Centration of Progressive Addition Lenses

Lenses that have the same prescription power over the whole lens. This power can be a sphere power, a cylinder power, or a spherocylinder power, and may also call for prism.

Single vision lenses

The ability to change the power of the eye's inner crystalline lens, enabling a person to see objects clearly at a near viewing distance.

Accommodation

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

Presbyopia

Lenses with more than one power, used to correct presbyopia

Multifocal lenses (or multifocals):

Lenses that have two or more distinctly divided areas of power. These areas are usually demarcated clearly by a visible bordering line (with the exception of blended bifocals, which have a nearly invisible border).

Segmented multifocal lenses

Two major categories of multifocal lenses

1. Segmented Multifocal Lenses

2. Progressive Addition Lenses

The key zones of a progressive lens

1. Distance Portion

2. Progressive Zone

3. Near Portion

One of the key zones of progressive lens that is the upper area of the lens

Distance Portion

One of the key zones of progressive lens that is the intermediate area where power gradually increases from distance to near. This zone typically ranges from 12 mm to 17 mm in length, depending on the lens design.

Progressive Zone

One of the key zones of progressive lens that is the lower area where the full near power is achieved.

Near Portion

Also known as the Prism Reference Point (PRP). This is the location on the lens where the prescribed prism is measured.

Major Reference Point

The specific place where the distance power is measured. It is located far enough above the MRP to avoid the power changes of the progressive corridor.

Distance Reference Point

The location where the full near power (the "add") of the lens is measured.

Near Reference Point

Used for alignment; because power changes gradually, there are no distinct line segments like in bifocals.

Fitting Cross

Power begins changing at the MRP and increases in plus power down the corridor until it reaches the full near addition.

Power Graduation

Most of these points are not immediately visible; they are lightly marked or engraved and can only be seen under optimal lighting

Invisible Markings