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Contact Lens year 1 > unit 4 > Flashcards

unit 4 Flashcards

1
Q

Aberrations

A
  • Property that causes light to be spread out over an area rather than focused on a particular point
  • Image is blurred or distorted
  • Types
    ◦ Monochromatic
    ◦ Chromatic
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2
Q

Monochromatic Aberration

A
  • Present with a single wavelength of light * Can arise from a variety of causes
  • Classical Seidel aberrations:
    ◦ Spherical aberration
    ◦ Coma
    ◦ Oblique astigmatism
    ◦ Field curvature
    ◦ Distortion
  • Spherical aberration most significant for contact lenses
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3
Q
  • Basic method for obtaining RMS wavefront error:
A

◦ Divide the pupil into equal small areas
◦ Add the squared values of the wavefront error for each small area
◦ The sum is divided by the number of areas and the square root gives the RMS wavefront error.

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4
Q

Chromatic Aberration

A
  • The refractive index of all the ocular media varies with wavelength (colour)
  • Types
    ◦ Transverse chromatic aberration
    ◦ Longitudinal chromatic aberration
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5
Q

Transverse Chromatic Aberration

A
  • Occurs when the size of the image changes with wavelength
  • Red, yellow, blue light focus at separate points in the vertical plane
  • Most of an issue with glasses
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6
Q

Longitudinal Chromatic Aberration

A
  • Occurs when different wavelengths focus at different points on the horizontal axis due to the dispersion properties of the lens material
  • Significant for multifocal contact lenses
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7
Q

Optical Properties

A
  • Contact lens magnification
  • Accommodative demand
  • Convergence demand
  • Field of view
  • Apparent eye size
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8
Q

Magnification

A

CLM= Image size corrected wither contact lens / Image corrected with spectacles
CLM= contact lens magnification
* Comparison of 2 different images
* Contact Lens Magnification (CLM)

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9
Q

CLM for Minus Powered Lens

A
  • Will not go over complex calculation
    ◦ Depends upon vertex distance
  • CLM for -10.00 D lens is 1.14 ◦ Meaning?
    ◦ The image size is larger when wearing contact lenses compared to glasses for myopia
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10
Q

CLM for Plus Powered Lens

A
  • CLM for +10.00 D lens is 0.86 ◦ Meaning?
    ◦ The image size is smaller when wearing contact lenses compared to glasses for hyperopia
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11
Q
  • Accommodation
A

The process of the eye changing its focus to maintain a clear image at near

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12
Q
  • Myopia
A

Accommodative demand is HIGHER with contact lenses compared to spectacles

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13
Q
  • Hyperopia
A

Accommodative demand is LOWER with contact lenses compared to spectacles

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14
Q

Convergence demand

A
  • Prismatic effect with spectacles when off optical center
  • No prismatic effect with contact lenses (bc the lenses are on your eyes)
    ◦ Contact lens moves with the eye
  • Myopia
    ◦ INCREASED convergence demand with CL compared to spectacles
  • Hyperopia
    ◦ DECREASED convergence demand with CL compared to spectacles
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15
Q

Convergence Demand count…

A
  • Spectacle minus lens creates BI prism
    ◦ Image moves towards apex or outward
  • Spectacle plus lens creates BO prism
    ◦ Image moves towards apex or inward
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16
Q

Field of View

A
  • Increased field of view with contact lenses compared to spectacles
  • Contact lenses move with the eye
  • Spectacle edge of lens and frame reduce field of view
17
Q

Apparent Eye Size

A
  • Cosmetic issue
    ◦ Affected by vertex distance and power
  • Plus powered lenses
    ◦ Eyes look larger with spectacles compared to contact lenses
  • Minus powered lenses
    ◦ Eyes look smaller with spectacles compared to contact lenses
18
Q

Lens Effectivity

A
  • Different effective powers for a contact lens versus a spectacle lens
  • The contact lens power will NOT be the same as the glasses power for higher prescriptions
  • Due to distance from the cornea
    ◦ Contact lens = 0 vertex distance
    ◦ Spectacles = may be 12 or 13 mm vertex distance
19
Q

Closer Add Plus (CAP) Rule

A
  • When a lens is moved from the spectacle plane to the corneal plane, plus power needs to be added to the contact lens in order to maintain the same effective power
20
Q

Closer Add Plus (CAP) Examples - minus lens

A
  • Minus lens (myopia): Moving towards cornea increases effective power so you need to add +0.25 D to the contact lens power
    ◦ Result = lower minus powered lens in a contact lens compared to the glasses
21
Q

Closer Add Plus (CAP) Examples. - plus lens

A
  • Plus lens (hyperopia): Moving toward cornea decreases effective power so you add +0.25 D to the contact lens power
    ◦ Result = higher plus powered lens in a contact lens compared to the glasses
22
Q

CAP rule notes..

A
  • Note: Only applies to lens powers that are +/-4.00D or higher
    ◦ Lower lenses do not create enough of a power change

Contact Lens year 1 (9 decks)

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