Topic 3: Aspheric Lenses

Question 1

define best form lens

Answer 1

a lens in its best form is a lens without oblique astigmatism

best combination of front (base curve) and back curvature (base) powers for a lens

can be found using tscherning’s ellipse (power - x, bfl - y)

Question 2

describe limitations of tscherning ellipse for bfl

Answer 2

(1) bfl too expensive
(2) plus bfls too curved to fit into frames
(3) plus lenses > +7.50D will always have smaller field of clear vision/OA

Question 3

explain aspheric lenses

Answer 3

unlike spherical lenses, the surface changes shape from centre to periphery.

does not have same radius of curvature over the entire surface (based on surface curvatures from different CONIC sections)

Question 4

explain asphericity to thin lenses

Answer 4

minus: lens curvatures progressively steepen at front surface but progressively flatten at back surface
plus: front surface flattens towards the periphery while back surface remains spherical/constant radius

lenses becomes lighter and thinner with aspheric surfaces vs spherical

Question 5

explain asphericity to eliminate OA

Answer 5

aspheric lens surfaces are spherical in its centre but gradually change its curvature (steepening/flattening) at certain distances from the lens OC

the rate of the change in curvature is specially calculated to offset OA

hence, aspheric is not only thinner than spherical but also free of OA at the lens periphery. meaning, a bigger field of clear vision

Question 6

explain dispensing aspheric lenses

Answer 6

aspheric lenses - central spherical zone & periphery aspheric zone

for the aspheric zone to neutralise OA properly, lens OC must not be at pupil centre but lowered if frame chosen has a pantoscopic tilt (1mm lowered every 2 degrees)

additional steps: 1) dot then measure to the lowest point of the frame 2) measure PT with pantometer (wearer in habitual head position) 3) apply formula

Question 7

describe lens selection for high hyperopes

Answer 7

(1) smaller lens diameter: wearer to choose frame with small eyesize, have to pay additional as stocks are bigger and thicker, diameter to order is via minimum lens size = longest axis + (fpx-pxpd) +2mm
(2) higher RI: frame can be slightly bigger BUT 2-3x the price of lower index lenses and image quality at periphery affected by OA
(3) aspheric design: when frame size bigger, better option to high index lens (eg. RI 1.7) also, lower cost and better image quality at the lens periphery

Question 8

describe frame selection for hyperopes

Answer 8

do not advise rimless or rimlon as the thinned edges cannot allow for groove to be made (rimlon) and it is too weak to allow for hole drilling (rimless)

Question 9

advice for new aspheric lens wearers

Answer 9

slowly increase the wearing hours for adaptation

Question 10

lens designs for super high powers ( > +10D & >-20D )

Answer 10

lenticular design: restrict lens power (optical zone) to only the centre part of lens. lens periphery can be plano. junction of optical zone and carrier can be smoothened