5 Spectacle Frame and Measurements

zylonite (zyl)

original material for plastic frames, not used anymore but sometimes still called that today

Pro and con of Monel (alloy = has different materials)

• Cheap

• may contain nickel, potential for allergy

What metal material is hypoallergenic?

titanium

pros and cons of titanium

• lightweight

• durable

• corrosion resistant

• hypoallergenic

con: more expensive

What is used to be more affordable than titanium?

titanium alloy

What is the Boxing System? What are the dimensions?

smallest “box” that can contain the lens

A dimension: horizontal, “eye size”

B dimension: vertical

How do you find the geometric center, given A and B?

Half of A and half of B

or, half the diagonals of the lens

DBL

distance between lenses aka bridge size

DBC

distance between centers (aka frame PD)

seg height

dist between top of bifocal bottom of lens

ED

effective diameter (2x longest radius)

Frame PD = _ + DBL

A + DBL

Why do we measure PD?

avoid prismatic effect, esp with higher powers (decentration, Prentice’s rule)

Why is decentration used?

so the patient looks through the optical centers of the lenses

When is no decentration needed?

when frame PD = patient PD

Decentration equations (not provided)

What is the question we ask to find decentration?

“where is the optical center of the lens relative to the geometric center?”

ED (frame measurement)

effective diameter = 2x longest radius of lens

Theoretical minimum blank size

= ED when there is no decentration; can be used to grind lens down for frame

Minimum blank size equations

we just use theoretical, the optician’s was for in the past when lenses were ground by hand