12 - Fresnel Prisms & Aniseikonia Flashcards
What is fresnel Prism? 3
-A “tower” of small, wide prisms
-equally powered prisms and gluing them, one above the other, onto a thin piece of plastic
-1mm thick.
What are 6 advantages of Fresnel Prism?
-Very thin
-Extremely lightweight
-Flexible
-Can be applied in house
-Can be cut into any shape
-Reduce magnification differences
What are 3 disadvantages of Fresnel Prism?
-Cosmetically different
-Harder to clean
-Slight loss of visual acuity caused by the reflections
What are 4 steps to applying Fresnel Prism?
-Check Base direction desired
-Cut the shape to match the lens with base direction in mind
-Peel the backside away
-Using backside of fresnel + warm soapy water adhere fresnel to lens. (Make sure their is no air bubbles).
When is fresnel used? 7
-High Prism Amounts
-Temporary prism to test the amount
-Sectional application
-Visual field defects
Prism in bifocal
-Cosemetic improvement(blind/turnt eye)
-Prism Bifocal areas only
-Treatment of nystagmus
What is Anisometropia?
Having two eyes have a different refractive power, so there is unequal focus between the two eye,
(slightly different shapes and sizes)
What is the relationship between spectacle lenses and anisometropia? 4
even if anisometropia is corrected with specs, problems arent always over
-spec lenses can create difficulties
-spec lenses worn at a distance from the eye will + or - objects viewed
-different lens powers magnify different amounts.
What is happening when the two lenses in a pair of glasses are of differing powers?2
The image viewed will not be the same size if the lens are of differing powers.
The brain tries to fuse these images into a single object
What is Aniseikonia?
A relative difference in the size/shape of the images seen by the right and the left eyes
Name the 6 types of Aniseikonia? (Name the subcategories)
-Physiologic - Natural/Anomalous
-Symmetrical- classic symmetrical/meridional
-Asymmetrical
-Anatomic
-Optical
-Relative spectacle Magnification
What is Physiologic Aniseikonia? 2 subcategories
Natural Aniseikonia occurs in all eyes regardless of rx and is dependant on the distance the object is from each eye.
Clinically significant aniseikonia = anomalous
What is Symmetrical Aniseikonia? 2 subcategories
-Symmetrical Aniseikonia -1 eye sees an image equally larger in all meridians than the other
-Meridional Aniseikonia - occurs when their is a size difference in a meridian of one eye compared with the other eye (found in any meridian)
What is Asymmetrical Aniseikonia?
Progressive increase/decrease across the visual field
Image gets progressively larger across the visual field
What is Anatomic Aniseikonia ?
Caused by unequal distrubution of rods and cones in one eye vs the other
What is Optical Aniseikonia? 2 subcategories
Inherent optical aniseikonia - aniseikonia because of optics of the eye
Induced Aniseikonia - because of outside source (correcting ophthalmic lenses)
What is Relative Spectacle Magnification Aniseikonia? 2
The amount of magnifcation produced by the eye relative to a standard eye.
Compares a corrected ametropic eye with a standard emmetropic eye.
How do we determine if it’s present in a px? Step 1
you are concerned but no clear evidence = First Pass Method
How do we determine if it’s present in a px? Step 2
Fairly certain A is present, want to address but cant measure it = directionally correct magnification changes to each lens individually.
How do we determine if it’s present in a px? Step 3
Estimate percent magnificationdifferences based on refractive prescription and change lens parameters accordingly
How do we determine if it’s present in a px? Step 4
Measure the percent magnification differences between the two eyes and change the lens parameters accordingly
What is the First Pass Method? 4
-Use a frame with a short vertex distance
-use a framer with a small eye size
-use an aspheric lens design-flattens base curve
-use high-index lens material-thins plus lens center thickness
What are the 3 categories Aniseikonia?
-Anisohyperopia - requires hyperopic corrective lenses
-Anisomyopia-requires myopic corrective lenses
-Antimetropia - One eye requires a myopic correction and a hyperopic correction.
How do we reduce the effects of Anisohyperopia?
2 then Higher+lower +rx lenses (3 points each)
-small vertex distance
-eyesize small
Higher Plus lens:
-flatten BC
-thin
-decrease vertex distance
Lower Plus lens:
-steepen the BC
-Increase centre thickness(not thicker than high plsu lens)
-move bevel towards the back of the lens, moves lens forward which increases magnifcaiton.
How do we reduce the effects of Anisomyopia?
3 then Higher+lower +rx lenses (2 points each)
-small vertex distance
-eyesize small
-avoid BC changes especially if over -2.00D
Higher Minus Lens:
-Move bevel towards front of lens decrease the vertex distance
-If BC needs to be changes the center thickness may need to be thickened
Lower Minus Lens:
-Edge the bevel back (increasing vertex distance)
- Do not thin the lens
How do we reduce the effects of Antimetropia? 2 then Plus (3 points) + Minus (2 points)
-small vertex distance
-eyesize small
Plus Lens:
-Flatten the BC
-Thin the lens
-Decrease the vertex distance
Minus Lens:
-Do not thin
-Decrease the vertex distance by moving bevel to front of the lens.
