Freeform: optimise or individualise? Part 2

Transcription

1 4 dispensingoptics November 2014 Freeform: optimise or individualise? Part 2 By Phil Gilbert FBDO CompetencIes covered: Dispensing opticians: Optical appliances, Refractive management Optometrists: Optical appliances In Part 1 of this article, published in July, we looked closely at the influence freeform or digital surfacing has with regard to both the industry and profession, together with the potential benefits that can be obtained by the spectacle wearer. However, we only looked at lenses that were optimised during the manufacturing process and did not consider freeform individualisation, which can further enhance the visual performance experienced by the wearer. In Part 2, the function of full individualisation will be explored in order to demonstrate the impact it can have on powers prescribed following a subjective refraction and relating it to the ultimate prescription taking into account the as worn position. In order to achieve this, a random Rx will be taken, which is considered to be bordering on the complex, and the lenses virtually dispensed into a frame that is also considered to be out of the ordinary with regard to its fitting parameters. We will then look at the impact that the three main individualised measurements have on the Rx in turn and, lastly, with all three measurements combined. There are increasingly more and more dispensing decisions to be made and measurements that can be taken depending on the complexity of the chosen manufacturer s fully individualised lens design. However, for the purposes of this article only the three main influencing measurements will explored, ie. vertex distance, pantoscopic angle and face form angle. The Rx chosen was completely random, albeit added complexity was used by introducing higher cylindrical powers with oblique axes. The frame fitting parameters were also chosen randomly but kept within the normal values that we are likely to encounter in our everyday lives. The values were then fed into the calculation programme used by Carl Zeiss Vision in order to look at the eventual verification powers given. The Rx chosen was: R +6.00DS -2.50DC axis 135 Add DS -3.25DC axis 75 Add As we are dealing with manufacturing production software, the Rx will automatically be transposed into a + cylinder form and in all cases, the verification power will be in cross cylinder form to aid the recording of the actual measured values on a manual focimeter. All of the findings in cross cylinder form have been recorded, as this will demonstrate the differences between the prescribed values and the verification power in the same format. This article has been approved for 1 CET point by the GOC. It is open to all FBDO members, including associate member optometrists. The multiple-choice questions (MCQs) for this month s CET are available online only, to comply with the GOC s Good Practice Guidance for this type of CET. Insert your answers to the six MCQs online at After log-in, go to CET Online. Questions will be presented in random order. Please ensure that your address and GOC number are up-to-date. The pass mark is 60 per cent. The answers will appear in the March 2015 issue of Dispensing Optics. The closing date is 7 February C-37579

2 Continuing Education and Training Note that by this stage some new terminology has been used, which may be unfamiliar to most readers. The International Standards Body is considering introducing the term Verification Power to describe the powers that manufacturers will issue with the lenses. The term Measured Values would then be used to denote the powers that are recorded once the lenses have been checked against the given verification power on a focimeter. These terms will find their way into BS EN ISO Terminology and ISO: TR18476, which is a Technical Report on Freeform currently being written. The refractive index chosen for this demonstration was n = 1.67 and the frame measurements were 52 x 30 x 18 with a centration distance of 32 R & and fitting heights of 18mm. The progressive corridor on the lens was of standard length at 14mm. The position of wear parameters chosen were vertex distance 16mm, pantoscopic angle 13, face form angle 10. In total, five separate entries were put into the calculation programme in order to make comparisons showing the entry of the following: 1. The verification power for an optimised but non-individualised progressive lens 2. The verification power introducing a measured vertex distance 3. The verification power introducing a measured pantoscopic angle 4. The verification power introducing a measured face form angle 5. The verification power introducing all three individualisation measurements It must be pointed out that the resultant verification power may well vary between manufacturers as they are produced using 100th dioptre steps, however, individualisation is in principle the same software function with all lens producers. Variations could be seen due to the manufacturer s individual freeform software calculations by lens design and other factors such as the chosen refractive index, corridor length and base curve which will have an influence on the result Entry 1. The verification power for an optimised but nonindividualised progressive lens Although verification powers are given by most manufacturers who produce freeform lenses, their use for non-individualised lenses are always based upon industry averages with regard to the three main parameters comprising: vertex distance, pantoscopic angle and face form angle. Over the years, since freeform technology was introduced, each manufacturer has accumulated a huge source of data with regard to these averages and most lens producers use parameters similar to the following as a basis for their real time freeform calculations: Vertex distance: 13mm Pantoscopic angle: 5 Face form angle: 5 There has been a change in the average pantoscopic angle, which used to be 9 but which has now been reduced due to the recent trend for thicker spectacle frame sides. This trend has led to lenses sitting flatter in front of the eyes as the thicker sides leave little leeway for adjustment to obtain a higher pantoscopic angle. These averages will, again, vary slightly between manufacturers depending on the software that they are using, however, as we will see from Table 1 the resultant verification power difference between these and the prescribed values can still be significant. On analysing the findings in Table 1, we can straight away see changes in the verification power that could cause the dispensing optician to warn R R Table 1: The verification power for an optimised but non-individualised progressive lens a patient who is moving to a freeform produced lens for the first time after having worn a conventionally produced semi-finished progressive lens, that there may need to be a period of adaptation. There are significant power and cylinder axis changes, particularly in the near portion which, although designed to give the patient improved vision, could cause initial adaptation issues. The cylinder axis changes in the near portion are as a result of the eye looking obliquely through the near portion of the lens, very often at an angle greater than 40, as shown in Figure 1. In fact, the longer the progressive corridor and the lower the pantoscopic angle, the more the near axis will need to change in order to overcome this issue. Viewing through any spectacle lens at an oblique angle will have an effect on power but none more so Continued overleaf Figure 1: ooking obliquely through a lens for near vision

3 6 dispensingoptics November 2014 Figure 2: Perpendicular lens position Figure 2: Oblique lens position Figure 3: Vertex distance measured in the as worn position than through the near portion of a progressive lens as the steepness of the change in viewing angle can be dramatic. In Figure 2 we can see a lens in the first picture that through the near optical area reads +2.75DS +1.07DC axis 169, but on tilting the lens in the second picture it now reads +3.00DS +1.60DC axis 169 together with altered prismatic implications. We will see as we go through the various individualised entries, and ending up with a fully individualised product, how these power factors can change. Entry 2. The verification power introducing just the measured vertex distance The first question we need to ask is, What is vertex distance? In the Standard BS:EN:ISO vocabulary, the vertex distance is stated as the distance between the back surface of the lens and the apex of the cornea, measured with the line of sight perpendicular to the plane of the spectacle front. spectacle frame when it is in position on the patient. When dealing with the back vertex power of lenses and how power modification is calculated, we need to appreciate that the total power of an optical system can change by adding distance between two optical/refractive surfaces. This means that the distance from the cornea of the eye to the back surface of the lens is crucial when it comes to fitting lenses into a frame, particularly if the power is high 1. As can be seen by the chart in Table 2, in this particular case there was little overall change in distance power even though the BVD has increased by 3mm. The production software has found either no or little change by the introduction of a different BVD. However, in the case of the reading power there is a significant increase in power values, due to negative vergence, that should be read on the focimeter and a higher cylinder swing in the near area right lens. Entry 3. The verification power introducing just the measured pantoscopic angle The definition of pantoscopic angle is contained in the International Standard BS:EN:ISO 13666: Vocabulary which states that the as-worn pantoscopic angle is the angle in the vertical plane between the normal to the front surface of the spectacle lens at its boxed centre and the line of sight of the eye in the primary position, usually taken to be the horizontal (angle between visual and optical axes). The angle is regarded as positive if the lower part of the lens lies closer to the face. An example of the pantoscopic angle is shown in Figure 4. The effect of pantoscopic angle can be even more pronounced for progressive lens wearers than for single vision wearers. This is because not only does it affect distance vision, It is a physical measurement that can be taken manually or electronically, which can then be used to alter a prescription to take into account the position in front of the patient s eyes of the glazed spectacle lenses in direct comparison with the position of the trial lenses that were used in the sight test environment. It is also used by lens manufacturers for fine tuning the resultant prescription with regard to modern individualised digital freeform generated lenses. In Figure 3 we can see the vertex distance that is measured with the Table 2: The verification power after introducing just the measured vertex distance R R

4 Continuing Education and Training Patient and practice management Figure 4: The pantoscopic angle Figure 5: Face form angle measurement but it can have a dramatic effect on the useable reading area. This is particularly true in cases of higher dioptric power in combination with higher cylindrical power where the progressive design of the lens and the reading addition are moulded on the front surface of a semi finished lens blank. This surface was rarely optically optimised with the power that was then generated on the back surface of the lens blank 2. Freeform production eliminates this problematic area. On analysing Table 3 we can now start to see the impact that individualised measurements can make to the distance power area where changes are seen by the axis direction swing of 3 in the right lens and power reductions of 0.25D in both distance power cylinders. This is in contrast to the axis directions at near, which are 4 and 5 different to the prescribed axis directions respectively and higher power increases at near than in the distance but in the spherical components only. Entry 4. The verification power introducing just the measured face form angle Firstly, we need to look at the standard frame and consider the face form angle and base curve of the lenses supplied; these need to match in order to give the patient good vision through their lenses. Taking a standard frame and placing it on a face form angle chart such as the one shown in Figure 5, we would expect the face form angle to be around 5 and the lenses to be worked with a standard base curve. A frame where optical considerations need to be made would be any frame that has a face form angle of between 8 and 20 degrees 3. In fact, care also needs to be taken with regard to some rimless frames that have a face form angle of between 0 and 4 degrees. Good quality wrapped Rx lenses should compensate for the following Continued overleaf Table 3: The verification power after introducing just the measured pantoscopic angle R R

5 8 dispensingoptics November 2014 errors, which are caused by the oblique positioning of the lenses if the frame is glazed with Rx lenses: Astigmatic error Prismatic error Axial error Centration error from mounting the lenses R In Table 4 we can see the impact that can occur on axis directions for near when a higher face form angle is introduced and here we are seeing a near vision axis swing of 7 and 9 respectively. Surprisingly, there is little difference in the overall distance powers but, again, higher spherical powers are required in the near area. The impact of a higher face form angle also often introduces manufactured base in prism, particularly on sport frames of 15 wrap and above. Although this entry did produce some required manufactured prism the values of R & 0.11 Δ were not significant enough to record or warrant overcomplicating the experimental entries. Entry 5. The verification power introducing all three individualisation measurements After having entered the three measured parameters singly, the final entry was to look at the verification power of the progressive lenses with all three combined. We have seen in the previous tables how the powers can change dependent on the specific parameters, causing changes both up and down in power and back and forth on cylinder axes. It is only now in Table 5 that we can see the combined effect and the true variation against the prescribed powers, and it is now where the significance of full individualisation becomes apparent parameters can produce radically increased difference in powers between optimised and individualised lenses. It will be appreciated that most phoropters and trial sets are in 0.25D steps and that the majority of subjective refractions are undertaken using these values. Most practitioners would like to use 0.12D steps but the subjective response from most patients in a test environment is not sufficiently reliable. Prior to digital freeform production, the surfacing tolerance on lens power was such that it was difficult to manufacture lenses to such high degrees of accuracy but these days lenses can be calculated and produced with tolerances well below the previous 0.06D levels. One may ask why manufacturers are aiming to produce lenses to such fine tolerances in the 100th dioptre range when patients find differentiating between 0.12D in the test room so difficult. It must be remembered, however, that the whole point of R Table 4: The verification power after introducing just the measured face form angle individualisation is the translation of a very good subjective refraction into powers that relate directly to the position of wear of the patients new spectacle frame. This position of wear is often very different in comparison to a phoropter head or trial lenses. The powers supplied are calculated together so that the combination all of these factors can have a major bearing on the eventual verification powers supplied and ultimately the visual performance of the lenses for our patients. In conclusion, for those practitioners who wish to see the impact position of wear parameters can have prior to dispensing and selecting the freeform design format that they wish to recommend, there is a very good website that will give instant evaluations regarding basic verification powers. It can be found by clicking the optical calculators link on By entering the refractive index together with the various measured parameters it can demonstrate the impact that Table 5: The verification power introducing all three individualisation measurements Relating this back to the question of optimisation or individualisation in Part 1 of this article, you will see from the values calculated in all of the examples that we are dealing with powers in 100th dioptre steps, which may not appear too significant to some. The comparison verification powers between the optimised version and the individualised version seen in Table 6 may not initially appear to be very different, however, given that different powers and fitting R R

6 Continuing Education and Training Patient and practice management DV Verification Values R DV Verification Values R NV Verification Values NV Verification Values R R Table 6: The verification power comparing optimised and individualised results non-average parameters will give on the prescribed lens powers and help the practitioner to decide whether to optimise or ultimately individualise. References 1. Gilbert P (2011) Back vertex distance explored. Eyes September: Gilbert P (2008) Dispensing high base curve lenses. Eyes September: Gilbert P (2011) Pantoscopic angle explored. Eyes August: Graphics reproduced, and calculations produced, courtesy of Carl Zeiss Vision. Phil Gilbert is a qualified dispensing optician with over 40 years experience. He currently works for Carl Zeiss Vision UK as an ophthalmic lens consultant. He is a committee member of BSI TC/172/WG3 Ophthalmic enses and the chairman of the Standards Panel of the Federation of Manufacturing Opticians. He has produced many articles for the benefit of educating ophthalmic professionals and is the editor of the ABDO publication, Ophthalmic enses Availability, which lists and describes every spectacle lens available in the UK. n Restoration to the GOC register Members restoring to the General Optical Council (GOC) register should note that any CET points accrued over the number of points specified by the GOC for restoration will not be carried over to their CET record on their new GOC number. It is only after your registration date that the points will count towards your general CET requirement. The current cycle runs from January 2013 to December Those joining the register during that period, by restoration or by new qualification, will have a reduced general points requirement equal to the number of whole months they have been registered by the end of the cycle. n