8.3 Multifocal Optics

What is the intermediate power normally (and if not specified)?

50%

How does adding distance plus power (decreasing the minus) affect the near power?

increased the near power

can keep add the same as before or decrease it

How does decreasing the distance plus power (increasing the minus) affect the near power?

decreases near power, may need to increase add

How is add power obtain in a one-piece bifocal?

by change in curvature of the near portion of the lens

What are the refractive surfaces of a fused bifocal?

has 3 refractive surfaces (front, back, and countersink)

How does a doctor specify segment position?

based on patient anatomy (at lid margin, at pupil margin, etc)

How does an optician specify segment position?

takes rx and specifies seg position by relation to frame

What is the equation for seg drop?

seg drop= B/2 - seg height

What is the average reading center?

11 mm below pupil

What is an ideal seg OC?

at reading center

Where do we place the seg OC and why?

between the reading center and seg top (above reading center) to allow for phone/computer reading while minimizing prismatic effect

What is segment inset?

displacement of seg OC from distance PRP

Why do we inset the segment?

so the fields of view through 2 segments coincide

prevent segment from producing horizontal prism

What is the seg inset equation?

(distance PD - near PD)/2

What does the seg inset depend on?

distance PD

vertex distance

fixation distance

If you increase distance PD, how does that affect inset?

increased inset (inset is farther in)

If you increase vertex distance, how does that affect inset?

increased inset (farther in)

If you increase fixation distance, how does that affect inset?

decreases inset (not as far in)

What is total inset?

lateral displacement of seg OC to GC

What are the equations for total inset?

TI= PRP decentration + seg inset

TI=(frame PD-near PD)/2

What is image jump?

vertical prismatic effect @ segment top from segment only

What is the equation for image jump?

image jump= (d)(Padd)

d=distance from seg top to seg OC

Image jump will always be in what direction?

base down

What bifocal has no image jump?

executive bifocal

Where is the OC for flat top bifocal?

5mm below top

Where is the OC for a round bifocal?

1/2 diameter

What is total displacement at the reading level?

total prismatic effect @ any point on seg

What is total displacement the sum of?

induced prismatic effect from distance lens + induced prismatic effect from seg

How do you calculate prismatic effect @ reading level for the segment?

=(d)(Padd)

d= distance between reading level + seg OC