PRESCRIPTION COMPANION ©2012Transitions Optical inc. ophthalmic lens technical reference JUBILEE YEAR 2012 E -Edition 7 www.norville.co.uk Introduction and Page Index The Norville Companion is a supporting publication for our Prescription Catalogue, providing further technical details, hints and ideas gleaned from everyday experiences. TOPIC Page(s) TOPIC Page(s) Index 2 - 3 Part II Rx Allsorts Lens Shapes 4 - 6 Lens Forms 49 Effective Diameter Chart 7 Base Curves 50 - 51 Simplify Rx 8 Aspherics 52 - 53 Ophthalmic Resins 9 Free-form Digital Design 54 Indices of Ophthalmic lenses - Resin 10 Compensated Lens Powers 55 - 56 Polycarbonate 11 Intelligent Prism Thinning 57 - 58 Trivex 12 - 13 Superlenti - Glass 59 Resin Photochromic Lenses 14 Superlenti - Resin 60 Transitions Availability Check List 15 V Value / Fresnels 61 Nupolar Polarising Lenses 16 E Style Bifocal / Trifocal 62 Drivewear Lenses 17 - 18 Photochromic / Glazing / Prisms 63 UV Protective Lenses 19 Lens Measures 64 Norville PLS Tints 20 Sports 65 Tinted Resin Lenses 21 3D Technology Overview 66 Mid and High Index Resins Tintability 22 Rx Ordering 67 Norlite Tint Transmission Charts 23 - 25 Order Progress 68 Norlite Speciality Tinted Resins 26 - 31 Rx Order Form 69 Norlite Mirror Coating 32 Queries 70 Reflection Free Coating 33 - 34 Optical Heritage 71 F.A.Q. Reflection Free Coatings 35 - 37 Rx House - Change afoot? 72 - 73 Indices of Ophthalmic Lenses - Glass 38 Remote Edging 74 Glass Photochromic Lenses 38 Remote edging - F.A.Q. 75 Speciality Absorbing Glass 39 Quality Assurance 76 Speciality Tinted Glass 40 Optical Standards 77 Companion Tint Transmission Charts 41 Protective Eyewear 78 Resin Bifocal & Trifocal Availability 42 Lenses for the relief of Heterophoria 43 Optical Chart Addenda Lenses for the relief of Anisometropia 44 - 45 Resin - Edge Substance Comparison A1 Prism Controlled Lenses 46 Resin - Centre Substance Comparison A2 Norbond 47 Glass - Substance Comparison A3 2 The Norville Rx Companion Page Index TOPIC Page(s) TOPIC Page(s) Part III Dispensing Update Part IV Specific Selling Advice The Versatile Reading Lens 83 Selling Points - Polarised Lenses 99 Fitting Guide to Enhanced Reading Lenses 84 - 85 Selling Points - Office Lenses 100 Occupational Progressive Powered Lenses 86 Selling Points - RF Coatings 101 Progressive Lenses 87 - 88 Successful Varifocal Dispensing 89 Dispensers Aware!! 102 Progressive Lenses: The latest generation 90 - 91 Prescription for success 92 - 95 Technical Booklets 103 Compensating Wrap-around Frames 96 - 97 Technical Publications and Catalogues 104 Optically useful websites: Association of British Manufacturing Opticians www.abdo.org.uk A.B.D.O. College www.abdocollege.org.uk Corning - SunSensors™ www.corning.com Drivewear® Lenses www.drivewearlens.com European Hard Resin Institute www.hardresin.com Federation of Manufacturing Opticians www.fmo.co.uk Ophthalmic Antiques International Collectors’ Club www.oaicclub.com The Norville Group Limited www.norville.co.uk Worshipful Company of Spectacle Makers www.spectaclemakers.com Younger - Transitions® - NuPolar® www.youngeroptics.com Text books mentioned throughout the Norville Rx Companion are available from the Norville Bookshop The Norville Rx Companion 3 Lens Shapes One of the most important hallmarks of an excellent pair of spectacles (besides of course the ability to see well!) is the finished substance of its lenses. Lens shape and decentration combined results in an effective diameter which determines the ultimate minimum size uncut (MSU) needed to glaze a spectacle frame to its speciied optical centration (see page 7). Nowadays stock inished single vision resin lenses are manufactured as circles and can vary from 75mm down to 55mm rounds. Today we even have a 50mm round stock lens in Trivex material which takes us almost full circle back to the earliest small stock (glass) lenses of the early 20th Century. Round "uncuts" Actual sizes 55mm 75mm Where lenses are surfaced to prescription there is usually a range of semi-inished blank diameters available. For single vision resin CR39 this would include 80, 74, 70, 65, 60 and 55mm round diameters, although for bifocals usually only one size is available due to the cost of maintaining all those stock keeping units (SKUs) which over a range of adds from 0.75 to 3.00 would make 10 individual units for each lens type as a minimum, having a range of three or four lens bases making 30 or 40 stock keeping units just for one lens type design. It would not be unlikely for a large prescription house to hold 16,000 stock items. Where progressives or a D Seg is concerned this is further complicated when held in separate right and lefts, this of course doubles up to 60 or 80 SKUs per lens type. This would even be higher if 4 or 5 diameters needed to be kept across every lens type, however the outcome of the lens surfacing process allows customisation of diameter, reducing from the maximum shown in this bar chart. + PLUS MINUS - 8.00 75 8.00 Progressive Semi-inished lens blanks most usually come in a diameter of 75mm (see above) which as you can note is of little help when we are to glaze an average frame size (50mm). Lens Shape 4 The Norville Rx Companion Lens Shapes To overcome this most surfaced lenses prior to the surfacing process are reduced in size. Tradiionally this was to a round shape (trepanning) which originally was in 5mm steps, today to the nearest 2mm increments down to 52mm minimum circle. Even then for some lenses especially progressives with oblique cylinders the reduced diameter blanks would waste away to knife edge, something that is not helpful to the subsequent hard coaing or vacuum coaing processes. To minimise this efect we can now oval trepan which results in shapes as these following allows the precise customizaion of substance to small frames and individual prescripions. Shape ET1 50mm x 35mm Shape ET2 55mm x 40mm Shape ET3 60mm x 45mm Shape ET4 65mm x 50mm Shape ET5 70mm x 55mm Shape ET6 75mm x 60mm The Norville Rx Companion 5 More About Lens Shapes All laboratory work instructions are designed around the spectacle frame parameters, eyeshape, DBL, patient’s Rx, optical centre position and if applicable, segment top/fitting cross position. Considering these ensures that lenses are made to the absolute thinnest possible centre and edge substances. This has obvious benefits for hypermetropes, even those with powers as low as +2.50DS. Myopes gain very little from this process as their lens edge substance is dictated by power and to a lesser degree the final centre thickness specification of the material. The power of modern computers, together with Norville’s advanced programming has brought huge benefits to the task of lens diameter and thickness computation. This is especially so for progressive lenses where in-built prism differences between the top and bottom of the lens design makes it difficult to manually calculate the finished lens substances (see pages A1 to A3). Electronic shape transfer technology may obscure the importance of always providing this data, but for absolute control of substance, it is essential that a shape is provided especially when an E-style or progressive lens thickness is to be calculated. Not to do so is “substance suicide”, so much so that it is better to select a guide shape rather than none. To this end we would use one of the standard design shapes below. Quadra shape Aviator shape 51 52 53 54 55 56 Oval shape 46 48 50 Panto shape 49 Rectangular Shape 51 51 53 Lens DIAMETER controls thickness which is a product of: refractive index worked curves eye shape Rx and decentration type of mount (i.e. rimless etc.) Given the same diameter, lenses can be produced proportionately thinner by increasing the refractive index of the material. Is it not strange that the current most popular lens material CR39 results in the thickest lenses as it has the lowest refractive index of all n = 1.498. Incorrectly calculating the minimum required diameter can result in considerably thicker lenses than necessary. The old craft adage “measure twice cut once” can be applied to lenses. In this case extra care in assessing lens diameter will result in thinner lenses. If you are unable to use an electronic frame tracer then the diameter can be calculated using the effective diameter chart on the next page. 6 The Norville Rx Companion Effective Diameter Chart MINIMUM SIZE UNCUT (MSU) 100 80 9 0 110 70 120 60 100 80 130 50 110 70 120 60 140 40 130 50 140 40 150 30 150 30 102 98 102 94 160 20 98 90 94 86 90 82 86 78 82 74 160 20 78 70 74 66 70 62 66 58 62 54 58 50 170 10 54 46 50 42 46 38 170 10 42 38 180 0 180 0 38 42 170 10 38 46 170 10 42 50 46 54 50 58 54 62 58 66 62 70 66 74 70 78 74 160 20 82 78 86 82 90 160 20 86 94 90 98 94 102 98 102 150 30 150 30 140 40 140 40 130 50 120 60 130 50 110 70 100 80 120 60 110 70 100 80 9 0 STEPS 1) Place the frame front down on chart and centralise by use of concentric circles and markings on 180 line so that, in the horizontal direction, an equal number of lines are showing within the eye rim. Repeat for the vertical direction. The frame should now be exactly centralised about its lens centre. 2) Should decentration be required in the finished prescription, then move the centralised frame by the required amount of decentration for that eye, but in the opposite direction to that specified e.g.