A practical guide for quality engineers and production managers using Rotlex optical measurement systems
Beyond the Green Light and Red Light
Every Rotlex measurement system delivers a clear verdict: Pass or Fail. A green indicator means the lens meets specifications and can proceed to the next production stage or final packaging. A red indicator means the lens falls outside acceptable tolerances and requires action.
But that binary result is only the beginning of the story.
Behind every Pass/Fail determination lies detailed measurement data that reveals not just whether a lens failed, but why it failed and what production variables likely caused the deviation. Quality engineers who understand how to interpret this data transform their measurement systems from simple gatekeepers into powerful diagnostic tools that drive continuous process improvement.
Rotlex systems-including the IOLA series for intraocular lenses, Contest series for contact lenses, and FFV/Mapper/SMC+ for spectacle lenses-generate comprehensive measurement outputs. The software compares each measured parameter against user-defined acceptance criteria, automatically flagging lenses that fall outside tolerances. This guide explains how to interpret those results, understand what each type of failure indicates, and take appropriate corrective action.
How Pass/Fail Determination Works in Rotlex Systems
User-Defined Acceptance Criteria
Rotlex systems do not impose fixed pass/fail thresholds. Instead, the software allows you to define custom acceptance criteria appropriate for each product specification. This flexibility accommodates the wide range of lens types, regulatory requirements, and quality standards across the ophthalmic industry.
When configuring acceptance criteria, you typically define tolerances for:
Primary optical parameters:
- Sphere power tolerance (e.g., ±0.25 D for standard IOLs, ±0.12 D for premium designs)
- Cylinder power tolerance (e.g., ±0.25 D)
- Axis tolerance for toric lenses (e.g., ±2° or ±5°)
- Addition power tolerance for multifocal designs
Quality metrics:
- MTF threshold at specified spatial frequency (e.g., >0.43 at 100 lp/mm per ISO 11979-2)
- Wavefront RMS limits
- Surface quality indicators
Geometric parameters:
- Center thickness tolerance
- Diameter tolerance
- Optical center location
Automatic Comparison and Flagging
During measurement, the system captures all relevant parameters and automatically compares each value against your pre-set tolerances. The software generates:
Overall Pass/Fail status: A single determination based on all criteria
Parameter-specific flags: Individual pass/fail status for each measured parameter
Deviation values: The numerical difference between measured value and specification
Detailed measurement record: Complete data for trending, analysis, and regulatory documentation
This automated approach ensures consistent, objective quality decisions regardless of operator. The same criteria apply to every lens, eliminating subjective judgment and ensuring repeatability.
Categories of Measurement Outcomes
Understanding measurement outcomes requires recognizing that failures fall into distinct categories, each with different implications for production and different corrective actions.
Category 1: Primary Parameter Failures
These failures indicate the lens does not meet its fundamental optical specification-the basic prescription is wrong.
Sphere Power Failure
When measured sphere power falls outside tolerance, the lens focuses light at the wrong distance. This is the most fundamental optical failure.
Typical tolerance: ±0.25 D to ±0.50 D depending on power range and regulatory requirements
Measurement systems: IOLA 4C, IOLA MP, IOLA MFD (for IOLs); Contest 2, Contest MP (for contact lenses)
Accuracy reference: IOLA systems deliver 0.04 D repeatability; Contest systems deliver 0.03 D repeatability
What sphere power failure indicates:
If measured power is consistently high or low across multiple lenses:
- CNC generator calibration drift
- Incorrect tool radius compensation
- Material batch variation (refractive index)
- Temperature affecting cutting parameters
If measured power varies randomly between lenses:
- Inconsistent blocking or fixturing
- Material inhomogeneity
- Process instability
Cylinder Power Failure
Cylinder (astigmatism) failures indicate the lens has incorrect astigmatic correction or unwanted astigmatism.
Typical tolerance: ±0.25 D for toric lenses; <0.12 D for spherical lenses
What cylinder failure indicates:
For toric lenses with incorrect cylinder:
- Wrong toric parameters entered
- Toric surface misaligned
- Incomplete or incorrect toric cutting
For spherical lenses showing cylinder:
- Warping from mechanical stress during processing
- Uneven curing in molded lenses
- Blocking wax shrinkage causing distortion
- Chuck pressure creating three-point stress (appears as trefoil in wavefront)
Axis Failure (Toric Lenses)
Axis errors indicate the astigmatic correction is oriented incorrectly, even if the cylinder magnitude is correct.
Typical tolerance: ±2° to ±5° depending on cylinder magnitude (tighter for higher cylinders)
What axis failure indicates:
- Misalignment during marking or printing
- Rotation during processing
- Incorrect orientation in holding fixtures
- Axis detection algorithm issues (rare with Rotlex automatic detection)
Category 2: MTF and Image Quality Failures
MTF (Modulation Transfer Function) failures indicate the lens cannot deliver adequate image contrast, even if primary parameters are within specification. This is the scenario described in Rotlex documentation as “the quality control paradox”-a lens that passes power testing but fails MTF.
ISO 11979-2 Requirement for IOLs: MTF > 0.43 at 100 lp/mm with 3 mm aperture
What MTF failure with correct power indicates:
The lens has hidden defects that power testing cannot detect. Rotlex wavefront-based systems identify the specific cause through Zernike decomposition:
Spherical aberration (Zernike Z₄⁰): Light from the lens periphery focuses at a different distance than light from the center. Creates halos. Caused by incorrect conic constant, radius error, or thermal expansion during cutting.
Coma (Zernike Z₃¹): Front and back surfaces are decentered relative to each other. Creates asymmetric blur. Caused by misaligned collet, uneven polymer curing, or wedge error.
Astigmatism (Zernike Z₂²): Present even in non-toric lenses when surfaces are warped. Caused by clamping stress, blocking wax shrinkage, or uneven chuck pressure.
Trefoil (Zernike Z₃³): Three-fold symmetric distortion. Caused by three-point clamping mechanism applying uneven force.
High-order aberrations / scatter: Surface roughness or mid-spatial frequency errors scatter light, reducing contrast at high spatial frequencies. Caused by tool wear, lathe chatter, or polishing defects.
Category 3: Geometric Parameter Failures
These failures indicate physical dimensions are outside specification, even if optical parameters are acceptable.
Center Thickness Failure
Measured using MCT-3000 with ±1 µm accuracy.
Typical tolerance: ±0.02 mm for IOLs; ±0.01 mm for contact lenses
What thickness failure indicates:
- Incorrect blank thickness
- Over-cutting or under-cutting
- Material hydration state different from specification (for hydrophilic materials)
- Swelling or shrinkage from environmental conditions
Diameter Failure
What diameter failure indicates:
- Incorrect blank size
- Edge processing errors
- Hydration state affecting overall size
Optical Center Decentration
What decentration failure indicates:
- Blocking misalignment
- Cutting not centered on optical axis
- Marking/printing offset from true optical center
Category 4: Surface Quality Failures
These failures indicate surface defects that may not affect primary optical parameters but degrade overall quality.
Surface Roughness / Scatter
Detected through MTF degradation at high spatial frequencies while low frequencies remain acceptable.
What surface quality failure indicates:
- Diamond tool wear or damage
- Lathe vibration (chatter marks)
- Polishing issues (orange peel effect)
- Contamination during processing
Localized Defects
Detected through power or cylinder maps showing isolated anomalies.
What localized defects indicate:
- Inclusions in material
- Coating defects
- Surface damage from handling
- Center nipple artifact from lathe cut-off
Interpreting MTF Curve Patterns
For systems that provide full MTF curves (IOLA MFD, Contest 2), the shape of the MTF failure provides diagnostic information before examining wavefront data.
Pattern: Low MTF Across All Spatial Frequencies
The entire curve is depressed from low frequencies to high.
Dominant cause: Defocus or spherical aberration-the basic lens shape is wrong.
Wavefront signature: Elevated Z₂⁰ (defocus) or Z₄⁰ (spherical aberration).
Production action: Recalibrate CNC generator. Verify tool radius compensation. Check for thermal drift during cutting.
Pattern: MTF Acceptable at Low Frequencies, Drops at High Frequencies
Low and medium frequency MTF is within specification, but performance falls sharply approaching 100 lp/mm.
Dominant cause: Surface finish issues-overall shape is correct but micro-roughness scatters light.
Wavefront signature: Elevated high-order aberrations without single dominant low-order mode.
Production action: Replace diamond tool. Check spindle vibration. Optimize polishing parameters.
Pattern: Sagittal and Tangential Curves Diverge
MTF is acceptable in one meridian but degraded in the perpendicular direction.
Dominant cause: Astigmatism from surface warping or mechanical stress.
Wavefront signature: Elevated Z₂² (astigmatism).
Production action: Check blocking process for uneven wax shrinkage. Reduce chuck pressure. For toric lenses, verify axis alignment throughout the process chain.
Pattern: Asymmetric MTF Across Field Positions
MTF varies unpredictably or shows asymmetric behavior.
Dominant cause: Decentration or tilt between optical surfaces.
Wavefront signature: Elevated Z₃¹ (coma).
Production action: Re-align collet and spindle. Check mold halves alignment. Verify lens centering in all fixtures.
Pattern: Three-Fold Symmetric Distortion
Spot diagram shows triangular rather than circular blur.
Dominant cause: Uneven stress from three-point clamping.
Wavefront signature: Elevated Z₃³ (trefoil).
Production action: Inspect lens holder for uneven force distribution. Ensure all contact points apply equal pressure.
Through-Focus MTF Interpretation for Multifocal IOLs
For multifocal, trifocal, and EDOF IOLs, the IOLA MFD provides through-focus MTF analysis showing performance across the designed focal range.
Expected Through-Focus Pattern
A properly manufactured multifocal IOL shows distinct MTF peaks at designed focal positions:
Far peak: Distance vision, typically at 0 D defocus
Intermediate peak: Computer/arm’s length vision (EDOF and trifocal designs)
Near peak: Reading vision, typically at +2.5 D to +3.5 D add
Through-Focus Failure Patterns
Missing or weak peak: One focal zone is not achieving design MTF.
Indicates: Zone geometry error, diffractive structure defect, or incorrect ring spacing (for diffractive multifocals).
Peak at wrong position: Focal zone exists but at incorrect vergence.
Indicates: Base power error shifting all zones, or incorrect add power.
All peaks reduced equally: Global degradation affecting all zones.
Indicates: Surface quality issue (roughness, scatter) affecting entire lens.
Asymmetric peak shapes: Individual peaks have unusual width or shape.
Indicates: Zone transition errors or localized aberrations within specific zones.
Contact Lens Specific Failure Interpretation
Contest 2 and Contest MP systems measure contact lenses with 0.03 D repeatability and provide complete power maps. Contact lens failures have unique characteristics related to their flexible materials and hydration requirements.
Base Curve Failures
Base curve (BC) tolerance is critical for fit and comfort.
Regulatory minimum: ±0.20 mm (FDA, ISO 18369-3, CE); ±0.10 mm (JIS)
Industry best practice: ±0.05 mm for standard lenses; ±0.03 mm for toric/multifocal; ±0.02 mm for scleral/custom
What base curve failure indicates:
- Material hydration state different from specification
- Temperature affecting polymer properties (0.01-0.02 mm per 10°C)
- Mold wear
- Post-cure settling
- Lathe cutting parameter drift
Hydration-Related Failures
For hydrophilic contact lenses measured in wet conditions at 35°C (simulating on-eye temperature), failures may relate to hydration state rather than manufacturing defects.
Measurement environment: Contest systems use temperature-stabilized saline at 35°C.
What hydration-related failure indicates:
- Insufficient equilibration time before measurement
- Storage solution contamination or incorrect composition
- Material batch variation in water content
- Environmental temperature or humidity affecting lens before measurement
Zone Power Failures (Multifocal Contact Lenses)
For multifocal contact lenses, power maps show center-near or center-distance designs with specified zone powers and diameters.
What zone power failure indicates:
- Incorrect zone diameter
- Transition zone profile error
- Add power outside specification
- Zone decentration
Spectacle Lens Failure Interpretation
FFV, Mapper, and SMC+ systems provide comprehensive power mapping for progressive, free-form, and specialty spectacle lenses.
Progressive Lens Specific Failures
Progressive lenses present unique interpretation challenges because they contain intentional power variation and inherent peripheral astigmatism.
Corridor Quality Failure
The progressive corridor should maintain low astigmatism (<0.25 D) to allow smooth transition between distance and near zones.
Failure threshold: Maximum corridor astigmatism >0.25 D indicates blocked corridor.
What corridor failure indicates:
- Manufacturing error in corridor zone
- Surface form deviation from design
- Design file mismatch (wrong design loaded)
Near Vision Zone Positioning Error
The optical near zone should align with the marked near reference point.
Failure threshold: Positioning error >1.5 mm requires review.
What positioning failure indicates:
- Marking/printing error
- Surfacing offset from intended position
- Design mounting error
Far Vision Field Width
The usable distance zone should provide specified clear width.
Typical values: Excellent >35 mm; Good 28-35 mm; Acceptable 20-28 mm; Narrow <20 mm
What narrow far field indicates:
- Design selection issue (inherent design characteristic)
- Manufacturing pushing astigmatism into distance zone
- Form error affecting distance zone
Design File Comparison
Rotlex systems can compare measured power maps directly against design files, distinguishing design characteristics from manufacturing deviations.
Deviations present in both design and measurement: Design characteristics, not manufacturing defects
Deviations present only in measurement: Manufacturing error requiring corrective action
Systematic Troubleshooting Framework
When a lens fails, use this systematic approach to identify root cause and corrective action.
Step 1: Identify the Failing Parameter
Review the measurement report to determine which specific parameter(s) caused the failure:
- Primary optical (sphere, cylinder, axis, addition)
- MTF / image quality
- Geometric (thickness, diameter, centration)
- Surface quality
Step 2: Determine Failure Pattern
Is the failure:
Systematic (consistent across multiple lenses): Indicates calibration, setup, or material issue affecting all production
Random (varies between lenses): Indicates process instability, handling variation, or inconsistent fixturing
Drifting (gradually worsening over time): Indicates tool wear, thermal drift, or environmental change
Step 3: Check Environmental Factors
Before investigating production process, verify measurement conditions:
- Temperature within specified range (18-28°C optimal for most systems)
- Humidity within range (30-70% RH)
- No recent environmental excursion
- Daily verification completed and passed
Step 4: Correlate with Wavefront Data (if available)
For systems providing wavefront analysis, identify dominant Zernike modes:
| Dominant Mode | Aberration | Production Cause |
| Z₂⁰ | Defocus | Radius/power error |
| Z₂² | Astigmatism | Warping, stress, toric error |
| Z₃¹ | Coma | Decentration, wedge |
| Z₃³ | Trefoil | Three-point stress |
| Z₄⁰ | Spherical | Conic constant error |
| High-order | Scatter | Surface roughness |
Step 5: Implement Corrective Action
Based on identified cause, take appropriate action:
For calibration/setup issues: Recalibrate equipment, verify settings, re-enter parameters
For tooling issues: Replace worn tools, verify tool geometry, check spindle alignment
For material issues: Verify material batch, check storage conditions, confirm hydration state
For process issues: Review process parameters, check fixturing, verify operator procedures
Step 6: Verify Correction
After corrective action, measure additional lenses to confirm the issue is resolved. Document the failure, root cause, corrective action, and verification results.
Configuring Effective Acceptance Criteria
The effectiveness of Pass/Fail determination depends on properly configured acceptance criteria.
Regulatory Requirements
Start with applicable regulatory requirements as minimum thresholds:
IOLs (ISO 11979-2):
- Sphere: labeled power ±0.30 D (powers ≤15 D) to ±0.40 D (powers >25 D)
- Cylinder: ±0.25 D
- Axis: ±5° (cylinders <1.5 D) to ±2° (cylinders >1.5 D)
- MTF: >0.43 at 100 lp/mm with 3 mm aperture
Contact Lenses (ISO 18369-3):
- Power: ±0.25 D to ±0.50 D depending on power
- Base curve: ±0.20 mm
- Diameter: ±0.20 mm
Internal Quality Targets
Many manufacturers set tighter internal tolerances than regulatory minimums to ensure margin and support premium product claims:
Example internal IOL tolerances:
- Sphere: ±0.12 D (versus ±0.30 D regulatory)
- Repeatability: 0.04 D (matching IOLA system capability)
- MTF: >0.50 at 100 lp/mm (versus 0.43 regulatory)
Product-Specific Adjustments
Different products may require different tolerance configurations:
Premium/Toric/Multifocal lenses: Tighter tolerances for axis, MTF, zone powers
High-volume disposable products: Tolerances matched to regulatory requirements for cost efficiency
Custom/Specialty products: Application-specific tolerances based on intended use
Documentation and Data Management
Measurement Record Retention
Rotlex systems automatically log every measurement with timestamp, lens identification, and complete parameter data. This supports:
Traceability: Link measurement records to specific production batches, lots, and serial numbers
Trending: Track parameter trends over time to detect drift before failures occur
Investigation: Access historical data when investigating field returns or quality issues
Regulatory compliance: Maintain records required by FDA 21 CFR Part 820, ISO 13485, and other quality system standards
Export and Integration
Measurement data can be exported in multiple formats:
- TXT files for simple data transfer
- Excel reports for analysis
- SQL database integration for quality management systems
- API connections for automated data flow to MES/LIMS
CFR 21 Part 11 compliant electronic signatures and audit trails support regulated environment requirements.
Statistical Analysis
Track key statistics over time:
- Mean and standard deviation of measured parameters
- Cp and Cpk process capability indices
- Failure rates by category and cause
- Trend analysis for drift detection
Set control limits (UCL, LCL) to identify when processes approach tolerance boundaries before failures occur.
Common Questions About Pass/Fail Interpretation
Why does my lens pass power but fail MTF?
Power testing measures only low-order aberrations (where light focuses). MTF responds to all optical imperfections including high-order aberrations and surface roughness. A lens with correct power but surface quality issues, decentration, or spherical aberration will pass power testing but fail MTF.
How do I know if a failure is a measurement error or a real defect?
Remeasure the lens. If the failure repeats consistently, it’s a real defect. If results vary significantly, investigate measurement conditions (environmental factors, lens positioning, cleanliness). Rotlex motion-free technology provides exceptional measurement stability, so true measurement errors are rare when systems are properly maintained.
What causes random failures that don’t show a pattern?
Random variation typically indicates process instability: inconsistent fixturing, handling variation between operators, material inhomogeneity, or environmental fluctuations. Review process control, operator training, and environmental monitoring.
Should I reject a lens that barely fails one parameter?
Follow your documented acceptance criteria consistently. If a lens fails by a small margin, the proper action is rejection per your criteria-then investigate whether your tolerances are appropriately set. Accepting marginally failing lenses undermines the entire quality system.
How often should I review my acceptance criteria?
Review annually at minimum, and whenever you introduce new products, change processes, or receive field feedback. Criteria that were appropriate when established may need adjustment as products, regulations, or quality expectations evolve.
Connecting Measurement Outcomes to Process Improvement
The ultimate value of understanding Pass/Fail results is using that knowledge to improve manufacturing processes, reduce scrap, and deliver better products.
Building a Failure Database
Track every failure with:
- Measured values and deviations
- Identified root cause (when determined)
- Corrective action taken
- Verification results
Over time, this database reveals patterns: which failure modes are most common, which are most costly, which respond to specific corrective actions.
Prioritizing Improvement Efforts
Focus improvement efforts on failures that:
- Occur most frequently
- Have highest cost impact
- Are most challenging to rework or salvage
- Affect product categories with highest margin or strategic importance
Predictive Quality Control
Advanced use of measurement data enables predictive rather than reactive quality control:
- Monitor parameter trends to detect drift before it causes failures
- Correlate upstream measurements (mold inspection, material testing) with final product outcomes
- Use statistical process control to maintain processes within capability
Summary: From Data to Action
Rotlex measurement systems provide comprehensive data for objective, consistent Pass/Fail determination. But the true value emerges when quality teams understand what those results mean and how to act on them.
Key principles for effective Pass/Fail interpretation:
Configure criteria appropriately: Set tolerances that match regulatory requirements, product specifications, and quality objectives.
Understand failure categories: Distinguish primary parameter failures from MTF/quality failures from geometric failures from surface issues.
Use wavefront data diagnostically: Zernike decomposition transforms MTF failures from mysteries into actionable production feedback.
Recognize patterns: The shape of failure (systematic, random, drifting) guides root cause investigation.
Document and trend: Maintain complete records, track statistics, and detect problems before they become failures.
Close the loop: Connect measurement outcomes to process improvements that prevent future failures.
With proper interpretation, every measurement-Pass or Fail-contributes to better understanding of your production process and drives continuous improvement in product quality.
Disclaimer: This document is intended for educational and operational guidance. It does not replace official Rotlex documentation or training. For specific regulatory requirements, consult with your quality assurance team and relevant regulatory authorities.
