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Sources of error in soft contact lensmeasurement
Sources of Error in Soft Contact Lens Measurement David Campbell Burns, FBCO, FAAO, ARPS
A contact lens practioner and a member of the B.C.L.A. Council, David Campbell Burns is Managing Director of WOlk.
Introduction Five years ago at a Contact Lens Society Summer Clinical Conference one of the main topics for discussion was the impossibility of checking the, then rather new, soft contact lenses which many practitioners were fitting for the first time. Since then the science and the technology have proliferated and few would consider their inventory of lenses complete without one or more of the hundreds of soft lenses available. In the intervening years most clinicians have mastered the techniques involved in assessing the basic parameters of flexible lenses and it was, therefore, felt appropriate to discuss some aspects of the errors inherent in some of the methods used. Criticism has been levelled at the instrumentation available, but it can be shown that satisfactory levels of accuracy have already been achieved in this area whilst insufficient attention has been paid to much larger potential errors. Lens measurement will be discussed under the following four headings: Dioptric Power, Overall Diameter, Centre Thickness and Posterior Radius, with emphasis on indicating the levels of accuracy attainable and possible sources of error.
Power Most practitioners will measure the back vertex power of their contact lenses on a conventional optical focimeter. Accuracy is dependent upon two main factors, image quality and vertex distance. The quality of the image is entirely dependent upon the care taken in the preparation of the lens. In this connection a tissue known as Josef paper has been found to give the best results as it is sufficiently absorbent and, at the same time, does not leave
60
particles or fibres on the surface of the lens. The lens to be measured is placed upon a clean and dry area of tissue, blotted and is then moved two or three times and the blotting repeated. The surface will now be clean and dry and slight movement of the tissue on the surface is sufficient to leave it bright and clear. The lens is placed on the focimeter where the quality of image obtained should be comparable to that produced by a PMMA lens. A setting accuracy within + / - one-eighth dioptre can be achieved with practice. Vertex compensation becomes important where the power of the lens exceeds + / - 5.00D. With high plus lenses this correction is vital as the errors involved can exceed one or two dioptres. Most focimeters are supplied with a contact lens stage which will also ensure that the lens does not become contaminated by marking ink where the focimeter is also used for spectacle checking. The special contact lens stage must, of course, be removed when reverting to spectacle lens checklng, or these results will be wrong! New types of electronic focimeters have been appearing over the last fews years, some of which are simply conventional optical focimeters linked to a digital read-out, but a number of advanced instruments, such as that offered by Rodenstock, are entirely automatic and print out the result. The laser focimeter from Acuity Systems employs new technological principles but has not, for us, been very reliable, is capable of producing accurate readings from lenses with less well prepared surfaces. This instrument, too, is able to display the result in plus or minus cylinder form corrected for spectacle or contact lens vertex distances and may be supplied with a printer, if required. It is particularly suited to the measurement of toric soft lenses.
Journal of the British Contact Lens Association
It is, of course, also possible to calculate the power of a contact lens and this has been made easier with the advent of programmable calculators, but the accuracy of the result is necessarily dependent upon the accumulation of errors in the data required. Accuracy of these methods can be considered totally satisfactory.
Diameter Although the diameter of a hydrated soft lens may be measured with a simple scale or caliper, this method is of limited accuracy compared with projection systems, a number of which are available. The first to be offered was manufactured by the Soehnges Company in Germany several years ago, although the instrument was primarily intended to measure the hydrated radius of soft lenses. Later instruments are the DL2 adaptation of a microfilm reader by Zeiss Jena, the 'contact analyser' by Teladown and a new instrument from Optimec. These instruments all employ front or back projection on to a calibrated screen. A measuring accuracy of + / - one-eighth of a millimetre is possible. The accuracy of the scales can be checked by a graticule in the focal plane and one is well advised to check the accuracy of a new instrument, as small variations in construction and focal lengths of lenses employed can produce significant errors, particularly with larger lenses. The DL2, which measures lenses in air, has an additional source of error in that the wet lens, when placed upon the glass plate, will produce a marked meniscus which must be allowed for when focusing and determining the diameter of the lens. The accuracy of these methods can be considered satisfactory for our purposes.
Thickness The conventional contact lens thickness gauge cannot, of course, be used for soft contact lenses as it tends to fenestrate the lens! An adapted form has been developed by Fatt and recently described by Pearson (World Contact Lens Congress Vienna 1980). This relies upon electrical contact between a probe attached to a micrometer and the lens. It is not known whether this instrument is in commercial production. A n alternative method, also described by Pearson, is by means of a radiuscope which is simply used as a travelling microscope. One focuses first on the stage a n d then interposing the lens, on the nearer lens surface, the dial indicating the lens thickness. If one focuses on each surface of the lens in turn one must remember to correct for the refractive index of the material. A method still largely in the developmental stage employs ultrasonics. The accuracy of these methods is considered satisfactory.
Journal of the British Contact Lens Association
Radius The measurement of the hydrated radius of a soft lens is the area responsible for most discussion amongst practitioners and manufacturers alike. Early methods using test spheres (CLM) or projection (Soehnges) have been largely superceded by more advanced instrumentation. The Soehnges projection method was designed to measure hydrated lenses in saline, but as the cell was situated in the gate of a powerful slide projector a considerable amount of temperature variation could be anticipated. It has been shown by Farlow that swell factors will vary with water content and temperature. It can be demonstrated that, for the 50% water-content material under test, the change in posterior radius will be of the order of 0.2mm, or one normal fitting step. Farlow has also shown that for Hema materials the changes are reduced to between 10 and 20% of those shown and are, therefore, only marginally significant over the temperature range normally encountered. Fortunately for the manufacturers of high water content materials, the progression does not seem to be linear! At least one new instrument does offer the facility of temperature control which will be of value to researchers and those working in environments where temperature is not controlled, especially if high water content materials are being measured.
Instrumentation It must be emphasized that there is little restriction on the theoretical limits of accuracy of modern measuring instruments; the majority of problems have their origin in the nature of the material being measured. It has been suggested that instrument designers should develop a foolproof measuring system, which would determine categorically the specification of a lens. I believe that this is possible but the equipment would almost certainly prove too expensive for all but the wealthiest practitioners and, even then, when the lens was removed from its reference solution in its temperature and Ph controlled standardized environment - - it would change again. The Ophthalmometer plus wet cell, preferably with high intensity illuminated and coUimmated mires, can give results to + / - 0.05mm, but is not very quick and is very sensitive to careful placement of the lens in the cell. The in-air mechanical spherometer is cheap, quick to use and, with practice, can give results within 0.1ram. Its main disadvantage is that, as one is working in air and the lens is drying, one must not take too long over the readings. The in-air electronic spherometer is expensive, but very quick and, with the advantage of a built-in processor, gives an immediate read-out accurate to 0.01mm. Both of these methods are poor at measuring high water content or super thin lenses.
61
The Optimec instrument is fairly expensive but, for its cost, and in the time taken gives a good level of accuracy in measuring the radius, diameter and centre thickness, all with temperature control, if required. It has an ingeneous centring device, which saves time positioning the lens and is very good indeed with super thin and high water content lenses.
which showed least deviation from the manufacturers stated values and less of the apparently illogical spread shown by other host media. All that one can do at this stage is to indicate the magnitude of the errors encountered. Reference lenses were checked in saline made from salt tablets and purified water.
Errors
Steep/Flat Spread
Sources of error already discussed include temperature, environmental and instrumental limitations. One factor which has received less attention is the influence of the host medium, that is the hydrating solution, on the dimensions of the lens. Although most laboratories hydrate their finished lenses in saline and measure them at this state, the lens is, thereafter, likely to find itself in any one of several hundred different solutions, all of which will have different and largely unpredictable effects upon lens dimensions. Even salines vary enormously and quite small deviations from the so-called 'normal' 0.9% solution, will produce some variation. We have shown differences between lenses hydrated in different proprietary salines, preserved and unpreserved, and two or three proprietary soaking solutions. It would be convenient if one could report that these differences were regular and consistent and that, for a change from solution A to solution B, one could just add or substract a given value from the stated specification. This was not found to be true, neither were the differences related in any predictable way to lens power, thickness or radius. The most consistent of the solutions tested was Amisol,
Flexsol 0.13f 0.05/0.27
Hydrocare 0.25f 0.15/0.42
Amisol 0.05f
(My thanks are due to Angela Whitworth of the W6hlk staff for assistance in preparation of the above data.) Lenses originally specified in saline then stored in Flexsol were found to measure significantly nearer to the stated specification if given a thorough rinse in saline or distilled water first. It is intended to continue with these measurements and to make this one aspect the subject of a future paper. In all of the foregoing it has been assumed that new or perfect lenses are being measured - - it is obvious that deposition or deformation can materially affect results. Address for further correspondence: W r h l k Contact Lenses, Eridge Road, Crowborough, Sussex TN6 2SJ
Announcement
Although Abatron will not be exhibiting this year at the BCLA conference in Birmingham, both John Evans and Derek Garrity will attend as delegates and they look forward with great pleasure to meeting their customers and friends.
abatron Abatron Limited
12A Churchyard, Hitchin, Hertfordshire, SG5 1HR, Great Britain. Tel. Hitchin (0462) 58601/2
62
Journal of the British Contact Lens Association
A contact lens practioner and a member of the B.C.L.A. Council, David Campbell Burns is Managing Director of WOlk.
Introduction Five years ago at a Contact Lens Society Summer Clinical Conference one of the main topics for discussion was the impossibility of checking the, then rather new, soft contact lenses which many practitioners were fitting for the first time. Since then the science and the technology have proliferated and few would consider their inventory of lenses complete without one or more of the hundreds of soft lenses available. In the intervening years most clinicians have mastered the techniques involved in assessing the basic parameters of flexible lenses and it was, therefore, felt appropriate to discuss some aspects of the errors inherent in some of the methods used. Criticism has been levelled at the instrumentation available, but it can be shown that satisfactory levels of accuracy have already been achieved in this area whilst insufficient attention has been paid to much larger potential errors. Lens measurement will be discussed under the following four headings: Dioptric Power, Overall Diameter, Centre Thickness and Posterior Radius, with emphasis on indicating the levels of accuracy attainable and possible sources of error.
Power Most practitioners will measure the back vertex power of their contact lenses on a conventional optical focimeter. Accuracy is dependent upon two main factors, image quality and vertex distance. The quality of the image is entirely dependent upon the care taken in the preparation of the lens. In this connection a tissue known as Josef paper has been found to give the best results as it is sufficiently absorbent and, at the same time, does not leave
60
particles or fibres on the surface of the lens. The lens to be measured is placed upon a clean and dry area of tissue, blotted and is then moved two or three times and the blotting repeated. The surface will now be clean and dry and slight movement of the tissue on the surface is sufficient to leave it bright and clear. The lens is placed on the focimeter where the quality of image obtained should be comparable to that produced by a PMMA lens. A setting accuracy within + / - one-eighth dioptre can be achieved with practice. Vertex compensation becomes important where the power of the lens exceeds + / - 5.00D. With high plus lenses this correction is vital as the errors involved can exceed one or two dioptres. Most focimeters are supplied with a contact lens stage which will also ensure that the lens does not become contaminated by marking ink where the focimeter is also used for spectacle checking. The special contact lens stage must, of course, be removed when reverting to spectacle lens checklng, or these results will be wrong! New types of electronic focimeters have been appearing over the last fews years, some of which are simply conventional optical focimeters linked to a digital read-out, but a number of advanced instruments, such as that offered by Rodenstock, are entirely automatic and print out the result. The laser focimeter from Acuity Systems employs new technological principles but has not, for us, been very reliable, is capable of producing accurate readings from lenses with less well prepared surfaces. This instrument, too, is able to display the result in plus or minus cylinder form corrected for spectacle or contact lens vertex distances and may be supplied with a printer, if required. It is particularly suited to the measurement of toric soft lenses.
Journal of the British Contact Lens Association
It is, of course, also possible to calculate the power of a contact lens and this has been made easier with the advent of programmable calculators, but the accuracy of the result is necessarily dependent upon the accumulation of errors in the data required. Accuracy of these methods can be considered totally satisfactory.
Diameter Although the diameter of a hydrated soft lens may be measured with a simple scale or caliper, this method is of limited accuracy compared with projection systems, a number of which are available. The first to be offered was manufactured by the Soehnges Company in Germany several years ago, although the instrument was primarily intended to measure the hydrated radius of soft lenses. Later instruments are the DL2 adaptation of a microfilm reader by Zeiss Jena, the 'contact analyser' by Teladown and a new instrument from Optimec. These instruments all employ front or back projection on to a calibrated screen. A measuring accuracy of + / - one-eighth of a millimetre is possible. The accuracy of the scales can be checked by a graticule in the focal plane and one is well advised to check the accuracy of a new instrument, as small variations in construction and focal lengths of lenses employed can produce significant errors, particularly with larger lenses. The DL2, which measures lenses in air, has an additional source of error in that the wet lens, when placed upon the glass plate, will produce a marked meniscus which must be allowed for when focusing and determining the diameter of the lens. The accuracy of these methods can be considered satisfactory for our purposes.
Thickness The conventional contact lens thickness gauge cannot, of course, be used for soft contact lenses as it tends to fenestrate the lens! An adapted form has been developed by Fatt and recently described by Pearson (World Contact Lens Congress Vienna 1980). This relies upon electrical contact between a probe attached to a micrometer and the lens. It is not known whether this instrument is in commercial production. A n alternative method, also described by Pearson, is by means of a radiuscope which is simply used as a travelling microscope. One focuses first on the stage a n d then interposing the lens, on the nearer lens surface, the dial indicating the lens thickness. If one focuses on each surface of the lens in turn one must remember to correct for the refractive index of the material. A method still largely in the developmental stage employs ultrasonics. The accuracy of these methods is considered satisfactory.
Journal of the British Contact Lens Association
Radius The measurement of the hydrated radius of a soft lens is the area responsible for most discussion amongst practitioners and manufacturers alike. Early methods using test spheres (CLM) or projection (Soehnges) have been largely superceded by more advanced instrumentation. The Soehnges projection method was designed to measure hydrated lenses in saline, but as the cell was situated in the gate of a powerful slide projector a considerable amount of temperature variation could be anticipated. It has been shown by Farlow that swell factors will vary with water content and temperature. It can be demonstrated that, for the 50% water-content material under test, the change in posterior radius will be of the order of 0.2mm, or one normal fitting step. Farlow has also shown that for Hema materials the changes are reduced to between 10 and 20% of those shown and are, therefore, only marginally significant over the temperature range normally encountered. Fortunately for the manufacturers of high water content materials, the progression does not seem to be linear! At least one new instrument does offer the facility of temperature control which will be of value to researchers and those working in environments where temperature is not controlled, especially if high water content materials are being measured.
Instrumentation It must be emphasized that there is little restriction on the theoretical limits of accuracy of modern measuring instruments; the majority of problems have their origin in the nature of the material being measured. It has been suggested that instrument designers should develop a foolproof measuring system, which would determine categorically the specification of a lens. I believe that this is possible but the equipment would almost certainly prove too expensive for all but the wealthiest practitioners and, even then, when the lens was removed from its reference solution in its temperature and Ph controlled standardized environment - - it would change again. The Ophthalmometer plus wet cell, preferably with high intensity illuminated and coUimmated mires, can give results to + / - 0.05mm, but is not very quick and is very sensitive to careful placement of the lens in the cell. The in-air mechanical spherometer is cheap, quick to use and, with practice, can give results within 0.1ram. Its main disadvantage is that, as one is working in air and the lens is drying, one must not take too long over the readings. The in-air electronic spherometer is expensive, but very quick and, with the advantage of a built-in processor, gives an immediate read-out accurate to 0.01mm. Both of these methods are poor at measuring high water content or super thin lenses.
61
The Optimec instrument is fairly expensive but, for its cost, and in the time taken gives a good level of accuracy in measuring the radius, diameter and centre thickness, all with temperature control, if required. It has an ingeneous centring device, which saves time positioning the lens and is very good indeed with super thin and high water content lenses.
which showed least deviation from the manufacturers stated values and less of the apparently illogical spread shown by other host media. All that one can do at this stage is to indicate the magnitude of the errors encountered. Reference lenses were checked in saline made from salt tablets and purified water.
Errors
Steep/Flat Spread
Sources of error already discussed include temperature, environmental and instrumental limitations. One factor which has received less attention is the influence of the host medium, that is the hydrating solution, on the dimensions of the lens. Although most laboratories hydrate their finished lenses in saline and measure them at this state, the lens is, thereafter, likely to find itself in any one of several hundred different solutions, all of which will have different and largely unpredictable effects upon lens dimensions. Even salines vary enormously and quite small deviations from the so-called 'normal' 0.9% solution, will produce some variation. We have shown differences between lenses hydrated in different proprietary salines, preserved and unpreserved, and two or three proprietary soaking solutions. It would be convenient if one could report that these differences were regular and consistent and that, for a change from solution A to solution B, one could just add or substract a given value from the stated specification. This was not found to be true, neither were the differences related in any predictable way to lens power, thickness or radius. The most consistent of the solutions tested was Amisol,
Flexsol 0.13f 0.05/0.27
Hydrocare 0.25f 0.15/0.42
Amisol 0.05f
(My thanks are due to Angela Whitworth of the W6hlk staff for assistance in preparation of the above data.) Lenses originally specified in saline then stored in Flexsol were found to measure significantly nearer to the stated specification if given a thorough rinse in saline or distilled water first. It is intended to continue with these measurements and to make this one aspect the subject of a future paper. In all of the foregoing it has been assumed that new or perfect lenses are being measured - - it is obvious that deposition or deformation can materially affect results. Address for further correspondence: W r h l k Contact Lenses, Eridge Road, Crowborough, Sussex TN6 2SJ
Announcement
Although Abatron will not be exhibiting this year at the BCLA conference in Birmingham, both John Evans and Derek Garrity will attend as delegates and they look forward with great pleasure to meeting their customers and friends.
abatron Abatron Limited
12A Churchyard, Hitchin, Hertfordshire, SG5 1HR, Great Britain. Tel. Hitchin (0462) 58601/2
62
Journal of the British Contact Lens Association