Optical Performance of Multifocal Lenses Flashcards
where is downgaze compared to distance OC
10 mm
the distance between the GC (or FPD) and the DOC in each eye is called the
inset
formula for inset
(FPD - DPD) /2
the distance between the DOC and the NOC in each eye
segment inset
formula for segment inset
(DPD - NPD) /2
the lateral displacement of the segment optical center (NOC) from the geometric center of the lens
total inset
formula for total inset
(FDP - NPD) /2
total inset=
inset + seg inset
what inset is most common?
+ inset, because usually eyes nasal to GC
segment inset is always ___
IN (because we converge)
2 purposes of segment inset
- to ensure the fields of view through the 2 segments coincide
- to prevent the segment from producing horizontal prismatic effects
linear field of fixation formula
FOF = [ (d +2.7) x (2h)] / 2.7
in linear FOV calculation, what does d and h stand for
d= viewing distance (in cm) h = 1/2 seg size (in cm)
the linear field of fixation through a segment ____ as the segment diameter and viewing distance increases
increases
the linear field of fixation through a segment _____ as the vertex distance decreases
increases
why do we use 2.7 cm in FOF?
average distance between the spectacle plane and the eye’s center of rotation
why does image jump occur?
vertical prismatic effect at the line between the carrier lens and the segment
how can the image jump increases?
increases the farther the segment OC is away from the top of the segment line
image jump calculation
image jump = F(a) x dcm
F(a) = add power
dcm = distance the seg line is from the seg OC
define apparent scotoma
the differential displacement of light at the top of a seg results in an apparent scooter (blind spot)
apparent scotoma size formula
size (cm) = (vd/100) x image jump (prism diopter)
when does image displacement at the reading level occur?
if the reading level is NOT coincident with the seg OC
if reading level is below the optical center, _____ is induced
BU
if reading level is above the optical center, ____ is induced
BD
formula for image displacement at reading level
= Fa x ds
Fa= add power
ds= distance from the reading level to the seg OC
total image displacement is the sum of:
the prismatic effects from both the carrier (distance) and reading seg
total image displacement =
(dc x Fc) + (ds x Fa)
