Optics and Low Vision Devices

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Optics and Low Vision Devices

• Marsha Swanson, O.D.
• UAB School of Optometry
• UAB Center for Low Vision Rehabilitation

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• Optical Devices are the tools of the trade,
  and the more one understands how they work, the
  more effective intervention will be.

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Purpose of optical devices

• Provide magnification
• Enhance visual field, side vision

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What is Low Vision?

• Decrease in visual acuity, visual field, visual
  function that leads to visual disability
• Loss of independence
• Loss of some ability that once brought them
  pleasure
• Different for different people

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Fundamentals

• Devices are not a cure
• Provides better function
• Makes images larger but not clearer
• Devices are prescribed by O.D.s or M.D.s
• Task specific
• How the device works

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Light, the basics

• Individual rays

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Electromagnetic Spectrum

• UV---Visible Light---IR
• 400nm-700nm violet-red

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Changing the direction of rays

• Prism

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Changing the direction of rays

• Lenses
• Refraction
• Index
• Surface curvature

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Convergence
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Divergence
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Plano
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Types of Lenses

• Plano-convex
• Plano-concave
• Bi-convex
• Bi-concave

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Types of lenses

• Notice areas of thickness
• Curvature and material determine how light rays
  are refracted
• Concave
• Convex magnification
• Vision rehab typically uses convex lenses

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Lenses

• Focal point and focal length
• Shape and material determines these
• Power the unit that specifies the focal length
  of a lens
• Diopter units of lens power
• The stronger the convex lens, the shorter the
  focal length

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Refractive Errors

• Best correction
• Refractive error
• Light does not focus on the retina
• Myopia a.k.a nearsightedness
• Hyperopia a.k.a. farsightedness
• Astigmatism
• Emmetropia

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Correcting Refractive Errors

• Concave lenses
• Correction for myopia
• Convex lenses
• Correction for hyperopia
• Cylindrical
• Correction for astigmatism
• Refraction
• Determines best correction
• Which is better, 1 or 2

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Presbyopia

• Loss of ability to focus up close
• Loss of flexibility of natural lens
• Additional lens is needed to correct
• Add or Near Add
• Bifocal, trifocal, multifocal

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A Basic Prescription

• OD 2.50-1.00 x 090
• OS 2.00
• Add 1.50 OU

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Magnification

• Making things larger
• Print
• 4 types

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4 types of Magnification

• Relative distance
• Moving closer to an object
• Relative size
• Keeping distance constant, enlarging what you
  want to see
• Angular
• Magnification by optical means
• Electronic

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Designation of magnification

• 3x, 4x, 5.6x
• Diopters
• Total magnification
• Not additive
• Multiplicative

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Convex lenses

• Objects appear larger
• Power is designated with a plus sign
• Recall f 50cm, then Power 2D
• Field of view
• Increasing power decreases the field of view

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Increasing power of a convex lens

• Shortens the focal length
• Decreases the field of view
• Increases thickness of lens
• Increases the weight

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Hand-held magnifiers

• Object just inside f
• Patient instruction
• Where to hold the system
• Example 10D
• Keep glasses on
• Look through top
• Illuminated vs. non-illuminated

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Hand-held magnifiers

• Summary
• Just inside f
• Distance
• Look thru top of spectacles

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Hand-held magnifiers

• Practitioner knowledge musts
• Which eye
• Which side is up
• Smallest print read comfortably
• Distance from lens to print
• Distance from lens to eye
• With glasses or not

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Stand magnifiers

• Stands on a surface
• Guided by hand
• Patients that benefit
• Patient look through add _at_ specific distance
  stated by prescribing doctor
• Illuminated and non-illuminated

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Stand-magnifiers

• Summary
• Base rests on page
• Distance from eye
• Look thru add

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Stand magnifiers

• Practitioner knowledge musts
• Which eye
• Smallest print read comfortably
• Distance eye to lens
• With or without glasses

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Microscopes

• Convex lens mounted in glasses
• Single vision, bifocal, half-eye
• Distance of print still held just inside f
• Example 20D f 5cm
• Hands free
• Monocular, binocular

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Microscopes

• Summary
• Convex lens in specs
• Full lens, bifocal, half-eye
• Usually monocular

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Microscope

• Practitioner knowledge musts
• Which eye
• Smallest print read comfortably
• Working distance

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Telescopes

• Distance viewing
• 2 or more lenses that provide angular
  magnification
• Hand-held or mounted in glasses
• Most are Rxed monocular
• Focusable
• 10-20ft

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Telescopes

• Summary
• Distance viewing
• Monocular
• Hand-held or mounted in specs
• May use without specs

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Telescopes

• Practitioner knowledge musts
• Which eye
• Size of objects seen
• What distance
• How to focus
• Scanning and spotting techniques

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Telemicroscopes

• Combination
• Distance telescope with a plus (convex) lens aka
  microscope reading cap
• For near mounted in spectacle
• Advantage greater focal length
• Disadvantages decreased field of view, increases
  weight, , more noticeable (?)

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Telemicroscopes

• Hand-held
• Mounted
• Monocular, binocular
• Reading cap
• Held at focal length of cap
• 3.00 100/333 cm
• Built into the system

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Telemicroscopes

• Summary
• Combination of telescope and microscope
• For near
• Longer working distance

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Telemicroscopes

• Practitioner knowledge musts
• Which eye
• Smallest print read comfortably
• Working distance
• Cap

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Field Enhancement

• Use remaining field more effectively
• Scanning
• Training
• Reverse telescopes
• Increases field of view, decreases image size
• Enhancement magnification of scope
• Hand-held or mounted
• Basic instruction to patient

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Prisms

• Base direction
• Image jump
• Include scanning with prism
• Incorporated into glasses
• Ground in
• Fresnel

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Mirrors

• Reflects images from area of field loss
• Disadvantages
• Conspicuous
• Blocks part of remaining field
• Reversed image

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Field Enhancement

• Summary
• All types of field enhancement
• Will not restore lost visual field
• Practitioner knowledge musts
• Extent of field loss
• How device works
• Techniques for scanning

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Summary

• Devices Rxed by eye care professional
• Best-corrected vision
• Devices dont provide clear vision
• Dont restore lost vision
• Using remaining vision more effectively
• X does not have the same meaning from device to
  device

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Summary

• More effective after instruction and training
• Communication between doctor and practitioner
  should be clear and concise
• Reading is not always the only goal.

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Thats it!

• Reference Low Vision, Occupational Therapy
  Intervention with the Older Adult. M. Warren, et
  al. Lesson 4. Basic Optics and Optical Devices.
  R. Nowakowski, OD, PhD. 2000