B-Free is a backside free-form invisible bi-focal that is digitally designed for every Rx. It is more advanced than conventional bi-Focals.
The Seg Height for the Next Generation Bifocal (NGB) is measured the same way as for a Flat Top bifocal else using the Box method; the distance from the lowest point not he lens to the lowest point on the lower eyelid.
The Layout Reference Point (LRP) for the NGB is at the entrance of the gate of the Near Visual Field for vertical dimension, which is the Seg Height (SGHT).
The LRP for the horizontal dimension, which is the Seg Inset, is set default to 2.5 mm due to these factors:
The near reading habits have changed due to the frequent use of hand held devices like cell phones which are generally held at much closer distance from the eyes than conventional reading distance that is considered to be 35 to 40 cm.
The NGB free form lens design shares much of the characteristics of free form progressive lenses where the set inset is commonly set by default at around 2.5 to 3 mm.