Quantitative analysis of optical transmittance change is a critical materials testing technique that measures how transparent and translucent materials lose or alter their light-transmitting properties after environmental exposure, aging, or mechanical treatment. By comparing spectral transmittance values before and after testing using internationally recognized standards such as ASTM D1003, ISO 13468, and GB/T 2410, manufacturers can objectively evaluate material durability, optical stability, and long-term performance. This article explains the measurement principles, sample requirements, testing procedures, calculation methods, and typical applications across the photovoltaic, automotive, construction, and electronics industries.
Technical Overview
Optical transmittance change quantitative analysis is a laboratory measurement technique used to determine the degree to which a material's light transmission performance varies after exposure to environmental stressors such as ultraviolet radiation, xenon arc weathering, damp heat, thermal cycling, chemical contact, or mechanical abrasion. Transparent and semi-transparent materials—including polymer films, sheets, glass, coatings, and adhesives—may undergo yellowing, haze formation, surface crazing, or microstructural degradation during service, all of which reduce or modify light throughput.
The analysis is performed by measuring the luminous or spectral transmittance of a specimen before treatment (initial value, T0) and after treatment (final value, T1), then quantifying the difference. The result is typically expressed as an absolute change (ΔT = T1 − T0) or a relative percentage change (ΔT% = (T1 − T0) / T0 × 100). This quantitative approach replaces subjective visual comparison with reproducible, instrument-based data suitable for quality control, R&D verification, and certification support in accredited third-party laboratories.
Key measurement parameters covered under this analysis include total luminous transmittance, spectral transmittance distribution across the ultraviolet, visible, and near-infrared regions, haze, and yellowness index. The combination of these parameters provides a complete picture of optical degradation mechanisms and their magnitude.
Testing Samples
Polymer Sheets and Films
Rigid and flexible polymer materials such as polycarbonate (PC), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polyolefin films are among the most frequently tested sample types. These materials are widely used in glazing, protective windows, and flexible packaging, where transmittance stability directly affects product appearance and function.
Photovoltaic Encapsulant Films
Ethylene-vinyl acetate (EVA) and polyolefin elastomer (POE) encapsulant films used in solar modules are tested for transmittance retention after UV preconditioning and damp-heat exposure. Because encapsulant yellowing reduces the light reaching solar cells, transmittance change is a decisive acceptance criterion in module reliability evaluation aligned with IEC 61215 requirements.
Glass and Coated Glazing
Architectural glass, automotive glazing, and coated or laminated glass products are evaluated for transmittance stability after weathering, abrasion, or chemical exposure. Measurements follow glass-specific methods such as ISO 9050 and relevant EN standards for light and solar transmittance determination.
Optical Coatings and Adhesives
Transparent coatings, hard coats, anti-reflective layers, and optically clear adhesives applied to substrates are tested to verify that the coating or adhesive layer does not introduce unacceptable transmittance loss, haze, or yellowing after environmental conditioning.
Display and Lighting Components
Light guide plates, diffuser films, protective covers, and lamp lenses made from transparent resins are tested to ensure optical performance is maintained throughout the product life cycle.
Testing Items
Typical measurable items in a transmittance change study include:
- Total luminous transmittance (Tt) before and after treatment, measured over the photopic visible range (380–780 nm)
- Absolute transmittance change (ΔT) and relative transmittance change (ΔT%)
- Full spectral transmittance curves across UV, visible, and near-infrared wavelengths (typically 200–2500 nm)
- Haze (diffuse transmittance ratio) before and after aging
- Yellowness index (YI) and color coordinates to characterize yellowing effects
- Transmittance retention after UV aging, xenon arc exposure, damp heat, thermal cycling, or chemical resistance testing
- Abrasion-induced transmittance loss for hard-coated surfaces
Testing Methods
The core instrument for transmittance change analysis is a UV-Vis-NIR spectrophotometer equipped with an integrating sphere, which collects both regular (specular) and diffuse transmitted light. Measurements are referenced against air or a calibrated standard, and specimens are conditioned at standard laboratory atmosphere prior to each measurement to ensure comparability.
The general test procedure is as follows:
- Prepare and condition specimens, typically flat samples of at least 50 mm × 50 mm with representative surface finish and thickness
- Measure the initial transmittance spectrum and record the total luminous transmittance T0
- Expose the specimen to the specified treatment, such as UV fluorescent lamp aging per ISO 4892-3 or ASTM G154, xenon arc exposure per ISO 4892-2 or ASTM G155, damp-heat exposure per IEC 61215, or other agreed conditioning
- Re-measure the transmittance under identical instrument settings to obtain T1
- Calculate ΔT = T1 − T0 and, where required, ΔT% = (T1 − T0) / T0 × 100
- Report spectral curves, haze values, and any visual defects observed on the specimen
Primary reference standards for the measurement itself include ASTM D1003 and ISO 13468 for total luminous transmittance of transparent plastics, ISO 14782 and JIS K 7361 / JIS K 7136 for haze and transmittance of plastics, GB/T 2410 as the corresponding Chinese national method, ASTM E903 for spectral solar transmittance using an integrating sphere, and ISO 9050 for light transmittance of building glass. Instrument calibration, wavelength accuracy verification, and photometric accuracy checks using certified reference filters are performed regularly to guarantee traceable results.
Reporting should clearly state the illuminant and observer conditions used for luminous calculation (commonly D65 illuminant and the 10-degree standard observer), specimen thickness, measurement geometry, and the aging conditions applied, since all of these factors influence the reported change values.
Applications
Quantitative transmittance change analysis supports material selection, product qualification, and failure investigation across many industries:
- Photovoltaics: verifying that encapsulant films, backsheets, and front covers retain high transmittance after UV and damp-heat aging, ensuring long-term module power output
- Automotive: evaluating headlamp lenses, light guides, glazing, and interior trim materials for yellowing and haze development after accelerated weathering
- Building and construction: assessing glass, polycarbonate roofing, and window films for light transmission stability after solar exposure and surface wear
- Consumer electronics and displays: confirming that cover materials, touch panel layers, and adhesives maintain optical clarity after reliability testing
- Packaging: monitoring transmittance of transparent films and containers that protect light-sensitive contents such as pharmaceuticals and food products
- Research and development: comparing additive packages, UV stabilizers, and resin formulations by quantifying their effect on transmittance retention
FAQ
What wavelength range is used for transmittance measurement?
Total luminous transmittance is calculated over the visible spectrum from 380 nm to 780 nm using the photopic response of the human eye. For solar and energy-related applications, the spectral scan is often extended from 200 nm to 2500 nm to cover the ultraviolet, visible, and near-infrared regions.
What does a negative ΔT value mean?
A negative change indicates that transmittance decreased after treatment, which is the most common result of photo-oxidative yellowing, haze formation, or surface degradation. A positive change, while less common, can occur when certain additives or surface contaminants are removed during exposure, and should be investigated together with haze and yellowness data.
What sample size and form are required?
Flat specimens of approximately 50 mm × 50 mm are typical, cut from sheets or films at the actual product thickness. Specimens must be clean, free of fingerprints and scratches, and large enough to fully cover the instrument aperture. Curved or rigid parts may require specially adapted fixtures.
How is haze related to transmittance change?
Haze is the percentage of transmitted light that is scattered by more than 2.5 degrees from the incident beam. A material can maintain nearly constant total transmittance while developing significant haze, which severely degrades visual clarity. For this reason, transmittance change analysis is often performed together with haze measurement per ISO 14782 or ASTM D1003.
What acceptance criteria are commonly applied?
Acceptance limits depend on the product specification. For photovoltaic encapsulants, a transmittance change of no more than 2% to 3% after damp-heat or UV preconditioning is a common requirement derived from IEC 61215-based qualification sequences. Automotive and display specifications typically define maximum allowable ΔT and haze increase values agreed between supplier and customer.
Why should testing be performed in an accredited laboratory?
Accredited third-party laboratories provide calibrated instrumentation, traceable reference standards, controlled environmental conditioning, and test reports that are widely recognized for supplier qualification, regulatory compliance, and customer submission purposes.