Ophthalmic Prisms OPTOM FASLU MUHAMMED

Ophthalmic Prisms

A prism is defined as a portion of a refracting medium bordered by two plane surfaces which are inclined at a finite angle.

The angle between the two surfaces is called the refracting angle or apical angle of the prism.

A line bisecting the angle is called the axis of the prism.

The opposite surface is called the base of the prism.

When prescribing prisms ,the orientation is indicated by the position of the base , ‘base in’, base up’, base down, base out.

A prism consists of two angled refracting surfaces.

The simplest form of a prism is two flat surfaces that come together at an angle at the top. The point is called the apex of the prism; the wider bottom of the prism is called the base.

Ophthalmic Prisms

REFRACTION THROUGH PRISM

Light passing through a prism obey snell’s law at each surface.

The ray is deviated towards the base of the prism.

This causes objects to be displaced away from the base of the prism towards its apex.

The net change in direction of the ray , angle D is called the angle of deviation.

All varieties of spectacle lens has the effect of a prism when viewed through a point away from the optical center.

The further the away from the optical center, the greater is the prismatic effect

For a prism in air ,the angle of deviation is determined by three factors

1. The refractive index of the material of which

the prism is made.

2. The refracting angle of the prism

3. The angle of incidence of the ray considered.

Light rays entering and leaving a prism are bent towards the base of the prism.

This causes objects to be displaced away from the base of the prism towards its apex.

Base down prism - upward. Base up prism - downward

To the right or left when seen through a base-out or base-in prism

CHARACTERISTICS OF PRISM A prism has a thickest edge, the base and a

thinnest edge, the apex.

A prism displaces the incident rays towards the base of the prism.

A prism displaces the image towards the apex of the prism.

CHARACTERISTICS OF PRISM A prism does not change the vergence of the

rays.

A prism does not magnify or minify the image.

A prism also disperses incident pencil rays into its component colours.

Angle of Minimum Deviation

For any particular prism, the angle of deviation D is least when the angle of incidence equals the angle of emergence. Refraction is then said to be symmetrical and the angle is called the Angle of minimum deviation.

Image Formation

The image formed by a prism is

Erect

Virtual

Displaced towards the apex of the prism.

There are two primary positions in which the power of a prism may be specified

The position of minimum deviation

The prentice position.

Units of prisms The power of a prism can be expressed in

various unit.

1. The prism dioptre

2. The centrad

The prism dioptre (Δ) A prism of one prism dioptre power produces

a linear apparent displacement of 1 cm ,of an object O, situated at 1 m.

Denoted by the symbol Δ.

The centrad

This unit differs from the prism dioptre only in that the image displacement is measured along an arc 1m from the prism.

The centrad produces a very slightly greater angle of deviation than the prism dioptre, but the difference ,in practice is negligible.

Refracting angle

A prism may also be described by its refracting angle.

DEVIATION PRODUCED BY PRISM The deviation so produced can be measured by the

Prentice’s Rule:

Δ = cF is commonly known as Prentice’s

c = image displacement in cm F =lens power

PRISM ORIENTATION

Prism can be oriented in front of the eye using notations

Base-in Base out

Base up Base down

All other base directions require 360° notation

PRISM ORIENTATION

PRISM ORIENTATION

Use of prisms

Diagnostic prisms

Therapeutic prisms

Diagnostic prisms

1. Assessment of squint and heterophoria(a) Measurement of angle objectively by prism cover test.

(b) Measurement of angle subjectively by maddox rod.

(c) To assess likelihood of diplopia after proposed squint

surgery in adults.

(d) Measurement of fusional reserve.

(e) Four dioptre prism test. (microtropia)

2. Assessment of simulated blindnessIf a prism is placed in front of a seeing eye, the

eye will move to regain fixation.

Forms of prism used in diagnosis Single unmounted prisms

The prisms from the trial lens set

Prism bars These are bars composed of adjacent prisms

of increasing power.

Therapeutic prism1. Convergence insufficiency commonest therapeutic use of prisms in

orthoptics,for building up the fusional reserve of patients with C.I.

Base out prisms are using in exercise periods.

They are not wore constantly.

2 .To relieve diplopia in certain cases of squint

Include decompensated heterophorias, small vertical squint and some paralytic squint with diplopia in the primary position.

Prisms are reserved for those patients for whom surgery is not indicated.

Forms of therapeutic prismTemporary wear

prisms used in treatment include clip-on spectacle prisms for trial wear.

Eg:-Fresnel prism (pronounced fre-nell') prisms,)

Fresnel prism

To understand how a Fresnel prism works, imagine cutting off the tops of a large number of equally powered prisms and gluing them, one above the other, onto a thin piece of plastic.

A Fresnel prism is only 1 mm thick.

Fresnel prism

It consists of plastic sheet of parallel tiny prisms of identical refracting angle.

The overall prismatic effect is the same as that of a single large prism.

The sheets are lighter than a glass prism and can be stuck on to the patient’s glasses.

Fresnel prism

Advantages of a Fresnel Prism

It is very thin and extremely lightweight. It is flexible and can be applied to an existing

spectacle lens.

It can be cut to any shape with scissors or a razor blade.

Reduce magnification differences considerably.

Disadvantages

They are harder to clean than conventional lenses.

Fresnel prisms also cause a slight loss of visual acuity caused by reflections for prisms greater than 10Δ.

WHEN ARE FRESNEL PRISMS USED?

High Amounts of Prism

Use and Reuse

Sectorial Application

Slowing of Nystagmus

Permanent wear

Prism can be mounted in spectacles permanently .

Vector addition of prisms

Sometime a patient requires a prismatic correction in both the horizontal and vertical directions.

This can conveniently be achieved by using one stronger prism mounted at an oblique angle. This is calculated by vector addition, either graphically or mathematically.

Risley prism

There is an application of obliquely crossed prisms that is used on a regular basis in ophthalmic practice.

That application is called a rotary or Risley’s prism

A rotary prism is a combination of two prisms.

These prisms are placed one on top of the other. Initially, their base directions are exactly identical, but as the prisms are rotated, their bases move by equal extents in opposite directions.

For example, suppose two prisms of 10Δ each are placed on top of one another base-to-base. The total prismatic effect is 20Δ .

But if they are placed base-to-apex, then their total prismatic effect is zero

Risley prism

Interpretation of orthoptic reports

1 prism dioptre = ½ degree

eg: 20 degree = 40 prism diopter

Prescription of prisms

Generally when prescribing prisms, the correction is split between the two eyes.

The apex of the prism must always be placed towards the direction of deviation of the eye.(apex towards deviation )

Convergence or eso deviation Base out prism ,e.g. 8 prism diopter base out R and

L.

Divergence or exo deviation prisms must be base -in , e.g. 6 prism diopter

base in R and L.

Vertical deviation

To correct vertical deviation the orientation of the prism is opposite for the two eyes.

2 base - down RE 2 base – up for R hypertropia

Prisms in optical instruments

Slit lamp bio microscope

Applanation tonometer

keratometer

Right angle prism

Porro prism

Dove prism

DETECTION OF PRISM IN AN OPTICAL LENS Hold the glass up between the eye and an object

which forms a straight line.

If the continuity of the line in broken as seen in Figure ,it implies that the prism is present and since the line appears to be deviated towards the apex, we know the direction of apex of the prism.

The amount of displacement produced is the strength of the prism, and can be measured by neutralising.

•To do so put the prism lens of known strength in contact with the lens under checking with its apex in opposite direction. •The prism lens with which the continuity of the lens is again established, represents the strength of the prism.

Lensometer Lensometer can also be used to detect the prism

dioptre. To check this get the best focus point of the target image and read the prismatic power.

An additional accessory called ‘prism compensator’ is to be attached to the lensometer for higher dioptre of prism.

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