L5 Physical Characteristics of Opthal lens Flashcards
explain lens forms
the front and back surfaces/curvatures powers determine the lens form/shape and the lens thickness determines what TYPE of lens
state the different lens forms
- biconvex/biconcave
- plano-convex/plano-concave
- positive/negative meniscus (convex front, concave back)
break down the lenses
lens forms are single vision lenses (either distance or near) -> spherical (simple myo/hyperopia) and cylindrical (astigmatism)
explain spherical lenses
- all meridians will have the same curvature and power
- vision will not be affected by lens rotation
- produces focal point image
- each surface has only has one surface power
explain astigmatic lenses
- different radius of curvature in the meridians
- different thickness in meridians (thicker=higher power, thinner=lower power)
- produces focal LINES
- vision of the wearer will be affected by lens rotation
explain line images formed by astigmatic lens
- 2 line images formed
- the greater the difference in powers of the 2 meridians, the greater the separation of the 2 images
- power meridian and axis meridian perpendicular
- line image formed follows axis
types of astigmatic lenses
plano-cylindrical, bi-cylindrical, spherical-cylindrical, toric lens
explain plano cylindrical
- used during regular refraction and retinoscopy
- in the lab, we use minus cyl hence one plano surface, the other is a minus surface. plano-concave.
eg. -1.50 x 90, axis meridian is 90, power -1.50 is along horizontal axis since axis and horizontal meridian are perpendicular.
explain bi-cylindrical (cross cylinder)
- used in lab for subj refraction (refining cylinder JCC)
- 2 cyl surfaces, one at front one at back
- curves placed at right angles to each other
- one convex the other concave
explain spherical cylindrical
flat lens with one spherical surface one cylindrical surface (usually minus)
explain toric lens
- lens with both curved surfaces. one spherical and one toroidal (sphero-cyl)
- front is always sph, back is always toroidal
- none of surfaces will have a zero power meridian
positive to negative cyl transposition
new sph: sum of old sph and old cyl
new cyl: change sign of old cyl
new axis: change by 90
summarise ophthalmic lenses
- sph power: lenses in meniscus form
- astig power: lenses in back surface toric form, minus cyl form.
- ocular(back) surface is caved in, -ve power. front surface bulging out, +ve
explain minimum size blank
- uncut lenses must be sufficient size so lens required for frame can be cut
- size of lens must be determined correctly to avoid delays and additional cost and an oversized lens
state minimum lens size formula
MLS=longest axis + (frame pd - px pd) + 2mm (for grooving)
frame pd/size=A+DBL
