1. Lens Charateristics Flashcards by J C

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)

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Cylindrical surface - Min curvature vs. Max curvature

Cylindrical surface - Min curvature (zero, // to axis) vs. Max curvature (perpendicular to axis)

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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

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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

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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)

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Surface power - Formula

P = (nc-1) / r where P = surface P, nc = lens clock index = 1.53 (std), r = radius (of curvature)

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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

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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)

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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)

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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)

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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)

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Possible lens forms - Determination

To det possible lens forms - Use two P crosses that can be added to get ideal resultant

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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

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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

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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

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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)

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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&raquo_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&raquo_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&raquo_space; Resultant w/ meridians of same P (+) and (-) cyl

Optic axis vs. Optical center vs. Lens poles

Optic axis (line joining centers of curvature of front and back surfaces, perpendicular to both surfaces) vs. Optical centers (pt on optic axis intersected by path of light incident on front, which is // to path of light after it is refracted by the back surface, BUT not necess on actual front and back surface of lens) vs. Lens poles (pts on optic axis actually on front and back surfaces of lens, Anterior pole - On convex side, Posterior pole - On concave side)

Linear motion viewed through Minus vs. Plus lens

Through Minus lens (WITH motion) vs. Through Plus lens (AGAINST motion or WITH motion, dep on where observer's eye is in relation to 2ndary focal pt of lens)

Circle of least confusion (blur circle) vs. Interval (Conoid) of Sturm

Circle of least confusion (at dioptric midpt btwn two focal pts) vs. Interval (Conoid) of Sturm (linear distance btwn front and back focal pts, vert meridian >> horiz line image, horiz meridian >> vert line image)

Image formation for spherical vs. cylindrical vs. toric lens

Image formation for spherical lens (pt image) vs. cylindrical lens (line image // to axis) vs. toric lens (two line images perpendicular to each other w/ Primary focus - most plus, if both meridians + >> primary image - focused closest to lens, Secondary focus - least plus, if both meridians + >> secondary image - focused furthest from lens) - (similar corresponding rules if minus)

Size of blur circle - Formula

h / b = y / v where h = height of blur circle, b = distance (blur circle to F2, image), y = diameter of lens, v = focal length of lens

Spherical equivalent - Def

Best avg overall Rx (where see Circle of Least Confusion, if circle imaged through sphero-cylindrical lens)

Spherical equivalent - Formula

SE = Sph + 1/2 (cyl), using given sign for cyl

Rx modification when change cyl

>> For every (-) 0.50 change in cyl, (+) 0.25 change in sph >> For every (+) 0.50 change in cyl, (-) 0.25 change in sph

Off axis P in toric lens - Formula

P(alpha) = Ps + Pc sin^2 (alpha) where P(alpha) = off-axis P, not a true P b/c P doesn't really exist in oblique meridian = cyl P (diff btwn P of axis meridian and P of oblique meridian) >> used to help calc edge (center) thicknesss, Ps = sph P (most + meridian) of surface, Pc = cyl P (diff btwn two Ps of meridians) of surface = more minus meridian - less minus meridian, (alpha) = angle btwn axis meridian and oblique meridian, so off axis meridain = axis + or - (alpha) or 180 deg away from that

Combine two sphero-cylindrical lenses w/ axes NOT // or perpendicular to each other… Rules for location of resultant axis

If cyl Ps w/ same sign (btwn orig axes), If cyl Ps are equal (midway btwn orig axes), If cyl Ps are equal AND axes are 90 deg apart (sph lens), If cyl Ps are unequal (resultant axis more towards axis of greater cyl P)

Combine two sphero-cylindrical lenses w/ axes NOT // or perpendicular to each other… Rules for resultant cyl P

If axes of cyl Ps close to each other (resultant cyl P close to sum of two cyls), If axes of cyl Ps almost 90 deg apart (resultant cyl P close to diff of two cyls w/ axis closer to higher cyl P)

Pantoscopic tilt vs. Face form tilt (facial wrap)

Pantoscopic tilt (along 180 deg, in sagittal plane, // to tangential focus) vs. Face form tilt (facial wrap), (along 90 deg, in tangential plane, // to sagittal focus)

Lens tilt effect - Martin's formulas

Pns = Po [ 1 + sin^2 (alpha) / 2n ], Pc = Pns tan^2 (alpha) where Pns = new sph P due to tilt, Pc = cyl P induced due to tilt w/ same sign as sph, Po = old sph P (P along 090 for pantoscopic tilt, since x 180 OR P along 180 for face form tilt, since x 090), (alpha) = angle of tilt, n = index of lens

Lens tilt effect - Det total Rx

Pns + Pc x 180 (for pantoscopic tilt) OR x 090 (for face form tilt) + Orig cyl w/ x 180 (for pantoscopic tilt) OR x 090 (for face form tilt)

Optical center adjustment, taking pantoscopic tilt into account

For every 2 deg of pantoscopic tilt - Lower optical center by 1mm (for optical axis of lens to intersect center of rotation of eye)

Typical P range (sphere and cyl) for CR-39 stock-finished lens blanks

-6.00 D to +2.00 D w/ cyl up to -2.00 D