AREA 2: Fitting single-vision lenses
• Topic 4: Alignment, decentration, and minimum
diameter
• Topic 5: Calculating thickness in spherical lenses
• Topic 6: Calculating thickness in astigmatic lenses
• Topic 7: How lenses affect vision
• Topic 8: Prismatic effects
• Topic 9: Base rules
• Topic 10: How alignment affects binocular vision
Topic 8: Prismatic effects
• Prentice's rule
• Calculating prismatic effect
• Measuring prismatic effect using a
lensometer
Prentice's rule
When a ray of light passes through a prism, it is bent
towards the base of the prism. If an object is viewed
through the same prism, however, it is seen to be
displaced towards the apex of the prism.
Prentice's rule
Prism dioptre (Δ):
The deviation of a light ray by 1 cm at a distance of 1
metre
Prentice's rule
Orientation of the prism
base at a given point on
the lens
• In plus lenses, the base of
the prism is oriented towards
the centre of the lens.
• In minus lenses, the base
of
the
prism
is
oriented
towards the edge of the lens.
Prentice's rule
The prismatic effect at any point other than the optical
centre (CM) can be calculated as follows:
  d P
CM
UNITS:
• d (cm)
• P (D)
OC
Calculating prismatic effect
It is easy to calculate
the prismatic effect at
any point on a lens if
you know the distance
between this point and
the optical centre and
the power of the lens in
this direction.
d
Calculating prismatic effect
Example:
BVP: +2.00 D
distance OC – CM: 10 mm
CM
10 mm
OC
ΔCM =1 cm · 2.00 D = 2 Δ
Calculating prismatic effect
Prismatic
effect
astigmatic lenses
in
The prismatic effect of a
point on either of the two
principal meridians can be
calculated using Prentice's
rule and the power of the
corresponding meridian.
Calculating prismatic effect
Prismatic effects
astigmatic lenses
in
If the point is not on
either of the two
principal meridians, we
cannot apply Prentice's
rule directly, as we do
H  x ·E  C·sin( )·(x ·sin( )  y ·cos( ))
not know the power of
V  y ·E  C·cos( )·(x ·sin( )  y ·cos( ))
the meridian it is on.
Measuring prismatic effect using a
lensometer
1
+ 3.50
2
3
4
1
- 1.75
2
3
4
Measuring prismatic effect using a
lensometer
When the prismatic effect
is oblique, it can be
expressed by its
4
compound form, or by its
3
2
1
horizontal and vertical
components separately.
H  T ·cos( )
V  T ·sin( )
+ 4.25
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