Handout 3 Flashcards
The angle though which the surface turns in a unit length of arc
Curvature (R)
The smaller the radius the ___ the curvature
Larger
The larger the radius the __ the curvature
Smaller
Units for curvature
Diopters/ reciprocal meters
The distance between a point on a circle and midpoint of a chord of a circle
Sagitta, sag, s
A geometric line segment whose endpoints both lie on the circle
Chord
When chord diameter is kept constant and curvature is decreased / radius is increased
Lens power and sag decrease
Sag is associated with
Power of the surface
Lens clock AKA __, measures
Lens measure/ lens gauge aka Geneva lens measure
- measures surface curvature using sagitta
- chord diameter :constant
Smaller index of refraction than the lens clock will give you
A weaker RX
Greater index than lens clock will give u
Stronger RX
Refers to lens shape due to front and back surface curvatures of lens
Lens form
Spherical lens forms include
Plano- convex Plano- concave Biconvave Bioconvex Modern: bent or meniscus lenses
Lens form used in spectacle lenses
Bent or meniscus
Non spherical lenses include
Cylindrical, toric/spherocylindrical
Cylindrical lens is
A cylinder cut in half describes by 2 perpendicular axis
- power axis= cylinder meridian
- cylinder axis =power meridian
Meridian that has 0 power
Axis meridian
Axis position of lenses is in a ___ notation
Degrees
Counterclockwise
Lens form for patient who has myopia or hyperopia with astigmatism
Toric, spherocylindrical
The lens curve on the surface of a lens, from which the other curves are determined
Base curve/ BC
Always on the front surface of single vision lenses
BC of a segmented multi focal is always on what side?
Same side of lens as the segment
We measure base curve with what?
Lens clock
Careful selection of base curves by the manufacturer minimizes
Monochromatic aberrations/ distortions
Lenses designed to minimize these aberrations are called
Corrected curve or best form
Why optometrists need to know the base curve?
- affects how shapes and straight lines appear
- change the way peripheral objects are seen
- some are sensitive to changes in BC
When to consider measuring BC?
- Replacement of only one lens
- ordering identically powered second pair
