3. • The previously used datum system for
measuring lenses was established as
a system of reference points for
frames & lenses so , that placement of
lens optical centers & bifocal
segment heights would be consistent.
• Horizontal lines were drawn at the
highest & lowest edges of the lens.
3
4. A line drawn halfway between the two
horizontal lines & parallel to them was known
as the “ datum line”.
The width of the lens along this line was called
the datum length or eye size.
The point along the datum line halfway
between the edges of the lens is called datum
center.
The depth of lens, measured as the vertical
depth through the datum center, was mid –
datum depth.
4
6. Boxing system
DISTANCE B/W LENSES OR BRIDGE SIZE
EFFECTIVE
DIAMETER
FRAME
DIFFERENCE
GEOMETRIC CENTRE DISTANCE
SIZE
SEG HEIGHT
HORIZONTAL MIDLINE
GEOMETRIC CENTRE MEASUREMENT
6
8. • Halfway between the top and
bottom of the lens.
• In the datum system this was called
datum line.
• This line referred to as the
horizontal midline or the 180 degree
line.
8
9. GEOMETRIC CENTRE
• The centre of the lens is the point on the
horizontal midline hale way between the two
lens bordering vertical line.
• It is known as the geometric centre or boxing
centre of the edged lens.
• This term does not imply anything about the
optical positioning of the lens.
9
10. SIZE
• The size of the lens then is the length and
depth of the box containing the lens.
• The horizontal length is now commonly
referred to as the eye size when referring to
the frame and the lens size when referring to
the lens.
• Both are measure in millimeter.
10
12. • In determining the horizontal boxing
dimension of a frame that measurement
begins at the inside of the groove on the left
side of the imaginary enclosing box and
extends horizontally across the lens opening
to the farthest part of the groove on the right
side of the box.
12
13. • In measuring a lens, the measurement begins
at the apex, or point, of the bevel on the left
side of the box enclosing the lens and extends
to the apex of the bevel on the right side of
the box.
13
14. EFFECTIVE DIAMETER
• The effective diameter of a lens is found by
doubling the distance from the geometric
centre of the lens to the apex of the lens bevel
farthest from it.
• This measurement helps determine the
smallest lens blank from which the lens can be
cut.
14
16. FRAME DIFFERENCE
• The frame difference between the horizontal
& the vertical measurement is known as the
frame difference.
• It is measure in millimeter.
• Frame difference is sometime referred to as
lens difference.
16
18. DISTANCE B/W LENS OR BRIDGE SIZE
• The DBL is the distance between the two
boxes when both lenses are boxed off in the
frame.
• Bridge size or DBL is measured on the frame
as the distance from the inside nasal eye wire
grooves across the bridge area at the
narrowest point.
• This distance is measured in millimeters.
18
21. GEOMETRIC CENTRE DISTANCE
• The distance between the two geometric
centers of the lenses is known as the
geometric center distance.
• The GCD is also known by three other names:
• 1. Distance between centers (DBC)
• 2. Frame center distance
• 3. Frame PD
21
22. SEG HEIGHT
• When specifying bifocal or trifocal segment
height, the reference points are given in
millimeters as either (l) the distance below or
above the horizontal midline (called seg drop or
seg raise), or (2) the distance from the lower line
of the boxing system rectangle enclosing the lens
shape (called seg height).
• In the actual measuring process , the level of the
lower line of the box corresponds to the lowest
point in the eye wire groove.
22
25. OVER ALL TEMPLE LENGTH
• The overall temple length is the distance from
the center of the center barrel screw hole to
the posterior end of the temple, measured
along the center of the temple.
• Many times the center of the barrel screw
hole will match the position of the butt end of
the temple.
25
27. • Comfort cable temples are measured in terms
of overall length.
• The actual measurement is done by grasping
the tip and extending the temple along the
ruler.
27
34. LENGTH TO BEND
• An older method of measuring temple length
is in terms of the length to bend (LTB).
• This is measured from the center of the barrel
to the middle of the bend.
• The distance from the middle of the temple
bend to the end of the temple is known as the
length of drop.
34
36. FRONT TO BEND
• If the end pieces wrap around in a swept-back
manner, there is a distance between the plane
of the frame front and the actual beginning of
the temple.
• In this case, the temple length could be
specified as frame to bend which would be
slightly longer than LTB.
• This measurement method is seldom used.
36
38. EYE SIZE & DISTANCE B/W LENSES
• When a frame marking such as 50 20is seen,
it means that the eye size is 50 mm and the
distance between lenses is 20 mm.
• The box between the numbers means that the
eye size is measured according to the boxing
method; it also serves to separate the two
numbers and prevent confusion.
• The eye size and DBL are sometimes simply
marked 50-20 or 50/20.
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