LOW VISION DEVICES(OPTICAL)

RAJU KAITIOpTOmeTRIsT

DhUlIKhel hOspITAl, KAThmAnDU UnIveRsITy hOspITAl

Defining Low Vision• A person with low vision is one who

– has impairment of visual functioning even after

treatment and/or standard refractive correction ,

and

– has a visual acuity of less than 6/18 to light

perception , or a visual field of less than 10 degree

from the point of fixation, but who

– uses, or is potentially able to use, vision for the

planning and/or execution of a task

INTRODUCTIONA wide variety of rehabilitation options are available

to help people with low vision life and/or work more effectively, efficiently, and safely.

Most people can be helped with one or more low vision treatment options.

Unfortunately, only about 20-25 percent of those who could benefit from these treatment options have been seen by a low vision optometrist.

There is a wide variety of devices that can help people with low vision see better.

Some are used to see things that are near, like a newspaper, and others are used for seeing distant objects, like street signs.

Low vision optical aids as specific tools for specific uses.

These are not intended as all-purpose aids.

Instead, they make it easier to use one's existing vision for specific tasks.

Unfortunately, low vision aids do not replace vision that has already deteriorated or been lost.

But they will maximize one's remaining vision and help one to enjoy favorite activities and hobbies.

Low vision aids can:

correct refractive errors

be used with corrective lenses, other optical devices and regular and specialized technologies

provide magnification

be accepted or rejected

change as individual needs change

require training or time to adjust

be in optical or non-optical form

MAGNIFICATIONS

• Relative distance magnification

• Relative size magnification

• Angular magnification

Relative distance magnificationRelative distance magnification• Magnification obtained by moving closer to the

object of regard

• As the object moves closer, size of retinal image increases

• RDM=original distance \new distance

• If the object is originally at 50cm & the observer moves it closer to 25cm, the magnification is 2x.

• Original distance=reference distance

• It is the magnification obtained by comparing the angular subtense of the object at the shortened viewing distance due to the magnifier to the angular subtense of the object at the initial viewing distance

RDM=Y2\Y1=tanA1\ tanA =Y\u2 =u1\u2

Y\u1RDM=u1\u2

Uses:

Spectacle magnifier

Hand held magnifier

stand magnifier

closed circuit television (CCTV)

RELATIVE SIZE MAGNIFICATIONRELATIVE SIZE MAGNIFICATION

• Magnification achieved by changing the actual size of the object being viewed while it remains stationary & no lens is used

• RSM=enlarged image size/original object size

• Determined by comparing the angle subtended by the enlarged object at the entrance pupil of the eye to the initial object

• If 10mm tall letter is replaced by 20mm tall letter, RSM=2x

• Examples:– Large print books, – cheques– Large television sets– CCTV

RSM=Y2\Y1

Near Optical devices

Types

1. Spectacle-mounted Reading Lenses

2. Telemicroscopes

3. Hand magnifiers

4. Stand magnifiers

5. Electronic Devices- CCTV, HMD

Spectacle mounted reading lenses

Spectacle mounted reading lenses

Relative distance magnification

Objects held at the focal length of the lens in order to obtain a magnified, erect image.

ADD= Working distance-1/2AA

Convergence demand (add>4D, base in prisms should be prescribed for the added convergence demand placed on the system.

Generally add 2 to the power of the glasses.

Advantages & disadvantages• Advantages-

– Both hands are free

– Cosmetically appealing

– Wider FOV

– Can incorporate spec Rx

– Broad range of power

• Disadvantages-

– Must hold objects close to face, limited WD

– Limited tasks performed

– Weight on nose, expensive

– Proper illumination difficult to achieve

– Discomfort from head positioning needed

Handheld Magnifiers

Handheld Magnifiers

• Plus lenses mounted in a frame

with a handle.

• Provides both RDM and Angular

Magnification

• Object is held at the focal

length of the magnifying lens,

the emerging vergence is

parallel.

• Distance from the eye to the

magnifier can change without

affecting the accommodative

demand.

• FOV is greater for close eye to

lens distance.

Hand Held Magnifier

• Refractive error should be corrected

• As the magnifier & object are brought closer to the eye, RDM increases an angular magnification decreases

• Total magnification remains constant as long as the object remains at the focal point of the magnifying lens

• HHM should be held at the focal length of the lens--Magnified retinal image

• Equivalent power

Fe = Fm+Fa-dFmFa,

where, Fm = power of the magnifying lens,

Fa = power of the add or accommodation,

d = eye to magnifier distance

Fe is maximum when HHM is held in contact with an add

• Fe is always equal to power of magnifier alone, When magnifier held at it’s focal length from the add

Field of view

• FOV = A(f/d),

where, A=objective lens diameter,

f=focal length of lens and

d=eye to lens distance

Advantages & disadvantages• Advantages-

– Familiarity , inexpensive

– Flexible WD

– Wide variety available

– Illumination available

– More socially accepted

• Disadvantages-

–One hand tied up

– Difficult to hold steady in proper

position at all times

– Limited FOV, depends on eye to lens

distance.

Stand Magnifiers• Plus lenses mounted in a stand at a

fixed distance from the object of

regard.

• Object is viewed at a distance less

than that of the focal length of the

lens.

• Emerging light rays are diverging so an

add or some accommodation is

necessary to act to form a clear retinal

image.

• Total magnification results from RDM & angular magnification

• Two types of Stand magnifiers:-focusable

-fixed focus

• Fixed focus:Fixed focus:• It has its lens set at a fixed distance from the base

• Usually, the distance from the reading material to the lens is slightly less than focal length of the lens, so requires an add or acc.

• Focusable Stand magnifier:

• Have lenses that can be adjusted closer to or farther away from the reading material

• focusing compensate for uncorrected RE or accommodative demand

Magnification for stand magnifiers

• Equivalent power,

Fe=F1+F2-dF1F2,

where, F1= power of the magnifier,

F2= accommodative demand ,

d= eye-to magnifier distance

• . Fe formula is also applicable for handheld magnifiers where the

object is held within the focal length of the lens producing

divergent light and therefore requiring an accommodative

demand.

Advantages • Do not have to hold

• Can slide across page

• Constant lens to page distance

• They are inexpensive and readily available

• The focusing distance is set by simply placing the magnifier on the page

• They are helpful for individuals with poor motor control

• They can be used in combination with regular eyeglasses.

• Some stand magnifiers come with built-in lights.

Disadvantages

• Limited FOV

• Difficult to work with on some surfaces

• Difficult to perform tasks under the magnifier

• Book bindings may present a problem, as the magnifier may not remain in a stable or fixed position

• They can block the reading light and reduce the amount of illumination that reaches the page.

• They can be bulky and are not as portable as smaller hand-held magnifiers.

Telemicroscopes• Eyeglass mounted telescopes

can be made to focus at any working distance.

• Allows a person to work at a more normal working distance.– viewing the computer screen,

knitting, playing music or– when or performing any task that

is uncomfortable when done at a close distance.

• hands free

Electronic magnification system

• Electronic Devices-CCTV

– Provides projection

magnification along with RDM

– Provides magnified image

projected onto a monitor

screen.

CCTVCCTV

• An electronic magnification system for reading & writing

• Incorporates both projection & RDM • consists of three major components:• camera

monitormovable reading platform

• Best for pt. Having constricted VF, decreased VA decreased contrast sensitivity secondary to glaucoma, RP, ION.

Magnification

• Magnification =(X)(Y)

•– X= print size on the monitor divided by the actual size

– Y= reference distance

Distance optical devices

Telescopes

• An optical instrument used to magnify the

apparent size of a distant object

• In low vision rehabilitation,

• Used whenever approach magnification and conventional

optical correction is not possible

• Refracting type (positive objective lens)

• Uses angular magnification (image of distant object subtend

larger visual angle than by object)

Telescopes

1. Hand-held for quick spotting

2. Bioptic telescopes – mounted in spectacles

3. Telemicroscopes for intermediate distance

Hand-held Telescope

• Lightweight

• Inexpensive

• Quick and easy for spotting

• Shorter the scope, weaker the power

– Spectacle mounted or handheld

–“full diameter”

–“bioptic” (superior)

–“reading” (inferior)

• Good for extended viewing,

television or sporting event

Bioptic Telescope

• Spectacle-mounted so hands free

• Quick and easy access by tipping head

• Monocular vs binocular depends on purpose

Autofocus Telescopes

COMPONENTS• Objective lens:– Always +ve/ convergent lens– Placed towards object– Large in diameter– Noted as Dobj or D1

• Ocular/ Eyepiece lens:– Either +ve (Keplerian) / -ve (Galilean) lens– Placed closest to the eye– Noted as Doc or D2

TELESCOPESTELESCOPES

• It can be used in two different ways

• 1)Focally • 2)a focally• When used afocally• -produces only an angular magnification

• -objective & ocular lenses are separated by a distance equal to the absolute values of their focal length- normal adjustment

• -image & object are located at infinity

• The ocular lenses is positioned in such a way that its primary focal point is located coincident with the image formed by the objective lenses

• Two types of telescopesGalileanKeplerian

• Magnification=-Doc\ Dobj

• Equivalent power= 0

• Telescopes when used focally,Telescopes when used focally,

• Primarily due to RDM

• Have finite focal distance(FOR NEAR AND INTERMEDIATE)

• Can be used focally by three methods– Increasing the power of objective lens– Decreasing the power of ocular lens– Changing the separation of the lenses

•

• Examples:– Telemicroscopes

– Reading telescopes

– Surgical telescopes

– Near point telescopes

– Telescopic loupes

ComparisionKeplerian

• Objective lens - Positive• Ocular lens- Positive • Eyepiece system- compound• Weight- heavier• Length- Longer• Exit pupil location- outside system• Magnification- Up to 10X• Uncorr. Myopia- Increased magn

• Uncorr. Hyperopia- Decreased magn

• Field of View- Larger• Image Quality- Better• Cost Higher

Galilean PositiveNegativeSimpleLighterShorterInside systemUp to 4XDecrease Magn

Increase Magn

SmallerPoorerLower

Exit Pupil

• Image of objective lens as seen by the eyepiece

• Galilean – inside telescope

• Keplerian – floating outside

Exit Pupil (Galilean)

“With” Movement

Exit Pupil (Keplerian)

“Against” Movement

Optics of telescope

Galilean telescope Keplerian telescope

Keplerian Telescope

• Use prism to re-invert the image for Keplerian Telescope

Telescopes used by uncorrected Telescopes used by uncorrected spherical ametropiaspherical ametropia

• when uncorrected Myopes uses Galilean telescope magnification decreases.

• Still can have focused image on the retina by adjusting the tube length -shorten.

• Hyperopic eye has a minus refractive error – that is corrected by a plus lens.

Magnification of telescope

• M = Fe/Fo where, Fe= power of eyepiece, Fo= power of the objective OR, entrance pupil diameter/exit pupil diameter.

• FOV = objective lens diameter/mag. of telescope

Telescopes

• Incorporates angular magnification

• Most systems are afocal

• Increased magnification lead to

decreased FOV.

Prescribing telescope• Handheld Vs Spectacle mounted

– Temporary or occasional use – Handheld (with cords/straps or finger-ring)

• Reading chalkboard, street sign, traffic lights, sport, view scenery etc

– Long term / prolonged viewing- Spectacle-mounted

• Watching TV, driving etc• Spec mounted in carrier lens (full diameter/ full field position)• Positioned superiorly – Bioptic telescope• Positioned inferiorly – reading / surgical telescope

Prescribing telescope

Monocular Hand held Telescope Binocular Spectacle mounted Telescope

Prescribing telescope• Determining required magnification

– MTS= Best VA / Target Acuity

– Target acuity is usually 20/40, 20/50

• Binocular Vs Monocular– Binocular for prolong tasks– For binocular, should have equal VA & binocularity– Occasionally, use as biocular (alternate fixation)– Monocular if need to change fixation at various dist

Devices for Peripheral Visual Field Defects

• Prisms

• Minus lens

• Reverse Telescope system

• Amorphic lens

THA

NKYOU…

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