside down
what lenses are the Galilean telescope made up of
1 +ve lens and 1 -ve lens
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
telescopes produce a magnified _______ at _________
telescopes produce a magnified image at infinity
what tend to be limited with telescopes
field of view
what is the calculation of the separation (d) of both lenses in a telescope
Separation d = (fo’+ fe’/2)
o = objective e = eyepiece
list 3 properties of a Galilean telescope
- Shorter
- Erect image
- More aberrations = poor image quality
list 4 properties of a astronomical telescope
- Longer & heavier
- Needs prism - to flip image upright
- Better FoV - than Galilean
- Better quality
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
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
what type of affect does a Galilean telescope have on a myope px
a lower magnification (so they do worse)
what type of affect does a Galilean telescope have on a hypermetrope px
a higher magnification (they do better)
which type of telescope do myopes get a higher mag with
astronomical
which type of telescope to hypermetropes get a lower mag (hence do worse with)
astronomical
list the 4 forms/types that telescopes can come in
- Monocular (handheld)
- Binocular – better for some
- Spectacle mounted
- Implantable Miniature Telescope (US)
wich type of telescope is used for nearer tasks
spectacle mounted
which type of telescope is best used for AMD patients
Implantable Miniature Telescope (US)
what is a bioptic telescope
Minature Keplerian telescopes mounted towards the top of specs
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
what are Max glasses
Eschenbach telescopes
Galilean telescopes mounted on a spectacle frame
what are Max glasses used for
TV or sporting events etc
by who are Max glasses dispensed regularly by
the NHS
what are the three types of Max glasses
- Max detail
- Max TV
- Max events
what is the magnification and working distance of Max detail glasses
- M = 2x
- 400mm working distance
what is the magnification and working distance of Max TV glasses
- M = 2.1x
- 3m working distance
what is the magnification and working distance of Max events glasses
- M = 2.1x
- 3m - infinity working distance
list the 3 means that magnification can be provided by
- Increased object size
- Reduced distance to object
- Produce a larger image optically
