Ophthalmic Prisms & Vision Correction

Cylindrical Lenses

• Used for correcting astigmatism
• Astigmatism literally means "not-point forming"
• Patients with astigmatism notice vertical and horizontal lines appearing different
• Example: the bars in a window frame may be clear horizontally, but blurry vertically
• Correcting lenses have varying power along the lens' principal meridians
• A cylindrical surface is flat along a meridian, parallel to the axis of revolution, but circular at 90° to the axis

Types of Cylindrical Lenses

• Cylinder
• Plano-convex cylinder
• Plano-concave cylinder

Correction of Regular Astigmatism

• Cylinder lenses only refract light in a plane perpendicular to the cylinder's axis
• The axis defines the non-refracting plane
• Cylinder lenses can be positive (plus) or negative (minus)

Principal Meridians of Cylindrical Lenses

• The flat or plane meridian of a cylinder is called the axis meridian
• The meridian of maximum curvature (90° to the axis) is the power meridian
• The power of the cylindrical lens is highest at 90° to its axis
• The axis meridian and the power meridian are called principal meridians of the cylindrical lens

Three Forms of Cylindrical Lenses

• Plano-convex cylinder
• Cross cylinder
• Sphero-cylinder

Plano Cylindrical Lenses

• Used for correcting astigmatism in one meridian only
• Simple astigmatism is the type of astigmatism that needs correcting in a single meridian only

Cross Cylinders

• Correcting astigmatism in two meridians
• Also called compound astigmatism or mixed astigmatism
• Two different cylinders used at right angles to each other

Sphero-cylindrical Lenses

• One surface is spherical and the other surface is cylindrical
• The sphere component is 0.00, while the cylinder component is -2.00 DC and lies in the 90th meridian
• This type can be +2.00 DS/-1.00 DC X180 where sphere component is +2.00 DS and the cylinder component is -1.00 DC lying on the 90° meridian.

Notes on Cylindrical Lenses

• All classes of cylindrical lenses can be shown in diagrams as cross-cyliders, or sphero-cylinder forms(using transposition)
• In clinic, cylindrical lenses are always shown as sphero-cylindrical form
• Regular astigmatism forms are shown in power diagrams

Types of Regular Astigmatism

• Simple hyperopic astigmatism
• Simple myopic astigmatism
• Compound myopic astigmatism
• Compound hyperopic astigmatism
• Mixed astigmatism

Prescription of Optical Cross-Cylindrical Forms

• Fig 1: Pl X 90 / -2.00 X 180
• Fig 2: +2.00 X 90 / +1.00 X 180

Scissors Movement of Cylindrical Lenses

• A: Axis meridian parallel with the vertical limb of the chart
• B: Plus cylinder rotated clockwise causes vertical limb to rotate anti-clockwise
• C: Minus cylinder rotated clockwise causes vertical limb to rotate clockwise

Neutralizing Cylindrical Lenses

• Place a minus cylinder with its axis parallel to the 180th meridian to neutralize the power in the vertical meridian
• Then place a plus lens with its axis parallel to the 90th meridian to neutralize the power in the horizontal meridian
• The cross cylinder prescription is therefore +0.50DC X 180 / -0.50DC X 90

Properties of Cross Cylinders

• Two cylinders with parallel axes can be combined into a single cylinder with the power equal to the sum of the two cylinders
• Two cylinders with opposite signs and parallel axes cancel each other out(produce a plano lens)

Flat Transposition of Cylindrical Lenses

• Transposition is changing the form of a cylindrical lens prescription without affecting its refractive correction.

Cross Cylinders to Sphero-Cylinders

• Write one of the cross-cylinder powers as a dioptre sphere.
• Subtract the chosen cylinder from the other cylinder to get the second cylinder power.
• The axis of the new cylinder is the axis of the cylinder that wasn't chosen for the sphere

Sphero-cylinders to Cross Cylinders

• The sphere power becomes the first cylinder
• The axis of the second cylinder is perpendicular to the cylinder in the sphero-cylindrical prescription
• The algebraic sum of the sphere and cylinder in the sphero-cylindrical prescription gives the second cylinder in the cross-cylinder prescription

Flat Transposition (One sphero-cylinder to another sphero-cylinder)

• Add the cylinder to sphere algebraically and use as new sphere
• Change the algebraic sign of the cylinder, but leave the cylinder power unchanged
• Change the cylinder axis by 90 degrees

Flat Transposition(from optical cross to cross cylinder form)

• Take the power on either arm of the cross and give it the axis designation of the other arm
• Take the power on the second arm and give it the axis designation of the arm 90° removed
• Combine the two powers and axis designations into a cross-cylinder form

Flat Transposition (taking prescription off a power diagram in minus cylinder form)

• Designate the cylinder with greatest plus power as the sphere power
• Use algebraic difference between the sphere and other cylinder power as the new cylinder
• Express the new cylinder in minus-cylinder form and give it the meridian of greatest plus power

Flat Transposition (taking a prescription off an optical cross in plus-cylinder form)

• Designate the cylinder with the least plus power as the sphere power(the first cylinder power)
• Use the algebraic difference between the sphere power and the other cylinder power as the new cylinder power
• Express the new cylinder power in plus-cylinder form
• Give the new cylinder power the meridian of least plus power

Hand Neutralization Practical

• Analyze lens type (single vision, bifocal, progressive, photochromic, tinted, anti-reflective, etc)
• Represent lenses in optical crosses and cross cylinder forms
• Translate representations into sphero-cylinder forms
• Identify refractive errors from prescriptions

Oblique Astigmatism

• Two principal meridians are not at right angles to each other but are crossed obliquely
• Requires correction with cross-cylinders not at exactly 90°

Combining Two Cylinders that are Not 90 Degrees

• Graphical method: graph cylinder powers on "A" and "B" lines, transpose to plus, and graph. These can be combined using a parallelogram
• Formula method:Use formulas R2 = A2 + B2 + 2ABCos2a or, Sin b = (B/R) Sin2a

Toric Lenses and Transposition

• Periscopic and meniscus lens forms with powers of +/-1.25 DS or +/- 6.00 DS are best or bent forms of spherical lenses, and are called base curves.
• Toric lenses are the best or bent forms of astigmatic lenses with one surface spherical, and the other cross-cylinder
• To make a quality astigmatic lens, base curves are given, which are toric bases. -To write a prescription for toric lenses, the base curves must be given. -Example +3.00 DS / +1.00 DC X 90 on toric base -6 DC

Toric Lenses and Transposition (Toric surface)

• A toric surface is a cylinder-like surface consisting of two curves at right angles on the same surface.
• A toric lens has different optical power and focal length perpendicular orientations

Toric Transpositions

• Describes manner in which lens is made
• Toric transpositions depend on use of a definite base curve.
• All other curves are determined by using base curve as reference point.
• By varying base curve, several lenses with same net effect but different surface powers can be made.
• This leads to a need for toric transposition

Base Curves

• Standard base curves are +/- (4, 6, 8, 9, or 10)
• Two criteria for selecting base curve:least aberration and ample room for eyelashes

Single Vision Toric Transpositions

• Enables a toric astigmatic lens to be defined in terms of surface powers.
• Toric astigmatic lens made with one spherical and one cross-cylinder surface -Base curve is weaker power of cross-cylinder meridian
• Sphere curve is spherical surface of toric lens

Steps in Transposition

• Transpose prescription so that cylinder and base curve are the same sign
• Calculate the required power of the spherical surface by subtracting the base curve power from the spherical power in previous step
• Specify axis of base curve as its the weaker meridian from the toric surface, which is 90° to the axis of the cylinder in the previous step.
• Add the required cylinder to base-curve power with its axis as indicated in the previous steps

Bifocal Lenses

• Ophthalmic lens with two portions having two different focal lengths
• Primarily corrects presbyopia(diminishing plasticity of crystalline lens with age)
• Used for people with muscle imbalance (accommodative insufficiency)
• The two portions are distinct for distant and near vision

Bifocal Lenses-History

• Franklin bifocal developed in 1784
• Solid Upcurve Bifocal developed in 1837
• Improved versions include those made by August Marick in 1888; incorporating a segment with different refractive index cemented onto a major lens blank

Bifocal Lenses-Modern Designs

• Fused bifocals-crown glass major with higher-index segment fused onto designated countersink curve
• One-piece bifocals-change in curvature of one surface creates power change for near vision

Bifocal Prescription

• Bifocal lenses are specified in prescriptions by distant power and the additional power(add)
• Near power = Distant power + Add
• Distance power is power required for comfort distance vision
• Near power is power required for comfort near vision

Trifocal Lenses

• Lens with three foci(distance, intermediate, and near)
• Mostly for patients needing addition for near and intermediate vision
• Intermediate portion power is often 50% of near addition power

Quadrifocal Lenses

• Lens with four foci
• Rare, and combines bifocal segment over a trifocal design
• Additional segment is for near overhead viewing

Progressive Addition Lenses

• Invisible multifocal lens with gradual increase in power of near vision

Protective Lenses

• One piece design with three optical zones(distance,intermediate, and near zones)
• Intermediate zone rises gradually from zero add to near correction

Opthalmic Prisms

• Used to measure and correct deviations in the eye muscles or innervations.
• Used in vision therapy and orthoptics
• Two types: relieving and adverse
• Measured in prism diopters

Three Methods of Measuring Prism Power

• Lensometer
• Prism neutralizing set
• Tangent scale

Units of Ophthalmic Prism Measurement

• Prism dioptres: power required to produce a 1 cm deviation at a 1 m distance, measured on a tangent scale
• Centrad:prism required to produce a deviation of 1 cm on the circumference of a 1-m radius circle

Apical Angle or Degree Prism

• Prism with a 1 degree angle is said to be a 1°.
• Based on separation of faces of prism and the index of refraction of the used glass

Formula for Deviating Power of a Prism

• d = a (n-1)
• d= deviating power of the prism in degrees
• a = apical angle of the prism
• n = index of refraction

Compounding and Resolving Prisms

• A vector is a straight line with a specific number of units and arrowhead to denote direction
• Prismatic effect for two or more prisms can be mathematically or graphically determined by adding up components

Explore ophthalmic prisms, their properties in dispersing light, and their use in correcting eye deviations. Learn about meridional placement, prism types, and the necessity of toric transposition in lens design for effective vision care.