1. LOW VISION DEVICES(OPTICAL)
RAJU KAITI
OpTOmeTRIsT
DhUlIKhel hOspITAl, KAThmAnDU UnIveRsITy
hOspITAl

2. 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

3. INTRODUCTION
A 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.

4. 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.

5. 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.

6. 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

7. MAGNIFICATIONS
• Relative distance magnification
• Relative size magnification
• Angular magnification

8. 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

9. • 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

10. RDM=Y2Y1=tanA1 tanA
=Yu2 =u1u2
Yu1
RDM=u1u2

11. Uses:
Spectacle magnifier
Hand held magnifier
stand magnifier
closed circuit television (CCTV)

12. 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

13. • If 10mm tall letter is replaced by 20mm tall
letter, RSM=2x
• Examples:
– Large print books,
– cheques
– Large television sets
– CCTV

14. RSM=Y2Y1

15. Near Optical devices

16. Types
1. Spectacle-mounted Reading Lenses
2. Telemicroscopes
3. Hand magnifiers
4. Stand magnifiers
5. Electronic Devices- CCTV, HMD

17. Spectacle mounted reading lenses

18. 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.

19. 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

20. Handheld Magnifiers

21. 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.

22. • 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

23. • 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

24. • 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

25. Field of view
• FOV = A(f/d),
where, A=objective lens diameter,
f=focal length of lens and
d=eye to lens distance

26. 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.

27. 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

28. • 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.

29. • 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

30. 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.

31. 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.

32. 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.

33. 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

34. Electronic magnification system
• Electronic Devices-CCTV
– Provides projection
magnification along with RDM
– Provides magnified image
projected onto a monitor
screen.

35. CCTVCCTV
• An electronic magnification system for reading &
writing
• Incorporates both projection & RDM
• consists of three major components:
• camera
monitor
movable reading platform
• Best for pt. Having constricted VF, decreased VA
decreased contrast sensitivity secondary to
glaucoma, RP, ION.

36. Magnification
• Magnification =(X)(Y)
•
– X= print size on the monitor divided by the actual size
– Y= reference distance

37. Distance optical devices

38. 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)

39. Telescopes
1. Hand-held for quick spotting
2. Bioptic telescopes – mounted in spectacles
3. Telemicroscopes for intermediate distance

40. Hand-held Telescope
• Lightweight
• Inexpensive
• Quick and easy for spotting
• Shorter the scope, weaker the
power

41. – Spectacle mounted or handheld
–“full diameter”
–“bioptic” (superior)
–“reading” (inferior)
• Good for extended viewing,
television or sporting event

42. Bioptic Telescope
• Spectacle-mounted so hands free
• Quick and easy access by tipping
head
• Monocular vs binocular depends on
purpose

43. Autofocus Telescopes

44. 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

45. 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

46. • 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 telescopes
Galilean
Keplerian
• Magnification=-Doc Dobj
• Equivalent power= 0

47. • 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
•

48. • Examples:
– Telemicroscopes
– Reading telescopes
– Surgical telescopes
– Near point telescopes
– Telescopic loupes

49. Comparision
Keplerian
• 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
Positive
Negative
Simple
Lighter
Shorter
Inside system
Up to 4X
Decrease Magn
Increase Magn
Smaller
Poorer
Lower

50. Exit Pupil
• Image of objective lens as seen by the
eyepiece
• Galilean – inside telescope
• Keplerian – floating outside

51. Exit Pupil (Galilean)
“With” Movement

52. Exit Pupil (Keplerian)
“Against” Movement

53. Optics of telescope
Galilean telescope Keplerian telescope

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

55. Telescopes used by uncorrectedTelescopes 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.

56. 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

57. Telescopes
• Incorporates angular magnification
• Most systems are afocal
• Increased magnification lead to
decreased FOV.

58. 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

59. Prescribing telescope
Monocular Hand held Telescope Binocular Spectacle mounted
Telescope

60. 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

61. Devices for Peripheral Visual Field
Defects
• Prisms
• Minus lens
• Reverse Telescope system
• Amorphic lens

62. THANKYOU…..