MUNI ELPORTAL |§| Print version Spectacle technique I Practical exercises MASARYK UNIVERSITY Table of Contents 1 Pupillary distance measurement for far..........................................................................................1 2 Near pupillary distance measurement............................................................................................6 3 Spectacle lens minimal diameter calculation................................................................................10 4 Spectacle lens centering according to real center of rotation......................................................14 5 Spectacle lens centering according to pupillary position in natural distance view......................17 6 Bifocal lens centering....................................................................................................................20 7 Bifocal lens E-line centering for patient with accommodative strabismus...................................23 8 Trifocal lens centering...................................................................................................................26 9 Progressive lens centering.............................................................................................................30 10 Lenticular lens centering...............................................................................................................34 11 Anisometropia and aniseikonia calculation in aphakic eye...........................................................36 12 Size of retinal image calculation in consequences of vertex distance change..............................39 13 Normative centering tolerance in case of spherical lens centering..............................................42 14 Normative centering tolerance in case of cylindrical lens centering............................................45 References.............................................................................................................................................47 1 Pupillary distance measurement for far 1.1 Introduction Pupillary distance (PD) is distance between pupils. We can usually measure it in millimeters. It is individual value and it can change during the life. This value is very important because it can influence spectacle lens centering. PD can be measured when fixing on objects located far away, near or other specific distance. Near PD is usually smaller due to convergence. Expected values in population are 70 mm for men, 65 mm for women and 55 mm for children (Wikipedia). Anton (2006) shows values from 61 to 65 mm in men over 15 years and from 60 to 62 mm in women. Monocular measurement is distance from nose middle point to center of the pupil of the right eye and separately for the left eye. Picture 1.1: Pupillary distance and its measurement (Stingyspecs 2013) PD measurement is possible to do with hand PD ruler, digital pupillary meter, drawing on demo-foil of the spectacles or with the electronic video centering system. It is better to measure PD for right eye and for the left eye because these values can be different. in ii i| ii |i nil i|.n |Mi|ii i|'ii;|.n -» m . 4.2 Goals methods 4.3 Equipment Hand PD ruler, special pencil for demo-foil, spectacle lens with demo-foil 4.4 Methods Draw marks on spectacle frame demo-foils which shows horizontal pupillary position for distance Patient wears spectacle frame which should be correctly adapted. Use some method for measuring PD to find horizontal positions of the pupils and mark right and left pupillary position on the spectacle demo-foil (horizontal). Draw marks on spectacle frame demo-foils which shows vertical pupillary position for distance For this measurement you should position patient to look at a distant object. Your eyes should be at the patient's eyes level. Put the frame on the patient's head and move with the head to set pantoscopic angle to zero degree. Vertical axis of the frame should be perpendicular to ground (so called perpendicular view). After that mark positions of patient's pupils on the spectacle demo-foil. Finally with the hand PD ruler measure right and left pupillary height. Picture 4.2: Setting of so called perpendicular view with spectacle frame (Rutrle 2001). Calculate vertical pupillary position on spectacle's demo-foil and compare it with previous methods Calculate vertical decentration of the spectacle lens (yv). Patient wears spectacle frame but now looks in infinity with natural head position (pantoscopic angle differs from zero). Further measure the frame's pantoscopic angle (e.g. with hand angle-meter) and vertex distance. To vertex distance you should add 13 mm which means distance from cornea to real eye's rotation centre. Use lower formula to calculate vertical decentration for right and left spectacle eye. yv = dx tga [mm, °] (5) 4.5 Results Schematically draw spectacle frame shape and horizontal and vertical position of the pupils in the level of the spectacle frame. Result of vertical decentration of the pupil position in the level of spectacle frame: yvR = yvL = 4.6 Discussion Currently we can use different types of spectacle lens centration methods. The simplest method is drawing centration marks on demo-foil. Firstly you should correctly adapt the spectacle frame on patient's head. Many lens manufactures offers electronic centration devices which can be also use for spectacle lens centration. 4.7 Conclusion, notes, comments Compare spectacle lens vertical centration methods-graphical and calculation. Does it differ? 5 Spectacle lens centering according to pupillary position in natural distance view 5.1 Introduction Spectacle lens centering according to pupillary position in natural distance view is usually used in anisometropia correction, high refractive index lenses, lenticular lenses and prism lenses. Spectacle lens's optical center is placed on pupil if the patient's head is in natural position during distance view. Picture 5.1: Centering of spectacle lenses according the pupil center during natural distance view (C - real eye rotation center, Zr,l- distance between pupils (i.e. PDr,l), Oo - position of the distance optical lens center, Rutrle 2001). 5.2 Goals • Find pupillary position during natural distance view with graphic method. • Calculate pupillary position in horizontal direction right and left. • Calculate pupillary position in vertical direction right and left. • Calculate minimal lens diameter of the spectacle lens. 5.3 Equipment Spectacle frame with demo-foil, special pencil for demo-foil, hand PD ruler, calculator 5.4 Methods Find pupillary position during natural distance view with graphic method. Patient wears spectacle frame safely without its moving from head. Mark pupillary position on the spectacle's demo-foil if the patient looks into infinity with natural head position. If there is the difference between high of the patient and examiner you can seat the patient. If seat patient check the natural patient's head position for far. With PD ruler measure horizontal and vertical distance from pupil position to edge of the spectacle eye. Measurements follow the boxing system. Note final results. Calculate pupillary position in horizontal direction right and left. According the boxing system measure width of the right spectacle eye. Horizontal position of the right pupil in right spectacle eye subtract from the half of spectacle eye width. The same is realized for the left eye. If the decentration is plus you will move with the lens in nasal direction. Calculate pupillary position in vertical direction right and left. According the boxing system measure high of the right spectacle eye and its half subtract from vertical position of the right pupil in right spectacle eye. The same is realized for the left eye. If the value is negative you will move with the lens in down direction. c PB = PDD P P DR 1 DL ! f j > I c d C / 1 Picture 5.2: Schematic measurement of the important point in spectacle frame's eyes (c - distance of the rim centers, hR,i_ - horizontal decentration, vr,l - vertical decentration, xr,l - horizontal distance, yr,l vertical distance, aR,i_ - frame eye width, bR,i_ - frame eye high, d - nose width, C - center of the frame rims, Or,l- rim size, Rutrle 2001). Calculate minimal lens diameter of the spectacle lens. According to exercise 3 calculate minimal lens diameter. 5.5 Results Draw schematic drawing of spectacle frame with pupil's position marks. Graphically realize lens decentration in spectacle frame according the boxing system. hR = hL = Vr = vL = MIN0R= (min lens diameter) MIN0l= (min lens diameter) 5.6 Discussion Spectacle lens' optical centers are centered on pupils during natural head position in anisometropic correction, high refractive index lenses, lenticular lenses and prism lenses. In anisometropia we eliminate vertical prism difference for right and left eye. In high refractive index lenses we reduce effect of rays' dispersion. In lenticular lenses we allow to patient to use maximum of lenses' visual field. In prism lenses we can guarantee exact prism effect in natural distance view. 5.7 Conclusion, notes, comments Define differences between spectacle lens centering according to pupil position during natural head position and position with so called perpendicular view. Calculate additional prism effect during distance viewing if the patient wears +5 on OD and +2 on OS. Lenses were centered on the real eye rotation centre, i.e. on perpendicular view. Vertical decentration was 3 mm. Is this prism effect important? 6 Bifocal lens centering 6.1 Introduction Bifocal lens is usually used for correction of ametropia and presbyopia. We distinguish mineral lenses with additional near segment, with grinded near segment and plastic bifocal lenses. Further we can use bifocal lens type E-line which are used for accommodative strabismus correction. According the type of near segment we distinguish type C, P, D, O, etc. Typical near segment decentration is 2.5 mm nasally. Picture 6.1: Different types of bifocal lenses - type D and O (Spanish alibaba 2013). The most used bifocal lens these days is plastic with D near segment. According to Rutrle (2001) we have to properly centre distance optical segment center (0D), geometric centre of the near segment (GN), eventually optical centre of near segment (On), nasal decentration (x) and high of the step (y) between distance and near segment. DR DL Picture 6.2: Important data for centering bifocal lenses (PDDr,l - pupillary distance for far, PDNp,l -pupillary distance for near, 0D - distance optical centre, GN - near geometric centre, C - frame eye centre, yR,i_ - high of near segment, Rutrle 2001). Both segments should enable point imaging by respect real rotation eye centre. We should also induce minimal prism effect on the near segment edge and respect hygienic and aesthetic needs (smooth transition between segments). Near segment edge should be placed appropriate high. If patient has distance gaze pupils are above edge and if patient gaze in near pupils are bellow edge. According to Rutrle (2001) we recommend to center near segment edge on inferior part of the iris. Najman (2010) recommends putting the edge of the segment 2 mm below edge of inferior eyelid. In practice is sometimes used centration of the near segment edge on the edge of inferior eyelid. 6.2 Goals • Bifocal lens centering • Calculation of the near segment decentration (hR, hi) • Calculate prism difference at near segment edge 6.3 Equipment Hand PD ruler, bifocal lens, spectacle frame, calculator, special pencil on demo-foil. 6.4 Methods Bifocal lens centering Patient wears spectacle frame with sufficient depth of frame eye (height). Patient should minimally use 15 mm of near segment height. Mark the pupil position on demo-foil during natural distance head position and height of lower eyelid. Calculation of the near segment decentration (hR, hL) Measure patient's pupillary distance for far and near. Calculate right and left decentration for ideal bifocal lens. Calculate prism difference at near segment edge Measure vertex refraction of the distance and near segment. Further measure distance from far optical center to edge and from near optical center to edge in millimeters. According the Prentice formula calculate prism effect on the edge of near segment. We will calculate prism effect on the distance side (APD) and near side (APN). If the prismatic effect is the same power and same base orientation, there is no image skipping (only theoretical example). Parameter dD and dN will be measured from your spectacle lens. APD = P [pD], S' [D], d [mm] (6) " 10 6.5 Results Draw schematically spectacle frame and write measured values in the picture. hP = hL = AD = AN = 6.6 Discussion If we center bifocal lenses we should respect height of near segment's edge. We distinguish upper decentration as more critical. If we center the near segment edge higher than is recommended patient will look on distance through near segment. This situation can cause serious problems. 6.7 Conclusion, notes, comments How height is placed near segment edge in your spectacles? Is the near segment height appropriate? Was the horizontal decentration of the bifocal lenses similar on OD and OS? Calculate theoretical placement of the optical centre of the near segment if the prism effect on the edge is reduced to zero. 7 Bifocal lens E-line centering for patient with accommodative strabismus 7.1 Introduction Bifocal lenses called E-line are used in children with accommodative strabismus. Children with accommodative strabismus have convergence excess during accommodation. It usually leads to diplopia and monocular suppression and finally to amblyopia. We can decrease the convergence excess with decreasing of accommodation. We prescribe bifocal lenses with addition 2.5 to 3 D (Rutrle 2001). We have to use appropriate type of bifocal lens with appropriate decentration. Half of the spectacle eye should be filled with addition. For this purpose can be used Franklin bifocal which do not respect hygienic and aesthetic conditions. That is why we use so called E-line bifocals. E-line bifocals are centered on the pupil center during natural distance gaze. Optical centers are placed vertically in one line. Picture 7.1: Special type of bifocal lens called E-line (VD - centration distance point, 0N - near optical point, decentration in millimeters, Rutrle 2001). i t 7.2 Goals • Center special bifocal lens E-line • Calculate vertical decentration on both eyes • Calculate horizontal decentration on both eyes • Verify adequate pupil position during near working 7.3 Equipment Bifocal special lens E-line, spectacle frame, hand PD ruler, special lens for demo-foil and mirror with fixation point. 7.4 Methods Center special bifocal lens E-line Put on spectacle frame on the patient's face. Patient looks naturally at infinity distant object. Mark pupil position on spectacle demo-foil. Note distance x which means horizontal distance between pupil and spectacle frame eye periphery. Further note distance x which means vertical distance between pupil and spectacle frame eye periphery. We also need to know basic parameters of the spectacle frame. Use boxing system. Calculate vertical decentration on both eyes According to boxing system calculate vertical decentration of the important point V for the right and left eye (vR, vL). Calculate horizontal decentration on both eyes According to boxing system calculate horizontal decentration of the important point V for the right and left eye (hR, hij. Picture 7.2: Schematic calculation of the E-line horizontal and vertical decentration (xR -horizontal distance from pupil to nasal part of the rim, vr,l - vertical decentration, hR,i_- horizontal decentration, b - eye frame height, VD - distance important point, yR - distance from the pupil to the rim bottom, Rutrle 2001). xR Verify adequate pupil position during near working Use mirror to check the position of the pupil during near working. 7.5 Results Vr = Vl = hR = hL = vr = yR-/2b [mm] (7) vl = Vl - 34 b y ... height of the pupil, v ... vertical decentration, b ... vertical size of the rim hR = /2 FS-PDr [mm] (8) hL = Yi FS - PDL FS ... frame size, PD ... pupillary distance, h ... horizontal decentration 7.6 Discussion We centered bifocal lens E-line directly on the pupil's center during the natural distance gaze. This lens helps to patients with accommodative strabismus. In horizontal direction we centered on pupils. With mirror method we can check lens centration. 7.7 Conclusion, notes, comments What is the edge height in E-line centration? Which mechanism eliminates convergence strabismus? 8 Trifocal lens centering 8.1 Introduction Trifocal lenses are used in patients with presbyopia. During decrease of accommodation amplitude decreases also accommodation intervals. So we cannot use bifocal lenses anymore. If we use bifocal lenses patient can see so called death areas, i.e. areas which are not covered by patient's accommodation. Picture 8.1: Trifocal lens enables PC working (Rutrle 2001). Visual field of the trifocal spectacle lens is divided into 3 correction areas. We can use special segment which is designed for middle distance, e.g. PC working. Addition of this middle distance equals half of measured patient's addition. Height of the trifocal lens' centration is individual but in practice is recommended to put upper middle segment's edge on lower part of the pupil. It is done during natural distance gaze. Picture 8.2: View over trifocal spectacle lens (Rutrle 2001, adapted). 8.2 Goals • Measure height of trifocal lens centration • Calculate middle distance addition value • Calculate accommodation intervals with trifocal lens 8.3 Equipment Trifocal lens, spectacle frame, handy PD ruler, special pencil on demo-foil. 8.4 Methods Measure height of trifocal lens centration Patient wears spectacle frame. Measure distance from bottom part of the frame eye to bottom part of the pupil (yR, yij. Further measure height of the eye frame. Finally measure and calculate vertical decentration of the lens on both eyes (vR, vl). Calculate middle distance addition value You have reading addition 3 D. Calculate the middle distance addition of use bifocal lens. Calculate accommodation intervals (Al) with trifocal lens Example 1: Patient with hyperopia +5 D has accommodation amplitude (AA) 2 D. Near add was set on 2 D (addisi). Calculate middle distance addition (addM) and accommodation intervals on middle distance in meters (AIM). _ 1 addM= 1 D 27 AID = R;P AID = oo;P + ± AID = com; 0.5 m AID (distance accommodation interval) = from infinity to 0.5 m AI ~ 1 1 1 M addM'addM AA 111 ^ = I;I + 2 ^4/M = 1 m; 0.33 m AIM (middle distance accommodation interval) = from 1 m to 0.33 m AI ~ 1 1 1 N addN'addN AA 111 AlN = r2 + 2 AIN = 0.5 m; 0.25 m AIN (near accommodation interval) = from 0.5 m to 0.25 m Conclusion: In this case we don't have any death zones without accommodation. Example 2: Hyperop 5 D has accommodation amplitude 1 D. Near add (addN) is 4 D. Calculate middl distance add (addM) and accommodation intervals on middle distance and near in meters. Example 3: Hyperop 5 D has accommodation amplitude 1 D. Near add (addN) is 2 D. Calculate middl distance add (addM) and accommodation intervals on middle distance and near in meters. 8.5 Results Measure height of trifocal lens centration yR = Yl = vr = vL = Calculate middle distance addition value addM = Calculate accommodation intervals with trifocal lens Example 2: adcJM = AIM = AIN = Example 3: addM = AIM = AIN = 8.6 Discussion Trifocal spectacle lens is alternative lens to modern progressive spectacle lens. They are not so often used in practice. Trifocal lenses are recommended to use in patients with low accommodation amplitude. We can offer better and fluent transmission between each segment in comparison with bifocal lenses. 8.7 Conclusion, notes, comments Conclude in which cases we will not find so called accommodation death zones in trifocal lenses? Would you recommend trifocal lens rather than progressive lens? Why? 9 Progressive lens centering 9.1 Introduction Progressive lenses are used for correction of ametropia and presbyopia. As the eye is changing its position behind progressive lens it changes the addition. Progressive lens contains so called micro-signs (laser signs) and printed signs. These signs enable better lens centration. Between distance and near point we have centration point for prism. In progressive lenses is often used so called reduction prism with BD. Progressive spectacle lenses are constructed with aspheric surfaces so we have to center them very exactly. Picture 9.1: Progressive spectacle lens (VD, n - important centration point for far and near, C- real eye's rotation center, Rutrle 2001). Progressive lenses are centered with distance centration point (VD) placing in pupil during natural distance view. In conventional progressive lenses is defined horizontal and vertical near main point decentration (VN, 2 - 2.5 mm nasally, Rutrle 2001). • Center progressive spectacle lens • Verify near convergence with mirror method • Schematically draw progressive spectacle lens and mark all important point in it. • Schematically draw iso-spherical lines on progressive lens • Schematically draw iso-astigmatic lines on progressive lens Progressive spectacle lens, special pencil on demo-foil, spectacle frame, mirror with fixation point. 9.2 Goals 9.3 Equipment 9.4 Methods Center progressive spectacle lens Patient wears spectacle frame. Measure and test frame's pantoscopic angle to ensure if the frame is suitable for progressive lens. Further center progressive lens with distance centration point to be placed on pupil during natural distance view. Measure height from pupil to lower part of the spectacle eye and decide if the frame is suitable for this type of progressive lens. Verify near convergence with mirror method With help of centration chart (ditest) draw important points of progressive lens on spectacle frame's demo-foils. With help of mirror verify if these progressive lenses are suitable for the patient. Check the convergence through the mirror. If the convergence is adequate pupils during near gaze are placed on near important centration points. Picture 9.2: Mirror method for centration of progressive lenses (Rutrle 2001). Schematically draw progressive spectacle lens and mark all important point in it. Picture 9.3: Progressive spectacle lens with micro-signs and prints (VD - distance important centration point, VP - centration point for prism, VN - near centration point, e - horizontal decentration VN, or variable inset, Rutrle 2001). Schematically draw iso-spherical lines on progressive lens Schematically draw iso-astigmatic lines on progressive lens 9.5 Results Center progressive spectacle lens Pantoscopic angle (a) = Height of distance centration point (yD) = Recommended type of progressive lens - short, middle, long (14,18, 22 mm) Verify near convergence with mirror method Convergence adequate/excessive/insufficient Schematically draw progressive spectacle lens and mark all important point in it. 32 Schematically draw iso-spherical lines on progressive lens Schematically draw iso-astigmatic lines on progressive lens 9.6 Discussion Progressive lenses are very often used today. Except classical conventional progressive lenses we can use special individual progressive lenses which are usually made with free-form method. For centering of individual progressive lenses we need to know many anatomical parameters of the patient. For measuring these individual parameters we use electronic centration devices with cameras. 9.7 Conclusion, notes, comments Discuss which type of progressive lens is suitable for first wearer of the progressive lens? Why is so important standard pantoscopic angle for standard progressive lens? Specify average pantoscopic angle? Why is important to know position of iso-astigmatism lines? What is astigmatism and distortion? 10 Lenticular lens centering 10.1 Introduction Lenticular spectacle lenses are lenses with high vertex refraction. Concave lenticular lenses has peripheral lenticular cut to decrease their weight. In convex lenticular mineral lenses is affixed superior part of the lens to the base lens. Aesthetic appearance is not good. Picture 10.1: Mineral concave and convex lenticular spectacle lens (Rutrle 1993, adapted). Currently we use plastic lenticular spectacle lenses with polish transmission from base part to optic part. If we want to center lenticular lens we place the patient to look into infinity with natural head position. If we use aspheric lens we have to center it with respect to real rotation eye center. 10.2 Goals Center spherical lenticular spectacle lens Center aspheric lenticular spectacle lens Draw plastic lenticular spectacle lens and describe it parts 10.3 Equipment Lenticular spectacle lens, spectacle frame, special pencil for demo-foil. 10.4 Methods Center spherical lenticular spectacle lens Patient wear spectacle frame. Mark pupils position on frame's demo-foils during natural distance view. Measure distance between pupil and lower part of the spectacle eye. Center aspheric lenticular spectacle lens Patient wears spectacle frame. Move with the patient's head to set the perpendicular view. Patient's spectacles should be perpendicular to the ground. Pantoscopic angle is zero. Measure distance between pupil position and lower part of the spectacle eye. Draw plastic lenticular spectacle lens and describe its parts Draw lenticular lens and mark distance centration point, optical zone and peripheral base lens of the lenticular lens. 10.5 Results Calculate the difference between both types of centration (spherical and aspheric lens) Draw plastic lenticular spectacle lens and describe its parts 10.6 Discussion Lenticular spectacle lenses are used for high ametropia over 9 D. But we currently use usually spectacle frame with high refractive index 1.74. These lenses we use successfully for refraction errors around 10 D. Main advantage of lenticular lens is weight reduction. Main disadvantage of lenticular lens is bad aesthetic appearance and smaller visual field. This visual field would be more reduced if we center the lens according point imaging. 10.7 Conclusion, notes, comments Which phenomenon follows high ametropia correction and reduces good lens' appearance? Is there any solution how to center lenticular lens to ensure point imaging and full visual filed? 11 Anisometropia and aniseikonia calculation in aphakic eye 11.1 Introduction Currently we solve aphakic states with implantation of posterior intraocular lens or anterior IOL or the lens is implanted in eye lens capsule. Earlier we don't use any lOLs so patients had induces high anisometropia and resulted aniseikonia. Sometimes when surgery is complicated we have to use aphakia these days. Picture 11.1: Current correction of aphakia (Eyesitemd 2013). 11.2 Goals • Calculate axial refraction of the aphakic eye • Calculate adequate spectacle correction of the aphakic eye • Calculate anisometropia in aphakia • Calculate size of the aniseikonia in monocular aphakia 11.3 Equipment Writing equipment, calculator. 11.4 Methods Calculate axial refraction of the aphakic eye Ar = ~p y± y i s bi Further according Rutrle (1993) we can conclude s'b2 = s'bi ~ Ad s'B [m], Ad [m] (12) And we substitute from previous formulas = 1 - Ad. S'B1 S'B [D], Ad [m] (13) Measure vertex refraction of the minus spectacle lens. We suppose that we change vertex distance with 5 mm to move with the lens closer to cornea. According to above placed formulas calculate change in size of retinal images (Pdm) Calculate change in retinal image size during the position change of the plus spectacle lens Measure vertex refraction of the plus spectacle lens. We suppose that we change vertex distance with 5 mm to move with the lens close to cornea. According to above placed formulas calculate change in size of retinal images (Pdp) 12.5 Results Calculate change in retinal image size during the position change of the minus spectacle lens Pdm = Calculate change in retinal image size during the position change of the plus spectacle lens Pdp = 40 12.6 Discussion Generally we can say that in myopia we can find larger image than in hyperopia. In axial hyperopia we usually measure smaller image than in emmetropia. The same size of the retinal image we get if we place corrective lens in eye focus in front of the eye. Picture 12.3: State when is there no change of retinal image size (S'B - vertex refraction of spectacle lens, d - vertex distance, R - distance point, y - perpendicular size, a - visual angle, No - node eye's point, F - main eye focus, Ho - main focal eye's level, Rutrle 1993). If you shift with minus lens into the eye this increases image before degreasing retinal picture in size. If you shift with plus lens into the eye this degreases image before increasing retinal picture in size. 12.7 Conclusion, notes, comments In which refractive error we can use spectacle lens shit to decrease aniseikonia? How we can change image size in case of systemic refractive error, i.e. systemic aniseikonia? 13 Normative centering tolerance in case of spherical lens centering 13.1 Introduction Association of Czech opticians and optometrist released in 2006 spectacle lens tolerance table (Benes et al. 2010). In incorrectly centered spherical spectacle lenses is induced unwanted prism effect. We distinguish more and less critical direction in incorrect spectacle lens centration. This errors cause problems with simple binocular vision. Table 13.1: Evaluation of unwanted prism and its effect on binocular vision (Benes et al. 2010) deviation of centration prism base vergence hyperotopia base in critical divergence critical in nasaly ,. . ,. direction direction correction with plus lenses base out critical convergence less out temporally ,. . . . , direction critical myopia . . , convergence less in base out les critical . . , critical correction with minus lenses base in critical divergence critical out direction direction Picture 13.1: Critical directions in spectacle lenses for correction of myopia and hyperopia (Or,l-optic center of the spectacle lens, Benes et al. 2010). In table 13.2 are presented maximal possible decentration which we can cause during centration of spectacle lens. Table 13.2: Maximal possible tolerance in millimeters during spectacle lens centration (Benes et al. 2010). vertex refraction horizontal direction vertical direction number including base out base in 1,0 5 5 2,5 2,0 3 2,5 1,25 3,0 3 1,5 1 4,0 2,5 1,25 1 5,0 2 1 1 10,0 1 1 1 20,0 1 1 1 50,0 1 1 1 13.2 Goals • Calculate prism effect induced on spectacle lens in horizontal direction. We suppose binocular prism effect. • Calculate prism effect induced on spectacle lens in vertical direction. We suppose binocular prism effect. 13.3 Equipment Spectacle lens, writing equipment, calculator 13.4 Methods For calculations use Prentice rule/formula (P - prismatic effect, S'B- vertex refraction of the lens, d -decentration in mm) P= S'B-d^m] P[pD],d[mm],S-B[D] (14) 13.5 Results Calculate prismatic effect induced on spectacle lens if we incorrectly center 3 mm horizontally out direction. Is it possible to use this lens to correction of refractive error? We suppose binocular prism effect. Pi = Calculate prism effect induced on spectacle lens if we incorrectly center 3 mm vertically. Is it possible to use this lens to correction of refractive error? We suppose binocular prism effect. 13.6 Discussion If we center spectacle lenses we want to center correctly. In standard single-focus lenses we choose centration which enables point imaging. But in anisometropia we induce unequal prism effect which can go over tolerated normative value. That is why anisometropic correction with single-focus lenses is centered on pupil during distance viewing. 13.7 Conclusion, notes, comments Which direction need more exact centration - vertical or horizontal? Is possible to accept error of horizontal centration in plus lens out direction? We shifted 5 mm out with lens +5 D? 14 Normative centering tolerance in case of cylindrical lens centering 14.1 Introduction Binocular tolerance in cylindrical lenses comes from the same norms according the need of Association of Czech opticians and optometrists (2006). Maximal possible decentration is presented in lower table. Table 14.1: Astigmatic cylinder rotation tolerance in degrees (Benes et al. 2010) corrective cylinder axis tolerance in degrees 0,25-0,75 ±5 <1,00 > 1,5 ±3 < 1,75-6 ±2 If we incorrectly centre the astigmatic axis in cylinder we can decrease visual acuity. Value of the vertex refraction of the cylinder induced during incorrect centration can be calculated according formula written below AS'B(cyl) = 2.S'Bcyl.sin/3 S'B [D], p [°] (15) S'B cyl = original cylindrical value, p = axis incorrectness 14.2 Goals • Calculate difference of induced cylindrical value on given cylindrical lens if we turn axis incorrectly 5 degrees • Calculate incorrect axis rotation if we induce difference astigmatic error 1.25 D • Decide if you can accept incorrectly turned lens with cylinder 2 D. Incorrect rotation is 3 degrees 14.3 Equipment Sphere-cylindrical spectacle lenses, calculator, writing equipment 14.4 Methods Calculate difference cylindrical value on given cylindrical lens if we turn axis incorrectly 5 degrees Calculate incorrect axis rotation if we induce difference astigmatic error 1.25 D Decide if you can accept incorrectly turned lens with cylinder 2 D. Incorrect rotation is 3 degrees To calculation use this formula: HS'B(cyl) = 2. S'Bcyl. sin/3 S'B [D], p [°] (16) 14.5 Results Calculate difference cylindrical value on given cylindrical lens if we turn axis incorrectly 5 degrees AS'B(cyl) = Calculate incorrect axis rotation if we induce difference astigmatic error 1.25 D P = Decide if you can accept incorrectly turned lens with cylinder 2 D. Incorrect rotation is 3 degrees YES/NO 14.6 Discussion If we induce unwanted difference astigmatism we should realize that in the lens raises incorrect astigmatic correction according the principal of Jackson's cylinder. For example if we induce astigmatism 0.26 D we also induce sphere error -0.13 D. It was empirically found that already 0.12 D of incorrect astigmatism lead to decreasing of visual acuity. So we can conclude that 1 D cylinder should be centered with exactness 2.5 degrees. 14.7 Conclusion, notes, comments Check marking device on lensmeter. Use transparent desk and mark it in the same axis from both sides. 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