Physical Characteristics of Ophthalmic Lenses

Curvature (R)

1/r

Bigger ROC → Flatter surface

Sagitta

Distance between a point on the circle and the midpoint of a chord of the circle

Sagittal depth

Use approximate sag formula when s is small compared to r (spec lenses)

Lens clock

Used to measure the sagittal depth of the front or back surface of an ophthalmic lens

Assumes material is n=1.53 (same as trivex)

Directly measures sag, indirectly measures power

How to check work if the lens is a different n than what the lens clock assumes

n (clock) > n (true index) → Overestimate

n (clock) < n (true index) → Underestimate

Lens form

Relationship between front and back surface curvatures

Types of spherical lens forms

• Plano-convex/concave

• Biconvex/concave

• Equiconvex

• Meniscus

Plano-convex/concave

One flat surface

Biconvex/concave

Both surfaces are convex or concave

Equiconvex

Half of total lens power at each surface

Meniscus

Convex front surface and concave back surface

Provides better visual function and reduces aberrations

Types of cylindrical/toric lens forms

• Plano cylinder

• Toric lens

• Bi-toric lens

Plano cylinder

One surface flat other cylindrical

Toric lens

One surface toric other spherical

Modern single vision lenses

Minus and plus lens forms

Bitoric lens

Both surfaces are toric

Used for anisocoria

Minus cylinder lens form

Spherical front surface, toric back surface

Nearly all single vision lenses

Plus cylinder lens form

Toric front surface, spherical back surface

Modern lenses are _____ in design

Meniscus

What do we try to maximize when choosing the correct lens form?

• Optical quality

  • Cosmetic appearance

Rough blank

Neither side is ground or polished

Semi-finished blank

One surface is ground and polished

In house labs will grind in 2nd surface

Uncut lens

Both surfaces are ground and polished but hasn’t be cut to fit in the frame

Base Curve (BC)

Standardized curvature ground onto one lens surface by manufacturer for a grouping of lens

Final lens power is determined by curvature ground onto the remaining surface

Refers to surface power not curvature

Single vision lenses BC

Spherical lens: Front surface curvature

Plus cylinder lens: Flatter front surface meridian

Minus cylinder lens: Front spherical curve (surface power)

Corrected Curve Lenses/Best-form lenses

Oblique astigmatism and curvature of image aberrations can be minimized by appropriate selection of base curve

Base curves are selected to…

Minimized lens aberrations

Compromise between lens thickness and image quality

Flatter lenses are thinner

What type of pts notice a change in BC?

Hyperopes

Does the Rx format describe lens form?

No