Labs: Lens Design (M2) Flashcards
What is the minimum center thickness for a back vertex power of less than +2.00D? 1. CT for greater than or equal to +2.00D? 2
- 2mm
2. Fv + 0.5mm
What is the spectacle magnification approximation formula?
%SM = tF1/15 + hFv/10 t = center thickness (mm) h = lens to entrance pupil distance (mm) = vertex distance + 3mm F1 = front surface power Fv = back vertex power
What problems does uncorrected anisometropia lead to?
- refractive amblyopia
- poor stereopsis
- headaches
- discomfort
What is more likely to cause distortions and discomfort in glasses: difference in sphere power or difference in cylinder power and/or axis?
difference in cylinder power and/or axis
How can you make an astigmatism correction more comfortable and reduce distortion?
bi-toric lens: cylinder ground into front and back of lens
What are lenses that are designed to correct the image size difference between two eyes and corrects the refractive error?
eikonic lenses
For axial anisometropia, do spectacles or contacts result in equal retinal image sizes? 1. For refractive anisometropes? 2
- spectacles
2. contacts
What is a plano lens that shape magnifies an image and can be overall or meridional?
afocal magnifier
What is a change in image size in an eye brought about by spec Rx called?
spectacle maginification
For a axial myope, do spectacles magnify or minify the retinal image (how does it compare to emmetropic eye)? 1. For axial hyperope (compare to emmetropic eye)? 2
- minify (same as emmetopic eye)
2. magnify (same as emmetopic eye)
What are the clinical tests that can be used to test for size difference?
- space eikonometer
- leaf room
- aniseikonia inspector
- Awaya test
- Brecher’s procedure
What problems does corrected anisometropia lead to?
- induced prism
- poor cosmetic appearance
- weight differences
- aniseikonia
For refractive ametropia, once corrected how does the retinal image size compare to an emmetropic eye?
not the same
What are the limitations of F1? 1. Limitations of F2? 2
- greater than or equal to +1.00
2. less than or or equal to -2.00
What does the ocular image depend on?
- retinal image (optics)
- retinal mosaic
- neuronal processing along visual pathway
- final representation in visual cortex
What is the law that explains when a corrective lens placed at the anterior focal point of the eyes creates a retinal image size equal to that of the standard emmetropic eye? 1. What is this only true for? 2
- Knapp’s law
2. axial ametropia
What percentage of anisometropia occurs per 1.00 D difference?
1%
How is a Brecher’s procedure performed to test for size difference?
neutralize size difference between lights and lines using 2 penlights, a Maddox rod and size lenses
In order to increase the spectacle magnification, what should be done?
- inc vertex dist for plus lens, dec vertex dist for minus lens
- make lens thicker (better for minus lens)
- make base curve steeper (better for plus lens)
- decrease refractive index (minimal)
What is the tolerance limit for anisometropia for the sphere? 1. Cylinder? 2. Oblique Cylinder? 3
- 1.50D
- 1.00D
- 0.50D
What is the amount of size difference that can lead to aniseikonia with symptoms?
over 0.75%
What is the rule of sixes of the limits of spectacles?
- less than or equal to 6D difference btw BC’s btw the two eyes
- less than or equal to 6D cylinder on the front surface
- less than or equal to 6D from the corrected curve lens design
What is a relative difference in size and/or shape of the ocular images called?
aniseikonia
What are the symptoms of aniseikonia?
- headaches and asthenopia
- photophobia, tearing, fatigue
- space distortion
- size difference (not common)
What is the equation for the “ideal BC” according to Vogel’s corrected curve for plus lenses? 1. Minus lenses? 2
- BC = SphEq + 6D
2. BC = SphEq/2 +6D
