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PRISM
PRISM
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.
How the user perceives the prism image
Real position of the object.
Position perceived by the observer.
Apex
Base
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.
Simplified representation of the
deviation caused by a prism.
Apex
ou
Thin Edge
BF
Base
ou
Thick Edge
BG
BG – BF = DB → Edge Difference
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.
Prismatic Diopter
∆ = , ∆
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.
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
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.
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
Prism
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.
Deviation versus Vergence
Fresnel
Fresnel
Prism
Simplified view of the lens
One way to
explain the
operation of the
positive lens is
to consider
them as two
prisms joined at
the base.
Simplified view of the lens
One way to
explain the
operation of the
negative lenses
is to consider
them as two
prisms joined by
the apex.
DECENTRALIZATION
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.
Prism
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.
Fitting decentralization
When the optical center is not centered in the
frame looked a fitting decentralization.
= =
×
dm – fitting descentralization
Frame center
Pupil of User
Fitting decentralization
Block left is the difference between the diameter of the block and the larger the rim
Block Left
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.
= −
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
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
Another way to understand the
decentralization
=
∅ − ∅
Bloco = 58 mm
MDA = 52 mm
PA = 72 mm
DP = 58 mm
∅ = − +
Ou
∅ = − . +
=
−
=
∅ = − + =
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.
EFFECT OF APPLICATION OF THE
PRISM IN THE LENS
PLUS LENS
Optical Axis
Optical Center
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
MINUS LENS
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
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.
∆=
.
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.
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
∆=
.
=
, .
= , ∆
= , → = ,
= × = , × , = ,
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
If our lens was centralized its schematic design would be :
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
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
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.
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.
Nasal relative
position in the
lens
2,4
1,2
0,5 + 2,4
3,4
0,5
3,4
1,2 + 0,5
2,9 0,5
1,7
1,7
5,1
2,9 0,5
1,7
1,7
5,1
+ 8,01
- 3,99
Final Result
Thickness
Curves
Prism
Prism

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Prism

  • 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
  • 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.
  • 19. Simplified view of the lens One way to explain the operation of the positive lens is to consider them as two prisms joined at the base.
  • 20. Simplified view of the lens One way to explain the operation of the negative lenses is to consider them as two prisms joined by the apex.
  • 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.
  • 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.
  • 25. Fitting decentralization When the optical center is not centered in the frame looked a fitting decentralization. = = ×
  • 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.
  • 33. EFFECT OF APPLICATION OF THE PRISM IN THE LENS
  • 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.
  • 47. Nasal relative position in the lens 2,4 1,2 0,5 + 2,4 3,4 0,5 3,4 1,2 + 0,5 2,9 0,5 1,7 1,7 5,1
  • 48. 2,9 0,5 1,7 1,7 5,1 + 8,01 - 3,99 Final Result Thickness Curves