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Fundamentals of Progressive Lens Design

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Fundamentals of Progressive Lens Design VisionCare Product News Volume 6, Number 9 • September 2006 Fundamentals of Progressive Lens Design By Darryl Meister, ABOM With the use of Plus-cylinder power at an oblique axis, it is possible to join a flatter distance zone curve into a steeper Progressive Lens Surfaces near zone curve without breaks in the surface. However, the geometry of a progressive surface is considerably more A progressive addition lens (or “PAL”) is a type of multifocal complex, with cylinder that varies in both magnitude and lens that employs a surface with a continuously smooth orientation. Traditional general-purpose progressive lenses increase in addition (Plus) power. The curvature of the possess four structural features (Figure 3): surface increases from its minimum value in the distance zone to its maximum value in the near zone (Figure 1). The 1. Distance Zone: A stabilized region in the upper portion total increase in surface power between these two zones is of the lens provides the specified distance prescription. equal to the specified Add power of the lens. This gradual 2. Near Zone: A stabilized region in the lower portion of increase in power also results in a variable focus the lens provides the specified Add power for reading. intermediate zone. 3. Progressive Corridor: A corridor of increasing power connects these two zones and provides intermediate or mid-range vision. 8 7 4. Blending Region: The peripheral regions of the lens 6 contain non-prescribed cylinder power and provide only minimal visual utility. DISTANCE 8 7 INTERMEDIATE 6 NEAR Progressive 8 Distance Corridor 7 6 Figure 1. The surface power of a progressive lens design Intermediate increases smoothly in order to produce a gradual change in Blending Add power from distance to near. Region Progressive lenses provide the desired Add power without Near any breaks, ledges, or lines by “blending” the transition between the distance and near zones. In fact, the transition Figure 3. The structural features of a general-purpose between these zones is “smooth” enough to prevent abrupt progressive lens changes in prism and magnification—or image jump—as well. This blending is achieved by incorporating varying Characterizing Progressive Optics amounts of cylinder power, oriented at an oblique axis, in While the central regions of a progressive lens surface are the lateral regions of the surface (Figure 2). relatively spherical, most points across the lens surface actually have some degree of cylinder. This means that the Plus-Cylinder curvature actually varies locally from meridian to meridian at these points. The cylinder at each point on the lens surface is often referred to as surface astigmatism, since cylinder power produces an astigmatic focus instead of a point focus. Surface astigmatism varies across the surface a 90° Wedge progressive lens. It is virtually zero along the progressive corridor, but increases into the lateral blending regions of the lens. In these regions, the surface astigmatism Figure 2. The ledge at the junction between a flatter curve and a steeper curve can be eliminated using cylinder power, as produces significant levels of unwanted cylinder power, demonstrated by removing a 90° wedge from an Executive- which—in sufficient quantities—is perceived by the wearer style bifocal and replacing it with a section of a Plus-cylinder. as blur, distortion, and image swim. Copyright ® 2006 Darryl J Meister and VisionCare Product News Since each point in the blending region of a progressive functions whose terms represent quantities such as mean lens can be thought of as a small cylinder, it makes sense power error, astigmatism, coma, and so on. to evaluate the optics of a progressive lens by measuring Surface astigmatism and mean power contour plots provide the amount of cylinder power present at these points. a convenient way to represent various optical quantities, These measurements are often represented using a but they are only indicative of performance. Furthermore, contour plot, which is a map that indicates how the levels of contour plots of surface power are usually less visually an optical quantity vary across the lens (Figure 4). meaningful than contour plots of ray-traced optical Astigmatism contour plots indicate regions of potential blur, performance. These plots are calculated using lens-eye image swim, and distortion, and are useful for predicting modeling to determine how the wearer actually perceives the size of the distance, intermediate, and near zones. the optics of the lens. Minkwitz’s Theorem 0.5 In well-designed progressive lenses, unwanted cylinder 1.0 0.5 0.5 power in the periphery is generally reduced to its 1.0 1.5 0.5 mathematical limits. However, some level of unwanted 2.0 1.5 1.0 cylinder power is ultimately necessary in order to blend any 2.5 2.5 1.5 2.0 3.0 surface with Add power. Minkwitz showed that the rate of 2.0 3.0 change in unwanted cylinder power (∆Cyl) at a small distance away from the centerline of the progressive corridor is nearly equal to twice the rate of change in Add Astigmatism Plot Mean Add Power Plot power (∆Add) over an equal distance along the centerline Figure 4. Contour plots show the distribution of an optical 2 of the corridor (Figure 5). quantity across the lens by indicating its levels in fixed intervals (e.g., 0.50 diopters). Generally speaking, progressive lenses have a unique astigmatism plot, so these plots serve as a kind of “fingerprint” of the lens design. It is also useful to evaluate ΔCyl ≈ 2×ΔAdd the distribution of Add power across the lens. Since most 0.50 points across the surface contain cylinder, the spherical 1.00 1.50 ∆Add equivalent, or mean power, is measured, which is the 2.00 +0.50 average power of the lens surface at each of these points. ∆Cyl Mean power contour plots indicate the location of the near +1.00 zone, as well as regions of excess Plus power that may interfere with clear distance vision. While plots of surface astigmatism and mean Add power are the most common measures of optical performance, Figure 5. Minkwitz’s theorem states that, in the vicinity of the progressive corridor, the cylinder power lateral to the corridor they fail to represent the combined interaction of these increases twice as fast as the Add power increases along the effects upon vision. Both unwanted cylinder power and corridor—or ∆Cyl ≈ 2 × ∆Add. excess—or insufficient—Add power contribute to blur. RMS (root-mean-square) power combines both the astigmatic The average rate of change in Add power along the and mean power errors into a single measure of power. progressive corridor is equal to the total Add power divided RMS power is a more clinically meaningful measure of by the corridor length of the lens (i.e., ∆Add = Add ÷ optical performance, and a useful predictor of blur and Corridor). This means that the average rate of change in visual acuity.1 Add power is proportional to the Add power and inversely proportional to the corridor length of the lens design. Although less common, it is also possible to characterize Therefore, Minkwitz’s theorem demonstrates two important the optics of a progressive lens using wavefront analysis. guidelines regarding the optics in the central regions of a Wavefront analysis evaluates “higher order” aberrations, progressive lens surface: including spherical aberration and forms of coma, in addition to mean power and astigmatism, which are the 1. The rate of change in cylinder power away from the “lower order” aberrations that are usually more detrimental progressive corridor increases as the length of the to vision. Wavefront aberrations are often represented progressive corridor decreases. This means that lens using a Zernike series, which is a series of mathematical designs with shorter corridors will produce more unwanted cylinder power or smaller viewing zones. Page 2 2. The rate of change in cylinder power away from the harder designs offer the kind of utility current bifocal progressive corridor increases as the Add power of the wearers enjoy (Figure 7). lens increases. This means the unwanted cylinder power in the periphery increases at roughly the same rate as the specified Add power (Figure 6). Close Spacing: Wide Spacing: High Gradients Low Gradients 1 × Astigmatism 3 × Astigmatism “Hard” Type Design “Soft” Type Design Figure 7. Astigmatism plots of “softer” lens designs exhibit more widely spaced cylinder power contours than plots of +1.00 Add +3.00 Add “harder” designs. Figure 6. The surface astigmatism of a progressive lens is proportional to Add power, and the unwanted cylinder power Modern progressive lenses are seldom strictly "hard" or of a +3.00 Add lens is roughly equal to three times the "soft," but instead represent a balance between the two in cylinder power of a +1.00 Add lens. order to achieve better overall utility. This balance may be tuned differently for certain Base curves and/or Add Distribution of Surface Optics powers. The lens designer may also choose to combine both design philosophies. For instance, a softer distance One of the most fundamental aspects of progressive lens periphery may be combined with a harder near periphery in design is the distribution of surface optics, including surface order to improve dynamic distance vision while ensuring a astigmatism and mean Add power. The magnitude, wide field of near vision. distribution, and rate of change of unwanted cylinder power and Add power define the gross performance of the lens Vision and Progressive Lenses design. Progressive lenses are often classified as “hard” or “soft” design based on the distribution of this astigmatism: Progressive lenses must satisfy several visual requirements in order to deliver sufficient performance and • Harder designs.
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