NCLE Basic Exam Study Guide Questions And Answers Rated A+

"K" is the ____ of the two meridians - flatter 4 refractive mediums - -Cornea -Aqueous -Crystalline Lens -Vitreous A ____ fitting lens is fitted flatter than K and will touch at the corneal apex with excessive movement over the cornea. It is not uncommon for this lens to fall off the eye or become easily decentered. Such a lens may cause abrasions at the corneal apex and may also cause corneal edema. - loose A lens with a steep CPC that vaults the corneal apex and touches the cornea at its peripheries is considered a ____ fittinglens. This lens will not ride freely over the cornea and will generally cause corneal edema and subsequent spectical blur. - tight Back Surface Toric Lens - -A contact lens that has two different and perpendicular radii of curvature on its posterior surface only -Has a spherical anterior surface -Only functional when manifest cylinder is 1.5 times greater than corneal astigmatism -Only used when the corneal toricity is so great that alignment in all meridians is not possible -Toric curves on the posterior surface are required when a spherical lens results in areas of excessive clearance or bearing example: -2.00-4.00x180 42.00@180/45.00@90 Refractive Cyl: 4.00D Corneal Cyl: 3.00DBitoric Lens - A contact lens that has two different and perpendicular radii of curvatures on both the anterior and the posterior surfaces Central Anterior Curves (CAC) - -Radius of curvature of the anterior portion of the lens -Determines power of the finished lens -Varies from lens to lens -Must compute with the CPC to yield the pre-determined power -Not relative to the fitting of the lens, only to its manufacture -Does not have to relate to the surface power of the cornea -Expressed in mm of radius of curvature -Manufacturers have slide rules/charts available to relate the CAC to a required power from a given CPC and thickness Central Posterior Curve (CPC) - -The Posterior radius of curvature designed to conform to the apical zone of the cornea -Must be highly polished and of definite optical quality -Referred to in terms of the surface power of the cornea to which it is related -Should be expressed in diopters, followed with its radius in mm enclosed in parenthesis. -AKA base curve or radius Complex formula for vertex distance - Fᴮ = ____Fᴬ____ *WVtx is less than RVtx 1-(dFᴬ) Fᴮ = ____Fᴬ____ *WVtx is greater than RVtx 1+(dFᴬ) Fᴬ = primary focal length of the RVtx Fᴮ = focal length of the compensated power at the WVtx (eyewear or CL) d = distance in meters that the lens has been moved*always round answer to the nearest 0.125D Corneal Topography - Axial Map - -Most widely understood map of the cornea for many CL fitters -Overall shape of the cornea -Colors relate to steepness/flatness -Sagittal reading measures the curvature of the cornea in diopters and is called the axial power map -Calculates curvature rather than power Corneal Topography - Elevation Map - -Shows the difference in the elevation of the cornea -Measured in microns Corneal Topography - Irregularity Map - -Similar to elevation maps -Uses best fit toric surface as a reference -Eliminates any toricity from the eye and displays the elevation differences