Introduction: The Same Physics, Different Objectives
Diffractive optics have been used in IOL design for decades. Traditional multifocal IOLs use concentric diffractive ring structures to split incoming light into two or three discrete focal points-typically distance, intermediate, and near. The ring spacing follows the Fresnel zone plate equation, and the step height at each ring boundary determines how light energy is divided between diffractive orders.
Diffractive EDOF IOLs use the same physical structures-concentric rings etched into the lens surface-but with a fundamentally different objective. Instead of creating discrete focal peaks, the diffractive EDOF distributes light energy across a continuous range of defocus positions. The result is a through-focus plateau rather than separate peaks. The difference in clinical outcome is significant: less dysphotopsia from reduced inter-focal scatter, smoother intermediate vision, and a through-focus profile that more closely matches the natural accommodation the lens replaces.
Achieving this distribution requires modifications to both the ring spacing progression and the step height profile relative to traditional multifocal designs. The ring spacing controls where the diffractive power is directed on the defocus axis. The step height controls how much energy each zone contributes to the overall distribution. Together, they shape the through-focus profile with a specificity that the refractive wavefront-shaping approach achieves through aspherical surface modification.
This article examines the design variables that the R&D engineer controls in a diffractive EDOF-ring spacing progression, step height profile, echelette shape-and the measurement feedback loop that connects the physical structure (measured by interferometry) to the optical output (measured by through-focus MTF). The design is not complete until both measurements confirm that the physical structure produces the intended optical performance.
From Multifocal to EDOF: What Changes in the Diffractive Profile
Traditional multifocal: Constant step height, Fresnel zone spacing
A classic bifocal diffractive IOL uses a uniform step height across all ring zones. Each ring boundary introduces the same optical path difference (OPD), typically designed so that the zeroth diffractive order (distance focus) and the first order (near focus) receive the intended energy split. For a 50/50 energy split, the step height produces a half-wave OPD. The ring spacing follows the Fresnel zone plate equation: rₙ² = 2nλ f, where rₙ is the radius of the nth zone, λ is the wavelength, and f is the diffractive focal length corresponding to the add power.
This design produces two sharp focal peaks separated by the add power value, with a valley of reduced MTF between them. The energy distribution is bimodal-concentrated at two points on the defocus axis with relatively little energy in between. This is precisely what the design intends: clear distance vision and clear near vision, with intermediate as a compromise.
Diffractive EDOF: Modified step height and aperiodic spacing
A diffractive EDOF modifies both the step height and the ring spacing to redistribute energy from discrete peaks to a continuous range.
The step height modification is the primary tool. Instead of a constant step height across all zones, the diffractive EDOF uses a step height that varies radially-typically decreasing from center to periphery (apodization) or following a more complex profile that redistributes energy between the zeroth and first orders as a function of radial position. In the central zones, a larger step height directs more energy into the first order (near). In the peripheral zones, a smaller step height directs more energy into the zeroth order (distance). The combined effect is a through-focus profile where the distance peak and near peak are broadened and merged into a single continuous range.
The ring spacing modification is the secondary tool. By departing from the strict Fresnel zone progression-making the spacing slightly aperiodic-the designer can shift where each zone’s diffractive contribution falls on the defocus axis. When all zones contribute at precisely the same add power, the result is a sharp near peak. When zones contribute at slightly different add powers (because their spacing varies), the near peak broadens into a range.
The combination of apodized step height and aperiodic spacing is the engineering recipe for converting a discrete multifocal into a continuous EDOF. The challenge is that both parameters interact: changing the step height profile changes the energy distribution, which interacts with the spacing-induced focus broadening. Optimizing both simultaneously requires iterative simulation and-critically-physical measurement to verify that the manufactured structure matches the design intent.
The Design Variables: What the R&D Engineer Controls
Step height and the energy balance
The step height at each ring boundary determines the phase difference that incoming light experiences at that boundary. For a material with refractive index n in a medium with refractive index nₙ, the OPD is: OPD = step height × (n − nₙ). For common IOL materials in aqueous humor, the RI difference is approximately 0.10–0.12, so a step height of 2.5µm produces an OPD of approximately 0.25–0.30µm-roughly half of the 550nm design wavelength.
In a traditional multifocal, the step height is chosen to set the energy split: a half-wave step height provides a 40/40 split between zeroth and first orders (with ~20% in higher orders). In a diffractive EDOF, the step height is designed to produce an intermediate energy balance that distributes energy between a broadened zeroth order and a smeared first order, creating continuous coverage across the defocus range.
Reducing the step height below the half-wave value shifts more energy into the zeroth order (distance). The first-order contribution weakens but does not disappear-it persists as a low-energy extension toward near. The through-focus profile transitions from two discrete peaks to one dominant peak with a broad shoulder extending toward intermediate distances. Further step height reduction produces an “enhanced monofocal” profile with minimal diffractive contribution.
The step height profile across the radial zones determines the shape of this energy distribution at each pupil size. A monotonically decreasing step height (apodized) concentrates the diffractive contribution in the central zones, making the EDOF effect pupil-dependent. A non-monotonic step height profile (e.g., decreasing in the center, slightly increasing in a middle zone, then decreasing again) can create more complex through-focus shapes for specific clinical objectives.
Ring spacing and the focus distribution
The ring spacing determines the diffractive add power-the defocus distance between the zeroth-order and first-order focal points. For a standard Fresnel zone plate, all rings contribute to the same add power. The first-order focus is a sharp peak.
By making the ring spacing aperiodic-varying the zone widths systematically across the aperture-the designer distributes the first-order contributions across a range of add powers. Central zones with slightly wider spacing contribute at a lower effective add power (closer to distance). Peripheral zones with slightly narrower spacing contribute at a higher effective add power (closer to near). The sum of all zone contributions is a broadened first-order focus that, when combined with the broadened zeroth-order from apodized step heights, produces the continuous EDOF range.
The magnitude of the aperiodicity determines the breadth of the distribution. A 5% variation in zone width broadens the first-order peak by approximately 0.3–0.5D. A 10% variation broadens it by approximately 0.5–1.0D. Beyond 15% variation, the diffractive efficiency drops significantly as the zones become too far from resonance.
Echelette profile shape
The transition between step levels-the echelette profile-affects both the diffractive efficiency and the scatter characteristics. An ideal echelette has a sharp step at each ring boundary and a smooth curved surface between boundaries. Manufacturing reality produces a rounded step transition with a finite width.
The rounding radius at the step transition scatters a fraction of the light into non-designed directions, creating stray light that contributes to halos and glare. For traditional multifocals, this scatter adds to the inherent dysphotopsia from the bimodal energy distribution. For diffractive EDOF-which is marketed partly on reduced dysphotopsia relative to multifocal-scatter from imperfect echelettes is a more significant design concern because it undermines the clinical positioning.
Table 1: Diffractive EDOF Design Variables and Their Through-Focus Effects
| Design Variable | Typical Range | Through-Focus Effect | Manufacturing Sensitivity | Measurement Method |
| Step height (central zone) | 1.0–3.5µm depending on material RI and design wavelength | Controls energy balance between distance and extended range. Higher step = more energy in first order = broader extension. | High: ±0.1µm error shifts energy balance by 3–5% | MCT-3000: LCI measurement of step height with ±1.0µm accuracy |
| Step height apodization (radial profile) | Monotonic decrease from center to edge; ratio of center-to-edge height typically 2:1 to 5:1 | Shapes the pupil-dependent through-focus profile. Steeper apodization = more pupil-dependent EDOF effect. | Moderate: profile shape error changes pupil dependency | MCT-3000: multi-zone thickness profiling to verify apodization gradient |
| Ring count | 8–20 rings within 6mm optic (design-dependent) | More rings = higher diffractive efficiency but tighter spacing tolerance. Fewer rings = more robust but less efficient. | Low: ring count is a design choice, not a manufacturing variable | IOLA MFD: power map resolves individual ring positions |
| Ring spacing aperiodicity | 0–15% deviation from Fresnel zone spacing | Broadens the first-order focus distribution. 5% → 0.3–0.5D broadening. 10% → 0.5–1.0D broadening. | Moderate: spacing errors compound with designed aperiodicity | IOLA MFD: high-density power map reveals ring spacing from diffractive modulation pattern |
| Echelette rounding radius | 0.5–5.0µm (manufacturing-dependent) | Larger rounding = more scatter = reduced efficiency + increased dysphotopsia. Critical for EDOF’s low-dysphotopsia positioning. | High: directly affects light scatter and halo energy | MCT-3000: transition zone profiling at each step boundary |
| Design wavelength | 546nm (standard green) or 550nm; some designs optimize for 555nm (photopic peak) | Determines the wavelength at which the OPD is exactly as designed. Off-wavelength performance introduces chromatic through-focus effects. | Low: fixed by design, not a manufacturing variable | IOLA MFD: monochromatic measurement at specified wavelength |
[Note: Step height values and their effects depend on the specific material RI, the measurement medium RI, and the design wavelength. The ranges given are representative for hydrophobic acrylic IOLs in aqueous humor at 546nm. Verify against your specific material and design parameters.]
The Measurement Feedback Loop: Structure → Optics → Correction
The diffractive EDOF design exists in two domains: the physical domain (step heights, ring positions, echelette profiles) and the optical domain (through-focus MTF, energy distribution, wavefront profile). The two domains are connected by diffraction physics, but manufacturing errors in the physical domain do not always produce predictable effects in the optical domain because of the nonlinear interaction between ring zones.
This is why the measurement feedback loop requires two instruments measuring two different aspects of the same lens, with the results correlated to close the design loop.
Physical structure measurement: MCT-3000
The MCT-3000 uses Low Coherence Interferometry to measure the physical structure of the diffractive surface with ±1.0µm accuracy. For diffractive EDOF, the MCT-3000 provides three critical measurements.
Step height at each ring boundary. The MCT-3000 measures the actual step height at each transition, resolving up to 20 layers within the lens structure. For a diffractive EDOF with 12 rings, the system measures all 12 step heights and compares each to the design specification. A step height error of 0.2µm at one ring changes the local energy distribution for that zone.
Apodization profile verification. By measuring step heights across the radial profile, the MCT-3000 verifies that the designed apodization-the systematic variation of step height from center to edge-has been correctly executed. A flat apodization (all steps equal) when the design calls for center-to-edge decrease means the lens will behave more like a multifocal than an EDOF.
Echelette transition width. At each step boundary, the MCT-3000 resolves the transition profile-the rounding radius that determines how sharp the step is. A measurement of the transition width at representative ring boundaries confirms that the machining or molding process has produced steps that are sharp enough to achieve the designed diffractive efficiency.
Optical performance measurement: IOLA MFD
The IOLA MFD captures the complete wavefront transmitted through the lens and computes the through-focus MTF that represents the optical output of the diffractive structure. For diffractive EDOF, the IOLA MFD provides the functional verification that the physical structure produces the intended optical performance.
Through-focus plateau verification. The through-focus MTF confirms whether the apodized step height and aperiodic ring spacing have produced the intended continuous range. The plateau width, minimum MTF, center position, and symmetry are all evaluated against the design reference.
Diffractive ripple assessment. Diffractive structures inherently produce periodic modulations in the through-focus MTF. In a traditional multifocal, these modulations create the distinct peaks and valleys. In a diffractive EDOF, the design aims to minimize these modulations to produce a smooth plateau. Residual ripples within the plateau indicate imperfect energy distribution-possibly from step height errors or spacing deviations.
Multi-aperture performance. The IOLA MFD computes through-focus MTF at any aperture from a single wavefront capture. For diffractive EDOF with apodized step heights, the pupil-dependent behavior is a critical design characteristic. The through-focus profile at 3mm (where the central high-step zones dominate) differs from the profile at 4.5mm (where the peripheral low-step zones contribute). Both must match the design intent.
Power map ring structure. The high-density power map from the IOLA MFD can resolve the diffractive ring pattern as periodic power modulations superimposed on the base refractive power. The ring spacing can be verified from the power map by measuring the radial period of these modulations. Any missing ring, doubled ring, or spacing anomaly appears as a deviation in the power map pattern.
Closing the loop: Structure-to-optics correlation
The design loop is closed when the MCT-3000 physical measurements and the IOLA MFD optical measurements are correlated for the same lens.
If the through-focus plateau is narrower than designed: check the MCT-3000 step height data. If central step heights are low, the diffractive contribution is reduced. If the apodization is flatter than designed, the pupil-dependent energy distribution is wrong.
If the through-focus shows residual ripples: check the MCT-3000 ring spacing. If specific rings are mispositioned, the energy from those zones contributes at the wrong defocus position, creating a local peak or dip in the through-focus curve.
If the through-focus is correct at 3mm but collapses at 4.5mm: check the MCT-3000 peripheral step heights. If the peripheral zones have excessive step height (not apodized enough), they contribute too much diffractive energy at large pupils, creating near-focus ghosting. If the peripheral zones have zero step height (over-apodized), they contribute only refractive (monofocal) energy, eliminating the EDOF effect at large pupils.
Table 2: Structure-to-Optics Diagnostic Correlation
| Optical Finding (IOLA MFD) | Physical Cause (MCT-3000) | Design or Manufacturing Issue | Corrective Action |
| Plateau narrower than designed at all apertures | Step heights uniformly below design; all zones under-delivering OPD | Manufacturing: mold depth too shallow, or material RI lower than nominal (reducing OPD) | Increase mold step depth; verify material lot RI; remeasure after correction |
| Residual through-focus ripples within the plateau | Individual ring step heights deviating from design profile; some zones over-height, others under-height | Manufacturing: non-uniform mold wear or polishing across zones | Identify specific rings with step height error; rework mold at those radial positions |
| Good plateau at 3mm, collapses at 4.5mm | Peripheral step heights below design; apodization over-executed; outer zones contributing monofocal only | Manufacturing: peripheral mold zones machined too shallow, or design: apodization too aggressive for clinical target | If manufacturing: correct peripheral mold depth. If design: reduce apodization slope to maintain peripheral diffractive contribution. |
| Excessive halo energy around point source | Echelette transition rounding radius too large; step boundaries not sharp enough | Manufacturing: tool rounding or mold polishing has rounded the step transitions | Improve step transition sharpness; inspect mold tooling; consider alternative machining approach for step boundaries |
| Near-focus ghost peak on EDOF through-focus curve | One or more ring zones have step height close to half-wave; those zones are operating as traditional multifocal rather than EDOF | Manufacturing: specific ring step heights not apodized; or design: apodization profile has a local step height peak at that radial position | Identify zones with near-half-wave step height from MCT-3000 data; reduce step height at those zones to eliminate the discrete near peak |
| Through-focus shifted asymmetrically toward near | Ring spacing systematically compressed (narrower than designed); effective add power higher than intended | Manufacturing: mold shrinkage or thermal contraction during molding compressed the ring pattern | Apply shrinkage compensation to the mold design; verify ring spacing under production thermal conditions |
| Low overall MTF but correct plateau shape | Step heights correct, spacing correct, but echelette surface roughness elevated; micro-scatter within each zone | Manufacturing: surface finish within diffractive zones below specification; polishing or molding surface quality insufficient | Improve surface finish within zones; inspect mold surface quality; reduce surface roughness without affecting step height |
Design Optimization: Balancing Range, Efficiency, and Dysphotopsia
The diffractive EDOF designer navigates a three-way tradeoff: wider through-focus range requires more aggressive diffractive modification (larger step heights, more aperiodicity), which increases diffractive scatter and reduces peak MTF. The optimization is finding the combination that maximizes usable range while keeping dysphotopsia below the clinical threshold and peak MTF above the regulatory threshold.
Range vs efficiency
Diffractive efficiency-the fraction of light that contributes to the designed through-focus distribution rather than to stray scatter-decreases as the design departs from the ideal Fresnel zone plate. Aperiodic spacing reduces the coherent addition of zone contributions, lowering the peak efficiency. Reduced step heights shift energy toward the zeroth order but do not completely eliminate first-order contributions-resulting in residual diffractive energy that is too low to create a useful near focus but high enough to create stray light.
The practical consequence: there is a minimum step height below which the diffractive contribution transitions from “useful focal extension” to “pure scatter.” Finding this threshold for your specific material and ring geometry requires iterative measurement. The MCT-3000 confirms the physical step heights. The IOLA MFD confirms whether the optical output at those step heights is useful extension or scatter.
Chromatic considerations
Diffractive structures are inherently wavelength-dependent: the OPD at each step varies with wavelength. This chromatic dispersion is opposite in sign to the chromatic dispersion of the refractive lens material-a property that traditional multifocals exploit for chromatic aberration correction.
For diffractive EDOF, the chromatic behavior adds complexity. At the design wavelength (typically 546nm), the through-focus profile matches the simulation. At shorter wavelengths (blue light), the diffractive contribution shifts toward higher add power-extending the through-focus range but at reduced efficiency. At longer wavelengths (red light), the diffractive contribution shifts toward lower add power-narrowing the effective range. The polychromatic through-focus profile is the weighted sum of these wavelength-dependent contributions.
The measurement implication: monochromatic bench testing at 546nm provides the design-wavelength performance. Polychromatic performance-which is what the patient experiences-must either be simulated from the monochromatic wavefront data (using known material dispersion) or measured directly under broadband illumination. The ISO 11979-2 standard specifies measurement at defined wavelengths; polychromatic evaluation is an R&D-level analysis that supplements the regulatory measurement.
Manufacturing Methods and Their Structure Signatures
The manufacturing method used to create the diffractive structure determines what types of structure errors are likely-and therefore what the measurement feedback loop should focus on.
Diamond turning (lathe-cut)
Direct machining of the diffractive rings into the lens surface using a diamond turning tool. This method produces sharp step transitions but is limited in minimum feature size by the tool radius. Step height accuracy is typically ±0.1–0.2µm. Ring spacing accuracy depends on the CNC positioning accuracy. The characteristic error signature: tool marks within the diffractive zones (mid-spatial frequency roughness) and slight rounding at each step boundary proportional to the tool nose radius.
Mold-based (cast or injection)
The diffractive structure is created in a precision mold, and lenses are cast or injection-molded. The mold carries the negative of the diffractive pattern. This method enables high-volume production but introduces mold-specific error signatures: shrinkage-induced ring spacing compression, mold surface degradation over production cycles, and replication fidelity limits at the step transitions. The MCT-3000 measurement of step heights should be performed on both the mold (using the reflected-wavefront capability) and the finished lens to separate mold errors from replication errors.
Hybrid approaches
Some manufacturers machine the diffractive structure into a mold insert using diamond turning, then produce lenses by casting. The error signature combines the diamond turning characteristics (tool marks, step rounding) with the molding characteristics (shrinkage, replication fidelity). The measurement feedback loop must address both the mold quality (MCT-3000 measurement of the mold insert) and the lens quality (IOLA MFD through-focus verification of the finished lens).
Iteration: From First Prototype to Production Design
Diffractive EDOF design converges through an iterative loop that alternates between simulation and measurement.
Iteration 1: Simulation-driven prototype. The initial design is optimized in simulation. The ring spacing, step height profile, and echelette shape are defined. The mold or lathe program is generated. The first prototype lenses are produced.
Iteration 2: Structure measurement. The MCT-3000 measures the physical structure of the prototype. Step heights, spacing, and transition widths are compared to the design. Deviations are documented. If the physical structure does not match the design, the manufacturing process is adjusted before optical testing begins-there is no point evaluating the optical output of a structure that was not correctly fabricated.
Iteration 3: Optical measurement. The IOLA MFD measures the through-focus MTF, wavefront profile, and power map of the prototype with verified physical structure. The through-focus profile is compared to the simulation prediction. Any gap between simulation and measurement is analyzed.
Iteration 4: Gap analysis. If the optical output does not match the simulation despite the physical structure matching the design, the simulation model requires refinement-typically incorporating manufacturing realities like echelette rounding, surface roughness within zones, or material dispersion that the simulation idealized. If the physical structure deviates from design and the optical output reflects those deviations, the manufacturing process requires adjustment.
Iteration 5: Refined design or refined process. Based on the gap analysis, either the design is modified (adjusted step height profile, modified spacing, refined echelette specification) or the process is improved (sharper tooling, tighter mold control, better surface finish). New prototypes are produced and the loop repeats.
The typical design converges in 3–5 iterations when both the MCT-3000 structure verification and the IOLA MFD optical verification are available in the loop. Without structure verification, the designer cannot distinguish design inadequacy from manufacturing inadequacy, and the iteration count increases significantly.
Conclusion
Diffractive EDOF design is multifocal design with a different objective function. The same ring structures that create discrete focal peaks in a multifocal create a continuous focal range in an EDOF-when the step height profile and ring spacing are modified to redistribute energy from peaks to plateau.
The design variables-step height, apodization profile, ring spacing aperiodicity, and echelette shape-interact in ways that simulation predicts approximately and that only measurement verifies definitively. The physical structure (step heights, ring positions, transition profiles) is measured by interferometry. The optical output (through-focus plateau, energy distribution, dysphotopsia indicators) is measured by wavefront analysis and through-focus MTF.
The two measurements answer different questions. The structure measurement answers: “Did manufacturing build what the designer specified?” The optical measurement answers: “Does what manufacturing built produce the intended optical performance?” Both questions must be answered affirmatively before the design is validated. The structure measurement without optical verification confirms fabrication accuracy but not optical function. The optical measurement without structure verification confirms that the lens works but not why-making root cause analysis impossible when the next prototype does not.
The diffractive structure is the hardware. The through-focus plateau is the software output. The MCT-3000 reads the hardware. The IOLA MFD reads the output. The designer who uses both closes the loop in three iterations. The designer who uses only one closes it in ten-if at all.
Disclaimer: This document is intended for educational use only. It does not represent legal, regulatory, or certification advice, and should not be interpreted as a declaration of compliance or approval by Rotlex or any regulatory authority. Diffractive design parameters, step height values, and efficiency estimates are illustrative and depend on specific material properties, design wavelength, and manufacturing method.
