Electronics Guide

Fizeau Interferometry

Fizeau interferometers represent the most common method for surface figure testing in optical manufacturing. The technique compares a test surface against a reference surface by analyzing the interference pattern formed when monochromatic light reflects from both surfaces. Modern Fizeau interferometers use phase-shifting algorithms to extract quantitative surface data from multiple interferograms captured at different reference positions.

Key specifications for Fizeau systems include measurement accuracy (typically lambda/20 or better for high-quality instruments), repeatability, spatial resolution, and dynamic range. Different configurations address various surface geometries: transmission spheres for concave surfaces, transmission flats for planar surfaces, and custom nulling optics for aspheric surfaces.

Twyman-Green Interferometry

The Twyman-Green interferometer offers advantages for testing complete optical systems and components with varying geometries. Its separated reference and test arms provide flexibility in accommodating different test configurations. While requiring more complex alignment than Fizeau systems, Twyman-Green interferometers excel in situations where the test surface cannot be positioned adjacent to a reference surface.

Shack-Hartmann Wavefront Sensing

Shack-Hartmann sensors measure wavefront slope across an aperture using a lenslet array that samples the wavefront at discrete points. The displacement of each focused spot from its reference position indicates local wavefront tilt. Integration of slope data yields the complete wavefront shape. These sensors offer high dynamic range and insensitivity to vibration, making them valuable for production testing environments.

Subaperture Stitching

Large optical surfaces often exceed the aperture of available interferometers. Subaperture stitching techniques address this limitation by measuring overlapping regions across the surface and computationally combining data into a full-aperture map. Sophisticated algorithms compensate for positioning errors and reference surface imperfections to achieve stitched results approaching single-measurement accuracy.

White Light Interferometry

White light interferometric microscopes achieve nanometer-scale vertical resolution for surface roughness measurement. By scanning through focus and analyzing the coherence envelope of interference fringes, these instruments measure surface height at each pixel. Large fields of view enable statistical characterization of surface texture across representative areas.

Atomic Force Microscopy

Atomic force microscopy (AFM) provides the highest resolution surface roughness measurement, capable of detecting sub-nanometer features. A sharp probe scanning across the surface maps topography with resolution limited primarily by probe tip geometry. While measurement areas are small compared to optical techniques, AFM reveals fine surface structure invisible to other methods.

Stylus Profilometry

Contact stylus profilometers remain valuable for roughness measurement despite being a mature technology. A diamond-tipped stylus traces across the surface while a transducer measures vertical displacement. Advantages include absolute vertical calibration, insensitivity to material optical properties, and ability to measure steep slopes. Care must be taken to avoid surface damage on soft optical materials.

Power Spectral Density Analysis

Power spectral density (PSD) analysis provides a comprehensive view of surface roughness across spatial frequency. By transforming surface height data into the frequency domain, PSD reveals how roughness amplitude varies with feature size. This information is essential for predicting scattering behavior, as different spatial frequencies scatter light at different angles.

Zernike Polynomial Decomposition

Zernike polynomials provide a mathematical basis for describing wavefront errors over circular apertures. Decomposing measured wavefronts into Zernike terms separates aberrations into familiar categories: defocus, astigmatism, coma, spherical aberration, and higher-order terms. This decomposition facilitates comparison with design tolerances and identification of manufacturing error sources.

Peak-to-Valley and RMS Wavefront Error

Peak-to-valley (PV) wavefront error reports the maximum deviation from the ideal wavefront, while root-mean-square (RMS) error provides a statistical measure of overall wavefront quality. For many imaging applications, RMS error better predicts system performance because isolated peaks have less impact than distributed errors. However, PV specifications remain important for applications sensitive to worst-case distortions.

Transmitted Wavefront Testing

Transmitted wavefront testing measures the combined effect of surface errors, material inhomogeneity, and index variations on light passing through an optical element. Double-pass configurations with a return mirror simplify alignment while measuring twice the actual wavefront error. Single-pass measurements require additional reference elements but provide direct wavefront data.

Measurement Principles

MTF describes the ratio of image contrast to object contrast for sinusoidal patterns at each spatial frequency. At low frequencies, good optical systems transfer nearly all contrast (MTF approaching 1). As frequency increases, diffraction and aberrations reduce contrast transfer until MTF reaches zero at the cutoff frequency. The shape of the MTF curve characterizes system performance more completely than simple resolution metrics.

Slanted Edge Method

The slanted edge technique extracts MTF from the image of a sharp, slightly tilted edge. Oversampling provided by the tilt enables computation of the edge spread function at finer resolution than the detector pixel spacing. Differentiation yields the line spread function, and Fourier transformation produces the MTF. This efficient method requires minimal test equipment and provides accurate results when properly implemented.

Test Target Methods

Traditional MTF measurement using bar targets or sinusoidal patterns directly images periodic structures at known frequencies. Analyzing contrast in the image at each frequency builds up the MTF curve. While more time-consuming than slanted edge methods, target-based testing can reveal field-dependent performance variations and asymmetric aberrations.

Production MTF Testing

High-volume production requires rapid MTF testing without sacrificing accuracy. Automated systems combine precision positioning, consistent illumination, and efficient algorithms to test hundreds of lenses per hour. Statistical analysis across production lots identifies process variations before they produce out-of-specification parts.

Spectrophotometry

Spectrophotometers measure transmission or reflection as a function of wavelength, typically covering ultraviolet through infrared ranges. Double-beam configurations compare sample signals to reference signals, compensating for source intensity variations. Key specifications include wavelength accuracy, photometric accuracy, and baseline stability.

Coating Characterization

Optical coatings require detailed spectral characterization to verify performance against design specifications. Measurements must address both normal and angular incidence, as many coating types exhibit significant spectral shifts with angle. Spectral measurements during coating deposition enable real-time process control and endpoint detection.

Spectral Bandwidth and Linewidth

For narrow-band filters and laser components, precise measurement of spectral bandwidth is critical. High-resolution spectrometers or tunable laser sources characterize transmission peaks and blocking bands with sub-nanometer precision. Specifications may include center wavelength, peak transmission, bandwidth at various transmission levels, and out-of-band rejection.

Polarimetry Techniques

Polarimeters measure the complete polarization state of light, typically expressed as Stokes parameters or Jones vectors. Rotating analyzer polarimeters sweep through polarization angles while recording intensity. Division-of-amplitude and division-of-aperture designs enable simultaneous measurement without moving parts. Mueller matrix polarimeters fully characterize how samples transform arbitrary input polarization states.

Extinction Ratio Measurement

Extinction ratio quantifies polarizer performance as the ratio of transmitted intensity for aligned versus crossed polarization states. High-quality polarizers achieve extinction ratios exceeding 10,000:1, requiring careful measurement technique to distinguish component performance from measurement system limitations.

Retardance Measurement

Retardance measurements characterize waveplates and other birefringent components. The phase difference between orthogonal polarization components determines retardance, typically specified in degrees or fractions of a wavelength. Accurate measurement requires attention to wavelength dependence, field angle effects, and temperature sensitivity.

M-Squared Measurement

The beam quality factor M-squared (M2) compares actual beam propagation to an ideal Gaussian beam. An M2 of 1.0 indicates perfect Gaussian behavior, while higher values indicate degraded focusability. Measurement requires characterizing beam width at multiple positions through a focus, then fitting propagation equations to extract M2. ISO 11146 standardizes measurement procedures.

Beam Profiling

Beam profilers capture the intensity distribution across a beam cross-section using camera-based detection or scanning techniques. Analysis software extracts beam diameter, centroid position, ellipticity, and higher-order mode content. Real-time profiling enables laser alignment and monitoring of beam stability over time.

Far-Field and Near-Field Analysis

Near-field measurements characterize beam profiles at or near the source, revealing aperture effects and mode structure. Far-field measurements examine the beam after sufficient propagation for diffraction to dominate, indicating angular divergence and beam quality. Comparing near-field and far-field provides complete characterization of beam propagation.

Scratch-Dig Standards

The scratch-dig specification system defined in MIL-PRF-13830 grades surface defects using standardized comparison samples. Scratch numbers relate to apparent width under specified illumination, while dig numbers correspond to diameter in hundredths of a millimeter. Modern automated systems provide more consistent and quantitative results than manual inspection against comparison standards.

Dark Field Inspection

Dark field illumination reveals surface defects by detecting scattered light against a dark background. Only light scattered by defects reaches the detector, providing high sensitivity for small scratches and particles. Automated dark field systems scan surfaces while image processing algorithms classify defects by size, type, and severity.

Confocal Scanning

Confocal microscopy provides three-dimensional defect characterization with high resolution. Optical sectioning capability distinguishes surface defects from subsurface damage and bulk inclusions. Automated confocal scanning enables systematic inspection of surfaces and bulk material for defects at all depths.

Automated Vision Inspection

Machine vision systems combine camera-based imaging with sophisticated algorithms to detect, classify, and grade defects at production speeds. Training on reference samples establishes acceptance criteria, while statistical tracking identifies process variations causing increased defect rates. Integration with manufacturing execution systems enables closed-loop process control.

Total Integrated Scatter

Total integrated scatter (TIS) measures the fraction of incident light scattered into all angles by a surface. Integrating sphere measurements capture scattered light from all directions, enabling rapid comparative assessment of surface quality. TIS correlates with surface roughness through well-established theoretical relationships.

BSDF Measurement

Bidirectional scatter distribution function (BSDF) measurements characterize the angular distribution of scattered light. Goniometric systems position detectors at multiple angles while illuminating samples from controlled directions. The resulting BSDF data enables accurate stray light modeling for optical system design.

Scatter Specification and Control

Scatter specifications derive from system stray light budgets, allocating allowable scatter among all surfaces contributing to unwanted background. Manufacturing process optimization targets root causes of scatter: surface roughness, subsurface damage, particulate contamination, and coating defects. Cleanliness protocols during handling and assembly protect surfaces from contamination-induced scatter.

Measurement Techniques

Stress birefringence measurements use polarimetry to detect phase differences between orthogonal polarization states induced by stressed materials. Polariscopes with crossed polarizers reveal stress patterns as colored or intensity variations. Quantitative measurements determine retardance magnitude and orientation at each point across an optical element.

Sources and Mitigation

Stress birefringence arises from thermal gradients during glass melting and annealing, mounting forces in optical assemblies, and environmental temperature variations during use. Careful annealing schedules minimize residual stress in optical materials. Mount designs that minimize clamping forces and accommodate thermal expansion reduce assembly-induced stress. Material selection considers stress-optical coefficient when birefringence requirements are stringent.

Optical Monitoring

Optical monitoring during deposition tracks coating thickness in real time by measuring transmission or reflection changes as layers grow. Turning point detection identifies thickness corresponding to extrema in the monitored signal. Direct optical monitoring provides inherent compensation for deposition rate variations.

Ellipsometry

Ellipsometry measures the change in polarization state when light reflects from or transmits through thin films. By analyzing amplitude ratio and phase difference between orthogonal polarizations, ellipsometry determines both thickness and optical constants of coating layers. Spectroscopic ellipsometry measures across wavelength, enabling characterization of complex multilayer structures.

Profilometry Across Step Edges

Step height measurement using stylus or optical profilometry provides direct thickness determination for witness samples with masked regions. The height difference between coated and uncoated regions equals coating thickness. This destructive technique validates optical measurements and provides calibration references.

Refractometry

Prism refractometers measure refractive index by analyzing the critical angle at a glass-sample interface or the minimum deviation angle through a prism sample. Abbe refractometers provide convenient measurement for liquids and solids with moderate accuracy. High-precision spectrometer configurations achieve index accuracy to the fifth decimal place.

Interferometric Index Measurement

Interferometric techniques determine refractive index from the optical path difference introduced by a sample of known physical thickness. Index accuracy depends on precise thickness measurement and interferometric path difference determination. This approach works well for plane-parallel samples where thickness can be accurately measured.

Dispersion Characterization

Optical materials exhibit wavelength-dependent refractive index (dispersion) described by Sellmeier or similar equations. Complete characterization requires index measurement at multiple wavelengths spanning the operating range. Dispersion data enables prediction of chromatic aberration and guides material selection for achromatic designs.

Interferometric Homogeneity Measurement

Transmitted wavefront interferometry through prepared samples reveals index variations as wavefront distortion. Careful sample preparation ensures that measured wavefront errors result from bulk inhomogeneity rather than surface figure errors. Multiple passes through the sample increase sensitivity to weak inhomogeneities.

Striae Detection

Striae are localized refractive index variations appearing as streaks or layers in optical glass. Shadowgraph or schlieren imaging techniques reveal striae by converting index gradients to intensity variations. Classification systems grade striae severity based on visibility under standardized test conditions.

Bubbles and Inclusions

Inspection for bubbles, stones, and other inclusions ensures bulk material quality. Transmitted light inspection under dark field conditions reveals inclusions as bright scattering sources. Specification limits on inclusion size and density ensure adequate material quality for the intended application.

Thermal Cycling

Thermal cycling tests subject components to repeated temperature excursions across the specified operating range. Testing reveals failures from thermal expansion mismatch between coatings and substrates, stress-induced cracking, and adhesion degradation. Test profiles typically include multiple cycles with controlled ramp rates and dwell times at temperature extremes.

Humidity and Moisture Resistance

Humidity testing evaluates coating durability and substrate stability in moist environments. Standard tests include constant humidity exposure and cyclic humidity with temperature variation. Post-test inspection checks for coating delamination, haze formation, and degradation of optical properties.

Abrasion and Durability

Abrasion testing evaluates surface durability using standardized rubbing procedures with specified materials. Eraser rub and cheesecloth tests assess resistance to cleaning and handling damage. Military specifications define specific test procedures and pass-fail criteria for coating durability.

Salt Fog and Corrosion

Salt fog testing evaluates corrosion resistance for components intended for marine or coastal environments. Samples are exposed to salt-laden mist under controlled conditions, then inspected for corrosion, coating damage, and optical degradation. Test duration and post-exposure evaluation methods follow application-specific standards.

Control Charts for Optical Parameters

Control charts track critical optical parameters over time, distinguishing normal process variation from assignable causes requiring investigation. Parameters such as surface figure RMS, coating center wavelength, and MTF at key spatial frequencies are monitored against control limits derived from process capability studies.

Process Capability Analysis

Process capability indices (Cp, Cpk) quantify how well manufacturing processes meet specifications. These metrics compare process variation to specification width, indicating whether current processes can reliably produce conforming parts. Capability studies guide decisions about process improvement priorities and specification negotiations.

Measurement System Analysis

Measurement system analysis (MSA) characterizes the contribution of measurement variation to observed process variation. Gauge repeatability and reproducibility studies quantify how much of the observed variation results from measurement uncertainty rather than actual part-to-part differences. Adequate measurement capability is essential for meaningful process control.

Sampling Strategies

Cost-effective quality assurance requires appropriate sampling strategies balancing inspection cost against quality risk. Variables affecting sampling include production volume, process stability, defect consequences, and measurement cost. Acceptance sampling plans specify sample sizes and acceptance criteria based on acceptable quality levels and lot sizes.

Root Cause Analysis

When SPC identifies out-of-control conditions, systematic root cause analysis identifies the source of variation. Techniques including fishbone diagrams, design of experiments, and process failure mode analysis guide investigation. Corrective actions address root causes to prevent recurrence rather than simply sorting defective parts.

Reference Standards

Reference standards provide known values for calibrating measurement instruments. Optical reference standards include certified reference flats and spheres for interferometry, wavelength references for spectrometers, and reflectance standards for spectrophotometry. National metrology institutes provide primary standards with documented uncertainty.

Calibration Intervals and Procedures

Calibration intervals balance the cost of calibration against the risk of measurement drift. Critical instruments may require daily verification against working standards, with periodic full calibration against reference standards. Documented procedures ensure consistent calibration practices and enable demonstration of measurement validity.

Uncertainty Budgets

Measurement uncertainty budgets identify all sources of uncertainty contributing to reported values. Combining individual uncertainty components yields expanded uncertainty representing a confidence interval around the measured value. Proper uncertainty reporting enables informed decisions about pass-fail determinations near specification limits.

ISO 9001 and Optical Industry Standards

ISO 9001 certification demonstrates implementation of a quality management system meeting international standards. Industry-specific standards address particular requirements: ISO 10110 specifies optical drawing indications, while ISO 14644 addresses cleanroom classification relevant to contamination-sensitive optical manufacturing.

Documentation and Records

Quality systems require comprehensive documentation of procedures, specifications, and measurement records. Traceability from finished parts back to raw materials, process parameters, and inspection data enables investigation of field failures and continuous improvement. Electronic systems increasingly replace paper records, improving searchability and analysis capabilities.

Supplier Quality Management

Optical manufacturers depend on supplied materials and components meeting specifications. Supplier quality programs establish requirements, conduct audits, and monitor incoming quality. Certified material test reports, incoming inspection, and supplier scorecards ensure that supply chain quality supports manufacturing objectives.

Environmental Control

Temperature and humidity control in measurement areas minimizes thermal drift and moisture effects on measurements. Vibration isolation protects sensitive interferometric measurements from environmental disturbances. Clean handling procedures prevent contamination that could be mistaken for surface defects or degrade measurement accuracy.

Operator Training and Certification

Skilled operators are essential for reliable optical measurement. Training programs should address instrument operation, data interpretation, and recognition of measurement artifacts. Certification testing verifies operator competence, while ongoing proficiency monitoring ensures continued measurement quality.

Continuous Improvement

Quality data should drive ongoing process improvement, not just parts acceptance. Trend analysis identifies gradual drifts before they cause failures. Correlation of quality data with process parameters reveals optimization opportunities. Regular quality reviews bring together manufacturing, engineering, and quality personnel to address systemic issues.