1. Lens Charateristics Flashcards
Physical lens characteristics vs. Optical lens characteristics
Phys lens characteristics (rep lens appearance, form, curves) vs. Optical lens characteristics (rep lens performance, image formation, optics)
Cylindrical surface - Min curvature vs. Max curvature
Cylindrical surface - Min curvature (zero, // to axis) vs. Max curvature (perpendicular to axis)
Toric surface - Base curve vs. Cross curve
Toric surface - Base curve (flatter meridian) vs. Cross curve (steeper meridian w/ greater overall P) - Doesn’t matter whether on concave or convex side of lens
Sag - Def
Sag = Saggital depth, Distance btwn midpt of chord and edge of circle, cent thickness of rigid CL (or cent thickness - edge thickness (plate height) ), height diff btwn outer pin and center pin, amount center pin travels past zero colinear pt (-s) = Center pin extends out from co-linear position
Sag - Formula
s = Pc h^2 / 2 (nc -1) = h^2 / 2r where s = sag, Pc = surface P det by lens clock = true base curve, h = 1/2 diameter / eye size / distance btwn two fixed outer pins of lens clock = 10.4 mm (std), nc = lens clock index = 1.53 (std), r = radius (of curvature)
Surface power - Formula
P = (nc-1) / r where P = surface P, nc = lens clock index = 1.53 (std), r = radius (of curvature)
Relationship btwn rigid CL thickness and P det by lens clock
Every 0.1- mm of rigid CL thickness rep 1 D of P det by lens clock
True base curve (TBC) vs. True (surface) ref P
True base curve (TBC) - (Measurement on lens surface det by lens clock, w/ lens index assumed equal to lens clock index = 1.53) vs. True (surface) ref P - (Actual P of lens surface w/ true lens index considered)
True (surface) ref P - Formula
Pt = Pc (nt -1) / (nc - 1) where Pt = true (surface) ref P, Pc = surface P det by lens clock = true base curve, nt = true lens index, nc = lens clock index = 1.53 (std)
Lens form - Def
Lens form - Relationship btwn front and back surface curvatures w/ each surface = Concave (-), Convex (+), or Plano (0) w/ infinite forms possible (spherical / toric / cylindrical concave / convex)
Spherical vs. Cylindrical vs. Toric surfaces
Spherical (same P in both meridians) vs. Cylindrical (P in one meridian, plano in other meridian) vs. Toric (diff P in diff meridians)
Possible lens forms - Determination
To det possible lens forms - Use two P crosses that can be added to get ideal resultant
Optimal lens form - Def
Optimal lens form - Best front and back surface curvatures of lens that reduce lens aberration, min lens mag problems, enhance visual performance (comfort, VA), ease of adaptability
Axis meridian vs. Power meridian
Axis meridian (aligned w/ axis in Rx, w/ uniform thickness) vs. Power meridian (90 deg from axis meridian, w/ thicker edges in (-) cyl lens), w/ thinner edges in (+) cyl lens
Base curve (for spectacle lens) - Def
Base curve (for spectacle lens) - Optimum surface P (often front surface P, in at least one meridian) of lens, resulting in min aberration for given Rx
Minus axis vs. Plus axis
Minus axis (w/ least minus P, so most minus is 90 deg away) vs. Plus axis (w/ least plus P, so most plus is 90 deg away)
Optimal base curve - Synonyms, Def
Optimal base curve (aka Best form aka Corrective curve) - (Base curve that best minimizes monochromatic lens aberrations for that particular lens surface and lens P
Rough lens blank vs. Semi-finished lens blank
Rough blank (both sides unfinished) vs. Semi-finished blank (large, thick blanks w/ front side finished by manu, back side unfinished)
Finished (uncut) lens blank vs. Stock finished lens blank
[Do NOT appear diff, just diff based on where they are finished] - Finished (uncut) lens blank (front side finished, back side unfinished»_space; finished by full service lab) vs. Stock finished lens blank (front side finished by manu, back side finished by manu)
Full service lab vs. Finishing lab
[Both complete finishing in-house, inc Edging - shaping lens to fit frame, Mounting - mount lens onto frame) Full service lab (completes surfacing in-house - turns semi-finished lens blanks (purchased from manu) into finished (uncut) lens blanks by surfacing back side) vs. Finishing lab (does NOT complete surfacing in-house - uses finished (uncut) lens blanks surfaced by full service lab, or uses stock finished lens blanks finished by manu)
Plus cyl form vs. Minus cyl form
Plus cyl form (cyl on front) vs. Minus cyl form (cyl on back - current, more common std, useful for making multifocal lenses where only have add on front surface where there is no cyl, useful for cosmetic appearance b/c hide variances in meridians on ocular side)
Convert Rx to P cross
Convert Rx to P cross - Use Rx: 1st # = Sph P (most plus meridian for minus cyl Rx) in both meridians of P Cross 1, 2nd # = Cyl P (on axis opp given axis, so 0 at given axis reserved for sph P in P Cross 2»_space; Add P Cross 1 + P Cross 2 = Resultant
Transpose (-) cyl to (+) cyl
Add sph + cyl, Change sign of cyl P, Change axis by 090
Crossed cyl - Def
Crossed cyl [like JCC] - Front surface w/ plus cyl, Back surface w/ minus cyl»_space; Resultant w/ meridians of same P (+) and (-) cyl