GEPY 6911: Functional Implications of Visual
ImpairmentSession 9: Wednesday, November 04, 2015
Optics and Low Vision Devices
Any questions from last week’s lecture?
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
Optics
Low Vision Devices
Agenda
OpticsLight and TheoriesRefraction/ReflectionVergence of LightLensesRefractive Errors
Sun is a natural source for all of the energy that moves by electromagnetic radiation◦ Is classified according to its specific wavelength
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
Understanding Light
The Electromagnetic Spectrum
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
Wave Theory
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
Particle Theory
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
Which is right?
Polarization◦ The process of restricting the vibration directions of the
electromagnetic wave to only one direction
Reflection◦ The turning back of light waves from the boundary of a
medium ◦ angle of incidence = angle of reflection
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
Basic Optics Definitions
Polarization
Refection
Refraction
Snell’s Law of Refraction
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
Snell’s Law of Refraction
Index of refraction =Speed of light in air
Speed of light in substance
Snell’s Law of Refraction 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
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 (∆)
Prisms
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
Prisms continued
D = 1/m
Divergence◦ Spreading apart of light rays as they leave an object◦ Minus lenses
Convergence◦ Light rays coming together at a point of focus◦ Does not occur naturally (needs optical device)◦ Plus lenses
Plano vergence◦ Light rays are travelling parallel
Vergence of Light
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
Lenses
Convex ◦ 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◦ Two prisms apex to apex◦ Diverges light◦ Minus power◦ Thinner in the middle and thicker at the edge◦ Objects appear smaller than true size
Types of Lenses
The focal length of a lens is the distance from the lens to the point of focus of that lens
The dioptric power (D) of the lens is reciprocal to its focal length in meters (m)
Focal Length
D = 1/m
What is the focal length of a +2.00D lens?
2 = 1/m 2m = 1
m = 0.5
The focal length is 0.5m or 50cm from the lens
Example
D = 1/m
Lenses have two focal points◦ One in front of the lens◦ One behind the lens
The focal points have the same focal distance
The primary focal point of a plus lens is in front of the lens
The primary focal point of a minus lens is behind the lens
Focal Points
Focal Points continued 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
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
Cylinders
Our optical system refracts light What two structures can do this????? Emmetropia
◦ Light rays from distant objects refract to a clear focused retinal image without accommodation
◦ The eye has no refractive error
Ammetropia◦ Eyes with refractive errors◦ Need to use accommodation or corrective lenses to correct
defect in optical system
Refractive Errors
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
Ammetropic Refractive Errors
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
Ammetropic Refractive Errors
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
Ammetropic Refractive Errors
Hyperopic Correction:◦ +4.00D
Myopic Correction:◦ -6.00D
Astigmatic Correction:◦ +2.25-1.25x090◦ -7.50+2.75x075
Prescriptions for Glasses
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)
Induced Prism
Patient is looking through base-out prism, which induces an exotropia!!!
Low Vision DevicesLow Vision AssessmentsOptical AidsElectronic AidsNon-Optical Aids
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?
◦ Discussion about lighting◦ Discussion about glare◦ Current low vision aids?◦ Help in selection of low vision aids
Low Vision Assessment
The main goal of a low vision
assessment?
To maximize their remaining vision!!!!
Optical
Electronic
Non-Optical
Categories of Low Vision Aids
Many different shapes and sizes
May be illuminated or non-illuminated
Can be used for distance or near viewing
Optical Aids
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.
Optical Aids: Generalizations
The closer the lens is held to the eye, the greater the area the user will see through (regardless of magnification)
Plastic is lighter than glass, but scratches
Glass has better optics, but it’s heavy
Optical Aids: Generalizations
Diopters (D) and Magnification (M)
American System European System
M = D/4 or D=4M
Example:◦ A 12D lens is what
magnification?
12 = 4MM = 3
Answer: 3X magnification
M = D/4 +1 or D = 4M-4
Example:◦ A 12D lens is what
magnification?
M = 12/4 + 1M = 4
Answer: 4X magnification
Optical Aids: Hand MagnifiersAdvantages Disadvantages
Variable work distance
Normal reading range Conventional aid
Spot-reading tasks
Readily available in low powers
Decreased field of view
Need to use two hands
Difficult to maintain focus
Difficult for patients with hand tremors/arthritis
Optical Aids: Stand MagnifiersAdvantages Disadvantages
Lens is mounted
Predictable focus
See details better because is always illuminated
Good for patients with arthritis/tremors
Decreased field of view
Requires coordination
Not extremely portable (some)
Bulky
Have to sit at desk/table
Strong microscopic glasses High hyperopic correction with base-in
prism Range in strength from +4.00D to +12.00D
Optical Aids: Spectacle Aids
Optical Aids: Spectacle AidsAdvantages Disadvantages
Hands free
Wide field
Prolonged reading
Monocular or binocular
Fixed reading distance
The stronger the glasses, the closer things need to be held
Poor posture
Illumination obstructed
Optical Aids: TelescopesAdvantages Disadvantages
Good for distance viewing
Monocular or binocular
Restricted visual field
Reduced focal range
May have to focus
Appearance
Not popular with elderly people!!
CCTV
Portable electronic magnifiers
Computer programs (e.g. Zoomtext)
Synthesized speech (e.g. EyePal)
Electronic Aids
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
Non-Optical Aids
CNIB Library◦ Reading club over the summer
Low Vision Service O&M Training Independent Living Skills Employment Assistive Technology Counselling/Peer Support Programs
Shameless CNIB Plug!
Next week: No Class(Remembrance Day)
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!!!
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