WHY ASPHERICS?
- Reduce aberrations
- Reduce magnification (flatter lens)
- Reduce thickness/weight
- Improve lens fit into frame (especially for plus lens)
- Progressive lenses are aspherics
Current aspherics
- Lens power, lens thickness, panto tilt, face form angle (dihedral angle), vertex distance
- Higher order polynomial: correcting beyond 2nd order
- “polynomial” aspheric lenses
aspheric surface
- creates a flatter lens (plus power: thinner centers; minus lens: thinner edges)
- flattening a lens results in power errors and astigmatic errors
- use of an aspheric surface results in negating or reducing the power and astigmatic errors
- deviates from conventional spherical best form
- front vs. back aspheric
aspheric lenses
- plus lens–to reduce the power error and astigmatic error, the lens front surface is made aspheric with
the lens flattening as one moves from the lens center towards the edge; - minus lenses–front surface made aspheric withlens steepening towards edge
- more base curves, each covering a smaller range
- asphericity increases as flattening increases
too much asphericity:
a great looking lens but potentially poorer optics if increasing asphericity
is not resolving peripheral aberrations
ATORIC LENSES
- extension of aspheric concept to toric surfaces
- greater cylinder powers, errors along each meridian are different (i.e. aberrations)
- need to reduce errors optimally utilizing aspheric concept along each meridian (or at least one of the
meridian is aspheric) - consequence: aspheric + toric = atoric lens design
Type I Atoric
atoric on cylinder side & spherical on
opposite side
-asphericity varies from one principal meridian to the other
Type II Atoric
atoric surface with no true cylindrical
correction at vertex & circular toric surface on opposite side
Type III Atoric
freeform atoric surfaces, used on complex or progressive type of lenses
-PAL front, complex atoric back
vision through circular torics vs atoric
atoric has obvious improvement in the flear FoV
ATORIC LENSES traits
- concept utilized with single vision and progressive lens designs
- visual field maximization
- not material dependent
- thinner lens
- less magnification
- better aberration reduction
atoric utilization
- high cylinder prescriptions
- anisometropia
- anisekonic lenses
- general reduction in lens thickness, lens volume, lens weight
Myopia Control Lenses
*Area of defocusing segments/lenslet (aka treatment zone): honeycomb appearance with each lenslet having a
+3.50D power interspersed with clear zones
* D.O.T (Diffusion optics technology)
* reduces myopia progression in
children by incorporating thousands
of micro-dots into lenses, which
scatter light and reduce contrast on
the retina
ASPHERIC LENS FITTING
- Rx consideration: >+2.00; >-3.00
- Lens index: 1.67, 1.701, 1.74; (typically aspheric)
- Lens index: 1.60, 1.586, 1.53 (Trivex), 1.498 (aspheric &
spheric) - Full aspheric vs “not so full” aspheric
- monocular PD’s (pupillometer)
- vertical OC placement
- OC height
- 1 mm drop per 2 degrees of tilt
- Alternative option: patient tilts chin up with frame on
until plane of frame perpendicular to floor, mark lens
with patient in primary gaze - no decentration (prism)
- vertex distance consideration (FOV)
- no greater than 12-13mm
