Cylindrical Lenses PDF

OPHTHALMIC OPTICS II OPT- 327 DR CHARLES DARKO-TAKYI DEPARTMENT OF OPTOMETRY AND VISION SCIENCE UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES Also called astigmatic lenses, they are used for correcting astigmatism. The word astigmatic literally means “not-point forming”. A patient with astigmatism may complain that vertical and horizontal lines look different. Example, the bars in a window frame may be clear horizontally but blur vertically. Eyes that suffer from astigmatism need a correcting lens with a power that differs along the principal meridians of the lens. UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES A cylindrical surface is plane (flat) along a meridian parallel with the axis of revolution of the cylinder but circular at 900 to the axis meridian. UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES The flat or plane meridian of a cylinder is called Axis meridian. The meridian of maximum curvature (900 to the axis) is the Power meridian. The power of a cylindrical lens is therefore at 900 to its axis. The axis meridian and the power meridian are called Principal meridians of the cylindrical lens. Principal meridians are represented on power diagrams and are perpendicular to each other. UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES THREE FORMS OF CYLINDERICAL LENSES Plano-convex cylinder Cross cylinder Sphero- cylinders UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES Plano cylindrical lens: For correcting astigmatism in one meridian only. This type of astigmatism is called Simple Astigmatism. Example is Fig 1: Pl / - 2.00 DC X 180 or -2.00 DC X 180 is simple myopic astigmatism. Fig 2: Pl / +2.00 DC X 90 or + 2.00 DC X 90 is simple hyperopic astigmatism Fig 1 Fig 2 -2.00 PL 180 PL 180 + 2.00 90 90 UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES Cross cylinders- Correcting astigmatism in the two meridians i.e. either compound astigmatism or mixed astigmatism. That is, two different cylinders at right angles to each other. E.g. fig 1: -2.00 DC X 180 / -3.00DC X 90 Fig 2: +1.00 DC X 180 / +2.00 DC X 90 Fig 1 Fig 2 -2.00 D + +1.00 180 -3.00 D 180 + 2.00 90 90 UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES Sphero- cylindrical lenses- One surface spherical and the other surface is cylindrical. Example is Pl / -2.00 X 180, where the sphere component is 0.00 and the cylinder component is – 2.00 DC and lies in the 90th meridian. Another one is +2.00 DS / - 1.00 DC X 180 where the sphere component is + 2.00 DS, and the cylinder component is – 1.00 DC and lies in the 90th meridian. Fig 1 Fig 2 -2.00 +1.00 180 PL 180 + 2.00 90 90 UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES NOTES - Every class of cylindrical lens can be put on power diagram - Technically, every class of cylindrical lens can be written in either cross cylinder forms or sphero-cylinder forms by means of transpositions - For easy Spectacle prescription writing in clinic, all cylindrical lenses are indicated in sphero-cylindrical forms. - The different forms of regular astigmatism are represented in power diagrams as follows UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES Types of regular astigmatism Simple hyperopic astigmatism Simple myopic astigmatism Compound myopic astigmatism Compound hyperopic astigmatism Mixed astigmatism UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES Prescription off an optical cross in cross - cylindrical forms: Fig 1: Pl x 90 / -2.00 X 180, Fig 2: +2.00 x 90 / +1.00 X 180 Fig 1 Fig 2 -2.00 +1.00 180 PL 180 + 2.00 90 90 UNIVERSITY OF CAPE COAST SCISSORS MOVEMENT OF CYLINDRICAL LENSES UNIVERSITY OF CAPE COAST NEUTRALIZING CYLINDRICAL LENSES Considering the example below +0.50 1800 -0.50 900 Place a minus cylinder with its axis parallel Then place a plus lens with its axis parallel to the 180th meridian to neutralize the power to the 900 meridian to neutralize the power in in the vertical meridian. Neutralizing lens the horizontal meridian. Neutralizing lens will be – 0.50 X 180 and the prescription in will be +0.50 X 90 and the prescription in that meridian will be +0.50 X 180. that meridian will be – 0.50 X 90. UNIVERSITY OF CAPE COAST NEUTRALIZING CYLINDRICAL LENSES The cross cylinder prescription will thus be +0.50DC X 180 / -0.50DC X 90 NB: Need to mark the principal meridians of the lens before neutralizing. Use cross line as target. The meridians can be found by rotating the lens about its own axes until any vertical or horizontal line viewed through it is no longer distorted obliquely. UNIVERSITY OF CAPE COAST PROPERTIES OF CROSS CYLINDERS Two cylinders placed with the axes parallel to one another can be replaced by a single cylinder whose power is equal to the sum of the two cylinder powers. Example +1.00DC axis vertical lens combined with +2.00DC axis vertical is equivalent to +3.00DC axis vertical ie. +1.00DCXV = + 2.00DCXV ≡ +3.00DCXV -4.50DCXH = -1.75DCXH≡ -6.25DCXH Two cylinders of equal power but opposite signs placed together with their axis parallel neutralize one another. Example +2.00DCXV = -2.00DCXV≡ Plano UNIVERSITY OF CAPE COAST PROPERTIES OF CROSS CYLINDERS Two identical cylinders placed together with their axis at right angles to one another are equivalent to spherical lens whose power is equal to either of the cylinders. Example +1.00DCXV= +1.00DCXH ≡ + 1.00 DS or -2.50DCXV= -2.50DCXH ≡ -2.50DS Any single cylinder can be replaced by a sphere of the same power as the cylinder combined with the cylinder of equal but opposite power to that of the original cylinder with its axis perpendicular to the axis of the first. E.g.1 +2.00DCXV ≡ +2.00DS= -2.00DC XH , E.g.2 -3.00DCXV≡ -3.00DS/+3.00DCXH E.g.3. +1.25DCXH≡ +1.25DS/-1.25DCXV UNIVERSITY OF CAPE COAST PROPERTIES OF CROSS CYLINDERS Two unequal cylinders placed together with their axis at right angles to each other can be replaced by a sphere and a cylinder. Example +2.00DCxV = +4.00DCxH ≡ + 2.00DS = + 2.00DCxH ≡ +4.00DS / - 2.00 DCXV UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION OF CYLINDRICAL LENSES Transposition is defined as changing the form of a cylindrical lens prescription without changing its net effect on refractive correction on the eye. By this, any cylindrical lens may be changed to plus-cylinder, minus cylinder, or cross- cylinder form without changing their net effect on the refractive correction. The rules for transposition are discussed under the following headings: Transposition from cross-cylinders to sphero-cylinders Transposition from sphero-cylinders to cross-cylinders Transposition from one sphero-cylinder to another sphero-cylinder To take a prescription off an optical cross in cross cylinder forms, minus cylinder forms and in plus cylinder forms UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION Cross Cylinders to Sphero-Cylinders 1. Write either of the cross cylinder powers as a sphere followed by its unit in Dioptre Sphere (DS). 2. Subtract the cylinder chosen as a sphere from the other cylinder to find the new cylinder. Add the unit Dioptre cylinder (DCyl or DC) to new cylinder 3. The axis of the new cylinder in the Sphero-cylindrical prescription is the axis of the cylinder in the cross cylinder prescription that was not chosen as the sphere. UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION Cross Cylinders to Sphero-Cylinders Examples 1. Transpose the cross cylinder +2.00DCxV/+3.00DCxH to sphero-cylindrical forms. 1. Step 1: +3.00DS / 2. Step 2 :( +2.00) – (+3.00) = -1.00 DCyl 3. Step 3: +2.00 (cylinder not chosen ) whose axis is Vertical Thus the sphero-cylindrical prescription will be +3.00DS/-1.00DCylxV. This is in ‘minus cylinder forms’. UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION from sphero-cylinders to cross-cylinders 1. Write the sphere in the sphero-cyl given as the first cylinder. This is followed by its unit DC or DCyl. 2. Its axis is perpendicular to the axis of the given cylinder in the sphero-cylinder prescription. 3. The algebraic sum of the sphere and the cylinder in the sphero-cylindrical prescription gives the second cylinder in the cross cylinder prescription. 4. Its axis is the same as the one in the prescription given. UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION Example. Transpose the sphero-cylinder prescription +2.00DS/+1.00DCxH to cross cylinder prescription. Answer 1. Step 1: +2.00DCyl 2. Step 2: xV 3. Step 3: (+2.00) + (+1.00) = +3.00 DCyl 4. Step 4: xH Thus the cross cylinder prescription will read +2.00DCylxV/+3.00DCylxH UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION Transposition from one sphere-cylinder to another sphero-cylinder. (From plus cylinder to minus cylinder or vice versa.) Steps are as follows: 1. Add the cylinder to sphere algebraically and use as the new sphere. 2. Change the algebraic sign of the cylinder, but leave the cylinder power unchanged 3. Change the axis of the cylinder 90 degrees. UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION Example 1. Change +2.50DS/-1.25DCyl X 90 to plus-cylinder form and represent on an optical cross. Answer 1. Step 1: (+2.50) +(-1.25) = +1.25 DS 2. Step 2: + 1.25 DCyl 3. Step 3: X 180 Degrees The new plus cylinder form is +1.25DS/+1.25DCyl X 180 UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION To take a prescription off an optical cross in cross-cylinder form Example: Given an optical cross below, write the prescription in cross-cylinder form 1. Take the power on either arm of the cross 300 1200 and give it the axis of the designation of the other arm. -2.50 +1.00 2. Take the power on the second arm and give Answer it the axis designation of the arm 900 Step 1: -2.50 DCX300 removed Step 2: +1.00 DCX1200 Step 3: -2.50 DCX300 = +1.00 DC X 1200 3. Combine these two powers and axis designation into cross-cylinder form UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION To take a prescription off a power diagram in minus cylinder form Examples. Given an optical cross below, write the prescription in minus-cylinder form. 1. Designate the cylinder with the greatest plus power as the sphere +2.50 power 1350 -1.00 2. Use the algebraic difference between 450 that sphere power and the other Answer cylinder power as the new cylinder, Step 1: +2.50 DS and express it in minus-cylinder form. Step 2: (+2.50) - (-1.00) = 3.50 DC = - 3.50 Step 3: X 450 3. Give the new cylinder power the Prescription in minus cylinder form is +2.50DS/ -3.50DC X450 meridian of greatest plus power.. UNIVERSITY OF CAPE COAST FLAT TRANSPOSITION To take a prescription off an optical cross in plus-cylinder form Example: Given the optical cross below, write it in plus cylinder form. 1500 -3.00 1. Designate the cylinder with the least plus power as the sphere power -4.00 2. Use the algebraic difference between 600 that sphere power and the other cylinder Answer power as the new cylinder power, and Step 1: -4.00DS express it in plus cylinder form. Step 2: Difference between 4 and 3 is 1; thus cylinder power is +1.00 DC Step 3: X 1500 3. Give the new cylinder power the Prescription will be -4.00 DS /+1.00 DC X 1500 meridian of least plus power. UNIVERSITY OF CAPE COAST HAND NEUTRALIZATION PRACTICALS PRACTICALS SESSION IN THE LABORATORY Determine the type of lens- single vision ( sphere or cylinder), bifocal, progressive, others: photochromic, tint, antireflective e.t.c. Representation of lenses on optical crosses Representation in cross cylinders Write them in sphero-cylinders forms Know the type of refractive error the prescription represents UNIVERSITY OF CAPE COAST OBLIQUE ASTIGMATISM UNIVERSITY OF CAPE COAST CYLINDRICAL LENSES FOR BI- OBLIQUE ASTIGMATISM Form of regular astigmatism Thus requires correction with cross-cylinders that are not exactly 90 degrees. UNIVERSITY OF CAPE COAST COMBINING TWO CYLINDERS THAT ARE NOT 90 DEGREES Can be resolved into sphero-cylinder forms Two methods Graphical method Formula method COMBINING TWO CYLINDERS THAT ARE NOT 90 DEGREES Graphical Method Cylinder powers are graphed along an “A” line and a “B” line. The A cylinder has the axis closest to the numerical value zero; the B cylinder has the axis further away from the numerical value zero All cylinders must be transposed to plus cylinders before graphing to obtain the resultant cylinders (in plus cylinders). The final plus sphero cylinder can then be transposed to minus sphero cylinders. COMBINING TWO CYLINDERS THAT ARE NOT 90 DEGREES Graphical Method “A” cylinder is plotted on the base line “B” cylinder is plotted 2α degrees away from the base line α the difference between the two axes of the oblique cylinders Use the two graphed cylinders to complete a parallelogram COMBINING TWO CYLINDERS THAT ARE NOT 90 DEGREES Graphical method Complete the parallelogram by drawing a dotted line from the point of origin to the opposite extremity of the parallelogram. The dotted line R represents the resultant cylinder and is measured. The new sphere is determined using the formula S = ( Acyl + Bcyl – Rcyl ) / 2 New axis is the axes of A cylinder plus γ i.e. “A”cyl axis + γ COMBINING TWO CYLINDERS THAT ARE NOT 90 DEGREES Formular Method Formula for the resultant cylinder is Rcyl =√ (A2 + B2 + 2ABCOS2α), where α is the difference between the two axes If 2α is equal to 900, the term 2ABCOS2α drops and the formula reads Rcyl =√ ( A2 + B2) (i.e. COS 90 = 0) If 2α is more than 900, the cosine term becomes negative. Formula now reads Rcyl = √ ( A2 + B2 - 2ABCOS2α) Sphere is determined by the formula S = (Acyl + Bcyl – Rcyl) / 2 COMBINING TWO CYLINDERS THAT ARE NOT 90 DEGREES Formular method New axis is determined by the formula “A” cyl axis + γ. γ is determined using the formula Sin2γ/Bcyl = Sin 2α/Rcyl From the above, the sphero-cylinderical prescription can now be written in plus cylinder forms To write in minus cylinder form, just transpose the prescription 5 above COMBINING TWO CYLINDERS THAT ARE NOT 90 DEGREES Example 1: What is the sphero-cylindrical equivalent of the prescription +3.00 DC x 550 / +1.00 DC x 50. Try this with both methods Answer Pl / +4.00 DC x 54 COMBINING TWO CYLINDERS THAT ARE NOT 90 DEGREES Example 2: What is the sphero-cylindrical equivalent of the cross cylinder + 3.00 DC x 60 / -4.00 DC x 25 Notes to solve example 2- if one of the original cross cylinders is in minus cylinder form, then the sphere formula now becomes: S = [(Acyl + B cyl –R cyl) / 2] + Original sphere after transposition (O1) Thus, to solve example 2, transpose -4.00 DC x 25 to plus cylinders as seen below: -4.00DS/+4.00DC x 115 The new plus cross cylinders now is +3.00 DC x 60/ +4.00 DC x 115 Thus “A” cylinder is +3.00 DC x 60 “B” cylinder is +4.00 DC x 115 Original sphere after transposition is -4.00DS Now follow the steps in transposition above either Graphical or Thompson’s formula to solve the problem. Answer for example 2 is -2.56DS/+4.11 DC x 93 IRREGULAR ASTIGMATISM Error in the eye exist in multiple directions; not two in regular astigmatism. Doesn’t lend itself to correction UNIVERSITY OF CAPE COAST Cornea Topographies UNIVERSITY OF CAPE COAST IRREGULAR ASTIGMATISM Caused by: Corneal disease such as keratoconus Corneal scaring following infections Cataract surgery complication Penetrating keratoplasty Elective kerato-refractive surgery UNIVERSITY OF CAPE COAST IRREGULAR ASTIGMATISM Keratoconus UNIVERSITY OF CAPE COAST IRREGULAR ASTIGMATISM Cornea scaring UNIVERSITY OF CAPE COAST IRREGULAR ASTIGMATISM Cataract surgery complication UNIVERSITY OF CAPE COAST IRREGULAR ASTIGMATISM Penetrating keratoplasty (PKP): corneal transplant or corneal graft UNIVERSITY OF CAPE COAST IRREGULAR ASTIGMATISM Refractive surgery complication: a surgical procedure performed to correct the refractive error. UNIVERSITY OF CAPE COAST DETERMING CYLINDER POWER IN AN OBLIQUE MERIDIAN The power along the 45th meridian is different from the powers along the two principal meridians of this cross cylinder +3.00 +2.50 1800 + 2.00 450 Thus if you have a cylinder lens, the power along specific meridians of the lens can also be determined. This also means that if you change the axis of any prescription when marking a cylindrical lens to fit a spectacle, you automatically change the power of the lens. DETERMING CYLINDER POWER IN AN OBLIQUE MERIDIAN The power of a cylinder in an oblique meridian thus varies with the square of the sine of the angle separating the oblique meridian from the cylinder axis. Formula is D1 = DSin2α, where D1 is the power of the cylinder in oblique meridian D is original power of the cylinder α is angle between the oblique meridian and the cylinder axis DETERMING CYLINDER POWER IN AN OBLIQUE MERIDIAN Example 1. Find the power of -3.00 DC x 180 cylinder in the 40th meridian. Answer. D1 = Dsin2α 90 D = -3.00 400 D1= -3.00 Sin240 α D1= -3.00 (41) 0 180 D1 = -1.23 D DETERMING CYLINDER POWER IN AN OBLIQUE MERIDIAN Example 2. Find the power of a + 2.00 DC x 900 in the 65th meridian. Answer. 250 90 D1= D sin2α 650 α = 900- 650 = 250 α D1= 2.00 Sin 2 25 0 0 180 D1= 2.00 (0.18) D1= 0.36 D TORIC LENSES AND TRANSPOSITION Periscopic and meniscus lens forms having powers of +/- 1.25 DS or +/- 6.00 DS respectively are best or bent forms of spherical lenses. +/- 1.25 DS or +/- 6.00 DS are called base curves. Toric lenses are the best or bent forms of astigmatic lenses with one surface spherical and the other surface cross-cylinder. To make them quality astigmatic lenses, bases are given in formula and they are usually +/- 3 D, +/-6 D, and +/-9 D Cylinders e.t.c. These are called toric base. In writing the prescription for toric lenses, this base curves must be given. For example, a +3.00DS/+1.00 DC x 90 can be written in a toric form only if a base curve is given. That is +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 is a lens with different optical power and focal length in two orientations perpendicular to each other. One of the lens surfaces is shaped like a "cap" from a torus (see figure at right), and the other one is usually spherical. Such a lens behaves like a combination of a spherical lens and a cylindrical lens. https://en.wikipedia.org/wiki/Toric_lens TORIC LENSES AND TRANSPOSITION A conventional barrel, for example is a torus (also called tore is a surface or its enclosed solid generated by the revolution of a conic section about any axis in its plane other than its diameter). Since the radius at the centre of the barrel is less than the radius of the curve along its sides, the curved surfaces of a barrel are toric surfaces. TORIC LENSES AND TRANSPOSITION Imagine that the cylindrical lens below is picked up by its ends and bent so that the axis XY becomes an arc of a circle. The previously cylindrical surface is now curved in both its vertical and horizontal meridians, but not to the same extent. It is now called a toric surface. The meridians of maximum and minimum curvature are called the principal meridians and in ophthalmic lenses these are at 90° to each other TORIC LENSES AND TRANSPOSITION The principal meridian of minimum curvature, and therefore minimum power, is called the base curve Every toric lens contains two principal meridians: the axis meridian for the cylinder and the power meridian for the cylinder. The meridians are always at right angles to each other and both of them always pass through the optical centre of the lens. Thus in toric lens manufacture, the spherical power of the lens can be ground on one surface and the cross cylinder on the other. The centre of pupil in primary position coincides with the centre of the lens. TORIC LENSES AND TRANSPOSITION TORIC TRANSPOSITIONS Definitely describes the manner in which the lens is made. Flat transpositions on the other hand define the lens power but not the manner in which the lens is made. Toric transpositions depend on the use of a definite base curve, with all other curves of the lens being determined by using the selected base curve as a point of reference. This base curve is a standard or uniform curve on which a series of lenses may be made. By varying the base curve, it is possible to make several lenses, all having the same net effect (lens power) Infront of the eye but each having different surface powers. This fact creates the need for a type of transposition called toric transposition. UNIVERSITY OF CAPE COAST TORIC TRANSPOSITIONS Base curves Standard base curves are +/- (4, 6, 8, 9, or 10) dioptres surface powers. Two principal criteria affect the selection of a base curve in any particular lens application namely: 1. To obtain the least aberration possible , 2. To provide lenses that leave ample room for the eyelashes to move without touching the lens UNIVERSITY OF CAPE COAST SINGLE VISION TORIC TRANSPOSITIONS Toric transposition enables a toric astigmatic lens to be exactly defined in terms of its surface powers. A toric astigmatic lens is made with one spherical surface and one cross cylinder surface. The principal meridian of weaker power of the cross cylinder surface is known as the Base curve of the lens. The spherical surface of the toric lens is known as the Sphere curve. The base curve must be specified if toric transposition of a lens prescription is required ( See Fig. below). UNIVERSITY OF CAPE COAST SINGLE VISION TORIC TRANSPOSITIONS UNIVERSITY OF CAPE COAST SINGLE VISION TORIC TRANSPOSITIONS The toric formula is written in two lines, as a fraction. The top line (numerator) specifies the power of the spherical surface- Sphere curve. The bottom line (denominator) defines: the surface power and axis of the base curve, followed by the surface power and axis of the other principal meridian of the toric surface- cross curve. For example: SINGLE VISION TORIC TRANSPOSITIONS Steps in transposition The steps of single vision toric lens transposition are now defined taking the case below as an example. to a toric formula of the base curve –6 D. (1) Transpose the prescription so that the cylinder and the base curve are of the same sign, for example: (a) becomes (b) UNIVERSITY OF CAPE COAST SINGLE VISION TORIC TRANSPOSITIONS (2) Calculate the required power of the spherical surface (the numerator of the final formula). This is obtained by subtracting the base curve power from the spherical power given in (b) in step 1. Put another way, to obtain an overall power of +4.0 D where one surface of the lens has the power –6D, the other surface must have the power +10 D (3) Specify the axis of the base curve. As this is the weaker principal meridian of the toric surface, its axis is at 90° to the axis of the required cylinder found in (b) in step 1. That is: SINGLE VISION TORIC TRANSPOSITIONS (4) Add the required cylinder to the base curve power with its axis as in (b) in step 1 The complete toric formula is thus Note: In manufacturing of the lens, the more concave surface is used as the back surface and the more convex surface is used as the front surface of the lens to ensure is comfortable for wear. This is the purpose for the manufacture of these best forms of cylinder lenses. UNIVERSITY OF CAPE COAST SINGLE VISION TORIC TRANSPOSITIONS Thus for above prescription: Front surface power is +10.00 DS Back surface power is -6.00 DC X 900/ -7.00 DC X1800 Base curve is -6.00 DC X 900 Sphere curve is + 10.00 DS The accuracy of the calculations can be verified by placing them on an optical cross: -7.00 +10.00 +3.00 1800 - 6.00 +10.00 +4.00 900 Taking the prescription off the plus cylinder form gives: +3.00DS/ +1.00 DC X 900 OR +4.00 DS / -1.00 DCX 1800 SINGLE VISION TORIC TRANSPOSITIONS Can be sketched as : SINGLE VISION TORIC TRANSPOSITIONS Try same question using on toric base of +6D Compare both prescriptions. Which one would be appropriate to manufacture for this patient. Assignment to be submitted on next lecture 1. Transpose +4.00DS/-2.00DCX180 to toric lens with base curve +6 D 2. Transpose -2.00DS/+3.00DCX90 to toric lens with base curve –6D 3. Given a +2.50DS/-1.00 DC X 1800 lens to be ground on a +8.00 base curve, show the single vision toric transposition 4. Given a base curve of +4.50, determine the surface curves for -4.00DS/ +1.50 DC X 200 UNIVERSITY OF CAPE COAST SINGLE VISION TORIC TRANSPOSITIONS Question 2. to a toric formula of the base curve +6 D. Steps Step 1: +3.00 DS /+1.00 DC X 90 Step 2: sphere curve is 3 – 6 = - 3.00 DS Step 3: Base curve + 6.00 DC X 180 Step 4: Cross curve 6 +1 = +7.00 DC X 90 Toric Prescription: -3.00 DS + 6.00 DC X 180 / +7.00 DC X 90 on toric base +6D UNIVERSITY OF CAPE COAST SINGLE VISION TORIC TRANSPOSITIONS +10.00 DS -6.00DC X 90 / -7.00 DC X 180 on toric base of -6D -3.00 DS + 6.00 DC X 180 / +7.00 DC X 90 on toric base +6D Summary: Comparing two toric prescriptions, one with toric base of +6D is more ideal for surfacing compared to one with base curve of – 6D. Reason: 1. Sphere curve is less curved, thus less spherical aberrations and can still give ample room for eyelash movement 2. Cross cylinder surface has same degree of curves even though opposite powers 3. Reduced curvature makes this has more cosmetic appeal UNIVERSITY OF CAPE COAST AXIS NOTATIONS AND DIRECTIONS OF CYLINDRICAL LENSES The axis direction of astigmatic lenses is specified in Standard Notations. Also referred to as TABO Notation – it was named after the technical committee in Germany (Technischer Ausschuss fur Brillen Optik) that first proposed its universal use. This was confirmed in 1917 and becomes the standard notation. Standard notation is illustrated in the figure below. 900 900 1350 450 1350 450 T N T 1800 1800 1800 1800 450 1350 450 1350 RE LE This assumes that the spectacle lens is being viewed from the front, for instance, as they would be seen when worn on a subjects face. The subject’s right eye is on the observers left and the left eye is on the observer’s right. AXIS NOTATIONS AND DIRECTIONS OF CYLINDRICAL LENSES TABO NOTATION The horizontal meridian is always referred to as the 180 meridian (not 0 meridian). The vertical meridian is always referred to as the 90 meridian. The notation is usually expressed in 50 steps but occasionally 2.50 or 10 steps may be stipulated. The degree itself is always omitted to safeguard against a carelessly written 50 being mistaken for 50. Figures represent axis directions of astigmatic lenses 700 1250 300 1250 300 A spectacle prescription under standard notation will thus read: R.E. +2.25 DS / -0.50DC X 170 SN or R.E. +2.25 DS / -0.50DC X 170 TABO UNIVERSITY OF CAPE COAST AXIS NOTATIONS AND DIRECTIONS OF CYLINDRICAL LENSES Bi-temporal notation Is a form of notation in which the axis direction commences on the temporal side of each eye and passes through to 180 on the nasal side of each eye. Illustrated below: 900 900 450 1350 1350 450 T N T 1800 1800 1800 1800 1350 450 450 1350 RE LE AXIS NOTATIONS AND DIRECTIONS OF CYLINDRICAL LENSES Bi-nasal notation A form of axis notation in which the axis direction commences on the nasal side of each eye through to 180 on the temporal side. Illustrated below: 900 900 1350 450 450 1350 T N T 1800 1800 1800 1800 450 1350 450 1350 RE LE BIFOCAL LENSES A bifocal lens is an ophthalmic lens with two portions having two different focal lengths. It is used primarily for the correction of presbyopia (term for the diminishing plasticity of the crystalline lens with aging). A presbyope needs separate lenses for both distant and near vision because of the receding accommodation with age. This condition necessitates additional plus power at near and usually becomes critical in the early to mid-forties. Bifocals are also used in cases of muscle imbalance where the patient needs more plus at near for comfortable vision e.g accommodative insufficiency. The two portions of a bifocal lens are for separate comfortable distant and near vision. BIFOCAL LENSES- History Franklin Bifocal (Split bifocal) in 1784 by Benjamin Franklin This is illustrated in the fig below: Franklin bifocal, front and side views. Top portion for distant vision and down segment for close or near work (e.g. for reading) Nose bridge frame rim Distance portion Near or reading portion Juncture of the segments (joint line) BIFOCAL LENSES- History In 1837, Isaac Schknaitman of Philadelphia patented the first one-piece bifocal called Solid Upcurve Bifocal. For a one-piece bifocal, change in power for near is achieved through a change in curvature of one surface. The lens was made by surfacing a flatter curve at the top of the lens to give a weaker plus correction for distance vision- i.e. top portion of a biconvex lens was ground flat on one surface. The finished lens was, therefore a combination of a plano-convex and a bi-convex lens. This bifocal presented a much improved appearance over the Franklin bifocal. BIFOCAL LENSES- History In 1888, perfection bifocal by August Marick an improved vision of the Franklin bifocal This had the same basic principle like the Franklin bifocal. It was made of two separate pieces of crown glass, with the distance portion much larger than in the Franklin model, thus affording a better field for distance vision. The juncture lines of the two pieces were either flat or grooved, the segment being semicircular. UNIVERSITY OF CAPE COAST BIFOCAL LENSES- History BIFOCAL LENSES- History 1888, August Marick again developed Cemented bifocals. This had a crown glass major lens to which was cemented a small section of crown glass called wafer- spherical power equivalent to the near add. The two were cemented with Canada balsam, an adhesive with almost the same index of refraction as crown glass. The wafer was usually cemented to the back of the major lens. Opifex bifocal was a neater vision of the cemented bifocal obtained by surfacing a cement bifocal wafer to a knife edge. BIFOCAL LENSES- History In 1899, John Borsch, Sr. made Cemented Kryptok bifocal. the first bifocal lens that had a segment with different index of refraction than the major lens. He named it Kryptok after the Greek word Kryptos- meaning hidden because the construction of the lens made the segment appeared inconspicuous. His bifocal was made by countersinking a curve on the convex surface of the major lens and fitting into it a small flint button having the same curve as the countersink, but opposite sign. The button was cemented with Canada balsam. BIFOCAL LENSES- History In 1908, John Borsch Jr. invented the first fused bifocals, retaining the name of his father’s lens- Kryptok. In his lens, a round segment button was fused into a major blank prepared by removing a section into which the higher index segment could be fused. It was made by embedding and fusing a segment or button on one refractive power into a major lens of different power. The kryptok was made by fusing a flint glass button into a crown glass major. The interface curves at the junction of the major lens and the segment was made identical. The segment provided an additional power for reading or close work. BIFOCAL LENSES- History The first one-piece bifocal to be available widely were manufactured in 1910 by continental optical co. this was distributed under the name Ultex. It is still produced today. In 1915, Henry Courmette patented a fusion process that combined the advantages of fused and cemented bifocals. This paved the way for the modern bifocal lenses. He used a segment made of two kinds of glass. Upper portion of the segment was always made of the same glass that is in the major lens. The lower portion was made of a glass of higher index of refraction to help provide the desired reading addition. When this two glass buttons were fused into the lens, the upper portion of the segment being of glass identical to the major blank, becomes invisible. The juncture between the two parts of the segment becomes the effective top of the segment. When ground and polished, the assembly presents a fused bifocal with a flat-top segment. This method, so practicable is used to manufacture almost all straight top or cut-off bifocals. UNIVERSITY OF CAPE COAST BIFOCAL LENSES- History In 1926, Univis bifocals were introduced by Univis Lens Co. This is a fused bifocal with a straight dividing line which eliminated the superfluous effect by round segments. The straight dividing line resulted in an optical centre located closer to the top of the segment. This therefore reduced the amount of image jump. In 1950, a bifocal which proofed extremely successful was a one-piece straight across design similar in appearance to the original Franklin Split bifocal. This was called Executive bifocal, manufactured by American Optical Corp. Executive bifocals are still in use in this modern time. Lastly, inconveniences in the dividing line lead to the manufacture of bifocals without dividing lines. The first successful one was the Younger Seamless Bifocal designed in the middle 1950s. The sharp line of demarcation was polished our. This bifocal lenses were called Invisible bifocals. However, invisible bifocals had their own drawbacks. The blending resulted in a blurred area with an uncontrollable cylindrical power at an oblique axis. MODERN BIFOCAL DESIGNS There are two basic forms of construction utilized in the manufacture of contemporary bifocals Fused bifocal constructions One-piece bifocal constructions Fused bifocals This features a crown glass major lens blank usually 1.523 index for distance correction. The near prescription is obtained by fusing a segment of higher index into a designated countersink curve in the major blank. Most quality bifocal manufacturers prefer barium segments whenever possible. This is because barium has a higher dispersion factor thus a low chromatism. MODERN BIFOCAL DESIGNS One-piece bifocal For this design, the change in near power results from change in curvature of one surface of the lens. It can either be the front surface or the back surface of the lens. Back surface ultex is also called back segment one-piece bifocal. Front surface ultex lens is also called Front segment one-piece bifocal. All one-piece bifocals with the exception of the Younger Seamless bifocal, can be identified by a ridge that divides the distance correction from the near. CLASIFICATION OF BIFOCAL BY SHAPE OF SEGMENT TOP Fused and one-piece bifocals of contemporary designs are further subdivided into five basic types depending on the shape of their segments. These include: Round-top bifocals Straight-top bifocals Curved-top bifocals Straight-across bifocals “Invisible” bifocals ALTERNATIVE CLASSIFICATION BY SEGMENT SHAPE Bifocal can also be classified by shape of segments round segments, D-Shaped segment, B- Shaped segments R-Shaped segments. DOUBLE VISION TORIC TRANSPOSITIONS This indicates the principles on which toric bifocal lenses are manufactured. To write in toric forms, a toric base is required. The base curve of a bifocal lens is the curve on the lens surface whereon the segment is placed. The principle for fused bifocals and ultex bifocals differ slightly. For fused bifocals the segment is attached onto the front surface only; some ultex or one-piece bifocals have their segments at the back surface of the lens. DOUBLE VISION TORIC TRANSPOSITIONS Fused bifocal The segments for fused bifocal are always place on the front surface of the lens. The base curve for fused bifocals is the plus spherical curve on the front of the lens. Thus Fused Bifocal Toric Transposition (FBTT) = Front Surface / back surface Front surface is a plus spherical base curve. The back surface is a toric concave surface. To manufacture a fused bifocal lens, base curve should always be positive (convex). Since the cylinder will be on the back surface, the lens is always ground as a minus cylinder. If prescription is not written in minus cylinder form, it is necessary to state it in minus cylinder form, using flat transposition principles. DOUBLE VISION TORIC TRANSPOSITIONS Example 1. Given a kryptok lens with a + 6.00 base curve and a + 2.50 Addition, show toric transposition to a +1.00 / +0.25 x 90 lens. Answer.- Procedures are as follows: Write the plus spherical base curve as your numerator. That is FBTT = +6.00 / Back surface. Ensure that your cylinder given is in minus cylinder form. If not, transpose it to minus cylinders using flat transposition principles. Thus + 1.25 / - 0.25 x 180 Determine what power will combine with base curve to produce the sphere in the minus cylinder form. If the power is indicated with x then x + 6.00 = + 1.25 x = - 4.75 Its axis is perpendicular to the axis in the minus sphero cylindrical form. Thus -4.75 x 90 Then complete the toric lens by adding the value of the – 4.75 cylinder to the original cylinder at its minus axis. If y is the power, then y = -4.75 + (-0.25) = -5.00 Its axis is the same as the axis in the minus cylinder form. Thus – 5.00 x 180 Thus FBTT + 6.00 DS _______________________________ -4.75 DC X 90 / -5.00X DC X180 ADD + 2.50 DOUBLE VISION TORIC TRANSPOSITIONS Example 2. Given a fused bifocal with base curve of + 8.50 and an addition of + 2.50. Find the toric transposition for the prescription + 2.50/ -1.00 x 10. DOUBLE VISION TORIC TRANSPOSITIONS Answer FBTT = + 8.50DS _________________________________________ -6.00 DC x 100 / -7.00 DC x 10 DOUBLE VISION TORIC TRANSPOSITIONS Ultex Bifocal Toric Transpositions Most ultex bifocals have their segments on the back surface. The base curve is the minus spherical curve on the back surface. The lens must always be ground in plus cylinders form. If prescription is given in minus cylinders, it must be transposed to plus cylinders using flat transposition principles. Ultex Bifocal Toric Transposition (UBTT) = Front Surface/ Back Surface Note- the front surface is a positive toric surface and the back surface is a minus spherical surface DOUBLE VISION TORIC TRANSPOSITIONS Example 1. Given an ultex B blank with a -6.00D base curve and an add of +2.00, state how to grind the prescription + 2.50 / - 1.50 x10 Solution Transpose to plus cylinder form: +1.00 / +1.50 x 100 UBTT = Front surface / back surface Minus spherical surface = - 6.00 DS What power added to the base curve will give the sphere in the plus sphero cylinder prescription. Let x be this power, then x + (-6.00) = + 1.00, x = + 7.00 Its axis is perpendicular to the axis in the plus sphero cylindrical prescription. Thus +7.00DC x 10 Other plus cross cylinder surface is generated by adding the + 7.00 to the cylinder in the plus sphero cylinder prescription. Thus +7.00 + (+1.50) = + 8.50 Its axis is the same as the axis in the plus sphero cylindrical prescription. Thus +8.50 x 100 DOUBLE VISION TORIC TRANSPOSITIONS The UBTT = +7.00 DC x 10 / + 8.50 DC x 100 ___________________________________________________ -6.00 ADD + 2.00 Proof for distant portion 1000 -6.00 +1.00 +7.00 + 1.00 100 -6.00 + 2.50 +8.50 + 2.50 For the near portion with ADD + 2.00, the power diagram will be + 3.00 +4.50 DOUBLE VISION TORIC TRANSPOSITIONS Trial Question- Given an ultex bifocal blank with -4.00 base curve and addition of +2.50, state how to grind the prescription +2.00/ +1.00 x 600 NOTE The power of the addition does not enter into the toric transposition calculation. This is because all the curves for the distance portion of the lens have already being fixed. Manufacturers of bifocal blanks frequently use an abbreviation to indicate the base curve and the addition. The package might be marked 8 225 for example. The first digit indicates the base curve, plus or minus, and the remaining digits designate the addition. If fused, then it reads like +6 +2.25; if ultex then -6 +2.25 BIFOCAL PRESCRIPTIONS Bifocal lenses are always specified in prescriptions by the power of the distant portion and the Add. The add is the additional plus power added to the distant power to obtain near power or prescription for near work. This is indicated below: Near power = Distant power + Add The distant power is the power required for comfortable distant vision. The near power is the power required for comfortable near work (example is reading). Example of a bifocal prescription is + 2.00 ADD + 3.25. BIFOCAL PRESCRIPTIONS Spherical bifocal lenses This type of bifocals are worn by people with spherical ammetropias such as, hyperopia or myopia, who have also attained the prebyopic age and needs an additional prescription for comfortable near work. It is also worn by emmetropes who have attained presbyopia. Both the distance portion and the near portions are spherical lenses. BIFOCAL PRESCRIPTIONS Example 1. A bifocal has a distance prescription of + 2.00D and a near prescription of + 4.00D. Write the prescription of the bifocal lens. Solution Distant power is + 2.00 Near power is + 4.00 Add = Near power – Distant power Add = + 4.00 – (+ 2.00) = + 2.00 The bifocal prescription is + 2.00 ADD + 2.00 BIFOCAL PRESCRIPTIONS Example 2. Find the distant prescription of a bifocal lens with a near prescription of + 3.00DS and an Add of + 1.50DS. Solution Distant SRX = + 3.00 – (+ 1.50) = + 1.50 The bifocal prescription is + 1.50 ADD + 1.50 Example 3. Find the near power of a bifocal with distant portion of – 1.25 DS and Add of +3.00 DS Solution Near power = - 1.25 + (+ 3.00) = + 1.75 DS BIFOCAL PRESCRIPTIONS Cylindrical bifocal prescriptions Cylindrical bifocal lenses are used by astigmatic patients who have attained the prebyopic age and thus will need an additional lens to their astigmatic prescription for comfortable near work. Such bifocal lenses are special order lenses because the astigmatic axis is factored into the manufacture or surfacing of the lens for each eye. For this type of bifocals, both the distance portions and the near portions are cylindrical (astigmatic lenses). BIFOCAL PRESCRIPTIONS An example of Cylindrical Bifocal prescription is written as + 2.00DS/-0.50DC X 180 ADD + 1.50 For these types of bifocal lenses the near prescription is obtained as follows: Add the “Add” of the bifocal to the spherical component of the distant portion The cylinder and the axis remain unchanged For the prescription above, the near prescription is thus + 3.50 DS / - 0.50 X 180. BIFOCAL PRESCRIPTIONS Distant prescription is represented in power diagrams as +1.50 1800 +2.00 900 900 Near prescription is represented in power diagrams as +3.00 1800 +3.50 900 The difference between the distant and the near portions as can be seen in the power diagrams is the Addition of + 1.50 to each meridian of the prescription for far. BIFOCAL PRESCRIPTIONS Example 1 A bifocal has a distant portion of +2.00/ -1.00 x 165 and a near prescription of + 5.25 / -1.00 x 165. Write the prescription of this bifocal lens. Solution ADD = 5.25 – 2.00 = + 3.25 Thus the prescription is +2.00/ -1.00 x 165 ADD + 3.25 BIFOCAL PRESCRIPTIONS Example 2 A bifocal has a near prescription of + 6.00 / - 1.50 x 113 and an Add of + 4.75 D. Write the prescription of this lens. Solution Distant portion = (+6.00 – 4.75) / - 1.50 x 113 Distant portion= + 1.25 / -1.50 x 113 The bifocal prescription is thus + 1.25 / -1.50 x 113 ADD + 4.75 TRIFOCAL LENSES Bifocals fulfill a need for the presbyope by providing clear vision for distance and near. An adequately prescribed bifocal lens can provide comfortable vision at both near and intermediate at an early presbyopic age. As the individual grows older, the accommodative mechanism becomes less flexible. Eventually, a point is reached when the bifocal segment does not provide adequate range for both near and intermediate clear vision. This is usually notice when the bifocal add reaches a power of +1.75 or more. At this stage, clear vision at all distances can be restored by the addition of an intermediate “add” of less plus power than the near segment. Trifocal lenses are thus prescribed. TRIFOCAL LENSES A trifocal lens has three foci: 1. a distance portion 2. an intermediate portion 3. a near portion TRIFOCAL LENSES Power of intermediate portion Most modern trifocal lenses are manufactured with the power of the intermediate segment 50% that of the near Add. For most patients, this 50% intermediate portion provides an excellent range of clear vision. Under this technique, a trifocal with an “add” of +2.00 for the near portion will have an “add” of +1.00 for the intermediate portion. In few cases where this power above is inadequate, the correct prescription is easily determined by trial frame refraction. The lens can be ordered in a design that fulfills the patient’s visual requirements. Under this condition, a patient could have an “add” of +0.75 for intermediate portion and an “add” of +1.25 for the near portion of the trifocal lens. TRIFOCAL LENSES Disadvantages of Trifocal lenses intermediate segment positioned too high, resulting in a dividing line that is difficult to ignore when using lenses for distance viewing. o To correct this, the upper line of the intermediate portion is positioned 1mm below the pupil ( in normal daytime illumination) Optical drawbacks such as object displacement, image jump and limitation of the near field are prominent in trifocals. o To correct this, trifocals segment should be at least 18mm high including the intermediate area. This height is sometimes difficult to obtain when frame eyesize is smaller or shorter. Thus, it is ideal that frames with longer eye sizes are used in fitting trifocals. TRIFOCAL LENSES Trifocal Designs fused trifocals Straight top fused trifocals curved top fused trifocals. One-piece trifocals straight across trifocals, ultex T trifocal. QUADRIFOCAL LENS This is a lens with four foci. Extremely rare. However, it is possible to prescribe a lens that has a bifocal segment over a trifocal design. Such a lens can bring added visual efficiency to the older presbyope needing an intermediate segment as well as a bifocal for overhead seeing. Thus in a quadrifocal lens, one segment is above the distance portion for overhead near seeing. This is to make the near targets over the head comfortable for reading. PROGRESSIVE ADDITION LENSES A progressive addition lens (PAL) is an invisible multifocal lens with a gradual increase in plus “adds” powers from distance to near. an uninterrupted series of horizontal curves link distance vision, intermediate vision, and near vision with no visible separation. Lens power increases smoothly from the distance vision area at the top of the lens, through an intermediate vision area in the middle, to the near vision area at the bottom of the lens. Vision is thus allowed to pass from distance to near in a gradual and progressive manner. This lens offers one of the options for correcting presbyopia and accommodative insufficiency. Progressive addition lens illustration. distance intermediat PROTECTIVE LENSES Progressive addition lens (varilux 1 and varilux 2) are in one-piece type of designs and features three optical zones. 1. The upper portion of lens contains distance prescription (distance zones) 2. The intermediate zone proceeds from zero add and gradually increases in power until the near correction is reached thus the name Progressive zone 3. Remainder of the clear optical zone consist of the full near power called near zone PAL Varilux progressive addition lenses Optical centres are factory positioned; the reading optical centre being 12mm below and 2.5mm in from the distance optical centre. For an adequate sized near correction, the manufacturer states that the overall vertical frame dimension should be at least 34mm and that the measurement form centre if pupil to lower lens edge be a minimum of 20mm. PROGRESSIVE ADDITION LENSES Another progressive addition lens is the Ultravue Progressive addition lens. This is a progressive addition lens manufactured by American Optical Company. The company identified it as a progressive power design. In this, the series of add powers through which the eye travels from distance to near is called Progressive Corridor. The term ‘optically pure” was coined to designate the areas of the lens that carry the prescribed corrections. This differentiated them from the distorted sections common to these designs. This progressive lens was made only of oversized hard resin CR 39 blank called Aolite Ultravue. The fitting conditions were the same as those for the varilux designs. PROGRESSIVE ADDITION LENSES Prescription of progressives The prescription of the progressive lens is taken from the optical pure areas namely distance optically pure area and near optically pure area where the distance optical centre and near optical centre are located respectively. The prescription is written in a similar way as bifocal prescriptions. The progressive powers fall within the power at the distance pure areas and the near pure areas. Example- A progressive lens with a distance optical pure area of +1.00 DS and a near optical pure area of +3.00DS will have a prescription of +1.00 ADD +2.00. In contrast to bifocals, the nature of the corridors whether short or long are also specified. PROGRESSIVE ADDITION LENSES A PAL ready for fitting for left eye is indicated below: N T NOTE: the addition may be indicated on the progressive convex lens surface. PROGRESSIVE ADDITION LENSES Advantages of Progressive lenses 1. Continuous field of clear vision- progressive addition lenses offer a continuous field of clear vision from distance to near. 2. Comfortable intermediate vision- progressive addition lenses provide clear and comfortable intermediate vision 3. Continuous support to the eyes accommodation- each point of the progressive lens meridians have powers corresponding to the eyes focusing distance. Disadvantages – reading assignment PROGRESSIVE ADDITION LENSES Surface Design of PAL Progressive lens provides the desired ADD powers without any breaks, ledges or lines. Achieved through blending the transition between the distance and near zone i.e intermediate zone. This transition between these zones is smooth enough to prevent abrupt changes in prism, magnification or image jump PROGRESSIVE ADDITION LENSES Surface Design of PAL Blending is the process of incorporating varying amounts of cylinder power, oriented at an oblique axis in the lateral region of the surface. Simply : Filling the holes between different powers in the intermediate zone PROGRESSIVE ADDITION LENSES Surface Design E.g With the use of a plus cylinder power at an oblique axis, it is possible to join a flatter distance zone into a steeper near zone curve The geometry of a progressive lens surface design to considerably more complex. Slightly simple for spherical lenses More Complex with cylinders that varies in both magnitude and orientation: PROGRESSIVE ADDITION LENSES Blending is done mostly at the lateral or peripheral portion of the intermediate zone. the central regions of a progressive lens surface are relatively spherical. However most points across the lens surface actually have some degree of cylinder. This means that the curvature actually varies locally from meridian to meridian at the points. PROGRESSIVE ADDITION LENSES The cylinder at each point on the lens surface is often referred to a as surface astigmatism. This produces an astigmatic focus instead of a point focus. Surface astigmatism varies across the surface of a progressive lens. Virtually zero along the progressive corridor, but increases into the lateral blending regions of the lens. PROGRESSIVE ADDITION LENSES In the lateral blending regions, the surface astigmatism produces significant levels of unwanted cylinder power, in sufficient quantities is perceived by the wearer as blur, distortions and image swim: Rocking Effect: Unwanted cylinder power, which in sufficient quantities is perceived by the wearer as blur, distortions and image swim. Felt most at the periphery PROGRESSIVE ADDITION LENSES Contour Plots Each point on the blending region of a progressive lens can be thought of as a small cylinder, Optics of a PAL can be evaluated by by measuring the amount of cylinder power present at this points. These different measurements are represented in a contour plot. A contour plot is a map that indicates how levels of an optical quantity vary across the lens. PROGRESSIVE ADDITION LENSES Structural features of a progressive lens Distance zone- A stabilized region in the upper portion of the lens provides the specified distance prescription. Near zone – A stabilized region in the lower portion of the lens provides the Add power for reading Progressive corridor – A corridor of increasing power connects the distant zone and near zone and provides intermediate or mid-range vision Blending region – The peripheral regions of the lens contain non-prescribed cylinder power and provides only minimal visual utility. PROGRESSIVE ADDITION LENSES Advantages of Progressive lenses 1. Continuous field of clear vision- progressive addition lenses offer a continuous field of clear vision from distance to near. 2. Comfortable intermediate vision- progressive addition lenses provide clear and comfortable intermediate vision 3. Continuous support to the eyes accommodation- each point of the progressive lens meridians have powers corresponding to the eyes focusing distance. Disadvantages ? PROGRESSIVE ADDITION LENSES OPHTHALMIC PRISMS Introduction Some eyes suffer from defects of the external muscles or the innervations controlling these muscles. These gives rise to manifest deviations called squint or strabismus or heterotropias. Latent or hidden deviations called heterophorias. Ophthalmic prisms are therefore used to measure and correct these deviations in the eye. They may be used alone or incorporated onto spectacle prescriptions. OPHTHALMIC PRISMS An ophthalmic prism is a transparent wedge of refracting material with triangular ends and three faces , two of which meet in a line called the apex and the rest upon a third face called the base. Prisms seem to displace objects that are seen through them. It refracts light towards its base, but the apparent displacement of objects is towards the apex. It also has the ability to disperse polychromatic light into its spectrum. This is illustrated below: OPHTHALMIC PRISMS Principal references of a prism B A Z C T 0 D R Faces ABTR and ABZO Base: plane ORTZ Apex: line AB Base-apex line: line AD Base-apex plane: plane ABCD Principal section: plane AOR perpendicular to apex line. Notes: the base can be any plane perpendicular to the base-apex plane OPHTHALMIC PRISMS Describing Prisms Meridional placement of the base-apex line The power of the prism The direction of its base The designation of the meridional placement of the base-apex line in an ophthalmic prism is the same as that used in cylinder axis placement. When prism is viewed from the ocular surface of the lens, the meridional readings are made clockwise from 0 to 180. When viewed from the front surface, the readings are made counterclockwise 90 0 from 0 to 180. 1350 450 1800 0 View from front surface OPHTHALMIC PRISMS Designation of prisms according to their use in vision care Relieving prisms- one prescribed to relieve or aid a patient who has a deficient eye muscle. positioned with its base over the paralysed or deficient muscle, and its apex is place opposite to the direction in which the muscle would normally turn the eye. An adverse prism prescribed to stimulate a muscle. It is placed with its apex over the muscle to be stimulated. Prescribe for training purposes in vision therapy or orthoptics. OPHTHALMIC PRISMS Designation of prisms according to their use in vision care A measuring prism- applied to ascertain the abducting or adducting innervational strength of a muscle or a group of muscles. I.e. they are used to measure deviations. Positioned in the same way as a relieving prism. A dissociating prism- is one applied to effect a temporary destruction of fusion. positioned in the same way as adverse prisms. OPHTHALMIC PRISMS Corrective ophthalmic prism power Corrective prism power as required may be placed before each eye in two ways: 1. Either entire power before one eye e.g. 8∆ Base Down and In along 135th meridian for OS. 2. Dividing prism power between both eyes. When power is divided equally between the two eyes, the prism powers would neutralize each other if superimposed. a. e.g. 8∆ base down and in at 135th meridian for OS can be divided as follows i. OD 4∆ Base Up and In along 135th meridian ii.OS 4∆ Base Down and In along the 135th meridian OPHTHALMIC PRISMS Units of measurement Prism Dioptre- the power required to produce a deviation of 1cm at a distance of 1m, measured on a tangent scale. Almost universally used in the clinic setting by doctors. Is fairly accurate for prisms up to 15∆. Centrad – The prism required to produce a deviation of 1cm on the circumference of a circle with a radius of 1m. This is more accurate than prism dioptres for measurements greater than 15∆. For powers up to 15∆, Dioptres and centrad are almost identical for all practical purposes. That is 1∆ ≡ 1 centrad OPHTHALMIC PRISMS Apical angle or Degree Prism- a prism with an apical angle of one degree is said to be a 10. This is common among ophthalmic laboratory technicians who grind ophthalmic prisms. It depends on separation of faces of the prim and index of refraction of glass used. E.g. For 1.523 glass, a 10 prism = 0.92∆ Deviating power of prism – this measures the power of a prism in terms of the angle of deviation it produces. 1 d0 = 1.745∆ OPHTHALMIC PRISMS Formula for deviating power of a prism This depends on two factors namely: 1. Index of refraction of the prism used. 2. Apical angle of the prism It is given by the formula d = a (n-1), where d is deviating power of prism in degrees, a is apical angle and n is index of refraction of the prism material. OPHTHALMIC PRISMS OPHTHALMIC PRISMS Three methods of measuring prism power By use of a lensometer. By using a prism neutralizing set By using the tangent scale Need to know the first two OPHTHALMIC PRISMS Prism neutralizing set Actions of prisms Fig. 1 - Prism placed with base-apex line parallel with the vertical meridian of a cross line chart and its base down displaces the horizontal meridian of the cross line chart upwards towards the apex. A prism placed base up with the base apex-line parallel to the vertical meridian is required as the neutralizing prism. This is illustrated below. Up Fig. 1 Neutralizer Down OPHTHALMIC PRISMS Fig. 2 - Prism placed with base up, and base-apex line parallel to the vertical meridian of a cross line chart displaces the horizontal meridian of the cross line chart downward towards the apex. A prism place base down with its base-apex line parallel to the vertical meridian of the cross line chart is required as a neutralization prism. Illustrated below: Up Neutralizer Down OPHTHALMIC PRISMS Example Fig 3. out in Fig 4.out in Neutralizer Neutralizer Fig. 3- Prism placed base out with base-apex line parallel with the horizontal meridian of a cross line chart displaces the vertical meridian of the cross line chart in towards the apex. A prism place with base in and base-apex line parallel with the horizontal meridian of the cross line chart is required as the neutralizing prism. Fig. 4 - Prism place base in with base-apex line parallel with the horizontal meridian of a cross line chart and displaces the vertical meridian of the cross line chart out towards the apex. A prism placed base out with base-apex line parallel with the horizontal meridian is required as a neutralizing prism. OPHTHALMIC PRISMS Compounding and resolving prisms In optics, a vector is a straight line of a definite number of units with an arrowhead to denote direction. Vector can be used to define a prism fully since a prism can be completely described by stating its power, meridional placement and direction of the base. When a prescription calls for two prisms, they may be compounded and resolved into a single prismatic effect for the eye. Resolution of prism powers can be calculated either mathematically or graphically by using vectors. OPHTHALMIC PRISMS Examples 1. Given 3∆ Base “Up” and 3∆ Base “In” for the left eye, calculate the resultant prism and the direction of its base. Solution R2 = 32 + 32 90 R2 = 18 3∆ R = 4.24 R Tan α = O / A = 3/3 N180 3∆ T 0 α α=450 The axis direction is = 45 + 90 = 1350 Thus, the resultant prism is 4.24∆ base “Up” and “In” along the 135th meridian. OPHTHALMIC PRISMS 2. Compound 3∆ Base “Up” and 4∆ Base “In” into a single prismatic effect for the right eye and for the left eye respectively. Solution For right eye R2 = 32 + 42 R=5 α Tan α = ¾ 3∆ R α= tan – 0.75 = 36.9 T 180 4∆ 0N The resultant prism for the right eye is 5∆ Base “Up” and “In” along the 36.90 meridian for the right eye. OPHTHALMIC PRISMS For left eye R2 = 32 + 42 R R=5 N 180 3∆ T 0 Tan α= 4/3; α = 53.10 4∆ α The meridian is 53.10 + 900 = 143.10 Resultant prism for the left eye is 5∆ Base “Up” and “In” along the 143.1th meridian. Trial question Given 4∆ Base Up and 2∆ Base Out for the right eye, calculate the resultant prism and the direction of its base. OPHTHALMIC PRISMS Resolving two obliquely crossed prisms The resultant for these prisms can either be obtained graphically or by employing the following formulae: R2 = A2 + B2 + 2ABCos α Sin b = (B/R) sin α Where R is the resultant prism A is first prism B is the second prism α is angle between prism powers b is angle opposite side B OPHTHALMIC PRISMS Example 1. Given 5∆ Base “In” at 00 and 2∆ Base “Up” and “In” at 600, find the resultant prism and the direction of its base for right eye. Solution R2 = A2 + B2 +2ABCos α R2 = 52 +22 + 2(5)(2)Cos 600 2∆ R R = 6.24 600 b Sin b = (B/R) sin α T 180 5∆ 0N Sin b = (2/6.24) sin 60 Sin b = 0.27; b = 160 meridian. The resultant prism is 6.24∆ Base “Up” and “In” along 160 meridian UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS Example 2. Given 2∆ Base “In” at 1800 and 4∆ Base “Up” and “In” at 1200 for the left eye, find the resultant prism and the direction of the base. Solution R2 = A2 + B2 +2ABCos α, A = 2∆, B= 4∆, α = 1800 – 1200= 600 R2 = 22 + 42 + 2(2)(4) Cos 600 R = 5.2920 Sin b = (4/5.292) sin 600 α Sin b = (4/5.292) (0.8660) R 4∆ Sin b = 0.66 b= 420 1200 Meridian = 1800 – 420 = 1380 N 1800 2∆ 0T The resultant prism is 5.292∆ Base “Up” and “In” along 1380 meridian. b UNIVERSITY OF CAPE COAST PRISMATIC EFFECT OF SPHERICAL LENSES Spherical lenses are made of prisms base-to-base or apex-to-apex Point of attachment of the separate prisms is the optical center. Base to base is plus spherical lens Apex to apex is minus lens Prismatic effect increases away from the optical centers UNIVERSITY OF CAPE COAST PRISMATIC EFFECT OF SPHERICAL LENSES Prismatic effect determined using Prentice rule, P = cF P is prismatic effect or prism power c is distance from optical center of lens in cm i.e. decentration F is lens power Prism base direction is given by direction of increasing lens thickness. UNIVERSITY OF CAPE COAST PRISMATIC EFFECT OF SPHERICAL LENSES Question 1 What is the resultant prism if a +4.00DS lens is decentered 4mm out in front of the right eye. Solution P = cf c = 0.4cm F = 4.00D P = 0.4 (4) = 1.6 prism diopters BO UNIVERSITY OF CAPE COAST PRISMATIC EFFECT OF SPHERICAL LENSES Question 2 If lens is -4.00DS Solution P = Cf = 0.4 (4) = 1.6 prism diopter BI Note the following Prisms are not in plus and minus signs Sign of the lens helps us set lens Infront of the eye to determine right directions of decentration UNIVERSITY OF CAPE COAST PRISMATIC EFFECT OF SPHERICAL LENSES Notes Eye is fixed, doesn’t move OC is moved either out or in by a magnitude The prism that falls infront of the eye after the decentration is the induced prism and its base is emphasized. The induced prism is desirable or wanted if prescription calls for it Is undesirable or unwanted if prescription doesn’t call for it. UNIVERSITY OF CAPE COAST PRISMATIC EFFECT OF SPHERICAL LENSES The eye must always look through the Major Reference Point (MRP) after fitting. For induced prism, MRP Infront of eye, OC away from eye If no induced prism, MRP and OC are at same point or is the same point i.e. coincides UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS Question 3. Describe how you will induce 3∆ base-in into a -5.00 DS lens for the right eye. Solution Using Prentice rule, P=cF P = 3∆ base-in , F = -5.00 DS , C = P/F = 3/5=0.6 cm = 6mm Answer: Decenter 6mm out UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS Question 4. Describe how you will induce 3∆ base-in into a -5.00 DS lens for the left eye. Solution Using Prentice rule, P=cF P = 3∆ base-in , F = -5.00 DS , C = P/F = 3/5=0.6 cm = 6mm Answer: Decenter 6mm out Summary Q3 and Q4: To induce Base-in prisms into minus lenses for right and left eyes, decenter out. UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS Question 5. Describe how you will induce 3∆ base-out into a - 5.00 DS lens for the right eye. Solution Using Prentice rule, P=cF P = 3∆ base-in , F = -5.00 DS , C = P/F = 3/5=0.6 cm = 6mm Answer: Decenter 6mm in UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS Question 6. Describe how you will induce 3∆ base-out into a -5.00 DS lens for the left eye. Solution Using Prentice rule, P=cF P = 3∆ base-in , F = -5.00 DS , C = P/F = 3/5=0.6 cm = 6mm Answer: Decenter 6mm in Summary Q5 and Q6: To induce Base-OUT prisms into minus lenses for right and left eyes, decenter IN. UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS Question 7. Describe how you will induce 3∆ base-In into a +5.00 DS lens for the right eye. Solution Using Prentice rule, P=cF P = 3∆ base-in , F = +5.00 DS , C = P/F = 3/5=0.6 cm = 6mm Answer: Decenter 6mm in UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS Question 8. Describe how you will induce 3∆ base-In into a +5.00 DS lens for the left eye. Solution Using Prentice rule, P=cF P = 3∆ base-in , F = +5.00 DS , C = P/F = 3/5=0.6 cm = 6mm Answer: Decenter 6mm in Summary Q5 and Q6: To induce Base-IN prisms into plus lenses for right and left eyes, decenter IN. UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS Question 9. Describe how you will induce 3∆ base-out into a +5.00 DS lens for the right eye. Solution Using Prentice rule, P=cF P = 3∆ base-in , F = +5.00 DS , C = P/F = 3/5=0.6 cm = 6mm Answer: Decenter 6mm out UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS Question 10. Describe how you will induce 3∆ base-out into a +5.00 DS lens for the left eye. Solution Using Prentice rule, P=cF P = 3∆ base-in , F = +5.00 DS , C = P/F = 3/5=0.6 cm = 6mm Answer: Decenter 6mm out Summary Q9 and Q10: To induce Base-OUT prisms into plus lenses for right and left eyes, decenter OUT. UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS SUMMARY PLUS LENSES Decenter in the same direction as the prism base E.g. to induce BI prisms, decenter in and vice versa MINUS LENSES Decenter in opposite direction to the base direction E.g. to induce BI prismatic effect, decenter out and vice versa. Use Prentice rule to calculate the amount of decentration. UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS NOTE DESIRABLE PRISMS / WANTED PRISMS UNDESIRABLE / UNWANTED PRISMS BI for correcting exophoria or exotropia- relieving prisms BI for training exotropia or exophoria – adverse prisms BO for correcting esophoria or esotropia – relieving prisms BO for training esotropia – adverse prisms UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST OPHTHALMIC PRISMS UNIVERSITY OF CAPE COAST

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