Transcript Optics and Low Vision Devices

Session 9: Wednesday, November 04, 2015
Optics and Low Vision Devices
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Any questions from last week’s lecture?
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Quizes!!
◦ Complete your “Syndromes Quiz” by November 8!
◦ Next quiz is on material from this lecture only
◦ Due Sunday, November 15, 2015 @ 11:55PM AST
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Optics
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Low Vision Devices
Light and Theories
Refraction/Reflection
Vergence of Light
Lenses
Refractive Errors
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Sun is a natural source for all of the energy
that moves by electromagnetic radiation
◦ Is classified according to its specific wavelength
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Light energy is the tiny portion of the
spectrum that is visible to the human eye
Of the visible spectrum:
◦ Red has the longest wavelength
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Light is composed of electric and magnetic
charges that leave from a source in waves to
radiate in all directions
The waves form concentric wave fronts, which
vibrate up and down perpendicular to the line
of travel
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Light is composed of invisible particles called
photons with characteristic frequencies
As electrons in an atom absorb photons they
get to an excited state with increased
vibrations, and gain energy from the photon
◦ This absorbed energy may be released as light
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Each theory is useful
Short waves behave like particles
Long waves behave more like waves
Light has characteristics of both, and behaves
as either, or both, depending on the
circumstance
When light is absorbed (i.e. Fluorescence) or
amplified (i.e. lasers) it behaves according to
particle theory
When light is refracted, reflected or polarized,
it behaves according to wave theory
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Polarization
◦ The process of restricting the vibration directions of the
electromagnetic wave to only one direction
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Reflection
◦ The turning back of light waves from the boundary of a
medium
◦ angle of incidence = angle of reflection
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Refraction
◦ The bending of light rays as they pass obliquely from
one medium to another of a different optical density
◦ angle of incidence ≠ angle of refraction
Snell’s Law
of Refraction
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Determines the speed of light through a
substance by comparing it with the speed of
light in air
This comparison is the index of refraction
The index of refraction of air is 1.00
Index of refraction =
Speed of light in air
Speed of light in substance
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Light rays travelling
obliquely from a lesser
optical density to a
greater optical density
are refracted toward the
normal
Light rays travelling
obliquely from a greater
optical density to a lesser
optical density are
refracted away from the
normal
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Wedge-shaped, transparent medium that
bends light rays toward it’s base
A prism can be used to break light up into its
constituent spectral colors
Measured in units called Prism Diopters (∆)
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1 prism diopter deviates light 1cm at a
distance of 1 meter
Diopter is the measure of the vergence of
light
◦ D: dioptric power
◦ m: distance light has travelled in meters
D = 1/m
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Divergence
◦ Spreading apart of light rays as they leave an object
◦ Minus lenses
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Convergence
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Plano vergence
◦ Light rays coming together at a point of focus
◦ Does not occur naturally (needs optical device)
◦ Plus lenses
◦ Light rays are travelling parallel
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A spherical lens is an optical device that
bends light rays with the same power in all
directions
The optical centre of the lens has no
prismatic power
Lenses can be thought of as a combination of
prisms
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Convex
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Two prisms base to base
Converges light
Plus power
Thicker in the middle and thinner at the edge
Objects appear larger than true size
Concave
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Two prisms apex to apex
Diverges light
Minus power
Thinner in the middle and thicker at the edge
Objects appear smaller than true size
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The focal length of a lens is the distance from
the lens to the point of focus of that lens
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The dioptric power (D) of the lens is
reciprocal to its focal length in meters (m)
D = 1/m
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What is the focal length of a +2.00D lens?
D = 1/m
2 = 1/m
2m = 1
m = 0.5
The focal length is 0.5m or 50cm from the lens
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Lenses have two focal points
◦ One in front of the lens
◦ One behind the lens
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The focal points have the same focal distance
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The primary focal point of a plus lens is in
front of the lens
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The primary focal point of a minus lens is
behind the lens
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A light source
emanating from the
primary focal point
emerges parallel from
the lens
Parallel light rays
striking a lens come to
a focus point at the
secondary focal point
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Complicated!!!
A cylindrical lens has a flat and a curved
surface
It has a refracting power in one direction
The other direction is 90⁰ away and has zero
power  is called the cylinder axis
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Our optical system refracts light
What two structures can do this?????
Emmetropia
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Ammetropia
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◦ Light rays from distant objects refract to a clear focused
retinal image without accommodation
◦ The eye has no refractive error
◦ Eyes with refractive errors
◦ Need to use accommodation or corrective lenses to
correct defect in optical system
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Hyperopia (farsighted)
◦ Eye is too short
◦ Without accommodation, parallel light rays come to
a focus behind the retina
◦ An uncorrected hyperope has an optical system
with too little plus power
◦ Can use correction or accommodate
 Patient pulls the image onto the fovea by adding plus
from their own lens
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Myopia (nearsighted)
◦ Eye is too long
◦ The optical system will refract parallel rays of light
from infinity to a focus in front of the retina
◦ An uncorrected myope has an optical system with
too much plus power
◦ There is no option to reduce the power in this eye
except by wearing refractive correction or refractive
surgery
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Astigmatism
◦ A refractive error caused by the nonspherical
surface of the cornea or lens
◦ Like a football, it is more curved in one direction
than in the direction 90⁰ away
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Hyperopic Correction:
◦ +4.00D
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Myopic Correction:
◦ -6.00D
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Astigmatic Correction:
◦ +2.25-1.25x090
◦ -7.50+2.75x075
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If a patient is not looking through the optical
center of their lens, they will be experiencing
induced prism
◦ Can be done on purpose (to put prism into an Rx)
◦ Sometimes done in error (can be problematic)
Patient is looking through
base-out prism, which
induces an exotropia!!!
Low Vision Assessments
Optical Aids
Electronic Aids
Non-Optical Aids
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An assessment will include:
◦ Thorough history
◦ Information about eye condition
◦ Determination of goals of assessment
 What do they want to do?
 Do they need referrals to other services/agencies?
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Discussion about lighting
Discussion about glare
Current low vision aids?
Help in selection of low vision aids
To maximize their remaining vision!!!!
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Optical
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Electronic
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Non-Optical
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Many different shapes and sizes
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May be illuminated or non-illuminated
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Can be used for distance or near viewing
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The larger the lens, the weaker the magnification
and the farther away the lens can be held away
from the material
The stronger the lens, the less the depth of field
and the more critical it becomes to hold the lens
at the correct distance to be seen clearly
The strong the magnification, the smaller the
area the user can see through at any given time.
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The closer the lens is held to the eye, the
greater the area the user will see through
(regardless of magnification)
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Plastic is lighter than glass, but scratches
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Glass has better optics, but it’s heavy
American System
European System
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M = D/4 or
D=4M
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M = D/4 +1 or
D = 4M-4
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Example:
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Example:
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◦ A 12D lens is what
magnification?
12 = 4M
M=3
Answer: 3X magnification
◦ A 12D lens is what
magnification?
M = 12/4 + 1
M=4
Answer: 4X magnification
Advantages
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Variable work distance
Normal reading range
Conventional aid
Disadvantages
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Decreased field of view
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Need to use two hands
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Spot-reading tasks
Readily available in low
powers
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Difficult to maintain
focus
Difficult for patients with
hand tremors/arthritis
Advantages
Disadvantages
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Lens is mounted
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Decreased field of view
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Predictable focus
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Requires coordination
See details better
because is always
illuminated
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Good for patients with
arthritis/tremors
Not extremely portable
(some)
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Bulky
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Have to sit at desk/table
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Strong microscopic glasses
High hyperopic correction with base-in prism
Range in strength from +4.00D to +12.00D
Advantages
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Hands free
Disadvantages
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Fixed reading distance
The stronger the
glasses, the closer
things need to be held
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Wide field
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Prolonged reading
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Poor posture
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Monocular or binocular
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Illumination obstructed
Advantages
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Good for distance
viewing
Monocular or binocular
Disadvantages
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Restricted visual field
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Reduced focal range
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May have to focus
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Appearance
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Not popular with elderly
people!!
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CCTV
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Portable electronic magnifiers
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Computer programs (e.g. Zoomtext)
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Synthesized speech (e.g. EyePal)
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Lighting
◦ Better light, better sight
◦ Position
◦ Intensity of light
Reading stands
Filters
Felt-tip pens
Large print calendars/address books
Signature/writing guides
Talking clocks/talking watches
Large print cards/games
Large button phones
Liquid level indicators
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CNIB Library
◦ Reading club over the summer
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Low Vision Service
O&M Training
Independent Living Skills
Employment
Assistive Technology
Counselling/Peer Support Programs
November 18th Guest Speaker: Peter Parsons (APSEA O&M Instructor)
Please come prepared to discuss, and have at least one question
ready to ask Peter!!!