"A" Measurement
- The
horizontal distance between the furthest temporal and nasal edges of the lens
shape or the distance between the vertical sides of the box. The A measurement
is also commonly known as the eyesize.
"B"
Measurement - The
vertical distance between the furthest top and bottom edges of the lens shape
or the distance between the horizontal sides of the box.
Datum Line - The horizontal line that runs through the
vertical center of the frame.
Geometric Center (GC) - The intersection of the Datum Line and
horizontal centers of each lens shape.
Distance Between Lenses (DBL) - The shortest distance between the nasal
edges of each lens or the distance between boxes. DBL is also commonly referred
to as bridge size.
Distance Between Centers (DBC) - The horizontal distance between the
geometric centers of the lenses. DBC is also know as the Geometric
Center Distance (GCD), but more commonly referred to as the frame PD. In
theory the DBC can be calculated by adding the "A" Measurement to the
DBL as marked on the frame, however in practice the calculation may differ from
the actual measurement.
Note: Frames are typically marked for size, for example: 54-18,
where 54 is the "A" Measurement and 18 is the DBL.
Effective Diameter (ED) - Twice the distance from the geometric
center of the lens furthest edge of the lens shape. The ED can also be defined
as the smallest diameter lens that would cutout, if the frame's geometric
center matched the lenses optical center. ED is used in combination with
decentration distance to select the minimum lens blank size required to fit a
given frame.
Seg Height - The vertical distance between the bottom
edge of the box and the top of the bifocal or trifocal segment
Seg Drop - The vertical distance between the Datum
line and the top of the bifocal or trifocal segment Overall
Temple Length (OTL) - The running distance between the middle
of the center barrel screw hole and the end of the temple.
Length to Bend (LTB) - The distance between the center of the
barrel and the middle of the temple bend.
Front to Bend (FTB) - The distance between the plane of the
front of the frame and the temple
bend. Used if there is a significant distance
between the frame front and the beginning of the temple.
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