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Prisms Done by: Othman Al- Abbadi, MD
35

Optical Prisms

Jan 21, 2015

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Optical prism for new ophthalmic residents
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Page 1: Optical Prisms

PrismsDone by: Othman Al-Abbadi, MD

Page 2: Optical Prisms

Reflection

• Occurs, at some degree, at all interfaces even when most of the light is transmitted or absorbed.

• Laws:– The Incident ray, the Reflected ray & the Normal..

All lie in the same plane.– Angle of Incidence = Angle of Reflection

Page 3: Optical Prisms

Reflection at spherical mirrors

C FP

F CP

Page 4: Optical Prisms

Clinical application

• The anterior surface of the cornea acts as a convex mirror.

Page 5: Optical Prisms

Light Refraction

• It’s the change in direction of light when it passes from one transparent medium into another of different optical density.

• The higher the density, the slower light and the higher the refractive index.

Page 6: Optical Prisms

• Absolute Refractive index: a comparison of light velocity in a vacuum to another medium.

• Refractive indices to some media:– Air= 1– Water= 1.33– Cornea= 1.37– Crystalline lens= 1.386-1.406– Crown glass= 1.52– Flint glass= 1.6

Page 7: Optical Prisms

Snell’s law

• To measure the refracting angle, we use the Snell’s law of refraction

Page 8: Optical Prisms

• On entering an optically dense medium from a less dense medium, light is deviated towards the normal.

• Medium 1nMedium 2 = n2 / n1

Page 9: Optical Prisms

• When passing obliquely from air through a glass plate then to air, light is deviated laterally.. Thus, the direction of the light is unchanged but is laterally displaced.

Page 10: Optical Prisms

Refraction on curved interfaces

• If n2 > n1 convergence

• If n1 > n2 divergence

• Refracting power= (n2 – n1 )/rr radius of curvature in meters

Page 11: Optical Prisms
Page 12: Optical Prisms

Critical angle

• As the rays meet the interface more obliquely, a stage is reached where the refracted ray runs parallel with the interface.

• More obliquely total internal reflection

Page 13: Optical Prisms

• Critical angle for the cornea?? 48.5

• How to overcome this reflection?? applying higher refractive index

material

Page 14: Optical Prisms

Light dispersion

• The dispersive power is not related to refractive index

• Shorter wavelengths more deviation• Rainbow total internal reflection with

dispersion– Primary– Secondary

Page 15: Optical Prisms

Prisms • A portion of a refracting medium bordered by

two plane surfaces which are inclined at a finite angle.

• The ray is deviated towards the base of the prism obeying the Snell’s law… forming erect, virtual & apically displaced image.

• The net change in direction is the angle of deviation.

Page 16: Optical Prisms

• Factors determining the angle of deviation in air:1. The refractive index of the prism material,2. The refracting angle of the prism,3. The angle of incidence of the ray.

Page 17: Optical Prisms

Primary positions

1. Angle of minimum deviation,

2. Prentice position.

Page 18: Optical Prisms

Angle of minimum deviation

• When the angle of incidence equals the angle of emergence.

• Symmetrical refraction.• Under these strict condition,

D= (n-1) * A• For glass prisms

D= A/2

Page 19: Optical Prisms

Prentice position

• When the incidence angle is zero, so that all the deviation takes place at the other surface of the prism.

• The “prentice position” power of the prism is greater than the “angle

of minimum deviation” power.

Page 20: Optical Prisms

In ophthalmic use

• The prentice position is specified for glass ophthalmic prisms… & the angle of minimum deviation power is specified for the plastic prism.

• Stacking prisms one on top of another doesn’t bring cumulative power of both prisms, so it’s not advised… except if used on different planes (one H & one V) because of perpendicular planes which result in independence refraction.

Page 21: Optical Prisms

Notations of prisms

1. Prism dioptre ( Δ )Linear of apparent displacement of 1 cm of an object that’s situated at 1 m.

2. Angle of apparent deviation ( θ )1 prism dioptre = ½ angle of apparent deviation

3. The centad ( Ϫ )The image is measured along an arc of 1 m from the prism.Slightly greater than the prism dioptre.

4. Refracting angle of the prism

Page 22: Optical Prisms
Page 23: Optical Prisms

Vector addition

• When the patient needs prismatic correction both H & V, we can give one stronger prism mounted at an oblique angle.

• Calculation:1. Graphically; by drawing a rectangle,2. Mathematically; by the Pythagoras’ theorem.

Page 24: Optical Prisms

Risley prism

Page 25: Optical Prisms

Orthoptic reports1. The synoptophore

Measuring the angle between the visual axes of the eyes in degrees… using (+) for XT & (-) for ETe.g: Synopt without gls +20

2. Prism cover testMeasuring the angle of squint by the alternating cover test while placing prisms of increasing power before one eye until movement is eliminated… measured in dipotrese.g: PCT= distance eso +40Δ

Page 26: Optical Prisms

Prescribing prisms

• The correction is split between the two eyes upon prescribing.

• E.g:– Convergence both base-out– Divergence both base-in

(the apex of the prism is directed toward the deviation)

– Hypper-/hypotropia one base-up & the other base-down

Page 27: Optical Prisms

1. The following are true about prism:

a. its orientation is defined by its apexb. light is deviated towards the apexc. light with shorter wavelength is deviated more

than light with longerwavelengthd. the angle of the prism apex is called the

refracting anglee. all the ophthalmic prisms are calibrated according

to the Prentice's position

Page 28: Optical Prisms

1. The following are true about prism:

a. its orientation is defined by its apexb. light is deviated towards the apexc. light with shorter wavelength is deviated

more than light with longerwavelengthd. the angle of the prism apex is called the

refracting anglee. all the ophthalmic prisms are calibrated

according to the Prentice's position

Page 29: Optical Prisms

2. The image formed by a prism is:a. erectb. magnifiedc. laterally invertedd. virtuale. deviated towards the apex

Page 30: Optical Prisms

2. The image formed by a prism is:a. erectb. magnifiedc. laterally invertedd. virtuale. deviated towards the apex

Page 31: Optical Prisms

3. The angle of deviation of a prism is determined by:a. the refracting angleb. the angle of incidence of the rayc. the refractive index of the prism materiald. the width of the basee. the thickness of the prism

Page 32: Optical Prisms

3. The angle of deviation of a prism is determined by:a. the refracting angleb. the angle of incidence of the rayc. the refractive index of the prism materiald. the width of the basee. the thickness of the prism

Page 33: Optical Prisms

4. The following are true about prisms:a. they can control torsional diplopiab. they can control diplopia caused by an eye which is deviated out and upc. the prism power can be calculated form the refracting angle aloned. a prism with 2 doptres will produce a linear displacement of 2 cm of an object situated at 1 me. a prism with 1 dioptre produce a stronger deviation than one with an angle ofapparent deviation of 1

Page 34: Optical Prisms

4. The following are true about prisms:a. they can control torsional diplopiab. they can control diplopia caused by an eye which is deviated out and upc. the prism power can be calculated form the refracting angle aloned. a prism with 2 doptres will produce a linear displacement of 2 cm of an object situated at 1 me. a prism with 1 dioptre produce a stronger deviation than one with an angle ofapparent deviation of 1

Page 35: Optical Prisms