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Similar a Progressive lens measurement
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Progressive lens measurement
- 1. PAL Fitting, facial measurement and marking GS Shrestha, B.OptM.Optom Gauri S. Shrestha, M.Optom, FIACLE
- 4. What should be systematic approach.. 1) Identify the problem --questing the presbyope --remark the lenses --check the frame fit 1.frame level 4.facial wrap 2.temple level 5.vertex distance 3.pantoscopic tilt
- 5. -- verify the lens fitting -- observation of use --confirm refraction and lens power 2) Possible signs and symptoms with PAL’s 3) Possible causes of problems with PAL’s What should be systematic approach..
- 12. Previous bifocal wearer who demonstrate need for a trifocal lens for occasional use may enjoy the flexibility of PALs PALs should not be recommended for all presbyopes however discretion should be exercised, especially with the following patients 1) currently bifocal or trifocal wearer who are completely satisfied with their lenses 2) Patients with large oculomotor imbalances who require prism Patient selection
- 13. 3) patients who are accustomed to wearing and are satisfied with single vision lenses for near vision only 4) Patients with a significant amount of anisometropia(3D or >) 5) Patient who need adds of high power and who have large pupils 6) Patients who are nervous, tense, impatient, and generally intolerant of health-care devices Patient selection
- 14. Fitting ...
- 25. Step 4 Monocular PD
- 26. Step 4 Monocular PD Monocular PD Monocular PD 32 29 61
- 27. Step 5 Monocular PD
- 28. Step 6 Marking the lens insert
- 29. Step 7 Fitting height
- 31. Step 8 Checking the cut out
- 33. Delivery .. On eye assessment Frame fit Fitting crosses
- 34. Delivery .. On eye assessment Distance Vision Near Vision
- 35. Delivery .. Delivery Instructions
- 37. Useful tip TAKE A BREAK !!!
Notas del editor
- 1 . Frame selection Frame shape: Progressive lenses are best suited to regular shapes, that is, round, oval and square and close variations of these. They are least suited to aviator style frames which cut off much of the reading zone. It is important that the shape allow the full use of all zones on the lens. Frame depth: All progressive lens manufacturers specify a minimum distance from the fitting cross to the bottom of the frame. This is also to allow full use of the near zone. For most progressive lenses this distance is between 18 and 23 mm, measured from the fitting cross to the inner rim directly below the fitting cross. It is important to note, though, this is not the point of reference for measuring the height of the fitting cross. We will consider this issue later. The frame chosen must at least allow for the manufacturer’s minimum depth. There is generally no recommended minimum height above the fitting cross, however, common sense should be used to avoid selecting a frame where the fitting cross is too close to the top of the frame. Some manufacturers do suggest a minimum of 12 mm. Frame size: Most frames are of adequate size for progressives, with the possible exception of depth as described above. The common problem with frame size is choosing a frame that is too large. The smaller the frame the more of the areas of astigmatic power that are removed from the lens. That is, the larger the frame the more of the areas of astigmatism that will be present in the finished spectacles. Fortunately, progressives are ideally suited to the modern regular frame shapes. Dispensing Progressive Lenses
- 1. Frame selection The next three frame requirements are all closely related in that they all affect the field of view through the various zones and they will be discussed further in the following section on frame adjustment: Back vertex distance: The best frames for progressive lenses are those that can sit comfortably with a relatively small back vertex distance. Pantoscopic tilt: The frame you select should also sit comfortably with a reasonable pantoscopic tilt. That is, a pantoscopic tilt of at least 10° - 12°. Facial wrap: The frame you select for progressives must not be too flat in its facial wrap. Dispensing Progressive Lenses
- 2. Frame adjustment The frames will need to be adjusted twice for progressive lens patients. First, when the measurements are being taken. It is not possible to take accurate facial measurements, particularly height, if the frames are not sitting exactly how they will when the patient wears the finished spectacles. Second, on final delivery of the spectacles. Good adjustment is critical not only for comfort but also to ensure that the lenses work to best effect. The normal frame adjustment procedures should be followed when adjusting the frame. This involves starting from the front of the frame and working back since any changes made to the frame front will affect the length to bend. Start with the facial bow and horizontal alignment of the frame, followed by the let back (the angle of the temples to the front when looking from above), the pantoscopic tilt, the length to bend and, finally, the angle down and anatomical bend. Dispensing Progressive Lenses
- 3. Facial measurements After deciding on progressives and choosing and adjusting the frame we must now take the facial measurements for the fitting of the lenses. The goal is to accurately centre the height of the pupil centres with the optics of the progressive lens when fitted into the spectacle frame. This necessitates a determination of the distance between the patient’s visual axes and ensuring that they are correctly aligned with the PAL’s optics in the spectacle frame, as recommended by the manufacturer. This is a very important part of dispensing progressives, poor facial measurements are probably the cause of more that 80% of problems. There are various ways of taking these measurements but they can be broken down to four basic methods. Whichever method is adopted it is important that it be carried out consistently and carefully. We will look at each method and then compare them. Dispensing Progressive Lenses
- 3. Facial measurements Distance method 1 Step 1: As mentioned earlier the first step is adjustment of the frame. It is important to consider the back vertex distance, the pantoscopic tilt and the facial wrap. Step 2 : Measure the monocular PDs with the pupillometer. The pupillometer will give an accurate, and consistent, reading of the distance between the visual axes. Where a pupillometer s not available the best alternative would be to take the PDs with a PD rule, using a pentorch to give the corneal reflex. It is important to maintain the pupillometer and have it checked for accuracy periodically. Remember, too, that the pupillometer will give a reading that is marginally narrower than a PD rule measuring the distance between the pupil centres. This is because the visual axes are slightly nasal of the pupil centres. The pupillometer reading should, though, coincide with the PD rule measuring the distance between the corneal reflexes produced by a pentorch (assuming that this measurement was taken accurately). Dispensing Progressive Lenses
- 3. Facial measurements Distance method 1 (continued) Step 3 : Mark the horizontal centre line (HCL) (datum) on the dummy lenses, using a parallel rule or layout chart, and draw a vertical line at the PD, marking every 2 mm above the HCL. Be careful, especially if using a parallel rule and clamping the outside of the frame, that it is the true mid-point between the tangents to the top and bottom of the lens that is found. A frame with a thick top rim and a thin bottom rim can create errors when using the parallel rule. Remember, when using the boxing frame measurement the three horizontal reference lines are the line tangent to the top of the lens (that is, the peak of the bevel of the lens or the depth of the groove), the tangent to the bottom of the lens (again the peak of the bevel) and the line mid way between them, formerly known as datum but now, more correctly, known as the horizontal centre line (HCL). Dispensing Progressive Lenses
- 3. Facial measurements Distance method 1 (continued) Step 4: Place the frame on the patient’s face and check the point on the marked vertical line that corresponds with the pupil centre. It is important that the practitioner’s head and the patient’s head are at the same level. Ideally an adjustable stool or chair should be used. The relative heights can be checked by looking into the wall-mounted mirrors and comparing the heights. Step 5: Check the height while talking to the patient. If the patient feels they are being measured, as in step 4, they will not be sitting naturally. If they are engaged in conversation they are likely to be sitting in a more natural position. Some practitioners would argue that the measurement should be taken while standing, however, it is unlikely that the practitioner and the patient will always be of similar heights. Therefore sitting in adjustable chairs ensures that head heights are equal. The practitioner may wish to observe the patient standing with the frame on to see if their head posture is noticeably different. Dispensing Progressive Lenses
- 3. Facial measurements Distance method 1 (continued) Step 6: Order the heights as per step 5 and the PDs as per the pupillometer reading. The heights can simply be measured with a PD rule or with the parallel rule. Before the lenses are ordered, though, it is important that the practitioner check that they will cut out by placing them on the layout chart and also ensure that there is sufficient depth below the fitting cross . Dispensing Progressive Lenses
- 3. Facial measurements Distance method 2 Step 1: Again the frame must be adjusted to fit. Step 2 : Use the pupillometer to measure the monocular PDs. The pupillometer will give an accurate, and consistent, reading of the distance between the visual axes. Some practitioners who adopt this method prefer not to use the pupillometer, using, instead, the PD rule or taking the PD from the dots obtained in step 3. If a PD rule is used then the preferred method of taking the PD would be to use a pentorch to obtain the corneal reflex. Dispensing Progressive Lenses
- 3. Facial measurements Distance method 2 (continued) Step 3: Place the frame on the patient. Then, using a fine-tipped marking pen, dot the dummy lenses at the point corresponding with the centres of the pupils. Once again, the practitioner must be on the same level as the patient. Use a fine tipped pen to en sure that the dot is small. A white inked pen is also useful since blue and black are difficult to see against the black pupil. Step 4: Check the height while talking to the patient. If the patient feels they are being measured, as in step 3, they will not be sitting naturally. This is particularly true when they are being approached by the tip of a pen. If they are engaged in a conversation they are likely to be sitting in a more natural position. If the dots no longer appear to be at the correct height the practitioner may need to re-dot the lenses. Dispensing Progressive Lenses
- 3. Facial measurements Distance method 2 (continued) Step 5: Order the heights as per step 4 and the PDs as per the pupillometer reading or the position of the dots at Step 4. Once again the lenses should be checked to ensure that will cut out by placing them on the layout chart. Dispensing Progressive Lenses
- 3. Facial measurements Near method 1 This is also referred to as the “mirror method”. The first five steps are identical to distance method 1. However, instead of ordering the positions of the fitting crosses after step 5 the practitioner should check the position of the reading zones. Step 6: First mark the distance and near visual point circles. These circles are drawn around the visual points. They are not the same as the checking circles marked on the layout chart. The distance visual point circle is a small circle (about 10 mm in diameter) centred at the fitting cross. The near visual point circle is a similar sized circle centred at the top of the near checking circle. Dispensing Progressive Lenses
- 3 Facial measurements Near method 1 (continued) Step 7: Put the frame with the visual point circles on the patient. Then a mirror is placed on the table between the practitioner and the patient at the normal reading distance. Alternatively, the practitioner may have the patient hold a mirror horizontally at that position. The practitioner looks at the mirror, observing the patient’s right eye with their right eye and their left with the practitioner’s left. The practitioner should see the centres of the patient’s pupils in the middle of the near circles. Some manufacturers produce mirrors with circular fluorescent tubes around them. These produce a small corneal reflection which can be seen in the circles. It is also possible for the practitioner to hold a pentorch immediately under each eye as they observe the patient. Step 8: If necessary the circles may need to be moved horizontally to coincide with the pupils when viewing through the mirror. If the pupils appear too high or low the practitioner should simply have the patient tilt their head more one way or the other. If they are at uneven heights then the practitioner should recheck steps 4 and 5, where an error must have occurred. Dispensing Progressive Lenses
- 3 Facial measurements Near method 1 (continued) Step 9: Place the frame over the layout chart (once the positions of the near visual point circles have been decided) with the near visual point circles centred at the top of the near checking circles and mark the positions of the fitting crosses. Dispensing Progressive Lenses
- 3. Facial measurements Near method 2 Step 1: Once again, the first step is to adjust the frame to fit, remembering to consider the back vertex distance, the pantoscopic tilt and the facial wrap. Step 2: Use the pupillometer to measure the monocular near CDs. This time the practitioner is not interested in the distance PDs but the near centration distance (CD). The reason for this is that all progressives have a pre-set inset for near (usually 2.5 mm, giving an overall inset from distance PD of 5 mm). It is clear from experience of taking PD measurements, that the inset for near varies and will not be 5 mm for everyone. Those who support this method, therefore, suggest that we measure for near, which is more critical, and then add the 2.5 mm each eye to obtain the ‘distance PD’ for ordering purposes. Dispensing Progressive Lenses
- 3. Facial measurements Near method 2 (continued) Step 3: (This step is the same as for Distance method 1) Mark the horizontal centre line (datum) on the dummy lenses, using a parallel rule or layout chart, and draw a vertical line at the PD, marking every 2 mm above the HCL. Be careful, especially if using a parallel rule and clamping the outside of the frame, that it is the true mid point between the tangents to the top and bottom of the lens that is found. A frame with a thick top rim and a thin bottom rim can create errors when using the parallel rule. Dispensing Progressive Lenses
- 3. Facial measurements Near method 2 (continued) Step 4: (Again, this is the same as Distance method 1.) Place the frame on the patient’s face and check the point on the marked vertical line that corresponds with the pupil centre. The practitioner must make sure that their head and the patient’s head are at the same level. Ideally the practitioner should be using an adjustable stool or chair. Once again, the relative heights can be checked by looking into the wall mounted mirrors. Step 5: Check the height by taking to the patient. If the patient feels they are being measured, as in step 4, they will not be sitting naturally. If they are engaged in a conversation, though, they are likely to be sitting in a more natural position. Dispensing Progressive Lenses
- 3. Facial measurements Near method 2 (continued) Step 6: Order the heights as per step 5 and the PDs as per the pupillometer near CD reading plus the manufacturer’s inset (usually 2.5 mm each eye). The heights can be measured simply with a PD rule or with the parallel rule. Before ordering the lenses, though, the practitioner should double check that the lenses will cut out by placing them on the layout chart and also ensure that there is sufficient depth below the fitting cross. Dispensing Progressive Lenses
- 3. Facial measurements Comparing the methods Pre-marking the dummy lenses: Pre-marking the dummy lenses allows both hands to be free during the observation of the heights, both the initial observation and the check while talking to the patient. It avoids the need for a steady hand while taking the measurements and allows adjustments to be made to the height without having to re-dot. It also avoids the problem of having to approach the patient’s eyes with a pen. The patient will certainly not be relaxed if the practitioner is poking a pen at their eyes. Pupillometer versus PD rules and dots: The pupillometer takes an accurate measurement of the distance between the visual axes, as opposed to the distance between the pupil centres, normally measured by PD rules and by dotting. A measurement of the distance between visual axes can be made with these other methods by using a pentorch but there is still the problems associated with head movement and parallax error cased by the position of the pentorch and the respective sizes of the patient’s and practitioner’s PDs. Care must be taken with pupillometers. As with any instrument they will not give an accurate reading if mistreated. They should therefore be checked and serviced regularly. It is important to remember, though, that the pupillometer will not agree with the PD rule, given that they are not measuring the same distance. Dispensing Progressive Lenses
- 4. Lens ordering PDs: Progressive lenses must be ordered with monocular PDs (regardless of the method used to measure them). Heights : Like PDs, heights must also be considered monocularly. It may well be, due to face asymmetry that the heights will be different in each eye. Measuring from the horizontal centre line (datum). many laboratories prefer the height to be ordered relative to ‘datum’. The reason is that its position is absolutely certain. All modern templates are cut with their locating holes on datum. Datum tends to leave less room for error. Measuring from the bottom of the frame: Unlike “datum”, there is less certainty about what is meant by the “bottom of the frame”. Some believe it to be the bottom of the frame at the monocular PD, some believe the bottom at the centre of datum, some the bottom at the centre of the box (from the boxing frame measurement) and some the absolute lens bottom. It is the last that is most correct (some countries and laboratories may use one of the other reference points). Heights referred to the bottom of the frame should mean a line tangent to the the bottom of the lens (that is, the peak of its bevel) or the inside of the groove. This is the bottom line of the boxing frame measurement’s “box”. Some newer edging machines require the distance from the bottom of the frame to be entered into the machine during processing, so this method is gaining popularity. Remember, too, if measuring from the inside of the rim 0.5 mm must be added for the depth of the groove. Dispensing Progressive Lenses
- 4. Lens ordering Checking the cut-out: As mentioned in the various facial fitting methods, once the position of the fitting cross is determined it should be checked against the appropriate layout chart to ensure that it cuts out. This must be done before the patient leaves so you won’t have to bring the patient back to complete the entire frame selection and measurement process again. Once this has been confirmed you will need to supply the laboratory with the following information: Lens powers Distance and add Frame details Distance monocular PDs (again, regardless of the fitting methods you have used) Fitting heights and point of reference (HCL or frame bottom) Progressive lens design Lens material Lens coating or treatments Dispensing Progressive Lenses
- 1. Pre-delivery verification Checking the markings: The finished spectacles will normally be returned from the laboratory with the ink markings still on the front surface. If these have been removed then relocate them using the micro-engraved horizontal locators. Once the markings have been found, check that the lens material and type are correct and that they are correctly aligned. Remember, the add will be engraved under the temporal marking and the lens type and material under the nasal marking. Checking the frame details: The frame details need to be checked, particularly if another colour has been supplied by the laboratory. In most cases, though, the frame selected from the display and adjusted before measurement will be the one to which the lenses are finally fitted. Checking the distance power and prism: The distance power should be checked in the normal way (back vertex power) through the distance checking circle. This circle will be ink marked on the lens. If the markings have been removed then relocated them as described above. In addition to checking the power through the focimeter, the locators must be checked to ensure that they are horizontal. The prism should be checked at the prism reference point. This is a dot under the fitting cross and on the same level as the horizontal locators. The fact that the target will not be clear at this point should be ignored, it is only the position of the target on the graticule that is important. It also important to note that there may not be zero prism at this point, due to prism thinning. It is the differential that must be zero. Many laboratories will introduce prism thinning to reduce the top edge thickness and overall thickness of the lens. This is done by grinding base down prism in both eyes. Delivery
- If a patient brings in a pair of progressives for checking, it is necessary to re-locate the inked markings to complete a focimeter check and to see if the fitting cross is positioned correctly. The steps you must follow to do this are: 1. Find the micro-engraved markings. This can be done by holding the lens up to the light, filling the lens with light (you may need to tilt the lens back and forward) and focussing on the surface rather than looking through the lens. Fogging the lens by breathing on it can also help to highlight the markings. The markings can also be located by using the “I see it” Micro-engraving Detector or the instruments mentioned earlier. 2. Once the permanent markings are found the horizontal locators should be marked with a soft tipped felt pen. 3. The permanent markings will indicate the manufacturer and the lens type. The correct layout chart should then be selected for marking the lens. Although the differences are not great between most lenses it is not acceptable to use a substitute for the real lens type, it must be not only the correct manufacturer but also the correct lens type. Re-locating Progressive Lens Markings
- 4. The lens with its two locators marked should then be positioned on the layout chart with the marked horizontal locators lined up with the matching markings on the chart. 5. Then the distance and near reference circles, the fitting cross and the prism reference point should be carefully traced. Make sure, also, that you are using the correct eye (and not, for example, marking up a right eye on the left eye chart). Re-locating Progressive Lens Markings