Physical Characteristics of Ophthalmic Lenses Flashcards
Sherical lens meridians
Constant curve in all meridians
Quick way to read bifocal add and actual way to read bifocal add
Quick way: use back vertex power and take diff between it and the near portion.
Actual way, especially if lenses have high plus or minus power, then flip around. Take diff between front vertex power up top and near add.
Meridians in cyl lenses
Constant curvature along each meridian but varying curvature between meridians
Meridians in aspheric lenses
Varying curvature along each meridian. Usually flatter in the near periphery. Helps make lens look flatter and decrease chromatic aberrations.
Lens form
Refers to the relationship between the front and back surface curvatures of the lens.
Examples of lens forms and when are they used?
Spherical lens form:
- plano concave. Used for high minus.
- Plano convex. Not used.
- Biconvex. Used often for high plus.
- Bi concave. Rarely used.
- periscopic. Rarely used today. One of the first bent lenses with convex front and concave back. Used + and - 1.25 BC on front and back.
- Meniscus. More recent version of a bent lens. Has BC of -6.00 and +6.00
Cylindrical and tonic lens form:
- Plano cyl. Patient with astigmatism but no myopia or hyperopia.
- Spherocylinder. Sphere on the front and toric on the back.
Most commonly used spherical lens forms
Plano concave for high minus
Biconvex for high plus
Meniscus. BC of +/- 6.00D. Altered based on Rx.
Optical axis
Imaginary line connecting the centers of curvature of 2 lens surfaces. Normal to both lens surfaces.
***Optical center of a lens
Point of no prism. For any bent ophthalmic lens, the optical center will fall outside the lens.
Optical pole
Points on the front and back of the surface of a lens that are penetrated by the optic axis. Also called the front and back vertices.
Types of base curves
Corrected curve or best form lenses are designed to minimize aberrations. Corrected curve lenses are the most common. Usually do not have to specify. Based on manufacturing.
When should you specify a base curve?
When replacing only one lens in a pair.
Sagitta
The distance between a point on the circle and the midpoint of a chord of the circle
What does a lens clock measure?
measures sagittal depth of lens surface.
Curvature (R)=
1/r (radius)
