Decentration and prismatic effect in lens (1)

Decentration and prismatic effect in lens

1. DECENTRATION AND ITS EFFECT IN LENSES By Prasamsha Dhungana

2. Introduction Spherical lenses are considered to be made of infinite number of prisms. Plus lenses have base to base and Minus lens have apex to apex arrangement of prism. Prismatic effect of the lenses increases with increasing lens power and also distance from the optical centre.

3. Optical centre The optical centre(OC) of the lens is the point at which light rays can pass with no deviation. When light goes through any other point on a lens, the ray of light is bent. It is important that a lens is ground so that its optical centre is directly infront of the patient’s pupil to allow optimum vision through the lens. There is least chromatic aberration and distortion in optical centre.

4. A Decentered Lens If the optical centre of the lens is positioned over the pupil, the lens is centered (there will be no prismatic effect). But if optical centre does not coincide with the line of sight of the eye, then it is called decentered lens and it induces prism. It may be good or bad depending upon the need of patient.

5. Induced prism Prism can be created intentionally or unintentionally. If prism is prescribed for a patient,(like in case of strabismus, convergence problems, hemianopia ) , then we induce it by decentering intentionally. But, if prism is not prescribed, errors in lab can create unwanted prism. With decentration, both prism power and prism base are manifested. The power of the prism depends on the amount of lens decentration and the refractive power of the lens being decentered. The prism base orientation depends on the direction of decentration and whether the lens is positive or negative.

6. Prentice’s rule It states that” the prismatic effect at any point on a spherical lens is equal to the distance of the point from the pole of the lens, in centimeters, multiplied by the power of the lens.” P=c*F where, p= prismatic power of dioptres c= amount of decentration in cm F= power of lens

7. Major reference point The point that has the desired amount of prismatic effect in a lens, when prism is prescribed. For non-prism prescription, the MRP and the optical centre of the lens are same.

8. Monocular eye movement in response to prism Image of object is displaced towards the apex. Eyes moves towards the apex through an angle equal to the angle of deviation of the prism. Eg . BO: eyes moves inward BI: eyes moves outward BU: eyes moves downward and BD: eyes moves upward

9. Binocular eye movements in response to prism When bases of prisms are in the same direction both eyes moves in the same direction(versions) Eg . Base in OD and Base out OS eye will turn to right.

10. When bases of prisms are in opposite direction. Eyes move in opposite direction ( vergences ) Eg . Base in OU: eyes moves outwards Base out OU: eyes moves inwards Divergence due to base in prism in both eyes.

11. Resultant horizontal prismatic effects When prisms make eye move in same direction,(base in different direction): The net effect is subtractive. Eg . Base in in one eye and base out in other eye. OD= 3∆ BO and OS= 5∆ BI (move eyes in same direction, left side) Net prismatic effect is 2∆BI.

12. When prisms make eye move in opposite direction,(base in same direction): The net effect is additive. Eg . Base out in both eyes. OD=3∆ BO OS= 4∆ BO (moves eye in opposite direction) Resultant prismatic effect will be 7∆ BO.

13. Resultant vertical prismatic effects When bases of prisms are in same direction(both prisms with base up or base down) Net effect is subtractive. Eg . OD= 4∆ BU OS= 2∆ BU Resultant prismatic effect is 2∆ BU.

14. When bases of prism are in opposite directions(one base up and other base down). Net effect is additive. Eg . OD= 2∆ BD OS= 2∆ BU Resultant prismatic effect is 4∆.

15. Prism incorporated by decentration can be either advantegeous or problematic, depending on the situation. For plus lenses, base of induced prism is towards the direction of decentration. For minus lens, base of the induced prism is towards the opposite directon of decentration.

16. Base of the induced prism in plus lenses is towards the direction of decentration. If plus lenses decentered nasally: Both eyes experience base in prism. IPD OCD

17. B. Plus lenses decentered temporally: Both eyes experience base out prisms. IPD OCD

18. Minus lens decentered upward in direction: IPD OCD Both eyes have base down prism. Base of the induced prism in minus lens is opposite to the direction of decentration.

19. When minus lenses decentered downwards: IPD OCD Here, both eyes will have base up prism.

20. Few examples to find resultant prismatic power and its base direction 1 . If power of both eyes = +3.00DS and Line of sight for eye passes 5mm nasal to the optical centre of lens. Prismatic effect=? P=c*F =0.5*3 = 1.5 ∆BO 5mm

21. 2. Prescription OD= -3.00DS Distance PD=64mm and near PD= 60mm Prismatic effect on each eye while reading= ?? PD difference at distance and near= 64-60mmi.e; 4mm Pd difference for each eye=4/2= 2mm(it is the amount of decentration as each eye moves 2mm inward for reading)

22. Prismatic power in OD= P=c*F =0.2*3 =0.6 ∆BI Prismatic effect in OS= P=c*F =0.2*3.5 =0.7∆BI Net prismatic effect= 0.6+0.7=1.3∆ BI

23. Compounding prism power When two prisms are combined in power and base orientation to form one prism that is the equivalent of both, the process is known as compounding prism. If two prisms are prescribed with prisms base on horizontal and vertical direction, we can compounded them into single oblique one. The resultant prism would be placed with its base between the base direction of two presribed prisms. We can calculate it by using power of prisms.

24. Examples on prism compounding Presriptiion : OD= 3∆ BU and 4∆ BI OS= plano In the given figure, OV represents vertical prisms, OH represents horizontal Prisms and OR represents resultant prisms. O V H R

25. The exact position of the resultant prism can be determined by using pythagoras prisms. Calculation for left eye: (OR)²= (OV)² + (OH)² (OR)² = 3² + 4² (OR)² =9+16 (OR)² =25 OR= √25 So, OR(power of resultant prism)= 5∆ Direction of base: Tan(ROH)= ¾ ROH= tan -1 ¾ =36.87º ~37º The resultant prism is: 5∆ base @ 37º

26. Resolving prism power The process of expressing a single oblique prism as two perpendicular componenets is known as resolving prisms. If prisms are precribed in oblique direction(like at 40˚),we can resolve it into horizontal and vertical prism. Example: Prescription: OD=4∆ Base @30˚ OS= plano

27. 4∆ 30˚ V H O Using simple trigonometry, we can resolve into two different component. For right eye, Sin30= RH/OR sin30= RH/4 RH= 4*sin30 =2∆ BU Cos30= OH/OR Cos30= OH/4 OH=4*cos30 =3.46∆ BI ~ 3.5∆ BI So, the resultant prismatic power is: R=2∆ BU and 3.5∆ BI L= plano 30º R V= PsinØ H= PcosØ

28. Decentration in spherocylindrical lenses Prismatic effect in cylindrical lenses is experienced only if decentration occur in power meridian. There would be no any effect on axis meridian as there is no power. If axis of the cylindrical lenses are in principal meridian i.e;90 or 180, prismatic effect due to decentration can be calculated same like in spherical lenses, using Prentice rule. But, if the axis of cylindrical power is oblique, the calculation becomes more complicated.

29. Examples Find the prismatic effect at a point 10 mm below and 2mm nasal of the optical centre of a +2.00/-1.00*90 lens ( inRE )? Horizontal prism: P=c*F =0.2*1 =0.2∆ BI Vertical prism: P=c*F = 1*2 =2∆ BD +2.00(V) +1.00(H)

30. THANK YOU