5/12/2014 1 Lynn Lawrence, CPOT, ABOC, COA Please prepare for certification testing with materials from the organization in which you want your certification granted i.e.: AOA JCAHPO ABO NCLE OT The Visual System Physiology Ametropias Ophthalmic Lenses Properties of Light ▪ UV, Visible, and Infrared Spectrums ▪ Reflection, Refraction, & Absorption ▪ Aberrations Lens Designs ▪ Fitting Basic Optical Formulas There are two main refractive bodies in the human eye... There are two main refractive bodies in the human eye... Cornea Cornea Crystalline Lens Crystalline Lens The cornea is the primary refractive element in the eye... The cornea is the primary refractive element in the eye... Cornea Cornea ... with a power of around +42-44 diopters ... with a power of around +42-44 diopters The tear film maintains both the health and optics of the cornea... The tear film maintains both the health and optics of the cornea... Cornea Cornea
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5/12/2014
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Lynn Lawrence, CPOT, ABOC, COA
� Please prepare for certification testing with
materials from the organization in which you want your certification granted i.e.:
� AOA� JCAHPO
� ABO
� NCLE� OT
� The Visual System� Physiology
� Ametropias
� Ophthalmic Lenses� Properties of Light▪ UV, Visible, and Infrared Spectrums
▪ Reflection, Refraction, & Absorption
▪ Aberrations
� Lens Designs▪ Fitting
� Basic Optical Formulas
There are two main refractive bodies in
the human eye...
There are two main refractive bodies in
the human eye...CorneaCornea
Crystalline LensCrystalline Lens
The cornea is the primary
refractive element in the eye...
The cornea is the primary
refractive element in the eye...
CorneaCornea
... with a power of around +42-44 diopters... with a power of around +42-44 diopters
The tear film maintains
both the health and optics of the cornea...
The tear film maintains
both the health and optics of the cornea...
CorneaCornea
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The crystalline lens
has a power of around +12-18 diopters...
The crystalline lens
has a power of around +12-18 diopters...
Crystalline LensCrystalline Lens
...it is primarily responsible for changing the eye’s focal point…a term referred to as accommodation
...it is primarily responsible for changing the eye’s focal point…a term referred to as accommodation
The retina is the “film” or sensory body...The retina is the “film” or sensory body...
RetinaRetina
Light is converted to electrical impulses which are sent through the optic nerve...
Light is converted to electrical impulses which are sent through the optic nerve...
RetinaRetina
...the “blind spot” is the point at which the optic nerve connects
...the “blind spot” is the point at which the optic nerve connects
FoveaFovea
MaculaMaculaOpticDisc
OpticDisc
The eye is connected
to the orbit by several muscles which control
movement...
The eye is connected
to the orbit by several muscles which control
movement...
Eye Movement or ocular motilityEye Movement or ocular motility
OrbitOrbit
AmetropiasAmetropias
In a “normal” eye, parallel light is brought
to a focus on the retina...
In a “normal” eye, parallel light is brought
to a focus on the retina...
...an eye free of refractive
error is called “emmetropic”
...an eye free of refractive
error is called “emmetropic”
AmetropiasAmetropias
An eye with excessive focal power for its length
focuses light in front of the retina...
An eye with excessive focal power for its length
focuses light in front of the retina...
...this refractive error is
known as “myopia”
...this refractive error is
known as “myopia”
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AmetropiasAmetropias
An eye with insufficient focal power for its length focuses light in behind the retina...An eye with insufficient focal power for its length focuses light in behind the retina...
...this refractive error is known as “hyperopia”
...this refractive error is known as “hyperopia”
AmetropiasAmetropias
Another refractive error can occur if every axis is not refracted evenly...Another refractive error can occur if every axis is not refracted evenly...
...this refractive error is known as “astigmatism”
...this refractive error is known as “astigmatism”
AmetropiasAmetropias
As the eye ages, the crystalline lens loses flexibility...As the eye ages, the crystalline lens loses flexibility...
...this results in a
condition known as
“presbyopia”
...this results in a
condition known as
“presbyopia”
PresbyopiaPresbyopia
The inability to focus on near objects becomes
noticeable around age 40 and steadily worsens thereafter...
The inability to focus on near objects becomes
noticeable around age 40 and steadily worsens thereafter...
� Ophthalmic Lenses� Properties of Light
▪ UV, Visible, and Infrared Spectrums
▪ Reflection, Refraction, & Absorption
� Lens Designs
� Lens Materials
Properties of LightProperties of Light
Like the artist’s paint, the eye requires light to see...Like the artist’s paint, the eye requires light to see...
...what is “light,” and how does it work? ...what is “light,” and how does it work?
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� particles in the form of a wave� electromagnetic radiation with wavelengths between 400
and 700 nanometers (a nanometer is 1/1,000,000th mm) is considered the visible spectrum
� white light is composed of all wavelengths
Properties of LightProperties of Light
So what is light?So what is light?
� visible light is a small portion of the overall spectrum of light� EM surrounding the visible spectrum is hazardous� Ultra-Violet is <390nm
� Infrared is >720nm
Properties of LightProperties of Light
So what is light?So what is light?
Properties of LightProperties of Light
Ultra-Violet RadiationUltra-Violet Radiation
UVA is less photobiologically active than UVB and consists of light just beyond the blue end of the visible spectrum...
UVA is less photobiologically active than UVB and consists of light just beyond the blue end of the visible spectrum...
AA
Properties of LightProperties of Light
Ultra-Violet RadiationUltra-Violet Radiation
UVB falls farther from the visible spectrum, is the most significant UV most of us experience, and is variably absorbed by ozone...
UVB falls farther from the visible spectrum, is the most significant UV most of us experience, and is variably absorbed by ozone...
AA BB
Properties of LightProperties of Light
Ultra-Violet RadiationUltra-Violet Radiation
UVC light is farthest from the visible spectrum, and is almost completely absorbed by ozone (except at very high altitudes)....
UVC light is farthest from the visible spectrum, and is almost completely absorbed by ozone (except at very high altitudes)....
AA BB CC
Properties of LightProperties of Light
InfraredInfrared
Infrared is >720nm- the most common source is blown glass...
Infrared is >720nm- the most common source is blown glass...
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Properties of LightProperties of Light
Movement of LightMovement of Light
Light moves in wave form along a straight line...Light moves in wave form along a straight line...
...the distance between peaks determines wavelength...the distance between peaks determines wavelength
Properties of LightProperties of Light
Movement of LightMovement of Light
As light spreads from a point, it forms a wavefront...As light spreads from a point, it forms a wavefront...
� Wavefronts interact with lenses in 3 ways
� Reflection
� Refraction
▪ prism
▪ dispersion
� Absorption
Properties of LightProperties of Light
Movement of LightMovement of Light
Properties of LightProperties of Light
ReflectionReflection
Every lens has a “critical angle”...Every lens has a “critical angle”...
...light striking the lens flatter than
this angle will be reflected
...light striking the lens flatter than
this angle will be reflected
Properties of LightProperties of Light
RefractionRefraction
Light striking at an angle steeper than critical is
refracted...
Light striking at an angle steeper than critical is
refracted...
...the amount of refraction depends
on the amount of prism
...the amount of refraction depends
on the amount of prism
Properties of LightProperties of Light
Refraction - PrismRefraction - Prism
Prism bends light
towards its base...
Prism bends light
towards its base...
...but displaces images towards its
apex
...but displaces images towards its
apex base
apex
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Properties of LightProperties of Light
Refraction - PrismRefraction - Prism
Light Rays
Plus lenses are prisms placed base to base...Plus lenses are prisms placed base to base...
...resulting in convergence...resulting in convergence
Properties of LightProperties of Light
Refraction - PrismRefraction - Prism
Light Rays
Minus lenses are prisms placed apex to apex...Minus lenses are prisms placed apex to apex...
...resulting in divergence...resulting in divergence
When light is not focused on the retina, an
ametropia is present...
When light is not focused on the retina, an
ametropia is present...
E
for example, a myopic
eye focuses light in front of the retina...
for example, a myopic
eye focuses light in front of the retina...
E
...a minus powered (diverging) lens shifts the
focus back to the retina
...a minus powered (diverging) lens shifts the
focus back to the retina
E
(unaided focal length of eye)(unaided focal length of eye)
(focal length of eye + lens)(focal length of eye + lens)
E
A hyperopic eye focuses light
behind the retina...
A hyperopic eye focuses light
behind the retina...
E
...a plus powered (converging) lens shifts the
focus up to the retina
...a plus powered (converging) lens shifts the
focus up to the retina
E
(unaided focal length of eye)(unaided focal length of eye)
(focal length of eye + lens)(focal length of eye + lens)
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E E
Astigmatism causes light along
different axes to focus at different
planes...
Astigmatism causes light along
different axes to focus at different
planes...
E E
...a cylindrical lens is prescribed to compensate
for astigmatism in the eye- creating a common focal point on the retina...
...a cylindrical lens is prescribed to compensate
for astigmatism in the eye- creating a common focal point on the retina...
(multiple focal lengths in unaided eye)(multiple focal lengths in unaided eye)
(common focal lengths of eye + lens)(common focal lengths of eye + lens)
E E
As previously mentioned, the crystalline lens
enables the eye to focus on near objects...
As previously mentioned, the crystalline lens
enables the eye to focus on near objects...
accommodationaccommodation
E E
...as the crystalline lens ages, it loses its
flexibility- and its ability to adjust focus...
...as the crystalline lens ages, it loses its
flexibility- and its ability to adjust focus...this condition is called presbyopia and typically
becomes noticeable around age 40
this condition is called presbyopia and typically
becomes noticeable around age 40
E E
To restore near vision, the required convergence
is supplied by a plus powered lens...
To restore near vision, the required convergence
is supplied by a plus powered lens...
Single Vision lens (NVO)Single Vision lens (NVO)
single point of focussingle point of focus
...this plus lens may stand alone (NVO) or may
work with distance correction (ADD)
...this plus lens may stand alone (NVO) or may
work with distance correction (ADD)
E E
ADDADDDistance RxDistance Rx
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Properties of LightProperties of Light
Refraction - DispersionRefraction - Dispersion
All prisms break white light into its
component colors...
All prisms break white light into its
component colors...
...this is called dispersion and results in chromatic aberration...this is called dispersion and results in chromatic aberration
W h i t e L i g h t
Dispersed Light
Properties of LightProperties of Light
Refraction - DispersionRefraction - Dispersion
Abbe value is used to describe the amount of dispersion a material will create...
Abbe value is used to describe the amount of dispersion a material will create...
...ranges are from 59 (CR-39) to 30 (polycarbonate)
...ranges are from 59 (CR-39) to 30 (polycarbonate)
Chromatic AberrationChromatic Aberration
Properties of LightProperties of Light
AbsorptionAbsorption
A dark lens absorbs light...A dark lens absorbs light...
White light is composed of all
colors in the rainbow- but all colors can be formed using a combination of three “primary
colors:”
White light is composed of all
colors in the rainbow- but all colors can be formed using a combination of three “primary
colors:”Red
Yellow
Blue
Red
Yellow
Blue
Properties of LightProperties of Light
AbsorptionAbsorption
Filters absorb light. Chemical compounds are used
to selectively filter single colors.
Filters absorb light. Chemical compounds are used
to selectively filter single colors.
To create a green filter, only
red light must be absorbed. The appropriate chemicals are applied, and a green filter is
created.
To create a green filter, only
red light must be absorbed. The appropriate chemicals are applied, and a green filter is
created.
Properties of LightProperties of Light
AbsorptionAbsorption
Properties of LightProperties of Light
AbsorptionAbsorption
To create a perfectly neutral- or gray- filter, energy from
all three primary points in the spectrum must be
absorbed to the same degree.
To create a perfectly neutral- or gray- filter, energy from
all three primary points in the spectrum must be
absorbed to the same degree.
When creating a filter by
absorbing dye into resin, this
process is further complicated by
the inconsistent nature of the
chemicals involved.
When creating a filter by
absorbing dye into resin, this
process is further complicated by
the inconsistent nature of the
chemicals involved.
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Properties of LightProperties of Light
AberrationsAberrations
The most common aberrations found in ophthalmic
lenses are:
The most common aberrations found in ophthalmic
lenses are:
� Power Error
� Material Distortion
� Marginal Astigmatism
� Chromatic Aberration
� Unwanted Prism*
Properties of LightProperties of Light
AberrationsAberrations
Aberrations occur due to various factors:Aberrations occur due to various factors:
� Refractive power
� Off-axis viewing of objects� lens tilt
� peripheral objects
� Vertex distance� Lens material
Properties of LightProperties of Light
Aberrations – Power ErrorAberrations – Power Error
Spherical lenses are stronger in the periphery...Spherical lenses are stronger in the periphery...
Properties of LightProperties of Light
Aberrations – DistortionAberrations – Distortion
The minus lens result is
barrel distortion...
The minus lens result is
barrel distortion...
...the periphery of an object will be minimized to a
greater degree than the center
...the periphery of an object will be minimized to a
greater degree than the center
Properties of LightProperties of Light
Aberrations – DistortionAberrations – Distortion
Plus lenses create the opposite
effect...pincushion effect
Plus lenses create the opposite
effect...pincushion effect
...the periphery of an object will be magnified to a
greater degree than the center
...the periphery of an object will be magnified to a