ABO Part 3: Lens Form & Analysis

prism

triangular shape with apex and base

apex

point of prism

base

flat surface in a prism

light passing through prism

redirects light towards its base

P=C/D

p=prism, c= displacement in cm, d= distance from prism in meters

plus lenses

combine prisms base to base

minus lenses

combine prisms apex to apex

focal length

f=1/F
f=focal length, F=lens power in diopters

optical center

single point on lens through which light may pass without being refracted

against motion

plus lens characteristic, image of black line viewed through lens moves in the opposite direction

with motion

minus lens characteristic, image of black line viewed as lens moves left moves in the same direction

minus lens power

becomes stronger as lenses are moved closer to the eye and weaker as lenses move away from the eye

plus lens power

becomes weaker as lenses are moved closer to the eye and stronger as lenses are move away from the eye

high plus lenses

pincushion distortion

high minus lenses

barrel distortion

aberrations

Spherical, marginal/oblique, distortion

Corrected curve theory

Formulas concentrate on correcting errors experienced when looking more than 30° away from center

Base Curve ANSI Standards

Within +/-0.75 D

Optical center

Single point on lens where light passes without being refracted

Base out prism in plus lens

OC are wider than PD

Base out prism in minus lens

OC narrower than PD

Base in prism in plus lens

OC closer together than patient PD

Base in prism minus lens

OC wider than patient PD

No prismatic affect

When OC and PD match

Base up prism in plus lens

OC are higher than pupil

Base up minus lens

OC lower than pupil

Base down plus lens

OC below pupil

Base down minus lens

OC above pupil

Excessive base down prism

Appears floor is concave or walking uphill

Excessive base up

Appears floor in convex or walking uphill

Prentices Rule

△=c x F

△=Prism diopters, c=amount of decentration in in cm, F=dioptric power in meridian being determined

Horizontal Prism ANSI Standards

0.67 prism diopters total or 2.5 mm difference between PD and OC if the rx higher is +/-2.75D or higher

Canceling prism

Base in & base out, base up & base up, base down&base down

Compounding prism

Base in&base in, base out&base out, base up&base down

Pantoscopic tilt

OC should be vertically below pupil 1mm for every 2° of pantoscopic tilts

Most frames average 10-12° of tilt

OC should be 5 to 6mm below pupil

Vertical imbalance

Power in one eye is significantly greater than the other eye (more than 1.5D), patients may experience vertical imbalance when they look down to read

Slab off/Bicentric grinding

Redirecting light by adding prism to one lens in a bifocal to eliminate double vision cause by difference in powers

Image jump

When the gaze is shifted across the segment line of a bifocal lens