Pt. 2 of Centration of Progressive Addition Lenses

the loss of this ability to focus well enough to see clearly and comfortably at near viewing distances

Presbyopia

the ability to change the power of the eye’s inner crystalline lens. Changing this power enables a person to see objects clearly at a near viewing distance.

Accommodation

lenses that have the same prescription power over the whole lens. That power can be a sphere power, a cylinder power, or a spherocylinder power.

Single vision lenses

Multifocal lenses are divided into the following two major categories:

1. Segmented multifocal lenses

2. Progressive addition lenses

lenses that have two or more distinctly divided areas of power. These areas of different powers are demarcated clearly by a visible bordering line

Segmented Multifocal Lenses

lenses that have different viewing areas—a distance viewing area and a near viewing area.

Progressive addition lenses

In progressive lenses, what are the two type that when no prism is prescribed, these two things are one and the same

1. Major reference point

2. Optical center

To differentiate the two places where the prism and the distance powers are measured

Prism Reference Point

The place where distance power is measured. The lens manufacturer chooses the location of this

Distance Reference Point

This is where the full near power of the lens should be measured. On the semifinished lens this area sometimes comes surrounded by a full circl

Near Reference Point

The fitting cross is a reference point on the lens that is usually ___ - ___ mm above the Major Reference Point

2-4mm

The is where the pupil center is located. It is the only reference point for both horizontal and vertical lens positioning for the dispenser and for edging laboratory

Fitting Cross

Another term for “Base Down Prism” and found at the Prism Reference Point

yoked based down prism