Optics of Low vision aids Flashcards
what Non - visual aids are avaible ?
1.Large digital talking watch
2.liquid level indicators
3.Portable daylight lamp
4.Big button phone
5.Talking tin lids
How can we make objects bigger?
- Objects which are bigger then normal
- Accessibility options ; Magnifer, contrast increases, color filters, screen narrators
What is a video magnifer?
They are more expensive and they are camera magnifiers.
It will read it and make it digital and bigger.
They can be portable or fixed in one location
How do video magnifiers work?
mag = linear size of the image on the screen / Linear size of the object
It makes a new object which is larger then the original.
The range can be from X2 - X70
They can also increase contrast as well as magnification.
what are some other Other low vision aids?
- Typoscope - reduces glare from the white background
- Low vision paper notebook
- Yellow filter glasses - this will reduce blue light therefore slow down glare.
How is mag of LVA defined?
retinal image size of object with mag (h’ with) / without mag (h’ without)
What is relative distance magnification?
Old distance / New distance = mag
when bringing the object closer it increases the angle subtended at the eye therefore the retinal image size is bigger
Note : this will require accommodation / add
how can we assume the formula M=F/4?
- baseline = 25cm = X1
- 25cm will require an accommodation of 4D
How about the working distance is 40cm?
Then we change the power to 2.5D!
what is nominal magnification?
Product of reference seeing distance in meters and the equivalent power
Agreed reference is 250mm (25cm)
Nominal mag = 0.25 x Fe
What is trade magnification?
The comparison of an object viewed at 25cm with the same object viewed through the magnifier.
M trade = 1 + F/4
Why is trade magnification not used?
It does not take into account the accommodation need (4D)
What is the range for spectacle magnification?
4D -> 14D
Why is base in prisms used in spec mags?
To reduce convergence demands and will encourage binocular vision.
What are the pros of spec mag?
- Good FOV
- Meets the px expectations
- hands free
- Cosmetics is good
Disadvantge of spec mags?
- short WD - will become tiring when reading
- Loss of light because of the close working distance
- Possibility of loss of BV
what is a plus Magnifier?
This is a high power positive lens to produce a magnified image of the object.
What does a higher power do to the depth of focus?
It will make it shallower. This is an optical property we cannot avoid
What is visual magnification?
The ratio of the angle subtended by the object seen through the magnifier to the angle subtended by the object at the reference seeing distance.
M = F/4
4D = because reference dist. is 250mm
What do we assume with visual mag?
- Image at infinity
- No accommodation or add needed
- Reference seeing distance is 250mm (25cm)
- The power used needs to be the equivalent power ; Back vertex power may be higher with high plus lens ….. false high mag indicated
Under these assumptions, the FOV will be affected but not the magnification
Why is field of view important?
If it is too small it can become unpractical.
What is field of view proportional to?
The ‘useful diameter’ of the magnifier.
The edges of the lens quality will be so poor it will not be useful —> that is where the term useful diameter comes from
What is the FOV inversely proportional?
The distance of the magnifier from the eye. The closer = bigger FOV
What if an add or acc is used when using a plus magnifier?
We can investigate this by treating them both as thin lenses
What is the equation?
FE = Fmag + Add/acc - dFmagA
the dFmagA is a correction factor for the separation between the two
Why would a younger person experience change in the magnification as well as FOV using a magnifier?
Because of the accommodation… go back to the equation
What are the positives of hand held?
- 10-12X - wide range of mags
- Low cost
- Cosmetically nice
- Easy to use
- Can get illuminated ones
- Aspheric surfaces used to reduce aberrations and increase FOV.
How does aspheric lenses increase FOV?
They will increase the useful diameter therefore increase the FOV
What is the negatives of hand held?
- One hand is now used ; spec mags was hands free n
- The magnifier has to be moved ; not practical for high volume of reading
- Joint problems / hands tremors can mean the px cannot use
How does colour temperature effect performance?
Lower temps are orange and higher temps are more blue.
It can effect the near acuity, reading speed and critical print size)
However, this is different for each px.
What can we used instead of a hand held if the px has a hand tremor?
Stand magnifier
What are positives of stand mags?
- Helps with shaky hands
- Higher mags
What are the disadvantages of stand mags?
- More bulky
- Need a flat surface
- need to get closer for a higher FOV (due to the higher mag)
What is a dome magnifier?
This will rest on the object and is described at flat-field magnifier
What is the pros for dome mag?
- Good image quality (aplanatic design)
- Gather light from surrounding –> the image is brighter
- can be used with spec mags
- Helps with hands tremors
- Normal reading distance can be used
Cons of dome mags?
- No access to the object ; the mag lies on top of it
- Heavy —> Acrylic can be scratched easier
- need a firm flat surface
What is a bar magnifier?
This is an adaptation of the dome magnifier ; good for large volume of text.
Pros of bar magnifier?
- 1 line at a time
- Normal reading dist
- Rest on a stable dist
Cons of bar Magnifier
- reflexes on the surface
- Limited mags (3X)
- The image is distorted ; the powers are different( Plano in one meridian and power in another meridian)
What are telecscopes?
They can be sued for near and distance
It uses angular magnification
5-15% dispensed
it ranges from 2-12X - low mags most successful
What are the cons of telescope?
- small FOV - due to the diamter of the lens and the seperation
- Cannot move around with using them due to distortion
- The cosmetics is poor
What are the two types of telescopes?
Galilean - one positive and one negative lens
Astronomical - two positive
They are normally A focal - there is no optical power —> this means they will produce as magnified image at infinity
How is angular magnification calculated?
The ratio of the power of the lens closest to the eye to the lens at the front of the telescope.
The separation between the two has to be equal to the focal distance due to the a focality of the lens.
This is bascially saying the length is important aswell
Galilean Telescope Postives
- Shorter
- erect image
Disadvantge of galilean?
Alot more aberrations
Pos of astronimical?
- Better FOV
- Good qaulity
Con of astronimical?
- Inverted image (needs a prism to make it erect)
- Heavier
- Longer
What happens when an uncorrected ametropia uses a telescope?
Galilean: Myope = Lower mag and hypermetropia = Higher mag
Astronomical : Myope = higher mag and hyper = lower mag
What are the diffrent type sof telescopes?
- Monocular - Used briefly for a particular task
- Binocular - Better for some
- Spectacle mounted - used for near work.
- Implantable
What are max glasses?
These are cost effective Galilean telescopes mounted onto a spectacle. Dispensed a lot with NHS
- sporting events
- TV
MAX detail = 40cm
MAX tv = 3m
MAX event = 3m to infinity
What is bioptics?
This is miniature Keplerian telescope mounted on top of glasses.
it is legal to drive with them in some states
How does this work?
Drivers can “sight” through them for a short period of time. —> reading road signs, pedestrians on the side ect
They will tilt their head slightly to see through it.
NOT accepted in the UK, only in USA
what amount of base IN prism can be incorporated in spec magnifiers?
5-16
in a plus magnifier, where is the object in relation to the focal point?
at or just in front of the focal point of the magnifier
