The document introduces the datum system, a British standard for measuring spectacle frames and lenses, explaining key components including the datum line, datum length, datum center, and mid-datum depth. It highlights that the datum system is an older method used for ensuring consistency in lens placement. Additionally, it notes that this system was used before the more commonly adopted boxing system.

1. DATUM SYSTEM
Presented by:
Mamitha Sravanthi,
2nd
year Bsc.Optometry,
GSL Medical College.

2. INTRODUCTION
 Datum system
 Datum line
 Datum length
 Datum center
 Mid-Datum depth

3. Systems of
Optical
dispensing
Datum system
of optical
dispensing
Boxing system
of optical
dispensing

4. WHAT IS DATUM SYSTEM?
 It is the British standard system which
is used for measuring spectacle frames
and lenses.
 It is the old method.
 It was established as a system of
reference points for frames and lenses
so that placement of lens optical
centers and bifocal segment heights
would be consistent.
 The lens placed as it should set in the

5. Datum system
of optical
dispensing
Datum line Datum length Datum center Mid-Datum
depth

7. What is Datum line?
 A line drawn halfway between the two
horizontal lines and parallel to them is
known as “Datum line”
What is Datum length?
Also called as eye size.
It is the horizontal measure of the lens shape
on its datum line.
In other words, Datum length is “The
width of the lens along the datum line”

8. What is Datum center?
The midpoint lying on datum line
denotes the datum center.
Datum center distance is distance
between the datum centers of right
and left lenses when the lenses are
fitted.

9. What is MID –DATUM DEPTH?
It is the depth of the lens,
measured as the vertical depth
through the datum center.
 The Datum system preceded the
currently used Boxing system.