Different Optical devices used in Low vision patients. Its very important to take proper assessment and calculations for giving Optical devices like Microscopes,Magnifier etc. The references are given.

1. LV Near Systems-I
Microscopes
&
Magnifiers
Harsh Jain
B.Optometry

2. Reference
 Essentials of Low Vision practice –

3. PREDICTING NEAR MAGNIFICATION
• When the patient brings specific reading material to
the examination and the print size is unknown, the
Rule of “1000” can help to determine the acuity
demand of material.
• This rule can be applied as follows:
a) The number of letters and spaces in 1 inch of text is
counted. Taking the average of several inches is
recommended.
b) By dividing the preceding number into 1000, the

4. METHODS OF DETERMINING THE
APPROPRIATE NEAR MAGNIFICATION
1. Lebensohn’s “reciprocal of vision”:
• This rule calculates the needed magnification by using
the patient’s best corrected distance acuity and a near
target acuity.
• By dividing the denominator of the distance Snellen
fraction by the denominator of the near Snellen fraction
of the estimated target acuity, the required
magnification is obtained.
• To convert the resultant magnification to diopters,the

5. 2. Kestenbaum’s rule:
• This rule also uses distance acuity to predict near
magnification. Using the Snellen fraction of best
corrected distance acuity, the denominator is divided
by numerator to obtain the dioptric power needed to
achieve 1M or 20/50 reduced Snellen.
3. Near magnification can also be determined by using
a simple ratio comparing near VA to a target acuity.
This method involves the following steps:
• The patient’s best near acuity(BNA) is determined
with a single character acuity card and the testing

6. • Any distance can be used, but it must be recorded
along with optotype.
• A target near acuity(TNA) is determined and a
ratio is set up (BNA/TNA=TD/?).
• The unknown number(?) is the new reading
distance that the reading material must be
brought to obtain the appropriate
magnification

7. MICROSCOPES

8. • A LV microscope can be described as a spectacle
Mounted convex lens.
• Microscopes enables the patient to take advantages of
the Principle of “Relative distance magnification”.
• As the patient brings the reading material closer, an
Increased retinal image size is produced.
• Microscopes does not produce increased retinal image
rather it, acts as a Converging system to neutralize the
diverging rays created by the close proximity of the
reading material.

9. Lens Options
• When considering a microscope,there are five basic
options available;
 Full-field microscopes.
 Half Eye microscopes.
 Bifocal microscopes.
 Loupe microscopes.
 Contact lens microscopes.

10. Full-field Microscopes
• These are mounted in conventional frames at a normal
vertex distance.
• These also offers largest field of view.
LENS DESIGN :-
 Spherical lens.
 Aspherical lens.
 Doublets.

11. Spherical lenses:-
• Have same power in all meridians.
• They can be prescribed in a Bi-convex & plano-
convex.
• These lenses are prescribed in Higher
powers,oblique astigmatism and curvature of
field begin to distort the Image.So we should use
upto the power range of approx +8.00DS

12. Aspherical Lenses :-
• Used to minimize Peripheral aberrations.
• The two types of lenses are Full-diameter &
lenticular.
• Full-diameter aspherical lenses have power range
of +10D to +20D.
• The aspherical lens design is an aspheric lens on a
plano base.

13. Doublets :-
• It is a combinations of Two convex lenses separated by a
Air space.
• These lens may be of Spherical or aspherical design and
can be made by plastic or glass.
• These lens provides High magnifications with minimal
spherical aberrations,coma,oblique astigmatism and
curvature of field in the peripheral area of lens.
• Magnifications ranges about 2x to 20x

15. Half Eye Microscopes
• These are convex lenses mounted in a half eye
frame worn at a normal or slightly longer vertex
distance.
• Classic Half eye microscopes are convex Sph lens
with base-in prism used for binocularity.
• Power range available +12D.

16. Bifocal microscopes.
• These are mounted in conventional frames at
normal vertex distance and the segments can be
placed at conventional height.
• These are of several types:-
One-piece molded plastic bifocal :-
• Flat-top segments upto range of +6D and round
segments upto +20D.

17.  Aspherical executive bifocals :-
• Range upto +32DS but not incorporated with cyl powers
• These are ground with Elliptical curves.
 Ben-Franklin Bifocals:-
• Design uses two separate lens that are cut in half,
cementated together and fit into a frame.
• The top can be the Distance or intermediate portion and the
bottom is the microscope.
• Range +20DS with cyl
• Range without cyl +48D.

18.  Doublet :-
• These doublets are composed of Two plano-convex lens
with convex side facing away from the patients.
• It has two styles:-
• Type E {13×23mm} and Type R {19×25mm}.
• Doublet bifocal range from +8D to +40D.
 Self-applied bifocals :-
• Allows the examiner to apply bifocal lens to a patients
prescribed lens.
• It can be placed in any position on the patients spectacles
and can be replaced or repositioned at any time.

19. • These are the variations of a microscopes that allows
sightly extended working distance.
Working Distances:-
• Defined as the distance from the spectacle plane to the
reading material.
Working Space:-
• It is the distance from the front of the optical device to
the reading material {usually focal length of the lens}.
• Loupes Increases the working distance by extending the
lens front of the spectacle plane.
Loupes
.

20. Head-borne Loupes:-
• These loupes are supported by a strap around the
patients head.
Clip-on Loupes:- { Jeweler’s Loupes }
• These are secured to either the temples,bridge or
along the top of the frame directly above the patients
eye.

21. Loupes

22. Contact Lens Microscopes
• The working distance of CL microscope would be
closer than using an equivalent powered microscopic
spectacle.
• Advantages :-Enhanced field of view.
Better cosmetics.
• Major problem is that the patient has to remove the
CL when he or she is not reading.

23. Advantages
• Easiest to adjust,because microscopes are most familiar
or conventional.
• Largest field of view.
• Allows both hands free.
• Useful for prolonged reading.
• Sometimes used for writing tasks.
• May be used binocularly {powers up to +12D} when
appropriate.
• Astigmatism correction can be incorporated in some

24. As lens power increases
 Working distance Decreases.
 Depth of focus Decreases.
 Possibility of binocularity
Decreases.
 Field Size Decreases.
 Reduced reading speed
generally occurs.
 Lighting become more critical.
 Aberrations Increases.
When learning to use microscope
{Instructions} to patients
 Fatigue of Neck,arm,and
shoulder muscles.
 Nausea and Dizziness.
 Eye fatigue.
 Headaches.
 No mobilty when wearing a
full-field microscopes.
Disadvantages

26. • When comparing one microscopes to another the
equivalent power should be used.
• Its predicted performance can be compared to any
other optical system by knowing equivalent power.
• With a plano-convex microscope, the equivalent power
will be equal or very close to equal to the front vertex
power & therefore can be easily measured with
lensometer.
• Biconvex microscopic lenses where front vertex power
will provide higher dioptric reading than the

28. • An in-office technique to determine equivalent
power is referred to as method of Triangulation.
• Other alternate methods of verifications that
provide an estimate of lens power, hand
neutrialization, lens clock, lensometer.

30. The recommendation of prescribing a
microscopic system after the equivalent power
is predicted based on Three Factor :-
1) The microscope {Reading lens} is generally the
most familiar near device for the majority of
patients.
2) The microscope is easier to use because the
practitioner need only be initially concerned with
the patients holding the reading material at the
proper focal points and that the proper illumination
is provided.
3) The microscope allows the practitioner to “Fine-

32.  The practitioner should always be positive and provide
appropriate encouragement.
 It should never be assumed that the patients knows how
to use the microscope properly.
 Discuss the importance of maintaining the correct focal
length.
 For lower power microscopes the patient should bring
the material close until the clear focus is found.
 Reading material should be flat to maintain the correct
focal distance.

33.  With high power microscopes ,patients should start
with the material touching their Nose & then push
it away with it is in clear focus.
 The patients visual axis should by perpendicular to
the lens surface passing through he optical centre &
also be perpendicular to the reading material
Material should be scrolled before the eye.
If the patient moves his head or eye or both,the focal
distance changes resulting in Blur or distortion.

35. S:N
O Problems Remedy
1
Material out of
focus a
The proper WD should be checked & reinforced;physically assisting the pt in
maintaining the WD may be required.
The patient should be told touch the reading material to his/her nose,then push it
out until clear.
b Material should be flat,a clip board to hold the material flat may be recommended
2
Postural
Fatigue a Reading stands should be investigated.
b Chairs with a firm back & arm support should be used.
3
Loses place
while reading a
The use of a typoscope {or} finger to mark pt place should be reviewed,a systemic
apparoach to scanning material should be Reinforced.
b Eccentric viewing should be practiced.
4 Asthenopia a The practice or reading times should be initially shortened.
b Eccentric viewing should be Re-inforced & practice.
c
If a binocular system is used, the amt of prism {or} the need to be monocular
should be investigated;if monocular,the poorer eye should be Occluded
d
If a pt has a astigmatic correction in his/her distance spectacle lens,it may have to
be evaluated for incorporation into microscopes.

36. 5 Double vision a The poorer eye should be occluded.
b If binocular,the near IPD of the pt & lenses should be checked.
c
If binocular,the need for Base-in prism or amount of prism should be
checked.
6 Distortion a The orientation & position of material should be checked.
b Lenses amy need cleaning.
c Frame may have to be adjusted.
d
Eye movements should be limited ,remind the patient to scroll material
before his/her eyes.
7
Dimness of
reading
material a
Direction of illumination should be changed or illumination should be
increased by bringing it closer to the object,or light source to be
Increased.
b Shadows should be eliminated
c Yellow acetate filter should be placed on material to increase contrast.
8 Glare a
Direction of illumination should be changed or illumination source should
be moved closer or further away from the object.
b A typoscope may be recommended.
c
Acetate filters may be recommended or a tinted filter may be worn over
the microscope,a visor may also useful.

37. HAND-HELD
MAGNIFIERS

38. • A hand-held magnifier is a convex lens that a
patient holds by means of a handle at various
distance from the spectacle plane.
• Ideally,the near object is held at the focal
distances of magnifying lens.
• The principle of Hand-held magnifier is
Relative distance magnification & Angular
magnification.

39. Lens Options
 Spherical lenses :-
• These are plano-convex or bi-convex power ranging from
+3DS to +14DS.
 Aspherical lens:-
• It can be spherical on One surface & the other surface can
be Aspheric or Bi-aspheric.
• Power range : +6DS to +40DS.
• The curved surface should face the patient and flatter or
spherical surface to the object.

40.  Aplanatic Lenses :-
• It consists of two plano-convex with the convex surface in
contact with each other.
• They are also termed as Doublets.
• Power range : +6DS to +40DS.
• These are very expensive then all.

41. Advantages
• Portability.
• Relatively In-Expensive.
• Patient familiarity and
acceptance.
• Easy to prescribe.
• Widely available with
shapes and sizes.
• Allows for an extended
working distance.
• No lens fabrication
required.
• Allows head movement.
• Binocularity possible in
Lower power with large
lens diameter.
• Helpful for patients with
reduced Peripheral field
who require magnifier for
near.
• Illumination available
{Halogen}.
• Used with or without
Add.

42. Dis-Advantages
• Requires a steady hand and co-ordination.
• Decreased field of view with increased working
distance.
• Need to replace bulbs and batteries for
magnifiers.
• Decreased reading speed.
• Must be held parallel to the reading material to
avoid print distortion.

44. • When the object is held a the focal point if the
magnifying lens, parallel light will leave the lens &
therefore patients best distance spectacle correction
should be placed.
• The equivalent power in this situation is equal to the
equivalent power of magnifying lens itself.
• The equivalent power or the total magnification of the
system is independent of the distance from the magnifier
to the spectacle plane as long as the object is held at the
focal point of the magnifying lens.

45. • When the hand-held magnifier is used in combination
with the patients add or accommodation the equivalent
power can be determining by:-
Deq = D1 + D2 – (d) (D1) (D2)
 Where, D1 = Dioptric power of hand-held magnifier.
D2 = Dioptric power of the ADD or Accommodation
used or uncorrected myopic refractive error at the
spectacle plane.
d = Separation between D1 & D2 {in meters}

47. Power of the magnifier should be
determined.
Flexibility.
Shape of the magnifier.
Size.
Illumination.
Coating and Tints.

49. • It is affected by separation between eye and magnifier
other factors affecting field f view include lens power,
size, distance of lens to the eye.
• Where, W = Linear width of visible field.
d = lens diameter.
f = Focal length of magnifier.
h = Distance from the lens to the eye.

51. 1 & 2 are same as instructions technique in
microscopes device.
Ensure the correct focal length patient should be
instructed to lay the magnifier on the page and pull
the magnifier to eye distance of the hand-held
magnifier.
Patients should be shown how field of view increases
as lens
7 object are brought closer to the eye.
Fie Non-illumination devices patients should be

52. • For illuminated magnifier patient should be switch off
the Bulb when magnifier is not in use
• The magnifiers lens should be held parallel to the
reading distance.
• Magnifier lens should be parallel to the spectacle plane
so the line of sight can be perpendicular to the lens.
• The patients should be instructed to move his/her eye
& the magnifier together as an unique.
• The minimize peripheral distortion most curved side
or convex side of the magnifying lens should be held

54. S:N
O Problems Remedy
1
Material out of
focus a Maintaining the proper focal distance should be stressed.
b The reading material must be kept stationary and flat.
c
It should be determined if the patient is using the distance or near correction
with magnifier.
2 Postural Fatigue a The patients arm and wrists should be braced.
b A reading stand may be suggested.
c
A microscopes or stand magnifier may be suggested instead of using the Hand-
held magnifier.
3
Loses place while
reading a A typoscope or finger to mark placed may be suggested.
b Instruction in scanning may be recommended.
c Instruction in Eccentric viewing may be recommended.
4 Distortion a The magnifier & object should be moved moved closer to the patients.
b
The magnifier must be held parallel to the object;the patient should be looking
through the center of magnifying lens.
c The most convex surface of magnifier must be facing patients eye.
d
the object should be held slightly inside the focal length of magnifier {pt may
have to use some add or acc}

55. 5 Small field of view a The magnifier & object should be moved closer the eye.
b May consider another magnifier of Eq power with larger lens.
c May consider an Aplanatic magnifier.
6 Inverted Image a
Object {or} reading material is being held outside of focal length of magnifier &
patient should bring the object closer to magnifier.
7
Reflections off
lens surface a An illumination magnifier may be considered.
b The direction of external light source may have to be changed.
c Hand-held magnifier may have to be Re-positioned.
d Magnifier with ARC msy be considered.
8 Glare a The position of light source should be changed.
b The illumination should be Decreased.
c A typoscope should be evaluated.
d Filters should be evaluated.
e Tinted magnifier may be considered.
f Hand-held magnifier may have to be re-positioned

56. STAND
MAGNIFIERS

57. • It is a convex lens that is mounted at a fixed
distance from the reading material.
• The patient is not required to hold the magnifier,
rather it is supported by legs or a housing that
stands on the reading material.
• Total magnification of a stand magnifier results
from Relative distance magnification and angular
magnification by the lens.

58. TYPE
 Variable focus:-
• Focusable stand magnifier have lenses that can be
adjusted.
• Closer to or farther away from the reading
material.
• Focusing can compensate for Uncorrected
refractive error or accommodative demands of the
stand magnifier.
 Fixed focus:-

59. Lens Options
 Spherical lenses :-
• Can be plano-convex or bi-convex power range +5DS
to +24DS.
• Bar magnifier is a variation having plano-convex lens
with a cylindrical component that only magnifier in
the vertical meridian power +2DC to +3.50DC.
 Aspherical Lenses :-
• Can be spherical on one surface, aspheric on the other
or Bi-aspheric.
• Power range :- +7DS to +40DS {upto +60DS for Bi-

60. Advantages
• Extended working
distance.
• Some designs may be
useful for writing.
• Portability.
• Good for patients with
tremors or poor motor
control because of its
stable base.
• Relatively in-
expensive.
• Large range of
powers.
• Available with or
without illumination.
• May be helpful for
patients with
constriction fields who

61. Dis-advantages
• Accommodation or
add needed for most
fixed focus stands.
• Decreased field of
view.
• Lens aberrations
induced if line of sight
is not perpendicular
to the lens optical
• Difficulty maintaining
proper illumination
unless the magnifier is
self illuminated has
clear housing.
• Some are heavy
specially those with
batteries.
• May create posture

62. Equivalent Power
Deq = D1 + D2 – (d) (D1) (D2)
 Where, D1 = Dioptric power of Stand magnifier.
D2 = Dioptric power of the ADD or
Accommodation used or uncorrected myopic
refractive error at the spectacle plane.
d = Separation between D1 & D2 {in meters}

63. Prescribing Strategies
Power
Flexibilty
Shape.
Size.
Illumination.
Coating and tints.

64. Instruction Techniques
• The presbyopic patient should be told of the need to use
his/her ADD while using the magnifier.
• The patient should be instructed to let the stand
magnifier rest on the reading material.
• The magnifier can simply slide along the page.
• Some patients lift the magnifier slightly off the page to
obtain a clear image or more magnification.
• The patients should be instructed to move his/her eyes
and the magnifier together as a unit.
• The patient should be reminded to turn off the
illumination source when finished using the magnifier.

65. S:N
O Problems Remedy
1
Material out
of focus a The power of the add or the magnifier should be modified.
b
The distance btw the magnifier & patients eye may require
modification.
2
Postural
Fatigue a Reading stands should be considered.
b Chairs with a back & arm support should be considered.
3 Asthenopia a Initial reading times should be Decreased.
b Eccentric viewing should be practiced.
c The bifocal may have to be Increased.
d
The magnification of magnifier Bifocal system may have to be
Increased.
4
Loses place
while reading a A typoscope can be taped can be taped to the base of the magnifier.
b
A systemic approach to scanning the material may have to be
demonstrated
c Eccentric viewing should be reviewed & practiced.

66. 5
Small field of
View a The magnifier should be moved closer to patients eye
b
Another stand magnifier of Equivalent lens power with a larger
diameter lens may be considered.
c A microscope of Equivalent power should be considered.
6 Distortion a
Patient should be instructed to look through the center of the
magnifying lens.
b Magnifier must be held parallel to material
c The patient should bring the magnifier closer to his/her eye.
7
Pulling
Magnifier off
Page a The power of the bifocal should be Increased.
b The power of stand magnifier should be increased.
8
Reflection off
lens surface a A self-illuminated magnifier should be considered
b The position of external light source should be Re-adjusted.
c The magnifier should be Re-positioned

67. 9 Glare a A magnifier with an ARC or tint should be recommended
b A typoscope may be recommended.
c A Filter over the Reading material should be demonstrated
d illumination level & position should be addressed.
10
Dimness of
Reading
material a A self illumination magnifier should be demonstrated.
b
The external illumination level & position of light source
should be investigated.
c
A yellow tint or filter may be demonstrated to increase
contrast.

68. LOW VISION NEAR SYSTEMS - II
Electronic Magnification Systems

69. • The most commonly recognized electronic
magnification system for reading & writing used by
visually im-paired children, youth & adults in the
CCTV.
• The principle includes both “Projection
Magnification & Relative distance magnification”.
• A no.of new electronic systems have been developed
that may revolutionize the field of low vision.
• These systems featured a Binocular, head mounted
display that uses “Liquid crystal display” {LCD}

70. • Using computer generated software, these systems
have the ability to provide Visual enhancement at
distance intermediate & near.
• Two such systems known commercially available
are;
a) Low vision enhancement systems {LVES}
b) V-max {enhanced vision system Costa mesa CA}

71. Reference
 Essentials of Low Vision practice –