Blank size ordering, prismatic effect

2.  Diameter of a lens in millimetres before it is cut
to the shape of the frame is known as its blank
size.
Or
 The size of the widest part of a spectacle lens
(centre), from one edge to other edge after its
manufacture in the labortary,measured in mm is
known as its Blank size.
 It is always measured in millimeters and denoted
by Ø.
 It also called Un-Cut size.
 It is always mentioned on the wrapper/envelope
of the spectacle lens in stock.

3.  The blank size readily available in the
spectacle market is 45Ø, 50Ø, 55Ø, 60Ø,
65Ø, 70Ø, 75Ø, 80Ø.
 Blank size are limited in glass lenses, usually
to a maximum 70Ø.
 The Blank size of high Index and aspheric
lens may be less then 45mm, because the
lesser the blank size of a lens, lesser is the
anterio-posterior thickness of the lens and
vice versa.

4.  One requires the following measurements for
ordering a blank size from the stockist
1. Frame PD
2. Patient PD
3. Widest part of eye-glass Rim.

6. Widest part=56mm

Frame PD=68mm

7. 1. Measure patient PD
2. Measure frame PD
3. Estimate the measurement for lens to be
decentered
4. Measure the widest part of eye-glass rim
5. Add widest part of frame to the amount of
decentration required.
6. Now add 2mm extra for the glazing purpose
to the result of step # 5.
7. Measurement of blank size required is
gained.

8.  Patient PD=62mm
 Frame PD=68mm
 Difference b/w Patient & Frame PD= 68-
62=6mm
 Required Decentration = 6/2=3mm(both
sides)
 Widest Part of eye-glass rim=56mm
 Extra addition for Glazing purpose=2mm
=> now
Blank Size Ordering=W.P+D+G
=> 56+3+2=61mm

9. INDUCED
PRISM

10.  Intentional induction
 Frame PD does not correlate with patient PD.
 Prismatic effect is required.
 Unintentional prism induction
 By error and mistakenly.

11.  To calculate the induced prismatic effect, we
use the relation
P = F x D
where P= prismatic effect
F= Dioptric power of the lens
D= Decentration in cm

12. CONVEX LENSES
Outward
decentration
Base out effect
Inward Decentration
Base in effect
CONCAVE LENSES
Outward
decentration
Base In effect
Inward Decentration
Base out effect