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 mm16,54 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

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

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.

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 descentralizationFrame 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.

�� = �� − ��

DecentralizationSurfacing calculation

�� =∅�� − ��

2

�� =�� − ��

2

�� = �� − ��

�� =58 − 52

2= 3 ��

�� =72 − 58

2= 7 ��

�� = 7 − 3 = 4 ��

Bloco = 58 mmMDA = 52 mmPA = 72 mmDP = 58 mm

You can use a lens of 58 mm in diameter in both eyes if confeccionarmos with a decentralization of 7 mm

DecentralizationSurfacing calculation

�� =∅�� − ��

2

�� =�� − 2��

2

�� = �� − ��

�� =58 − 52

2= 3 ��

�� =72 − 2�27

2= 9 ��

�� = 9 − 3 = 6 ��

Bloco = 58 mmMDA = 52 mmPA = 72 mmDNP = 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 mmMDA = 52 mmPA = 72 mmDP = 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 OpticalAxis

New position ofOptical 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.


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.


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

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