unit 4 Flashcards
Aberrations
- 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
Monochromatic Aberration
- 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
- Basic method for obtaining RMS wavefront error:
◦ 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.
Chromatic Aberration
- The refractive index of all the ocular media varies with wavelength (colour)
- Types
◦ Transverse chromatic aberration
◦ Longitudinal chromatic aberration
Transverse Chromatic Aberration
- 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
Longitudinal Chromatic Aberration
- 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
Optical Properties
- Contact lens magnification
- Accommodative demand
- Convergence demand
- Field of view
- Apparent eye size
Magnification
CLM= Image size corrected wither contact lens / Image corrected with spectacles
CLM= contact lens magnification
* Comparison of 2 different images
* Contact Lens Magnification (CLM)
CLM for Minus Powered Lens
- 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
CLM for Plus Powered Lens
- CLM for +10.00 D lens is 0.86 ◦ Meaning?
◦ The image size is smaller when wearing contact lenses compared to glasses for hyperopia
- Accommodation
The process of the eye changing its focus to maintain a clear image at near
- Myopia
Accommodative demand is HIGHER with contact lenses compared to spectacles
- Hyperopia
Accommodative demand is LOWER with contact lenses compared to spectacles
Convergence demand
- 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
Convergence Demand count…
- Spectacle minus lens creates BI prism
◦ Image moves towards apex or outward - Spectacle plus lens creates BO prism
◦ Image moves towards apex or inward
Field of View
- 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
Apparent Eye Size
- 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
Lens Effectivity
- 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
Closer Add Plus (CAP) Rule
- 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
Closer Add Plus (CAP) Examples - minus lens
- 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
Closer Add Plus (CAP) Examples. - plus lens
- 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
CAP rule notes..
- Note: Only applies to lens powers that are +/-4.00D or higher
◦ Lower lenses do not create enough of a power change
