Introduction: Meeting the Measurement Challenge of Modern Myopia-Control Lenses
The global myopia epidemic has transformed the ophthalmic lens industry. With nearly 30% of the world’s population currently affected by myopia – and projections reaching 50% by 2050 – manufacturers are racing to develop innovative lens designs that don’t just correct vision but actively slow myopia progression. These myopia-control lenses represent some of the most optically complex spectacle lenses ever manufactured, featuring hundreds or even thousands of distinct optical zones working together to create therapeutic effects.
This complexity creates an unprecedented measurement challenge. Traditional lens measurement instruments, designed to verify simple single-vision or progressive lenses, are entirely inadequate for myopia-control designs. A conventional focimeter measures power at a single point – but how do you verify a lens containing 400 individual micro-lenses, each requiring its own power verification? How do you ensure that peripheral defocus zones are positioned correctly when their therapeutic effect depends on sub-millimeter accuracy?
The answer lies in Moiré deflectometry, a wavefront-sensing technology that captures complete optical power distribution across the entire lens surface in a single measurement. This article explores the principles of Moiré deflectometry, examines the unique challenges of myopia-control lens measurement, and demonstrates how this technology enables manufacturers to verify the complex optical designs that are reshaping vision care.
The Myopia Crisis and the Rise of Control Lenses
Understanding the Myopia Epidemic
Myopia, commonly known as nearsightedness, has reached epidemic proportions worldwide. In East Asia, prevalence among young adults exceeds 80% in some populations. Beyond the inconvenience of requiring vision correction, high myopia (greater than -6.00D) significantly increases the risk of sight-threatening conditions including retinal detachment, glaucoma, and myopic macular degeneration.
This has driven intense research into interventions that can slow myopia progression during childhood, when the condition typically develops and worsens. While pharmaceutical approaches (such as low-dose atropine) show promise, optical interventions – specially designed spectacle and contact lenses – have emerged as practical, widely accessible solutions.
How Myopia-Control Lenses Work
Myopia-control lenses exploit a phenomenon called peripheral defocus. Research has shown that the peripheral retina plays a crucial role in regulating eye growth. When peripheral images focus behind the retina (hyperopic defocus), signals are sent that stimulate eye elongation – worsening myopia. Conversely, when peripheral images focus in front of the retina (myopic defocus), eye growth slows.
Traditional spectacle lenses correct central vision but often create hyperopic defocus in the periphery, potentially accelerating myopia progression. Myopia-control lenses are designed to provide clear central vision while simultaneously creating myopic defocus in the periphery, sending “stop growing” signals to the eye.
Types of Myopia-Control Lens Designs
Several distinct approaches to creating peripheral myopic defocus have reached the market:
DIMS (Defocus Incorporated Multiple Segments): Pioneered by Hoya in the MiYOSMART lens, DIMS technology incorporates a honeycomb array of tiny lenslets (approximately 400 segments) surrounding a central clear zone. Each lenslet adds +3.50D of defocusing power, creating a consistent myopic defocus pattern across the peripheral visual field. Clinical studies have demonstrated approximately 60% reduction in myopia progression.
DOT (Defocus Incorporated Technology): Developed by Essilor for the Stellest lens, DOT technology uses concentric rings of aspherical micro-lenses surrounding the central optical zone. These “highly aspherical lenslets” (HAL) create a volume of myopic defocus in front of the retina while maintaining clear central vision.
Peripheral Defocus Designs: Various manufacturers offer lenses with gradually increasing positive power from center to periphery, creating a smooth defocus gradient rather than discrete segments. These designs may be easier to manufacture but require precise control of the power distribution profile.
Extended Depth of Focus (EDOF): Some designs use extended depth of focus principles to create simultaneous myopic defocus at multiple distances, potentially providing myopia control while also reducing accommodative demand.
Each design presents unique measurement challenges that require full-surface power mapping rather than traditional point measurements.
The Measurement Challenge: Why Traditional Methods Fail
Limitations of Conventional Lens Measurement
Traditional spectacle lens measurement relies on focimeters (lensmeters) that measure optical power at discrete points. For a single-vision lens with uniform power, measuring the optical center provides sufficient verification. For progressive lenses, measuring distance, near, and fitting reference points captures the essential parameters.
Myopia-control lenses fundamentally break this paradigm:
Hundreds of optical zones: A DIMS lens contains approximately 400 individual lenslets, each with its own optical power. Measuring a single point – or even a dozen points – captures only a tiny fraction of the lens’s optical structure.
Sub-millimeter positioning requirements: The effectiveness of myopia control depends on the precise positioning of defocus zones relative to the pupil. Zone positioning errors of less than 1mm can significantly impact therapeutic effect.
Complex power gradients: Peripheral defocus designs require verification of power distribution across the entire lens surface, not just at discrete points.
Quality consistency: Every lens must match the design specification. Manufacturing variations that would be acceptable in conventional lenses may render myopia-control lenses ineffective.
The Inadequacy of Sampling-Based Approaches
Consider attempting to verify a DIMS lens using a multi-point measurement system that samples 20 locations across the lens surface. With 400+ lenslets, each sample point might happen to fall on a lenslet, between lenslets, or partially on both – creating highly variable and unreliable results. Even if every measurement is accurate, the sampling approach captures only 5% of the optical structure.
For meaningful verification of myopia-control lenses, manufacturers need technology that can:
- Capture the complete power distribution across the entire lens surface
- Resolve individual micro-lens elements with sub-millimeter spatial resolution
- Measure with sufficient accuracy to verify therapeutic power levels (typically ±0.03D)
- Complete measurements fast enough to enable 100% production inspection
Moiré deflectometry meets all of these requirements.
Understanding Moiré Deflectometry
The Moiré Effect
The Moiré effect is a visual phenomenon that occurs when two repetitive patterns (such as grids or gratings) are overlaid at a slight angle or with slightly different spacing. The interaction creates a new pattern of light and dark bands – Moiré fringes – that are much larger and more visible than the original fine patterns.
What makes Moiré fringes remarkable for metrology is their extreme sensitivity to small changes. A tiny shift in the spacing or angle of one grating produces a large, easily measurable change in the Moiré pattern. This amplification effect transforms subtle optical variations into clearly visible fringe shifts.
From Fringes to Power Maps
In Moiré deflectometry for lens measurement, the system exploits this sensitivity to detect how a lens bends light across its surface. The measurement principle works as follows:
Wavefront distortion: When collimated (parallel) light passes through a lens, the wavefront becomes curved according to the lens’s optical power. Higher power regions bend light more strongly, creating steeper wavefront slopes.
Grating interaction: The distorted wavefront passes through a pair of precision Ronchi gratings – fine parallel line patterns with precisely controlled spacing. The wavefront slope at each point determines how the light interacts with the gratings.
Fringe formation: The interaction between the distorted wavefront and the gratings creates Moiré fringe patterns. The fringe spacing and orientation at each location directly encode the local wavefront slope – and therefore the local optical power.
Single-shot capture: A high-resolution camera captures the complete fringe pattern in a single exposure, typically lasting only 20-50 milliseconds.
Algorithmic reconstruction: Sophisticated Fourier transform algorithms analyze the fringe pattern to extract the wavefront slope at every pixel location. Mathematical integration then converts slope data to optical power, producing a complete power map of the lens.
Technical Advantages of Moiré Deflectometry
Full-surface measurement: Every point across the lens surface is measured simultaneously, capturing the complete optical structure including all micro-lenses, zones, and gradients.
High spatial resolution: Modern implementations achieve spatial resolution better than 0.1mm, sufficient to resolve individual micro-lenses in DIMS and DOT designs.
Sub-diopter accuracy: Power measurement accuracy of ±0.02D to ±0.03D ensures that therapeutic power levels can be verified with confidence.
Motion-free operation: Because all data is captured in a single exposure with no moving parts, Moiré deflectometry systems maintain calibration stability over months or years without mechanical drift.
Speed: Complete measurement in seconds enables 100% inspection at production line rates.
Non-contact: The optical measurement does not touch the lens surface, eliminating any risk of damage to coatings or surface treatments.
Motion-Free Technology: The Foundation of Reliable Measurement
Why Motion-Free Matters
Moiré deflectometry is inherently a motion-free measurement technique. Unlike phase-shifting interferometry that requires precise mechanical movement of reference mirrors through fractional wavelengths, or scanning systems that translate samples across measurement fields, Moiré deflectometry captures complete data in a single static exposure.
This distinction has profound implications for production measurement:
Calibration stability: With no moving parts to drift, wear, or require adjustment, Moiré deflectometry systems maintain their calibration accuracy over extended periods. Annual verification typically confirms continued accuracy rather than requiring recalibration.
Reduced maintenance: Motors, actuators, bearings, and encoders are primary sources of maintenance requirements and failure modes in measurement equipment. Eliminating them dramatically improves reliability and reduces operating costs.
Environmental robustness: Motion-free systems are inherently less sensitive to vibration, temperature changes, and other environmental factors that affect mechanical positioning accuracy.
Consistent accuracy: Every measurement is captured under identical conditions, eliminating the measurement-to-measurement variation that can occur when mechanical systems don’t return precisely to their nominal positions.
The Motion-Free Advantage in Production
For manufacturers inspecting thousands of lenses per day, the practical benefits of motion-free operation are substantial:
Higher uptime: Systems that don’t require frequent recalibration or mechanical maintenance remain productive longer.
Lower operating costs: Reduced service requirements and spare parts consumption translate directly to lower cost per measurement.
Consistent quality data: Stable measurement systems provide reliable process feedback for continuous improvement.
Simplified validation: Quality system documentation is simpler when measurement systems maintain consistent performance over time.
Rotlex Solutions for Myopia-Control Lens Measurement
The SMC+ System
Rotlex has pioneered the application of Moiré deflectometry in ophthalmic lens measurement. The SMC+ (Surface Mapping Complete) system represents the state of the art for myopia-control lens verification, combining ultra-high resolution imaging with advanced analysis algorithms specifically developed for complex lens designs.
Measurement capability: The SMC+ captures over 500,000 measurement points across the lens surface in a single 16-second acquisition. This density is sufficient to resolve individual micro-lenses in DIMS designs (typically 1.03mm diameter) and to characterize the aspherical lenslet rings in DOT technology.
Accuracy: Power measurement accuracy of ±0.03D ensures reliable verification of both the central prescription zone and the peripheral defocus elements. For myopia-control lenses where the therapeutic effect depends on +3.00D to +3.50D defocus power, this accuracy provides confidence that every lens will perform as designed.
Analysis tools: Dedicated software algorithms detect and characterize micro-lens arrays, verify zone positioning relative to the optical center, calculate statistical distributions of lenslet powers, and compare measured power maps against design specifications.
Pass/fail determination: Automated analysis determines whether each lens meets specification, with detailed reporting of any deviations for process feedback.
Technical Specifications
| Parameter | SMC+ Specification |
| Measurement points | >500,000 per lens |
| Spatial resolution | <0.1mm |
| Power accuracy | ±0.03D |
| Power range | -20D to +20D |
| Cylinder range | 0 to 10D |
| Measurement time | 16 seconds |
| Lens diameter range | Up to 80mm |
| Data output | SQL, Excel, custom API |
Complementary Systems
While the SMC+ provides the ultra-high resolution required for myopia-control verification, Rotlex offers additional systems for comprehensive spectacle lens measurement:
FFV (Free-Form Verifier): Optimized for rapid verification of progressive and free-form lenses, the FFV completes measurements in just 4 seconds with ±0.02D accuracy. For manufacturers producing both conventional progressive lenses and myopia-control designs, the FFV handles high-volume progressive verification while the SMC+ addresses the specialized requirements of myopia-control products.
Mapper: Provides detailed power mapping for single-vision, bifocal, aspheric, and progressive lenses with production-oriented speed and comprehensive defect detection.
All Rotlex systems share the motion-free Moiré deflectometry foundation, ensuring consistent accuracy and reliability across the product line. Common data formats enable integrated quality management across multiple measurement stations.
Practical Implementation: From Laboratory to Production
Development and Design Verification
During lens development, Moiré deflectometry provides the detailed characterization needed to optimize designs and verify that manufacturing processes achieve design intent.
Design correlation: Complete power maps can be directly compared against optical design files, identifying any systematic differences between designed and manufactured lens performance.
Process development: As manufacturing parameters are refined, power mapping shows exactly how changes affect the optical result, enabling data-driven process optimization.
Design iteration: Rapid measurement feedback accelerates the development cycle, allowing more design iterations within project timelines.
Production Quality Control
In production, Moiré deflectometry enables quality control approaches that would be impossible with traditional measurement methods:
100% inspection: With measurement times of 16 seconds or less, complete inspection of every lens is economically viable even at substantial production volumes. This eliminates the statistical uncertainty of sampling-based quality programs.
Immediate feedback: Real-time power map data feeds directly into statistical process control systems, enabling rapid detection and correction of process drift before significant quantities of non-conforming product are produced.
Defect classification: The complete power map enables detailed classification of any defects – not just detection that something is wrong, but identification of what is wrong and often why.
Traceability: Complete measurement records for every lens support quality system requirements and enable investigation of any field issues.
Data Visualization and Interpretation
The power maps produced by Moiré deflectometry provide intuitive visualization of lens optical performance:
Color-coded power display: Optical power distribution is displayed using color mapping, with cool colors (blues) typically representing lower powers and warm colors (reds, oranges) indicating higher powers. For a well-made DIMS lens, this visualization immediately reveals the clear central zone surrounded by the regular honeycomb pattern of defocusing micro-lenses.
Statistical analysis: Summary statistics characterize the distribution of micro-lens powers, central zone uniformity, and zone boundary sharpness – all critical parameters for myopia-control effectiveness.
Trend visualization: Plotting measurement trends over time reveals process stability and enables prediction of maintenance or adjustment needs before they cause quality issues.
Comparison with Alternative Technologies
Hartmann-Shack Wavefront Sensing
Hartmann-Shack sensors use an array of micro-lenses to divide the incoming wavefront into spots, whose positions indicate local wavefront slope. While effective for many applications, Hartmann-Shack faces limitations for myopia-control lenses:
Limited resolution: Typical Hartmann-Shack sensors contain 1,000-2,000 micro-lenses, providing far fewer measurement points than Moiré deflectometry’s 500,000+. This resolution may be insufficient to fully characterize micro-lens arrays.
Spot overlap issues: High-power micro-lenses in myopia-control designs can cause Hartmann-Shack spots to overlap or merge, corrupting the measurement.
Dynamic range limitations: The combination of low central power and high peripheral defocus power can exceed the dynamic range of some Hartmann-Shack systems.
Phase-Shifting Interferometry
Phase-shifting interferometry achieves excellent accuracy and resolution but faces practical limitations:
Mechanical requirements: Phase shifting requires precise movement of optical elements, introducing the maintenance, calibration, and stability issues that motion-free systems avoid.
Environmental sensitivity: Multiple sequential measurements are required, increasing sensitivity to vibration and environmental changes.
Speed: The need for multiple phase-shifted images limits measurement speed compared to single-shot Moiré deflectometry.
Point Focimeters and Lensmeters
Traditional focimeters measure power at single points, making them fundamentally unsuitable for myopia-control lenses:
Sampling inadequacy: Measuring a few points cannot characterize hundreds of micro-lenses.
No spatial information: Point measurements provide no information about zone positioning or power distribution.
Operator dependency: Manual positioning introduces variability that undermines measurement repeatability.
Technology Comparison Summary
| Capability | Moiré Deflectometry | Hartmann-Shack | Phase-Shifting | Point Focimeter |
| Measurement points | >500,000 | 1,000-2,000 | High | 1-3 |
| Spatial resolution | <0.1mm | 0.3-0.5mm | <0.1mm | N/A |
| Power accuracy | ±0.02-0.03D | ±0.05D | ±0.01D | ±0.06D |
| Measurement time | 4-16 seconds | 1-5 seconds | 10-30 seconds | 10-30 seconds |
| Moving parts | None | None | Required | Manual |
| Micro-lens array suitability | Excellent | Limited | Good | Unsuitable |
| Production integration | Excellent | Moderate | Limited | Poor |
Quality System Integration
Data Management
The SMC+ provides comprehensive data management capabilities to support modern quality system requirements:
Automated data capture: Every measurement is automatically recorded with timestamp, system identification, and complete power map data. Manual transcription is eliminated.
Database integration: Direct connection to SQL databases enables integration with Laboratory Information Management Systems (LIMS), Manufacturing Execution Systems (MES), and Enterprise Resource Planning (ERP) systems.
Statistical process control: Built-in SPC capabilities monitor measurement trends, alerting operators to process drift before out-of-specification product is produced.
Supporting Customer Quality Requirements
The SMC+ provides features that support customers in meeting their regulatory and quality system obligations:
Data integrity: Secure user authentication, electronic signatures, and tamper-evident audit trails help manufacturers meet electronic records requirements within their regulatory frameworks.
Measurement traceability: Calibration using certified reference standards enables customers to establish measurement traceability chains as part of their quality system documentation.
Complete documentation: Automated reporting generates inspection certificates, batch summaries, and quality records that support quality management system requirements.
Note: These features support customers’ compliance efforts. Regulatory certification and approval remain the responsibility of the lens manufacturer.
Future Developments
Evolving Lens Designs
Myopia-control lens technology continues to advance rapidly. Emerging designs include:
Customized defocus patterns: Rather than one-size-fits-all defocus zones, future lenses may incorporate personalized patterns based on individual patient eye geometry and myopia progression patterns.
Combined functionality: Lenses that integrate myopia control with other features such as blue light filtering, photochromic response, or enhanced contrast.
Higher-density micro-lens arrays: Designs with smaller, more numerous micro-lenses may provide more uniform peripheral defocus coverage.
Adaptive designs: Lens designs that vary defocus patterns based on viewing distance or gaze direction.
Each evolution will place additional demands on measurement technology, requiring even higher resolution and more sophisticated analysis.
Advancing Measurement Capability
Moiré deflectometry continues to evolve to meet these challenges:
AI-assisted analysis: Machine learning algorithms can detect subtle patterns in power maps that indicate manufacturing issues, potentially identifying problems before they result in out-of-specification product.
Faster acquisition: Continued improvements in camera technology and processing speed enable even shorter measurement times for higher production throughput.
Enhanced visualization: Advanced rendering techniques make power map interpretation more intuitive, supporting rapid operator decisions.
Design integration: Direct links between measurement systems and optical design software enable automatic comparison of manufactured lenses against design specifications.
Frequently Asked Questions
What is the advantage of Moiré deflectometry over Hartmann-Shack wavefront sensing for myopia-control lenses? Moiré deflectometry provides significantly higher spatial resolution (>500,000 measurement points versus 1,000-2,000 for typical Hartmann-Shack systems), enabling complete characterization of micro-lens arrays. It also avoids the spot overlap and dynamic range issues that can affect Hartmann-Shack measurements of high-power peripheral defocus zones.
Can Moiré deflectometry measure all types of myopia-control lenses? Yes. The technology is suitable for DIMS designs with discrete micro-lens arrays, DOT designs with aspherical lenslet rings, continuous peripheral defocus gradients, and other emerging designs. The full-surface measurement approach captures whatever optical structure is present.
How does measurement resolution affect myopia-control lens verification? Spatial resolution determines whether individual optical features can be resolved and measured. For DIMS lenses with micro-lenses approximately 1mm in diameter, resolution better than 0.3mm is required to characterize each lenslet. The SMC+’s resolution below 0.1mm provides substantial margin for current and future designs.
What training is required to operate an SMC+ system? Standard operator training requires approximately one day and covers system operation, measurement procedures, and basic troubleshooting. The intuitive user interface and automated analysis minimize training requirements for routine production use. Advanced training covers customized analysis methods and integration with quality systems.
How do Moiré measurements correlate with clinical outcomes? While clinical efficacy depends on many factors including patient compliance, the optical performance verified by Moiré deflectometry – particularly the magnitude and distribution of peripheral defocus – directly determines the myopia-control signal presented to the eye. Lenses that meet design specifications for defocus power and zone positioning are expected to provide the clinical efficacy demonstrated in design validation studies.
How long does the SMC+ maintain calibration accuracy? The motion-free design maintains calibration stability over extended periods. Annual verification is recommended under normal operating conditions and typically confirms continued accuracy rather than requiring adjustment.
Summary
The emergence of myopia-control lenses represents both a therapeutic breakthrough and a manufacturing challenge. These complex optical devices – featuring hundreds of micro-lenses, precisely positioned defocus zones, and demanding tolerances – require measurement capabilities far beyond traditional lens verification methods.
Moiré deflectometry meets this challenge by capturing complete optical power distribution across the entire lens surface in a single, rapid, non-contact measurement. With spatial resolution sufficient to characterize individual micro-lenses and accuracy adequate to verify therapeutic power levels, this technology enables the quality control that myopia-control lens manufacturing demands.
The motion-free implementation pioneered by Rotlex provides additional advantages in calibration stability, reliability, and operating cost that matter for production environments. The SMC+ system combines these foundational capabilities with specialized analysis tools developed specifically for myopia-control designs, delivering a complete solution for this demanding application.
As myopia-control technology continues to evolve – with more sophisticated designs, higher micro-lens densities, and personalized defocus patterns – measurement technology must keep pace. The algorithmic foundation of Moiré deflectometry enables capability expansion through software evolution rather than hardware redesign, providing a path to meet future challenges.
For manufacturers committed to delivering effective myopia-control lenses, investment in appropriate measurement technology is not optional – it is essential for ensuring that every lens provides the therapeutic effect that patients and practitioners expect.
