Chapter 3: LENS FORM

two prisms apex to apex spreading light rays diverge light outward as they pass through the lens

resemble two prisms base to base converging light light rays as they pass through the lens

inward curve (-)

outward curve (+)

a flat or zero curve

correct for astigmatism

combines spherical and cylinder curves

These curves on the lens surface can easily be measured

• defined as lenses that are non-spherical
• This non spherical surface encompasses all kinds of lenses from aspheric, atoric, progressive, and aphakic.

ex.-2.75 -2.00 sphero-cylindrica

• the absence of the lens of the eye
• Aphakic lenses use aspherics because plus power lenses higher than +8.00 are outside of the Tersching ellipse and do not have a best form curve. This means that in order to provide the best vision the lens designer has no choice but to use aspherics.

• 1. Add the sphere and cylinder powers to determine the new sphere power.
• 2. Change the sign of the cylinder.
• 3. Change the axis by 90 degrees

• Example:
• Transpose -3.00 +2.00 x 30
• 1. Add the sphere and cylinder powers to determine the new sphere power.
• (-3.00) + (+2.00) = -1.00
• 2. Change the sign of the cylinder
• -2.00
• 3. Change the axis by 90 degrees.
• 120
• The transposed prescription is:
• -1.00 -2.00 x 120