3. PRISM
Under the conditions necessary for the use of
spectacle lenses, where the refractive index of
the prism is greater than the middle, the beam
was deflected toward the base.
As a result the image perceived through a prism
moves toward the apex.
4. How the user perceives the prism image
Real position of the object.
Position perceived by the observer.
Apex
Base
5. The clearest illustration shows how the person is
seeing the ladder, dark figure shows the real
position of the ladder,
There is here a fall hazard caused by incorrect
perception of the user.
This may be caused by the prismatic effect on the
glasses.
6. Simplified representation of the
deviation caused by a prism.
Apex
ou
Thin Edge
BF
Base
ou
Thick Edge
BG
BG – BF = DB → Edge Difference
7. Prismatic Diopter
To measure the power of a prism used a unit
called Prismatic Diopter .
A Prismatic Diopter produces a deviation of 1
cm per meter.
Two Prismatic Diopteres produce deviation of 2
cm per meter.
9. Prism Thickness
= , . ∆. ∅
Formula for calculating the thickness of the base of the prism,
which is recognized as Borders difference.
This formula is valid for the 1,530 index.
10. Prism Thickness
What is the thickness of the thick edge of a prism prism diopters 14.00
made in a diameter of 55 mm, CR39, whereas the thin edge with a
thickness of 1.0 mm?
∆ = × ∆ = 1,062 × 14,00 = 14,87
= 1,499 → = 1,062
= 0,019. ∆. ∅ = 0,019.14,87.55 = 15,54
1,0 mm1,0 mm
15,54 mm
16,54 mm
11. PRISM APPLICATION
The prism may have therapeutic applications in this
situation and its value is set by the professional who
prescribes certain value and the position of its base.
Ex: Near Rx +12.00 DE, Prism 14.00 prism diopters
with nasal base.
Probably the glasses are being used by the photo of
this woman.
12. With this type of lens and
the object's proximity you
could not converge
enough to preserve his
binocular vision.
So nasal base prisms are
needed to compensate for
the lack of convergence.
Prism in reading glasses
13.
14. Deviation versus Vergence
Lenses produce a convergence or divergence
effect of light rays.
The Vergence is usually called power and is
expressed in Spherical Diopters.
Prisms produce only deviation of light rays. The
deviation is expressed in Prism Diopters.
22. Why do we need?
Usually when the diameter of the lens is small
but sufficient for the assembly, we can through
the surfacing manufacture a lens positioning the
optical center of the lens in the best possible
way.
23.
24. If we want the centered lens
she must have a larger
diameter.
A block of smaller
diameter can meet the
needs since we can place
the optical center in an
appropriate manner.
In this situation the
geometric center and the
optical center are not in
the same position.
26. dm – fitting descentralization
Frame center
Pupil of User
Fitting decentralization
27. Block left is the difference between the diameter of the block and the larger the rim
Block Left
28. ds – surfacing decentralization.
SURFACING DECENTRALIZATION
The distance between the geometric center of the lens and the
optical center which we call "ds" is the surfacing
decentralization.
= −
29. Decentralization
Surfacing calculation
=
∅ −
2
=
−
2
= −
=
58 − 52
2
= 3
=
72 − 58
2
= 7
= 7 − 3 = 4
Bloco = 58 mm
MDA = 52 mm
PA = 72 mm
DP = 58 mm
You can use a lens of 58 mm in diameter in both eyes if confeccionarmos with a
decentralization of 7 mm
30. Decentralization
Surfacing calculation
=
∅ −
2
=
− 2
2
= −
=
58 − 52
2
= 3
=
72 − 2 27
2
= 9
= 9 − 3 = 6
Bloco = 58 mm
MDA = 52 mm
PA = 72 mm
DNP = 27/31 mm
If we choose the assembly by DNP, whereas the same 58 mm block will have a
different decentralized for each eye.
=
72 − 2 31
2
= 5
Olho direito:
Olho esquerdo:
= 5 − 3 = 2
31. Another way to understand the
decentralization
=
∅ − ∅
Bloco = 58 mm
MDA = 52 mm
PA = 72 mm
DP = 58 mm
∅ = − +
Ou
∅ = − . +
=
−
=
∅ = − + =
32. How to produce decentralization on
the lens
In our study we will consider only a need for decentralization in the horizontal
direction, it may also be necessary vertically.
You must add a prism lens, which will be recognized on the lens through a difference
of edges (DB).
We can apply this prism in two different ways depending on the location of
minimum thickness.
35. Thin Edge (Prism Apex)
Thick Edge (Prism Base)
On the positive lens
optical center moves
toward prism base.
Displacement of Optical Axis
New Optical Center
Position
37. Thin Edge (Prism Apex)
Thick Edge (Prism Base)
In the minus lens optical center
moves toward the prism apex.
Displacement of Optical
Axis
New position of
Optical Center
38. Optical Center Decentralization
You should calculate the prism should apply the
lens from the prescription and the desired
Decentralization.
This value is obtained by Prentice Rule.
∆=
.
39. Applying a spherical plus lens
decentralization
Consider the prescription:
OE Rx +4.00 DE,
• Made in CR39 (n = 1,499)
• Ø = 58 mm, block diameter available
• ds = 5 mm in nasal direction obtained from
the data of the frame and customer DP.
40. Usually decentralization occurs in the horizontal direction and the nasal direction.
Since this is a positive lens then the base of the prism must be positioned in the
nasal direction to ensure the desired decentralization.
We want to ready the lens center is shifted by 5 mm.
We will calculate the necessary prism to produce this decentralization.
ds
41. ∆=
.
=
, .
= , ∆
= , → = ,
= × = , × , = ,
As the lens is in CR39 need to apply a conversion factor to suit it to the laboratory
standards which instruments and tools are based on the index of 1.530
We can calculate the prism necessary to decentralize the lens
Let's check in the drawing as the positioning of this prism the lens
42. If our lens was centralized its schematic design would be :
43. We need to add a prism horizontally in order to produce the desired
decentralization and to know the basis of the position we need to indicate the
user's nasal position.
Nasal relative
position in the
lens
44. As is a plus lens the prism base will be in the nasal direction, we positioned it so as
to affect less as possible the thickness of the lens.
Nasal relative
position in the
lens
The prism added the lens will
result in an increase in
thickness.
DB
DB/2
EM + DB
SP
SP
EM
45. Edge Difference Calculation
= 0,019. ∆. ∅
Edges of the difference value from the prism calculated
from the prism and the block diameter.
The value of "0.019" is a constant related to the arc
tangent of prism apex angle and refractive index 1.530 of
a prism diopter of 1.00.
= 0,019. ∆. ∅ = 0,019 . 2,2 . 58 = 2,4
Faltam os demais cálculos que normalmente desenvolvemos para o cálculo da
lente.
46. Single Sagita = . = 4,24 . 0,79 = 3, 4
Sag Index ∅ = 58 → = 0,79
Minimum Thickness = 0,5
Let's return to our drawing to determine the thickness.
