Optics and Optical Devices

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Optics and Optical Devices
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Learning Objectives

• Understand basic optical principles of lenses
• How lenses manipulate light
• Distinguish between types
• Determine focal length and power of lenses
• Understand correction of refractive errors
• Identify the appropriate lens for each type
• Use magnifiers appropriately
• Describe properties of each type
• Describe correct technique for using each type

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Optical Devices

• Use lenses or prisms (or a combination) to focus
  light more precisely onto the fovea
• Enable retina to capture the highest quality
  visual data
• Enable CNS to see accurate and complete images

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Types of Optical Devices
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Types of Optical Devices

• Contact lenses
• Eyeglasses
• Magnifiers
• Binoculars
• Telescopes
• Microscopes
• Electronic magnifiers

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Optical devices correct for deficiencies in the
ocular system

• Inherited imperfections
• Acquired imperfections
• Imperfections caused by disease
• Imperfections caused by trauma
• Assist in situations where super vision is
  needed

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Differ in Price and Complexity

• Free magnifier in cereal box
• Basic 10 handheld from Walmart
• Precision made 200 magnifier
• 300 bifocals
• 1,000 plus electronic magnifiers
• Million dollar Hubble telescope

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Each device is only as good as the human using it
and the CNS receiving the image.
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Basic Optical Principles
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Measurement of Light
Long wave
Short wave

• Moves in electromagnetic waves
• See peaks and valleys
• Distance between peaks is a wavelength

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White Light

• Only component of electromagnetic waves that is
  visible
• Mixture of all the colors of the rainbow
• Each color has its own wavelength
• Red is the longest wavelength
• Violet is the shortest
• Beyond ends of this visible spectrum is invisible
  light
• Ultraviolet, x rays, gamma rays on short end
• Infra red radiation, radio waves on the long end

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Principles of Refraction

• A light ray striking the flat surface of a glass
  lens at a 90 degree angle will pass through
  without bending

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Principles of Refraction cont

• A light ray striking the surface of a lens which
  is curved will bend toward an imaginary line
  perpendicular to the edge of the surface of the
  glass

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Refractive Index

• Measurement reflecting the density of the lens
• Speed of light as it passes through the medium (
  the lens) will vary with the density of the
  medium
• The denser the medium, the slower the light
  passes through
• The slower the light passes through, the greater
  the refractive index

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• The greater the angle of the surface and the
  slower the index of refraction, the greater the
  refractive power of the lens

High refractive index
Low refractive index
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Why do we need refraction?

• At a distance of 20 feet or greater
• Light rays travel in parallel lines
• No refraction is needed to focus on fovea
• Closer than 20 feet
• Light rays begin to diverge
• And must be refracted to focus on the fovea

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Role of Ocular System in Refraction

• Ocular system ensures that light rays are focused
  on the fovea
• Light passes through 4 transparent structures
  before reaching retina

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Role of Ocular System in Refraction

• Ocular system ensures that light rays are focused
  on the fovea
• Light passes through 4 transparent structures
  before reaching retina
• Cornea
• Aqueous
• Lens
• Vitreous

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• As light passes through each structure, it is
  slowed down and bent

vitreous
cornea
lens
aqueous
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Cornea and lens provide the greatest refractive
power of the system

• Cornea
• High because air is less dense than cornea
• Lens
• High because it can change shape and be made very
  thick

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Total refractive power is determined by shape of
lens
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Greater the curvature of the lens the greater the
refraction
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Moving an image closer or farther away from the
lens will cause the focal point to move closer or
farther to the lens
E
E
F
F
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Reason for Accommodation

• When an object comes closer to the lens, the
  distance between the focal point and the lens
  will increase, causing the image to go out of
  focus
• Accommodation counters this by increasing
  refraction of the lens to shorten the focal point
  distance

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E
E
Accommodation
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A variety of lenses have been developed to help
persons achieve better focus and see larger images
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Optical Spherical Lenses

• Transparent, made of glass or plastic
• Have two sides, at least one of which is curved

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Concave lens
( minus lens)
Convex lens
(plus lens)
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Plano
Bi convex
Bi concave
Plano convex
Plano concave
Meniscus
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Aspherical lens

• There is greater light scatter at the edges of
  all lenses
• Causes chromatic aberration
• Image looks fuzzy on edges
• Aspherical lens have slight alterations made to
  curvature to reduce aberrations
• Person will experience fewer aberrations if
  he/she views directly through the CENTER of the
  lens
• Aka the optical center

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Prism changes direction of image as it passes
through the surface
Rays of light stay parallel and do not converge
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Prism

• Used to shift images on the retina
• Displaces image towards the apex of the prism
• Used in correction of strabismus
• Person wearing very strong plus lenses will need
  prism to assist in convergence

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Lenses have varying refractive strengths

• Refractive strength of lens is measured in
  diopters
• Magnifiers range between 2-60
• Stronger the lens, sharper the curvature and
  density of the lens material
• Also the shorter the focal distance between the
  rear of the lens and the focal point

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10 diopter lens
Focal point
50 diopter lens
Focal point
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Formula for determining dioptic strength of a
magnifier

• Divide 100 by the focal length of the magnifier
  (measured in centimeters)
• Example
• Lens brings image into focus at 5 cm
• 100 20 diopters of strength
• 5

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• If you know the focal length of the magnifier you
  can determine the diopters
• AND VICE VERSA
• If you know the diopters of the magnifier you can
  determine the correct focal length

Magnifier is 20 diopters in strength
100

5
centimeter focal length
20
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Important to Remember

• Strength of magnifiers (especially low power
  ones) is sometimes given in X units instead of
  diopters
• There is no standardization of X units but
• 1 X approximately 4 diopters

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Why is it important to know these formulas?
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Why is it important to know these formulas?
To determine the distance the patient should hold
the magnifier from the page to get the clearest
image
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Optical Correction of Refractive Errors

• With an emmetropic (perfect) optical system, the
  image comes into focus precisely on the retina
• If the optical system is imperfect, the image
  does not come into focus exactly on the fovea
• Person experiences a refractive error
• Objects appear blurry

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Myopia
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Correction with Concave Lens
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Hyperopia
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Correction with Convex Lens
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Astigmatism

• Image is blurred in one direction more than
  another
• WHICH TYPE OF LENS IS APPLIED?

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Astigmatism

• Image is blurred in one direction more than
  another
• WHICH TYPE OF LENS IS APPLIED?
• cylindrical

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Presbyopia

• Lens loses refractive power as it loses ability
  to change shape
• WHICH TYPE OF LENS IS APPLIED?

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Presbyopia

• Lens loses refractive power as it loses ability
  to change shape
• WHICH TYPE OF LENS IS APPLIED?
• Convex
• Applied as a reading add usually in a bifocal

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Reading Add

• Always a convex lens
• May be worn as a single lens
• Reading glasses found at Walmart
• When combined with a distance lens, it creates a
  bifocal
• Not always readily apparent to observer that
  patient has a bifocal if wearing progressive lens

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Bifocal Lens
Distance portion
Reading add
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Spectacle Prescription
OD
-1.25 -2.25 x 20 2.50
Spherical power to correct for myopia
Orientation astigmatism correction
Reading add
Amount of astigmatism correction needed
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Types of Magnification

• Relative Distance
• If you move closer to an object, the object
  becomes RELATIVELY larger

STOP
STOP
50 feet
10 feet
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Relative Size Magnification

• Object is made larger
• Viewed at the same distance, it appears
  RELATIVELY larger

Large print text is an example of relative size
magnification
Large print text is an example Relative size
magnification
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Angular Magnification

• Magnification achieved by lenses
• Makes objects at a distance appear closer to eyes
  and therefore larger

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Electronic Magnification

• A.k.a. Projection magnification
• Increases size of object through projection

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Convex Lenses

• When an object is between the convex lens and its
  focal point, the object will appear right side up
  and larger
• Closer the object is to the focal point, the
  larger the image will appear

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Convex Lenses
Field of view

• If an object is outside the focal range, it will
  appear upside down
• Only a limited area is visible inside the
  magnifier
• Called the magnifiers field of view

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Field of view varies with the strength of
magnifier
Weakest magnifier
Strongest magnifier
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General Rules of Magnifiers

• The stronger the power.
• The shorter the focal distance
• The smaller the field of view
• The heavier the magnifier

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You can hold a magnifier any where in relation to
the eye as long as the task material is held
within the focal distance of the magnifier
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Types of Magnifiers

• Pros and cons of these very important adaptive
  devices

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Hand Held Magnifiers

• Advantages
• Greater flexibility in placement
• Portable-can go into community
• Generally less expensive

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Hand Held Magnifiers

• Disadvantages
• More difficult to maintain proper focal distance
• Requires more hand strength and steadiness

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Stand Magnifiers

• Advantages
• Maintain a constant focal distance
• Can use stronger powers
• Less hand strength is needed
• Can be illuminated

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Stand Magnifiers

• Disadvantages
• Narrow field of view for stronger powers
• Has to be viewed monocularly
• Often awkward to position
• Can be expensive

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Head Worn/Hands Free

• Advantages
• Spectacles are familiar concept
• Portable
• Preserve more field of view
• Allow binocularity up to 8-10 diopters

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Head Worn.

• Disadvantages
• Difficult to adjust to and maintain close focal
  distance
• Heavy-can be uncomfortable
• Some types can be very expensive