Ophthalmic prisms

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Ophthalmic prisms

Ophthalmic prismsFakhruddin AliasgerLecturer, SPIO (a unit of Dr. Agarwals Eye Hospital)4/5/2017Fakhruddin Alliasger1

PrismA prism consists of two angled refracting surfaces The simplest form of prism is two flat surfaces coming together at an angle at the topThe point is called Apex of the prismThe wider bottom of the prism is called the base4/5/2017Fakhruddin Alliasger2

The base of all prisms are thicker than the apexThe angle between the two refracting surface of the prism is known as apical angle

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apical angle

BaseApex4/5/2017Fakhruddin Alliasger4

Ophthalmic PrismsOphthalmic prisms are, generally, thin prisms. They have an apical angle of less than 10 to 15.Thin prism have no dioptric power but can be combined with dioptric lenses in a refractive correction.A curved thin prism of Plano dioptric power has a front and back surface of equal and opposite power. 4/5/2017Fakhruddin Alliasger5

Thick Prism

Thin Plano Prism 4/5/2017Fakhruddin Alliasger6

The orientation of the prism in front of the eye will affect the position at which the eye perceives any object to be viewed through the prism. It is, therefore, important to specify accurately the orientation of the prism so that its effect on the eyes is known when incorporated into a refractive correction. 4/5/2017Fakhruddin Alliasger7

The orientation of a prism is specified in terms of the position of the base and axis.4/5/2017Fakhruddin Alliasger8

Properties of prismThe position of an object will appear to change when viewed through a prism.White light incident on a thick prism will appear to be dispersed into the colour spectrum when emergent from the second surface.A prism displaying this phenomenon is often known as a Newton prism.Dispersion is usually seen in thick prism i.e.. a prism whose apical angle is greater than 15 to 20.

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UNITS OF PRISM POWERThe apical angle and the refractive index of the prism determine its deviating power.Angular UnitsThe deviation produced by the prism is expressed in degrees or radians.Prism DioptresOne prism dioptre produces a deviation of one unit at a plane 100 units from the prismThe unit of prism dioptres is denoted . Three prism dioptres would be written as 3

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CentradsOne Centrad produces a deviation of one unit of arc at a distance of 100 units from the prismThe unit of Centrads is denoted . Thus, three Centrads would be written as 3 .4/5/2017Fakhruddin Alliasger11

Prism in spectaclesPrism is prescribed for various reasons, strabismus (the most common reason), convergence problems, hemianopia etc. Its purpose is to deviate the path of the incident light so that it corresponds with the visual axes of the eyes. So, for example, if one eye (the right in the side) has an exotropia base in prism can be prescribed to bring the path of light from the object being viewed back along the visual axis.

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Orientation of prismThe orientation of prism(s) in front of the eye(s) is given by the position of the base.When facing the patient, the patients right eye is on the examiners left side. Base out is denoted when the base is positioned on the temporal side. When, a base out prism is positioned in front of each eye, each prism will have their base orientated at the temples of each eye and the apices of the prisms will be pointing towards the nose.

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R DOWNL DOWN4/5/2017Fakhruddin Alliasger15

Similarly, base in prism is denoted when the base of the prism is orientated on the nasal side of the eye and the apex is pointing towards the temple4/5/2017Fakhruddin Alliasger16

Base orientationWhen prescribing prism it is, of course, necessary to indicate the direction of the prism base. While most cases will involve prism in one of the four main directions, up, down, in and out, oblique prism may also be ordered. There are two accepted methods for indicating the direction of an oblique prism.

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Base orientation

Standard notation36O notation4/5/2017Fakhruddin Alliasger18

Standard notation: This is the same axis notation as used for the axis of astigmatic lenses. This notation requires further indication of the direction of the base. For example, it is not sufficient to order RE: 4 at 135. This could be either up and out at 135 or down and in. So the prescription needs the direction as well as the angle.4/5/2017Fakhruddin Alliasger19

360 notation: This system of notation is the same as standard notation in the top two quadrants but continues to 360 in the bottom quadrants. This system requires no other notation that the angle. So, RE: 4 at 135 would mean up and out, there is no other possibility since down and in would be RE: 4 at 315.

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Clinical consideration

Due to the difference in thickness between the base and the apex of a prism, refractive corrections incorporating prism power for one eye only, the spectacles may be dispensed with the prism power split between the two eyes. This is usually due to a noticeable and cosmetically unacceptable difference in thickness between the spectacle lenses if they were made up as prescribed. 4/5/2017Fakhruddin Alliasger21

It is important that the effect on the eyes as a pair is maintained when the prism power is split between the spectacle lenses. This can be achieved using the following rules.If the prismatic power is prescribed monocularly in a refractive correction that is similar between the two eyes, the prismatic power should be split evenly.

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Prism power with horizontal base direction should have the same base direction in both eyesPrism power with a vertical direction should have opposite base directions in each eye, with the base direction for the eye in which the prism was originally prescribed remaining the same.

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Compounding prism powerThe following correction is prescribed:R Plano 3 UP 4 IN L PlanoThe two prisms could be compounded i.e.. replaced by a single oblique prism. The resultant prism would be positioned with its base between the base directions of the two prescribed prisms. The exact orientation of the single resultant prism is determined by the power of the two prescribed prisms.In the figure, OV represents the vertical prism, OH the horizontal prism and OR the resultant prism when the vertical and horizontal prisms are compounded.

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ORVH4/5/2017Fakhruddin Alliasger25

The exact position of the resultant prism can be determined using Pythagoras Theorem:(OR)2 = (OV)2 + (OH)2(OR)2 =( 3)2 + (4)2 = 25OR = 5 tan (ROH)= 3/4Angle ROH = tan-1(3/4) = 36.87The resultant prism power is 5 orientated at 37 dR Plano 5 UP @ 37 L Plano4/5/2017Fakhruddin Alliasger26

Resolving prism powerWe have seen that a horizontal and vertical prism prescribed in one eye can be compounded to a single oblique prism. In the same way, a single oblique prism can be simplified to two orthogonal prisms.The following correction is prescribed:R Plano 4 UP @ 030 L PlanoSince the position of the single oblique prism is known, simple trigonometry can be used to simplify the prism to a horizontal and vertical prism component:

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sin 30 = (OV) / (OR)OV = 4.sin 30 OV = 2 UPcos 30 = (OH) / (OR)OH = 4.cos 30 OH = 3.46 OUT = 3.5 INThe final result would be writtenR Plano 2 UP 3.5 IN L Plano

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Prentices rulePrentices rule is the formula used to calculate the decentration needed to create prism or the prismatic effect of a lens at a given point. For example, if we are required to find the prismatic effect of a +5.00 D lens at a point 4 mm below the optical centre we use Prentices rule. So,P = cF = 0.45 = 2 base up

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(A) The prismatic effect below the OC of a plus lens(B) The prismatic effect below the OC of a minus lens4/5/2017Fakhruddin Alliasger30

the prismatic effect at a point 4 mm below the optical centre of a -4.00 D lens.P = cF = 0.44 = 1.6 base downCalculation of the prismatic effect for sphero-cylinder lenses is the same if the prismatic effect is required in a principal meridian. It is a little more complex for oblique axes.

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Axes of 90 and 180The decentration required to create prism or the prismatic effect at any point on cylinders and sphero-cylinders can be determined using Prentices Rule, where:c is the distance from the optical centre for each of the vertical and horizontal meridians4/5/2017Fakhruddin Alliasger32

For example:Find the prismatic effect at a point 10 mm below and 2 mm nasal of the optical centre of a+2.00/-1.0090 lens.Horizontal prismP180 = c180 F180P180 = 0.2 +1.00 P180 = 0.2 Base Up

Vertical prismP90 = c90 F90P90 = 1.0 +2.00 P90 = 2 Base Up

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Calculation of prismatic effect in oblique cylinder

The calculation of prismatic effect in oblique cylinders requires a more complex set of formulae. Before these formulae can be applied certain notation and conventions need to be established.

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The notation:P is the point at which the prismatic effect is to be foundx is the horizontal distance from optical centre in cmy is the vertical distance from optical centre in cmS is the sphere power of the prescriptionC is the cyl power is 180 - axis for the