Manns , None; A. Ziebarth , None; A. Rosen , None; R. Amelinckx , None; E. Arrieta , None; R. Augusteyn , None; J. Parel , None. The publication costs of this article were defrayed in part by page charge payment. F igure 1. View Original Download Slide. B In vitro lens power measurement with a Scheiner system. A CCD chip mounted on a vertical position is used to visualize and record the location of the focus. In both cases, the anterior of the surface of the lens is up.
F igure 2. A sample sagittal side view shadowgraph image of a year-old isolated lens 2 days postmortem, with the anterior and posterior surface profiles overlaid. The surface profiles were fit with conic sections to calculate the anterior and posterior radii of curvature R a and R p and asphericities Q a and Q p.
The thickness was measured directly from the images with a ruler used for calibration. The image shows the crosshairs of the optical comparator and the supporting suture mesh of the immersion cell.
During the measurement, the camera was inverted. In this image the lens was photographed with the anterior surface resting on the suture wires. F igure 3.
The equations for the regression lines are given in Table 1. F igure 4. Surface power was calculated from surface curvature measurements obtained during isolated lens biometry. T able 1. View Table. Data ages in years are the in vitro human lens age-dependent linear regression equations of the measured optical properties.
The values from both eyes of each donor were averaged and used as a single point for the age-dependence linear regressions. T able 2. See Appendix for details. F igure 5. The equation for the regression line is given in Table 1. F igure 6. F igure 7. Comparison of our results with previously published data on the age dependence of the lens power A and the refractive index B.
F igure 8. The age-dependent data for the in vivo amplitude of accommodation from Duane 33 corrected to place the reference plane at the anterior corneal surface instead of the spectacle plane is superimposed on the graph. There is good agreement between the age-dependent loss of accommodation and the maximum amplitude of accommodation calculated from the isolated lens power.
AtchisonDA, SmithG. Optics of the Human Eye. A video technique for phakometry of the human crystalline lens. Invest Ophthalmol Vis Sci. Calculation of the radii of curvature of the crystalline lens surfaces. Ophthalmic Physiol Opt.
Clinical Visual Optics. A method of determining the equivalent powers of the eye and its crystalline lens without resort to phakometry. The equivalent refractive index of the crystalline lens in childhood.
Vision Res. Scheimpflug and high-resolution magnetic resonance imaging of the anterior segment: a comparative study. J Opt Soc Am A. Crystalline lens radii of curvature from Purkinje and Scheimpflug imaging. J Vision. Aspheric curvatures for the human lens. Spherical aberration of the crystalline lens. Refractive index gradient of human lenses. Optom Vis Sci. GlasserA, CampbellMC. Presbyopia and the optical changes in the human crystalline lens with age.
Biometric, optical and physical changes in the isolated human crystalline lens with age in relation to presbyopia. Central surface curvatures of postmortem-extracted intact human crystalline lenses: implications for understanding the mechanism of accommodation. Age-related changes in refractive index distribution and power of the human lens as measured by magnetic resonance micro-imaging in vitro. Refractive index distribution and optical properties of the isolated human lens measured using magnetic resonance imaging MRI.
Change in shape of the aging human crystalline lens with accommodation. Biometry of primate lenses during immersion in preservation media. Mol Vis. Numerical modelling of the accommodating lens. Insights into the age-related decline in the amplitude of accommodation of the human lens using a non-linear finite-element model. Br J Ophthalmol. Estimating the external force acting on the human eye lens during accommodation by finite element modelling.
Optomechanical response of human and monkey lenses in a lens stretcher. In vitro dimensions and curvatures of human lenses. Shadow photogrammetric apparatus for the quantitative evaluation of corneal buttons. Ophthalmic Surg. Radius of curvature and asphericity of the anterior and posterior surface of human cadaver crystalline lenses.
Exp Eye Res. Crystalline lens power in myopia. Initial cross-sectional results from the Orinda Longitudinal Study of Myopia. Change with age of the refractive index gradient of the human ocular lens. Ocular components measured by keratometry, phakometry, and ultrasonography in emmetropic and myopic optometry students. Comparison of ocular component growth curves among refractive error groups in children. Variation of the contribution from axial length and other oculometric parameters to refraction by age and ethnicity.
The shape of the aging human lens: curvature, equivalent refractive index and the lens paradox. Normal values of the accommodation at all ages. Axial growth and changes in lenticular and corneal power during emmetropization in infants. Changes in ocular dimensions and refraction with accommodation. The subjects' eyes were not dilated or cyclopleged before testing, which might have caused some degree of accommodation in some of the younger subjects, resulting in slightly more hyperopic refraction, increased lens thickness, and decreased anterior chamber depth.
All subjects gave written informed consent before participation. Subjects' refractions were determined monocularly using Jackson crossed cylinders in a phoropter.
For the emmetropic group, the radii of curvature of the anterior and posterior lens surfaces, as well as the lens equivalent refractive index, were obtained by analyzing Purkinje images, refraction, and biometry, by using a setup and calculations 13 similar to that described by Rosales and Marcos.
Bennett's method 6 calculates lens power P L when lens thickness T is available by keeping the distances from the surfaces to the principal planes of the lens in the same proportion as in the lens of the Gullstrand-Emsley eye model. Table 1. View Table. Overview of the Parameters Used. We modified the method by including the parameter c Sten , which is the estimated distance between the anterior lens surface and the first lenticular principal plane. Another approach to calculating P L without knowing T is to modify an equation proposed by Bennett and Rabbetts 11 for the purpose of calculating the spherical refraction of an eye when its biometry is known.
They replaced the lens with an equivalent thin lens located at the midpoint between the lenticular principal planes, using the Bennett-Rabbetts eye model.
Using the lens surface radii of curvature and lens refractive index determined using phakometry, along with the lens thickness, the lens equivalent power was calculated using the thick lens formula 16 :. Comparing Lens Powers with the Different Methods. To compare lens powers obtained with the methods detailed above, we determined the c constants c 1 , c 2 , c Sten , and c BR for both Gullstrand-Emsley and Bennett-Rabbetts eye models. As both eye models will differ from actual ocular biometry, we determined the optimal c constants also for each eye individually.
Means and standard deviations of these optimal c constants were called the customized c constants and are given in Table 2. Table 2. Using both the Gullstrand-Emsley and Bennett-Rabbetts eye models, the lens powers with the Bennett method were not significantly different from the phakometry powers.
Using the customized c constants did not improve the agreement. Figure 1. View Original Download Slide. Bland-Altman plots showing the differences between the phakometry lens powers and the lens powers calculated using a the Bennett method and b the Bennett-Rabbetts method.
Powers for the calculation methods are shown for both eye model c constants and customized c constants. The Pearson correlation coefficients between the calculated and phakometry lens powers were high Table 2 and independent of the eye model used. Figure 2 a shows lens powers calculated using the Bennett method with customized c constants calculated for the combination of the two datasets eyes as a function of axial length L.
Because phakometry was not available for the second dataset, this plateauing could not be confirmed experimentally. However, a similar trend was found in the raw data published by Sorsby et al. This choice is based on the observation of Dunne et al. Figure 2. The calculated powers use data from both datasets eyes , whereas the phakometry powers contain data only from the first dataset 66 eyes. Table 3. Using the customized c constants all three lens power calculation methods are in reasonable agreement with the phakometry lens power for emmetropic eyes.
This answers the first purpose of this article, which was to confirm the agreement that Dunne et al. However, for individual eyes, differences between calculated and phakometric power of up to 3. Using the argument of Bennett 6 and Dunne et al.
For the Bennett method, the choice of eye model did not influence the calculated lens power significantly, which may be a consequence of the fact that the method is based on ray tracing of a thick lens model rather than a thin lens approximation. It can be used accurately for emmetropic eyes with either the c 1 and c 2 constants of the two eye models or the customized constants derived in this work. For the Bennett method using customized c constants made little difference in the results, but in emmetropic eyes the customized c of 2.
If lens thickness is not available, both methods with the customized 2 constants may be considered to be good approximations of the Bennett method. Although the three calculation methods now match well with one another for a wide range of refractions, there are still theoretical issues to consider.
The more general case, when S pp and S CV are different from 0, could only be confirmed numerically because of the mathematically complicated equation 3. Although this seems to point at some common origin of both formulas, the meaning of this observation remains unclear. Again, a possible relationship between both methods could not be investigated further due to the complexity of equation 3. As lens power depends on lens refractive index, one could expect a correlation between the c constants and lens refractive index values n L determined from phakometry.
For this reason, the results of the lens power calculations were given for each eye model separately. However, a significant correlation with n L was seen only for c 1 of the Bennett method; the other c constants were either constant or randomly distributed. Tejas Mali. Garima Poudel. Zakir Hussain. Show More. Views Total views. Actions Shares. No notes for slide. Lens power measurement 1. Trial Lens Hand NeutralizationTrial Lens Hand Neutralization Two lenses neutralize each other whenTwo lenses neutralize each other when placed in contact with each other so thatplaced in contact with each other so that the combined power of the two lenses isthe combined power of the two lenses is equal to zeroequal to zero An unknown lens is neutralized by aAn unknown lens is neutralized by a known trial lens of equal power butknown trial lens of equal power but opposite in signopposite in sign 4.
Trial Lens Hand NeutralizationTrial Lens Hand Neutralization This is performed in the absence of aThis is performed in the absence of a lensometerlensometer It is used qualitatively as a means forIt is used qualitatively as a means for estimation in many clinical and dispensingestimation in many clinical and dispensing situationssituations It often involves simply identifying if it is aIt often involves simply identifying if it is a plus, a minus, or a toric lensplus, a minus, or a toric lens It more accurately estimates low powerIt more accurately estimates low power plus and minus lenses than toric lensesplus and minus lenses than toric lenses 6.
Trial Lens Hand NeutralizationTrial Lens Hand Neutralization It is used to measure theIt is used to measure the front vertexfront vertex powerpower of the lensof the lens 7. Trial Lens Hand NeutralizationTrial Lens Hand Neutralization For a plus or a minus lens, linear motion isFor a plus or a minus lens, linear motion is used to neutralize powerused to neutralize power Trial Lens Hand NeutralizationTrial Lens Hand Neutralization For a toric lens, rotational motion is used to findFor a toric lens, rotational motion is used to find the axisthe axis Trial Lens Hand NeutralizationTrial Lens Hand Neutralization Place known trial lens against front surface ofPlace known trial lens against front surface of unknown lensunknown lens No movement indicates neutralityNo movement indicates neutrality A minus or plus lens i.
LensometerLensometer LensometerLensometer It is used to measure theIt is used to measure the back vertexback vertex powerpower oror front vertex powerfront vertex power of the lensof the lens LensometerLensometer To find the back vertex power, place theTo find the back vertex power, place the concave side of lens against lens stopconcave side of lens against lens stop LensometerLensometer To find the front vertex power, place theTo find the front vertex power, place the convex side of lens against lens stopconvex side of lens against lens stop LensometerLensometer In the case that the lens is a sphero-In the case that the lens is a sphero- cylindrical prescription, the lensometer iscylindrical prescription, the lensometer is used to determine the cylinder axisused to determine the cylinder axis It is used to locate the optical center of theIt is used to locate the optical center of the lenslens The lensometer is used to measure theThe lensometer is used to measure the amount of prism in the lensamount of prism in the lens Lensometer SchematicLensometer Schematic Observation SystemObservation System The Keplerian telescope consists of anThe Keplerian telescope consists of an objective lens, an eyepiece, and a reticleobjective lens, an eyepiece, and a reticle The two plus lenses are positioned so thatThe two plus lenses are positioned so that their two focal points coincide with eachtheir two focal points coincide with each otherother The unknown lens whose power is to beThe unknown lens whose power is to be measured or neutralized is positioned atmeasured or neutralized is positioned at the lens stop the location of thethe lens stop the location of the secondary focal plane of the standardsecondary focal plane of the standard lens lens Lensometer OperationLensometer Operation When the lens of unknown power isWhen the lens of unknown power is introduced, the image of the illuminatedintroduced, the image of the illuminated target is thrown out of focustarget is thrown out of focus Lensometer OperationLensometer Operation The physical distance forward orThe physical distance forward or backward that the target moves indicatesbackward that the target moves indicates the power of unknown lens for thethe power of unknown lens for the meridian being measuredmeridian being measured LensometerLensometer reticule target Sphere line Cylinder lines Single Vision LensSingle Vision Lens MeasurementMeasurement To measure single vision lenses, eitherTo measure single vision lenses, either back vertex powers or front vertex powersback vertex powers or front vertex powers must be foundmust be found Single Vision LensSingle Vision Lens MeasurementMeasurement Determine which part of the target is usedDetermine which part of the target is used for determining the spherical componentfor determining the spherical component and which part of the target is used forand which part of the target is used for determining the cylindrical componentdetermining the cylindrical component Rotate the power wheel until the lines orRotate the power wheel until the lines or the spots are in clear focusthe spots are in clear focus If the power is spherical, all the lines orIf the power is spherical, all the lines or spots will be clearspots will be clear Note the power on the power wheelNote the power on the power wheel Types of TargetTypes of Target Single Vision LensSingle Vision Lens MeasurementMeasurement If the spherical and cylindrical lines do notIf the spherical and cylindrical lines do not come into focus at the same time, the lenscome into focus at the same time, the lens has a cylindrical componenthas a cylindrical component Rotate the power wheel until the sphericalRotate the power wheel until the spherical lines focus with thelines focus with the less minusless minus or more or more plus powerplus power Orient the target rotation dial axis wheel Orient the target rotation dial axis wheel so that the spherical lines are perfectlyso that the spherical lines are perfectly straightstraight Single Vision LensSingle Vision Lens MeasurementMeasurement Read the power and record as theRead the power and record as the sphericalspherical component of the prescriptioncomponent of the prescription Focus the cylindrical lines by rotating theFocus the cylindrical lines by rotating the power wheel topower wheel to more minusmore minus or less plus or less plus power 90 degrees away power 90 degrees away The difference in power between the twoThe difference in power between the two principal meridians is the amount ofprincipal meridians is the amount of minus cylinder powerminus cylinder power in the lensin the lens Read the axis of the cylinder from the axisRead the axis of the cylinder from the axis wheelwheel Single Vision LensSingle Vision Lens MeasurementMeasurement When both lenses have been measuredWhen both lenses have been measured and marked, measure the distanceand marked, measure the distance between optical centers of the lensesbetween optical centers of the lenses DBOC or geometric center distance DBOC or geometric center distance Multifocal Lens MeasurementMultifocal Lens Measurement Measure the distance portion of multifocalMeasure the distance portion of multifocal lenses, in the same way as with single visionlenses, in the same way as with single vision lenseslenses Turn the glasses around backward so that theTurn the glasses around backward so that the temples face the operatortemples face the operator Find theFind the distance front vertex powerdistance front vertex power Find theFind the near front vertex powernear front vertex power Record the addition power Add , which is theRecord the addition power Add , which is the difference between the distance and neardifference between the distance and near prescriptionsprescriptions
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