ASTM C1901: Flat Glass Optical Retardation Testing Services
Accredited ASTM C1901 flat glass optical retardation testing from Infinita Lab, performed to the exact standard requirements - accurate, reproducible results with full documentation for compliance, R&D, and quality control programs.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
What Is ASTM C1901 Flat Glass Optical Retardation?
ASTM C1901, Standard Test Method for Measuring Optical Retardation in Flat Architectural Glass, is published by ASTM Committee C14 on Glass and Glass Products and developed under Subcommittee C14.08. The current edition is C1901-21e2, which is listed as active; the earlier C1901-21 and C1901-21e1 versions are marked historical. The standard addresses measuring optical anisotropy in architectural glass. Here’s the physics in brief. Heat treatment leaves a non-uniform residual stress pattern in the sheet, and that stress makes the glass slightly birefringent. When polarized light passes through, the two polarization components travel at different speeds. The resulting phase difference is the optical retardation. It’s tied to local residual stress and is the quantity behind the faint quench pattern often called anisotropy or iridescence. The method measures retardation at many locations across a sheet, so the values can be used to calculate or predict how much visible pattern a panel may show. Scope is narrow. The method applies to monolithic heat-strengthened and fully tempered glass, clear, tinted, or coated. It excludes light-diffusing glass such as patterned, sandblasted, or acid-etched glass, as well as glass that isn’t optically transparent, such as mirrors, enameled, or fritted glass. The measurement is non-destructive. Run it on-line at the tempering furnace to tune the process, or off-line for quality assurance. The standard is a test method, not an architectural glazing specification, so it defines how to measure retardation but sets no pass-or-fail limit. Because the reading integrates through the thickness, it can’t report the level of temper, surface stress, or center tension in the glass.
Applications and Benefits of ASTM C1901 Flat Glass Optical Retardation Testing
Scope
ASTM C1901 covers the measurement of optical retardation in flat glass specimens using transmitted polarized light. The method quantifies birefringence caused by internal stress distribution through the glass thickness and at the surface.
The scope includes:
- ASTM C1901 covers the measurement of optical anisotropy in architectural glass by determining optical retardation.
- It’s a test method for measuring optical retardation, and it is explicitly not an architectural glazing specification.
- Retardation values obtained with the method may be used to calculate or predict the amount of visible pattern, commonly called anisotropy or iridescence, in heat-treated glass.
- Monolithic heat-treated glass is covered, meaning heat-strengthened and fully tempered product.
- Within scope: clear, tinted, and coated glass.
- Patterned, sandblasted or acid-etched glass, which diffuses light, falls outside the scope.
- Also excluded is glass that is not optically transparent, such as mirrors and enameled or fritted glass.
- Since the optical measurement is integrated through the glass thickness, it cannot be used to assess the level of tempering.
- No information on surface stress or center tension comes out of the method.
- SI units are regarded as the standard.
Applications
- Quality verification of tempered and heat-strengthened architectural glass
- Facade and curtain wall glazing stress evaluation
- Detection of inadequate tempering or annealing defects
- Structural glazing and overhead glazing safety assessment
- Laminated glass interlayer stress compatibility evaluation
- Glass processor quality control during production
- Failure investigation of prematurely fractured glass panels
- Research on residual stress distribution in specialty glass products
- Compliance testing for building code and glazing standard requirements
- Evaluation of glass after thermal or mechanical processing modifications
Benefits
- Non-destructive measurement with no specimen damage
- Directly quantifies residual stress through optical retardation values
- Applicable across a wide range of glass thicknesses and types
- Supports compliance with ASTM C1048 and related glazing standards
- Identifies under-tempered or non-uniform stress distributions early
- Suitable for both laboratory and in-line production quality control
- Rapid measurement with minimal specimen preparation
- Provides spatial stress mapping across the glass surface
- Reduces risk of in-service spontaneous breakage due to undetected stress anomalies
Our ASTM C1901 Testing Procedure
Specimen Preparation
Glass specimen is cleaned and positioned flat for unobstructed polarized light transmission through its thickness.
1Polariscope Setup
Instrument is calibrated with reference retardation standards before measurement begins.
2Retardation Measurement
Optical retardation is measured at defined points using transmitted polarized light.
3Reporting
Values are measured in nm, converted if required, and compared with acceptance criteria.
4ASTM C1901 Test Parameters and Requirements
| Parameter | Details |
|---|---|
| Measurement Parameter | Optical retardation (birefringence) |
| Reporting Units | nm or nm/cm |
| Glass Types Covered | Tempered, heat-strengthened, annealed flat glass |
| Applicable Thickness Range | Typically 3 mm to 19 mm |
| Measurement Method | Transmitted polarized light (polariscope) |
| Stress Type Measured | Residual internal stress from thermal processing |
| Measurement Locations | Edge zone, surface area, and mid-panel points |
- Optical retardation scanner – Measures retardation across the sheet, either on-line at the tempering furnace or off-line in a laboratory.
- Polarized light source – Supplies the polarized illumination that passes through the glass thickness so the phase difference can be detected.
- Polarization analyzer and detector – Reads the transmitted light. At each measurement point, that reading becomes a retardation value.
- Data acquisition and mapping software – Records the readings, then presents them as values or maps of retardation over the panel.
- Reference standards for verification – Confirm the instrument responds correctly before measurements are taken.
- Flat mounting fixture or conveyor – Holds or moves the panel in a controlled, flat position during measurement.
Equipment and Instrumentation Used for ASTM C1901 Testing
What You Receive: Test Report, Data, and Certification
- Optical retardation values in nm or nm/cm at measured locations
- Stress distribution map across the glass surface where applicable
- Pass/fail determination against specified retardation thresholds
- Edge and surface retardation comparison data
- Identification of non-uniform or deficient tempering zones
- Full test report with specimen dimensions, glass type, and test conditions
- Calibration records and instrument verification data
- Photographic or digital retardation map for automated systems
ASTM C1901 Flat Glass Optical Retardation FAQs
ASTM C1901 is a standard test method for measuring optical retardation in flat architectural glass.
Optical retardation affects how light passes through glass, impacting visual quality and aesthetic properties in architectural applications.
ASTM C1901 measures optical retardation. The test may not be capable of detecting many forms of optical defects. Other tests and inspections must be conducted to determine other defects, among which are not only excluded but also bubbles, inclusions, surface imperfections, etc.
The standard applies specifically to flat architectural glass, including tempered, laminated, and insulated glass units.
ASTM C1279 uses a similar polariscope approach but focuses on detecting stress and strain in flat glass for edge and surface evaluation. ASTM C1901 specifically addresses quantitative optical retardation measurement in architectural applications.
Phase retardation occurs when polarized light passes through a birefringent material, causing two perpendicular light components to travel at different speeds and develop a phase difference. This difference is measured using optical instruments to evaluate birefringence, material properties, or residual stress.
Why Choose Infinita Lab for Flat Glass Optical Retardation Testing
When your Flat Glass Optical Retardation results have to hold up - for compliance, a customer audit, or an engineering decision - ASTM C1901 accuracy and an unbiased third-party report matter more than price. Infinita Lab routes your ASTM C1901 flat glass optical retardation testing to ISO/IEC 17025-accredited U.S. partner labs with hands-on method experience, so you get defensible data, transparent reporting, and turnaround times built around your project deadline - not ours.
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