ASTM C1901 Test for Measuring Optical Retardation in Flat Architectural Glass

ASTM C1901 is used to measure optical anisotropy in flat architectural glass. Optical anisotropy is measured by measuring optical retardation. This test is used for quality control of the end use transparent glass products. The values stated in SI units are regarded as standard.

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ASTM C1901 Test for Measuring Optical Retardation in Flat Architectural Glass

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Overview
Scope, Applications, and Benefits
Test Process
Specifications
Instrumentation
Results and Deliverables

Overview

ASTM C1901 is a standard test method for measuring optical retardation in flat architectural glass using polariscope-based instrumentation. Optical retardation quantifies the degree of residual stress present in glass, arising from thermal tempering, heat strengthening, or improper annealing during manufacture.

This method is essential for verifying that architectural glass meets required stress levels for safety, structural performance, and optical clarity. It is applicable to tempered, heat-strengthened, and annealed flat glass used in facades, curtain walls, glazing systems, and structural applications, ensuring compliance with quality and safety standards before installation.

Scope, Applications, and Benefits

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 across the glass thickness and surface.

The scope includes:

Measurement of optical retardation in nm or nm/cm units
Applicable to tempered, heat-strengthened, and annealed flat glass
Testing of glass used in architectural glazing, facades, and curtain walls
Evaluation of edge and surface stress distribution
Applicable to glass thicknesses typically ranging from 3 mm to 19 mm
Compatible with both laboratory specimens and production-line quality control

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

Test Process

Specimen Preparation

Glass specimen is cleaned and positioned flat for unobstructed polarized light transmission through its thickness.

Polariscope Setup

Instrument is calibrated with reference retardation standards before measurement begins.

Retardation Measurement

Optical retardation is measured at defined points using transmitted polarized light.

Reporting

Values are measured in nm, converted if required, and compared with acceptance criteria.

Technical Specifications

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

Instrumentation Used for Testing

Transmission polariscope (automated or manual)
Compensator or wave plate for retardation quantification
Calibrated retardation reference standards
Light source with polarized filter assembly
Digital imaging system for stress mapping (automated systems)
Optical bench or flat support table for specimen positioning

Results and Deliverables

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

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Frequently Asked Questions

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.