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Objectives
To understand
β’ What are optical Low Vision Devices (LVD)
β’ Types, uses, advantages and disadvantages of
optical LVDs
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Introduction
β’ Optical devices sometimes referred
to as Low Vision Devices (LVDs)
β’ Consist of 1 OR > lenses placed between eye
and object to increase size of object
on retina
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Introduction
β’ Tasks done at far distances include watching
television, reading white boards, street signs, bill
boards and building numbers. They are often
performed beyond 1metre.
β’ Near tasks include reading books and newspapers,
writing and threading needles. They are often
performed within 40cm.
β’ Intermediate tasks are visual tasks performed at armβs
length, usually 50-100cm such as: computer work,
cards or board games, crafts, playing music and
cooking.
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Introduction
β’ There are basically 2 types of optical devices
β’ Distance:
β’ Telescopic devices (sometimes
intermediate distances)
β’ Near:
β’ Spectacle magnifiers
β’ Hand magnifiers
β’ Stand magnifiers
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Far optical devices
Distance magnification normally requires a
telescopic lens system. They represent an
effective way of providing magnification without
changing the working distance.
Telescopes are mostly afocal; the emergent rays
are parallel, needing no accommodation to view
objects.
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Far optical devices
Telescopes consist of two major elements;
οΆone Objective lens (always a convex lens)
which gathers the light from the object and
οΆone Eyepiece lens (can be either convex or
concave) which magnifies the image of the
objective lens
Telescopic devices are of two types;
β’ Galilean
β’ Keplerian
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Galilean telecopes
β’ Convergent(+) objective lens
β’ Divergent(-) eyepiece lens
β’ Parallel rays strike objective lens and image is formed
at second focal point of lens
β’ The eyepiece is positioned within the focal length of
the objective lens in such a way that its primary focal
point is coincident with the image formed by the
objective lens
β’ Upright virtual magnified image is formed
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Advantages and Disadvantages
Advantages
β’ Small compact light unit
β’ Easy to produce
β’ Upright image
Disadvantages
β’ Low magnification 2x β 3x
β’ Restricted field of view
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Keplerian telecopes
β’ The Keplerian telescope comprise of a convex objective lens
and a convex eyepiece lens.
β’ Parallel rays of light strike the objective lens and an image is
formed at its secondary focal point
β’ The eyepiece is positioned in such a way that its primary focal
point is coincident with the image formed by the objective lens
β’ This results in the formation of an inverted real magnified image
seen by through the eyepiece lens
β’ The inverted and reversed image of the Keplerian telescope is
re-inverted and re-reversed by a prism system included in the
telescope
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Advantages and Disadvantages
Advantages
β’ Larger field of vision
β’ High magnification 6x β 8x
Disadvantages
β’ Inverted image
β’ Heavy
β’ Complicated
β’ Difficult to use
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Comparing Galilean and Keplerian
telescopes
PROPERTIES GALILEAN KEPLERIAN
Objective lens Convex convex
Eyepiece lens Concave convex
Field of view Smaller larger
Exit pupil Inside system Outside system
Magnification Up to 4X Up to 10X
Tube length/ Weight Shorter/ Lighter Longer/ Heavier
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Telescope designs
β’ Galilean or Keplerian (according to lens/optical system)
β’ Hand-held, spectacle-mounted, or clip-on (according to
mode of wearing in use)
β’ Monocular or binocular (according to binocularity)
β’ Fixed focus, focusable telescope, or autofocus (according to
variability of distance in use)
ο Also worthy of note are the Bioptic telescope design
used as βfar additionsβ on spectacles on individuals
with visual impairment.
ο More so, the Reverse telescopes have been
prescribed to provide larger fields of view while
minifying objects in the field.
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Image brightness in telecopes
β’ The relationship between the size of the objective and
the eye piece determines the image brightness of a
telescope
β’ If the ratio is > 1
β’ larger the diameter of the lens ο greater will be the
amount of light transmitted
Diameter of objective lens
Diameter of eye-piece lens
fo
fe
Magnification of the unit
= =
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Field of view (FOV) of telescopes
β’ Greater magnification = smaller FOV
β’ Try to find smallest possible magnification that can
give patient desired acuity
β’ Not always possible to give largest FOV and
maximum brightness to all low vision patients
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Considerations when selecting a telescope
β’ Establish patient needs
β’ Explain, compare & contrast different units
β’ Manual dexterity
β’ Cost
β’ Clarify to patient what is optically possible and what is
not
β’ Train the patient in using the particular units
β’ Motivation of patient
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Determining the magnification to be
prescribed
One commonly practiced way is as follows;
Magnification=
π΅π΅π΅π΅π΅π΅π΅π΅ @ππππππ
ππππππππππππ ππππ
β’ A thorough refraction is needed to determine the best
corrected visual acuity BCVA
β’ The target VA should be at least 3 steps better than
the VA from where comfortable viewing can be
attained.
Another way to consider the Magnification is to know
that for every 3 lines of improvement on the standard
chart, a 2x magnification is needed
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Telescopes for intermediate distances
β’ Most keplerian telescope designs come in forms
where they can be focused for closer distances. This
happens by the change in the distance between the
objective and eyepiece lenses. With this focusable
and short focus mechanism, intermediate tasks can be
carried out.
These come as short- focus monocular telescopes or clip-on
binoculars.
However, the extent of near focusing can be limited
because of the limited extent this movement can be
made. Hence the advancements in the development of
telemicroscopes.
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Telescopes for intermediate distances
β’ Telemicroscopes have been designed to work as an
equivalent system to the Low Vision Telescope with
additional close-up lenses (caps put in front of the
objective lens corresponding to working distance).
Some designs can be used for distances as close as
10cm
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Comparing telemicroscope forms
Telemicroscope
form/ type
Field of view Ease in
binocularity
Size/ bulkiness Variability/
flexibility
Clip-on
Good field With IPD range of 62-
70mm and variable
positioning,
binocularity is easy
Light weight Fixed objective & eye
piece lens do not
support a range of WD.
Comfortable for flip-on
distant correction.
Spectacle
form
Restrictions in field Good IPD range, its
variable focus can be a
limitation
Light weight Variable focus but for a
limited range
Near
binoculars
with caps
Significant restrictions Could be clumsy most
times
Quite bulky,
detachable reading
caps can make it even
more
Can be used for a wide
range of WD using the
appropriate reading
caps
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Near optical devices
Individuals with visual impairments can profit from one of
the various types of magnifiers for their near work.
The effectiveness of a given magnifier would depend on;
β’ The nature of visual impairment
β’ The age of the patient
β’ The equivalent viewing power required by the
individual.
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Spectacle and hand held magnifiers
β’ Spectacle magnifiers are high plus lenses
mounted in spectacle frames. They provide
maximum magnification when objects are held
the focal distance of the lens.
β’ Similarly, the hand held magnifier produces
emergent parallel rays with image at infinity
when objects are at the focal length. Hence,
distance correction should be worn.
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Examples of spectacle magnifiers
In patients whose binocular vision is better than their
monocular vision (i.e., with similar visual acuity in both
eyes), a base-in prism can be added to facilitate
accommodative convergence.
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Comparing spectacle magnifier with hand
held magnifier
Characteristics Spectacle Hand held
Field of view Greater field of view but
reduces with higher power
Smaller field of view, hence lower reading
speed
Handling Both hands can be free Tricky and steady handling is required
Illumination Obstructs light when
working distance is closer
Have options of illuminated magnifiers
Eye to lens
distance
Worn at the spectacle plane Can be varied, though with the
consequence on the field
Comfort With the free hands come
comfort
Can be discomforting trying to maintain
focus
Combining
magnification
Possibility of combining with
other devices
Limited in combination to just the
spectacle
Short-term
tasks
May be quite clumsy Convenient for short term tasks like
reading a price tag
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Stand magnifier
β’ These are plus lenses designed with a
rigid mounting placed on the object.
β’ Rays emerging from them are not parallel
but divergent because objects are within
the focal length of the magnifiers.
β’ This implies that accommodative effort or
a near ADD is required to bring the image
into focus.
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Stand magnifier
β’ Person moves it around the print and can
see enlarged image of the print or any
object
β’ Available in different magnification and
sizes
β’ Can be used with distance and near
glasses for providing extra magnification
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Advantages and disadvantages of stand
magnifier
ADVANTAGES DISADVANTAGES
Device of choice in tremors &
reduced fields
Possibility of poor posture unless a
reading stand is used
Has a wide range of sizes
&magnification
Reduced field of vision
May have its own light source
as in LED
Poor illumination in non-illuminated
Usually portable Image distortion often a feature
Its fixed focus works well for
children
Requires accommodation or near ADD
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Determining equivalent viewing power
(EVP) in magnifiers
EVP= π΅π΅π΅π΅π΅π΅π΅π΅
ππππππππππππ ππππ
Γ 100
ππππππππππππππ ππππππππππππππππ
β’ A thorough refraction is needed to determine the
best corrected visual acuity BCVA
β’ The target VA should be at least 3 steps better
than the critical print size
The magnification is consequently;
M= β
πΈπΈπΈπΈπΈπΈ
4
β’ The magnification indicated by the manufacturer
can differ. It can be with reference to a working
distance of 33 or 40cm.
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Definitions of other LVD
β’ Foldable magnifier: A
magnifying aid which is compact
and can be folded, and is held in
the hand in front of the eye for
viewing small objects at close
range. It is good for spot viewing
or reading, with usual powers of
2x to 5x.
β’ Dome or Bar magnifier: A type
of stand magnifier that is a
transparent magnifying aid held
on the reading material. It
provides good contrast and is
usually available in 1.5x to 2.5x.
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Conclusion
β’ Low vision patients have a range of options in far
and near low vision devices to help with activities of
daily living.
β’ Telemicroscopes are also available in different forms
foe intermediate tasks.
β’ It is important to consider the features of a given
optical aid in selecting devices that match the
peculiarities of a person.
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REFERENCES
β’ Brien Holden Vision Institute, Global Optometry
Resources: Low Vision.
β’ Christine Dickinson (1998). Low Vision:
Principles and Practice. Butterworth Heinemann.
β’ Ndukuba, S.O. (2020). Near, yet far: meeting the
intermediate distance needs of individuals with
visual impairment- case report. Annals of Case
Reports, 14, 430-440.
β’ Vasconcelos, G. & Fernandes, L.C. (2015). Low
vision aids. https://www.aao.org/pediatric-center-
detail/low-vision-aids.
