Ophthalmic prisms

prism in ophthalmic practice

1. Ophthalmic prisms Fakhruddin Aliasger Lecturer, SPIO (a unit of Dr. Agarwal’s Eye Hospital) 4/5/2017 Fakhruddin Alliasger 1

2. Prism A prism consists of two angled refracting surfaces The simplest form of prism is two flat surfaces coming together at an angle at the top The point is called Apex of the prism The wider bottom of the prism is called the base 4/5/2017 Fakhruddin Alliasger 2

3. The base of all prisms are thicker than the apex The angle between the two refracting surface of the prism is known as apical angle 4/5/2017 Fakhruddin Alliasger 3

4. apical angle Base Apex 4/5/2017 Fakhruddin Alliasger 4

5. Ophthalmic Prisms Ophthalmic prisms are, generally, thin prisms. They have an apical angle of less than 10º 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. 4/5/2017 Fakhruddin Alliasger 5

6. Thick Prism Thin Plano Prism 4/5/2017 Fakhruddin Alliasger 6

7. 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. 4/5/2017 Fakhruddin Alliasger 7

8. The orientation of a prism is specified in terms of the position of the base and axis . 4/5/2017 Fakhruddin Alliasger 8

9. Properties of prism The 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 15º to 20º. 4/5/2017 Fakhruddin Alliasger 9

10. UNITS OF PRISM POWER The apical angle and the refractive index of the prism determine its deviating power. Angular Units The deviation produced by the prism is expressed in degrees or radians. Prism Dioptres One prism dioptre produces a deviation of one unit at a plane 100 units from the prism The unit of prism dioptres is denoted  . Three prism dioptres would be written as 3  4/5/2017 Fakhruddin Alliasger 10

11. Centrads One Centrad produces a deviation of one unit of arc at a distance of 100 units from the prism The unit of Centrads is denoted  . Thus, three Centrads would be written as 3  . 4/5/2017 Fakhruddin Alliasger 11

12. Prism in spectacles Prism 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. 4/5/2017 Fakhruddin Alliasger 12

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14. Orientation of prism The orientation of prism(s) in front of the eye(s) is given by the position of the base. When facing the patient, the patient’s right eye is on the examiner’s 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. 4/5/2017 Fakhruddin Alliasger 14

15. L UP R UP R OUT R & L IN L OUT R DOWN L DOWN 4/5/2017 Fakhruddin Alliasger 15

16. 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 temple 4/5/2017 Fakhruddin Alliasger 16

17. Base orientation When 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. 4/5/2017 Fakhruddin Alliasger 17

18. Base orientation Standard notation 36O notation 4/5/2017 Fakhruddin Alliasger 18

19. 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. 4/5/2017 Fakhruddin Alliasger 19

20. 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. 4/5/2017 Fakhruddin Alliasger 20

21. 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. 4/5/2017 Fakhruddin Alliasger 21

22. 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. 4/5/2017 Fakhruddin Alliasger 22

23. Prism power with horizontal base direction should have the same base direction in both eyes Prism 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. 4/5/2017 Fakhruddin Alliasger 23

24. Compounding prism power The following correction is prescribed: R Plano 3  UP 4  IN L Plano The 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. 4/5/2017 Fakhruddin Alliasger 24

25. H O R V H 4/5/2017 Fakhruddin Alliasger 25

26. 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 = 25 OR = 5  tan (ROH)= 3/4 Angle ROH = tan-1(3/4) = 36.87º The resultant prism power is 5  orientated at 37 d R Plano 5  UP @ 37 L Plano 4/5/2017 Fakhruddin Alliasger 26

27. Resolving prism power We 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 Plano Since 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: 4/5/2017 Fakhruddin Alliasger 27

28. sin 30 = (OV) / (OR) OV = 4.sin 30 OV = 2  UP cos 30 = (OH) / (OR) OH = 4.cos 30 OH = 3.46  OUT = 3.5  IN The final result would be written R Plano 2  UP 3.5  IN L Plano 4/5/2017 Fakhruddin Alliasger 28

29. Prentice’s rule Prentice’s 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 Prentice’s rule. So, P = cF = 0.4 5 = 2  base up 4/5/2017 Fakhruddin Alliasger 29

30. (A) The prismatic effect below the OC of a plus lens (B) The prismatic effect below the OC of a minus lens 4/5/2017 Fakhruddin Alliasger 30

31. the prismatic effect at a point 4 mm below the optical centre of a -4.00 D lens. P = cF = 0.4 4 = 1.6  base down Calculation 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. 4/5/2017 Fakhruddin Alliasger 31

32. Axes of 90 and 180 The decentration required to create prism or the prismatic effect at any point on cylinders and sphero-cylinders can be determined using Prentice’s Rule, where: c is the distance from the optical centre for each of the vertical and horizontal meridians 4/5/2017 Fakhruddin Alliasger 32

33. For example: Find the prismatic effect at a point 10 mm below and 2 mm nasal of the optical centre of a +2.00/-1.00 90 lens. Horizontal prism P 180 = c 180  F 180 P 180 = 0.2  +1.00 P 180 = 0.2  Base Up Vertical prism P 90 = c 90  F 90 P 90 = 1.0  +2.00 P 90 = 2  Base Up 4/5/2017 Fakhruddin Alliasger 33

34. 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. 4/5/2017 Fakhruddin Alliasger 34

35. The notation: P is the point at which the prismatic effect is to be found x is the horizontal distance from optical centre in cm y is the vertical distance from optical centre in cm S is the sphere power of the prescription C is the cyl power  is 180 - axis for the right eye and the axis for the left eye A,B and D are values used in the calculations of the prismatic effect H is the horizontal prismatic effect V is the vertical prismatic effect It is H and V that we are aiming to find. That is, the horizontal and vertical prismatic effects. 4/5/2017 Fakhruddin Alliasger 35

36. Calculation of prismatic effect in oblique cylinder The convention and condition are P is the position of measurement x is positive if P is in from the Optical centre x is negative if P is out from the Optical centre y is positive if P is up from the Optical centre y is negative if P is down from the Optical centre . 4/5/2017 Fakhruddin Alliasger 36

37. Prismatic effect at p If H is positive the prismatic effect is base out If H is negative the prismatic effect is base in If V is positive the prismatic effect is base down If V is negative the prismatic effect is base up. 4/5/2017 Fakhruddin Alliasger 37

38. Example: Calculate the prismatic effect 8 mm down and 5 mm in from the OC of a RE:+5.00/-2.00 60 The formulae are: A = S + Csin² = 5+ -2.00sin²120 = +3.5 B = Csincos  = -2.00sin120cos120 = 0.866 D = S + Ccos² = 5+ -2.00cos²120 = +4.5 H = Ax + By = 3.50.5 + 0.866 -0.8 = 1.057 V = Bx + Dy = 0.8660.5 + 4.5 -0.8 = -3.167 Therefore the prismatic effect at P is: 1.057  base out 3.167  base up Following from the conventions given in the previous slide, since H is positive the prism direction is base out and V is negative so the direction is base up. 4/5/2017 Fakhruddin Alliasger 38