DIMS vs. HAL: Measuring the Difference in Myopia Control Lens Designs

The Engineering Anatomy – Discrete vs. Aspherical Geometries

The optical industry is currently undergoing its most significant paradigm shift in decades: the transition from Vision Correction to Myopia Management. We are no longer simply moving the focal point to the retina; we are actively engineering the peripheral wavefront to retard the elongation of the axial length of the eye.

Two dominant technologies have emerged as the standard-bearers of this revolution: DIMS (Defocus Incorporated Multiple Segments) and HAL (Highly Aspherical Lenslet). While both aim to induce “Myopic Defocus” on the peripheral retina, their geometric structures are radically different. For the optical metrologist, these differences are not merely academic-they dictate two completely different measurement strategies.

This section deconstructs the physical anatomy of these lenses to understand why standard metrology fails and what we are actually trying to measure.

The Shift from Single Vision to Micro-Structures

Traditional single-vision lenses are smooth, continuous surfaces defined by global parameters (Sphere, Cylinder, Axis). A standard lensmeter measures these easily because the wavefront is consistent over a 3mm-5mm aperture.

Myopia control lenses, however, are Micro-Structured Optics. They consist of a central “Clear Zone” (typically 9mm) for distance vision, surrounded by a “Treatment Zone” populated by thousands of microscopic lenslets.

Technical Definition: Myopic Defocus

The physiological mechanism relies on the hypothesis that the eye grows toward the blur. By focusing peripheral light in front of the retina (myopic defocus) rather than behind it (hyperopic defocus), these lenses signal the sclera to stop elongating. The challenge is to provide this defocus signal while simultaneously allowing clear foveal vision.

DIMS: The “Honeycomb” Geometry

DIMS technology (popularized by designs like HOYA’s MiYOSMART) relies on Discrete Spherical Segments.

The Geometry:

Imagine a standard spherical base curve. Now, imagine boring thousands of tiny holes into the mold and filling them with a steeper curvature.

• Structure: A “honeycomb” array of non-contiguous lenslets.
• Lenslet Profile: Each lenslet is a discrete spherical lens with a constant power (typically +3.50D relative to the base correction).
• Fill Factor: The lenslets do not touch. There is a “gap” or transition zone between them that retains the distance correction power. The ratio is typically 50/50 (50% treatment area, 50% correction area).

The Engineering Implication:

From a measurement standpoint, DIMS is a binary surface. As you scan across the lens, the power jumps: Base $\rightarrow$ +3.50D $\rightarrow$ Base $\rightarrow$ +3.50D. This creates a high-frequency square-wave pattern in the power profile.

HAL: The “Volume” Geometry

HAL technology (popularized by designs like Essilor’s Stellest) relies on Contiguous Aspherical Rings.

The Geometry:

Instead of discrete dots, HAL uses concentric rings of lenslets.

• Structure: 11 to 13 concentric rings radiating from the clear zone.
• Lenslet Profile: The lenslets are Highly Aspherical. They do not have a single focal point. Instead, the power varies continuously across the diameter of each tiny lenslet.
• Contiguity: In many HAL designs, the lenslets are contiguous (touching), creating a “Volume of Myopic Defocus.” The light is not focused on a single plane in front of the retina but is spread across a 3D volume of space.

The Engineering Implication:

HAL presents a gradient challenge. The power map is not binary. It is a complex, oscillating wave of asphericity. Measuring the “Add Power” of a HAL lens is difficult because the “Add” is not a single number-it is a slope function.

The “Averaging” Problem in Standard Metrology

Why can’t you just put a DIMS or HAL lens in a standard automated lensmeter?

1. The Aperture Mismatch:

Standard lensmeters use a measurement beam of roughly 3mm to 5mm diameter. A typical DIMS lenslet is approx. 1mm in diameter.

• If the beam hits the treatment zone, it covers approx. 4 lenslets and 3 gaps simultaneously.
• The sensor averages the light from the +3.50D lenslets and the 0.00D gaps.
• Result: The machine reports an erratic, average power (e.g., +1.75D) and massive distortion/scatter, often flagging the lens as “Bad.”

2. The Scattering Effect:

The boundaries of the lenslets act as phase steps or diffraction edges. In a Hartmann-Shack sensor, this scattering destroys the focal spots. In a standard focimeter, the target image becomes blurry and unreadable.

Comparative Analysis: DIMS vs. HAL Structures

The following table summarizes the structural differences that dictate our measurement approach:

Feature	DIMS (Defocus Incorporated Multiple Segments)	HAL (Highly Aspherical Lenslet)
Micro-Structure	Discrete, round islands.	Concentric, contiguous rings.
Lenslet Profile	Spherical (Constant Power).	Aspherical (Gradient Power).
Defocus Type	Single Plane (+3.50D).	Volume of Defocus (Non-linear).
Fill Factor	~50% (Lenslets + Gaps).	High (Contiguous rings).
Metrology Goal	Measure Peak Power & Gap width.	Measure Slope RMS & Gradient.
Primary Defect	“Missing dots” or mold flow lines.	Ring concentricity & axis error.

 

Metrology Strategies – Mapping the Micro-Structures

To measure DIMS and HAL, we must abandon “Point Measurement” and adopt “High-Resolution Surface Mapping.” Technologies like Moiré Deflectometry are uniquely suited for this because they offer high spatial resolution and continuous phase mapping, unlike the discrete sampling of microlens arrays.

Strategy A: Measuring DIMS (The “Island” Approach)

When mapping a DIMS lens, we are essentially performing “Particle Analysis” on an optical surface. The software must identify each island, measure its peak, and verify the space in between.

1. The Power Map Visualization

A high-resolution power map of a DIMS lens looks like a starry night.

• Blue Background: The base curve (Distance Correction).
• Red/Yellow Dots: The lenslets (+3.50D Add).

2. Critical Parameter: Peak-to-Valley Extraction

The QA pass/fail criteria usually require the lenslet to achieve a specific “Add” power.

• The Challenge: Because the lenslet is small, the transition zone (the edge of the dot) has a steep slope. If the measurement pixel size is too large, the edge blurs into the center, lowering the measured peak power.
• The Solution: Use a “Local Maxima” algorithm. The software isolates each dot and finds the single pixel with the highest dioptric value.

3. Critical Parameter: The Fill Factor (The 50/50 Rule)

The efficacy of DIMS relies on the ratio of clear vision to defocused vision.

• Measurement: The software creates a binary mask (Thresholding). All pixels above +1.00D are “Treatment”; all below are “Gap.”
• Calculation: $\text{Fill Factor} = \frac{\text{Area}_{Treatment}}{\text{Total Area}}$
• Failure Mode: If the injection molding pressure is too low, the lenslets might not fill completely, reducing the Fill Factor to 40% and compromising clinical efficacy.

Strategy B: Measuring HAL (The “Gradient” Approach)

Measuring HAL is more complex because the lenslets are aspherical. A simple “Peak Power” measurement is misleading because the lenslet is designed to have variable power.

1. Slope Analysis (Derivative Mapping)

For HAL, we often look at the Slope Map (First Derivative) rather than just the Power Map.

• HAL lenslets create a specific “Ripple” pattern in the slope map.
• We verify the periodicity of the rings. Are they perfectly concentric? Is the spacing between Ring 1 and Ring 2 consistent?

2. Volume of Defocus Quantification

Instead of looking for a single +3.50D number, advanced software integrates the total “Defocus Signal” over the treatment zone.

• Metric: RAE (Relative Added Energy).
• Process: The system integrates the wavefront error of the treatment zone relative to the base curve. It confirms that the total volume of asphericity matches the theoretical design file.

3. Ring Centration

Unlike DIMS, where a slight shift in the honeycomb pattern is negligible, HAL rings are concentric to the optical center.

• Metrology Check: The software finds the geometric center of the concentric rings and compares it to the optical center of the clear zone.
• Tolerance: Typically <0.5mm. Misalignment causes induced prism and astigmatism in the transition zone.

The Resolution Imperative: Avoiding Aliasing

A common mistake in measuring these lenses is Aliasing.

If your metrology sensor has a resolution of 0.5mm (typical for some Hartmann-Shack sensors), and the lenslets are spaced 1.0mm apart, you might encounter the Nyquist Limit.

• The sensor might capture one lenslet but miss the gap, or measure the gap and miss the lenslet.
• Result: A Moiré pattern artifact (a false wavy pattern) appears on the map, which is a Ghost image of the sensor’s inability to resolve the feature.

Pro Tip: The “4-Pixel” Rule

To accurately characterize a micro-lenslet, you need at least 4 measurement points across its diameter.

• Lenslet Diameter: 1.0mm.
• Required Resolution: < 0.25mm per pixel.

Rotlex systems typically offer resolutions down to 0.05mm – 0.1mm, allowing for roughly 10-20 data points per lenslet. This density is non-negotiable for accurate HAL/DIMS metrology.

Workflow: Step-by-Step Mapping Protocol

1. Preparation: Clean the lens thoroughly. Dust looks remarkably like a defective lenslet on a high-res map.
2. Transmission Setup: DIMS/HAL are almost always measured in transmission.
3. Global Scan: Capture the full 40mm-50mm aperture.
4. Region of Interest (ROI) Segmentation:
   • Software automatically detects the central 9mm Clear Zone.
   • Action: Verify Base Power (Sphere/Cyl) in this zone only. Do not include lenslets in the base power calculation.
5. Micro-Structure Analysis:
   • Apply a “High-Pass Filter” to remove the base curve (Sphere/Cyl).
   • The remaining map shows only the micro-structures (the +3.50D bumps).
6. Statistics: Calculate Mean Add, Std Dev of Add, and Fill Factor.

Quality Control Protocols & Production Challenges

In the final part, we move from the lab to the factory floor. Manufacturing DIMS and HAL lenses (usually via injection molding of Polycarbonate or casting of CR-39/MR-8) presents unique challenges that do not exist in standard lens production.

The complexity of the mold inserts-featuring thousands of negative dimples-creates nightmares for yield management.

The “Mold Flow” Challenge

The biggest enemy of micro-structured lenses is Viscosity.

When injecting molten polycarbonate into a DIMS mold, the plastic must flow into thousands of tiny dimples.

• Short Shots: If the pressure is too low or the material cools too fast, the plastic doesn’t reach the bottom of the dimple.
• Result: The lenslet has lower power (e.g., +2.00D instead of +3.50D) or an irregular shape.
• Metrology Sign: The Power Map shows “Weak Dots” at the periphery of the lens (furthest from the injection gate), where pressure is lowest.

The “Void” and “Bubble” Confusion

In casting (MR-8), air bubbles love to get trapped in the mold dimples.

• False Positive: A bubble looks like a high-power lenslet on a map.
• False Negative: A bubble prevents the resin from filling the dimple.
• Discrimination: High-resolution mapping combined with intensity imaging (Shadowgraph) allows the system to distinguish between a Lenslet (refractive feature) and a Bubble (scattering feature).

Venting Issues and the “Diesel Effect”

A subtle but destructive phenomenon unique to DIMS molding is the “Diesel Effect” (or Dieseling). When molten polycarbonate rushes into the mold at high speed, the air trapped inside the thousands of blind micro-dimples must evacuate instantly. If the mold venting is inadequate or clogged, this trapped air cannot escape.

• The Physics: The rushing plastic compresses the trapped air bubble so violently that it superheats (similar to a diesel engine piston). This extreme heat scorches the polycarbonate exactly at the tip of the lenslet.

• Metrology Sign: Unlike a “bubble” which refracts light, a Diesel burn absorbs it. A high-resolution Transmission Map will reveal a tiny dark spot (a sharp drop in transmission amplitude) centered precisely at the peak of the lenslet.

• The Diagnosis: The system must distinguish this from a refractive error. If the Power Map shows a peak but the Transmission Map shows a burn, this is a material defect, not an optical one. It signals an urgent need to clean the mold’s vacuum channels or optimize the venting profile to prevent permanent damage to the expensive insert.

The “Bridging” Defect in HAL Designs

While DIMS lenses suffer from “short shots” in discrete dots, HAL (Highly Aspherical Lenslet) designs face a unique topological failure known as “Bridging.” In contiguous ring designs, the mold features a sharp V-groove to define the boundary between two concentric rings. If the polymer viscosity is too high or packing pressure too low, the plastic fails to reach the bottom of this groove, effectively bridging across the gap and merging two rings into a single plateau.

• The Stealth Failure: Standard Power Maps (First Derivative) often miss this defect because the general refractive curvature of the rings themselves remains correct. The measurement looks “smooth,” which is usually good-but in this case, it’s fatal.

• Detection Strategy: The key is switching the metrology view to a “Curvature Map” (the Second Derivative of the surface). The software algorithm must verify the presence of a sharp negative valley (or zero-crossing) between rings.

• Clinical Impact: The absence of this valley confirms a bridging defect, which collapses the distinct ring structure and compromises the calculated Volume of Myopic Defocus, potentially rendering the lens clinically ineffective.

The Impact of Hard Coatings: Quantifying “Planarization”

A critical blind spot in many production lines is measuring only the uncoated lens. While the molded “semifinite” lens might be geometrically perfect, the subsequent application of Hard Coating (typically 2-4 microns thick) introduces a significant variable: the “Planarization Effect.”

Myopia control micro-structures act like a rough surface to the viscous coating lacquer. Due to surface tension and capillary action, the liquid coating tends to pool in the concave valleys of DIMS lenslets or bridge the gaps between HAL rings.

• The Physics: The coating behaves like a liquid filler, smoothing out the sharp high-frequency features of the micro-structures.
• The Consequence: This physical leveling reduces the effective sagittal depth of the lenslets. Since optical power is a function of curvature, the planarization effectively “flattens” the lenslets, reducing their dioptric power. A lenslet molded at +3.50D might measure only +3.25D after curing.

Best Practice: The Bi-Phasic Protocol To counteract this, engineers must abandon the “measure once” mentality.

1. Bi-Phasic Measurement: Implement a protocol that correlates Pre-Coat measurements with Post-Coat results. Determine the “Planarization Factor” for your specific lacquer viscosity and spin/dip process.
2. Mold Compensation: Do not cut the mold to the final nominal target. If your process consistently loses 0.20D due to coating, you must engineer a Power Offset into the mold insert. For example, target a molded power of +3.70D to ensure the final, coated medical device lands precisely on the +3.50D specification.

The Trap of Micro-Concavities: Why Wiping Fails

A unique operational challenge with DIMS geometries (specifically those with concave lenslets) is that they are essentially dirt traps. Unlike a smooth single-vision lens where a quick wipe with a microfiber cloth clears the aperture, micro-structured lenses resist standard cleaning protocols.

• The Physics of Debris: Polishing slurry, coating overspray, and ambient dust tend to pack tightly inside the 1mm micro-dimples.
• The Wiping Failure: When an operator wipes the lens, the cloth bridges over the top of the dimples due to surface tension and fabric stiffness, leaving the debris packed at the bottom undisturbed.
• Metrology Consequence: To a wavefront sensor or a camera, this dried residue looks identical to a “dead dot,” a bubble, or a scattering defect.

Operational Requirement: Ultrasonic Cavitation Relying on manual wiping for DIMS lenses is a statistical suicide. It often results in a False-Fail Rate exceeding 15% due to “phantom” defects.

Best Practice: The metrology station must be immediately preceded by an Ultrasonic Cleaning bath. The frequency must be tuned to induce cavitation specifically within sub-millimeter features (often requiring a higher frequency sweep) to physically dislodge packed particulates that manual wiping cannot touch. Only a verified clean lens yields a verified power map.

Establishing Pass/Fail Criteria

How do you define a “Good” DIMS lens? It is impossible for every single one of the 3,000 dots to be perfect.

QA Managers must define statistical acceptance levels (AQL).

Sample QA Specification for DIMS:

1. Clear Zone: Must be free of any lenslets or distortion (Diameter > 8.8mm).
2. Lenslet Power: Mean power must be +3.50D ± 0.25D.
3. Lenslet Uniformity: No single lenslet shall deviate by >0.50D from the mean.
4. Defect Density: No more than 5 “Dead Dots” (unfilled or missing) in the central 30mm zone. No clusters of >2 dead dots.

Troubleshooting Matrix: Diagnosing Production Defects

When the metrology system flags a failure, use this guide to find the root cause in the molding machine:

Metrology Symptom	Probable Production Cause	Corrective Action
“Weak” Lenslets (Low Power)	Low injection pressure / Low mold temp.	Increase holding pressure; check mold heater.
“Smeared” Lenslets (Astigmatic)	Ejection stress / Hot demolding.	Increase cooling time before opening mold.
Flow Lines in Clear Zone	Gate positioning / Turbulent flow.	Optimize gate size; check injection speed profile.
Concentricity Error (HAL)	Mold insert misalignment.	Re-center the nickel shim in the mold block.
Variable Power across Lens	Non-uniform mold cooling.	Check water channels in the mold base.

Predictive Maintenance: Tracking Mold Erosion

In the high-volume production of DIMS and HAL lenses, mold inserts are consumable items. However, micro-structured molds rarely fail suddenly; they die a slow, statistical death. The abrasive flow of molten polycarbonate over thousands of cycles gradually erodes the sharp peaks of the nickel shim and rounds the crisp edges of the lenslets.

The “Slope Decay” Strategy Instead of treating quality control as a binary Pass/Fail gate, advanced manufacturers use metrology data for Trend Analysis.

• The Metric: Track the “Mean Peak Slope” (or Gradient Magnitude) of the lenslets for every batch produced.
• The Signal: As the mold wears, the lenslets become shallower and “softer.” You will observe a steady, linear downward trend in the slope values long before the lens actually fails optical tolerances.
• The Action: This data serves as an early warning system. It allows the production manager to predict exactly when the mold will drift out of spec (e.g., “Insert A4 will fail in 3,000 shots”) and schedule replacement during planned maintenance, rather than facing an emergency line stoppage when quality suddenly drops below the ISO threshold.

The Future: DOT and SAL

The industry is not stopping at DIMS and HAL.

• DOT (Diffusion Optics Technology): Uses light scattering micro-dots rather than refractive lenslets to lower contrast. Metrology requires measuring Haze/Scatter rather than Power.
• SAL (Spherical Aberration Management): Lenses with gigantic spherical aberration profiles.

Metrology systems must remain flexible. The “Hard Coded” algorithms of the past (that only looked for Sphere/Cyl) are obsolete. The future belongs to Raw Wavefront Analysis, where the software can be updated to recognize whatever new geometry the optical designers invent next.

Conclusion: Precision as the Treatment

Myopia control lenses are medical devices. A standard spectacle lens corrects vision; a DIMS/HAL lens treats a condition. The stakes are higher. If a child wears a defective myopia control lens for a year, the treatment efficacy is lost, and the eye grows irreversibly.

Therefore, the role of metrology in this segment is critical. It is the guardian of the treatment. By utilizing high-resolution mapping, understanding the specific anatomy of discrete vs. gradient structures, and implementing strict mold-flow validation protocols, manufacturers can ensure that every lens delivers the precise optical signal required to protect the vision of the next generation.

3 Key Takeaways for the Lab Manager

1. Spatial Resolution is King

Do not attempt to measure DIMS/HAL with a standard lensmeter or low-res Hartmann-Shack sensor. You need at least 10-20 pixels per lenslet to accurately verify the optical power.

2. Context-Aware Algorithms

Your software must be smart enough to separate the “Treatment Zone” from the “Clear Zone.” Averaging the two yields useless data. Use ROI (Region of Interest) segmentation.

3. Monitor the “Fill”

In injection molding, the presence of the dots isn’t enough; their shape matters. Monitor the “Peak Power” distribution to detect short-shots and cooling issues before they affect an entire batch.

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.