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!!!