Progressive Umbilical line Characteristics
- Progression length (vertical distance between the pupil and 90% of the add)
- -It determines the corridor width. Two lenses with the same length, will have the same corridor width.
- -Vertical position of the addition depend on progression length.
- -Shorter progression length Higher unwanted astigmatism.
- Vertical position (position of 10% of the add with respect to the fitting cross)
It determines the lens functionality, the balance between the far, intermediate and near vision.
It also determines the relationship between the head position and the orientation of the visual axis.
- Short (smaller than 9 mm)
Add will be higher: better ergonomics.
Astigmatism will grow faster at either side of the corridor → Narrower corridor, “harder” lens.
- Long (larger than 9 mm)
Add at a lower position: worse ergonomics.
Astigmatism will grow slower → Wider corridor, “softer” lens.
Vertical position of the progression
- High (at fitting cross or higher)
- Higher position helps using the PAL at intermediate distances (computer displays).
- Higher position may not be good for driving.
- It allows the same MFH with longer corridor → wider intermediate field.
Astigmatism enters the far region → The user must lower the head to have a wide enough far field.
- Low (below fitting cross)
- Lower position more common in european lenses
- Low position helps using the progressive as outdoor lenses.
- Astigmatism does not enter the far region → wider field in far vision (immediate, without lowering the head)
- Not good for continued near vision unless corridor is short → narrower corridor