The optical transmittance change rate quantifies how much light transmission through a transparent material degrades after exposure to UV radiation, weathering, abrasion, heat, or humidity, making it a critical quality metric for glazing, films, plastics, and photovoltaic products. This article explains the measurement principle, applicable international standards, sample types, laboratory test procedures, and typical acceptance criteria used by accredited third-party laboratories.
Technical Overview
Optical transmittance change rate determination measures the relative variation in light transmittance of a transparent or translucent material before and after a defined treatment, such as accelerated weathering, UV irradiation, thermal cycling, damp-heat exposure, chemical contact, or surface abrasion. The result is normally expressed as a percentage using the relationship: ΔT = (T0 − T1) / T0 × 100%, where T0 is the initial transmittance and T1 is the transmittance after treatment.
A significant loss of transmittance is an early indicator of material degradation. Photo-oxidation causes yellowing, haze formation, micro-cracking, and coating delamination, all of which reduce light transmission, compromise visual clarity, and in photovoltaic, lighting, and glazing applications directly impair energy performance. Core reference standards include ISO 13468-1 and ASTM D1003 for luminous transmittance measurement of plastics, ISO 3537 for safety glazing irradiation resistance, ISO 9050 and EN 410 for light and solar characteristics of glazing, GB/T 2410 for transparent plastics in Chinese-market products, and the ISO 4892 series together with ASTM G151, G154, and G155 for the laboratory aging exposures used to generate the change condition.
Testing Samples
Transparent and Translucent Plastics
Sheets and films of polycarbonate (PC), polymethyl methacrylate (PMMA), PET, polystyrene, PVC, TPU, and cyclic olefin copolymers used for glazing, lenses, light guides, and protective windows.
Glass and Coated Glazing
Architectural glass, laminated safety glass, tempered glass, and glass carrying anti-reflective, low-emissivity, or self-cleaning coatings, where long-term coating durability must be verified.
Optical and Functional Films
Polarizers, brightness enhancement films, diffusers, hard-coated films, anti-glare films, ITO conductive films, and window films for displays, instruments, and vehicles.
Photovoltaic Materials
EVA and POE encapsulant films, backsheet materials, and front covers, for which transmittance stability after UV and damp-heat aging is essential to module power output.
Packaging and Consumer Products
Transparent packaging films, bottles, blister packs, goggles, face shields, helmet visors, and display cover windows requiring clarity retention throughout service life.
Testing Items
- Total luminous transmittance in the visible range (380–780 nm) using standard illuminant D65 with a 2° or 10° standard observer
- Regular (specular) transmittance and diffuse transmittance measured with an integrating sphere
- Transmittance change rate after xenon-arc or UV fluorescent lamp accelerated weathering
- Transmittance change rate after damp-heat (85 °C / 85 % RH), thermal cycling, or high-temperature storage
- Transmittance change rate after Taber abrasion, RCA abrasion, or steel wool rubbing
- Transmittance change rate after chemical exposure to cleaning agents, oils, salt solutions, acids, or alkalis
- Spectral transmittance curves across the UV–visible range (200–800 nm) or the full solar range (300–2500 nm)
- Haze change determined in conjunction with transmittance testing per ASTM D1003 and ISO 14782
Testing Methods
Instrumentation and Reference Standards
Measurements are performed with a UV-visible spectrophotometer, preferably equipped with an integrating sphere, calibrated against certified reference standards in accordance with ISO 13468-1, ASTM D1003, ISO 9050, or GB/T 2410, depending on the material and end use. The spectral transmittance is integrated over the photopic response of the human eye under standard illuminant D65 to yield the luminous transmittance value.
Specimen Preparation and Conditioning
Specimens are cut to the required dimensions (commonly 50 mm × 50 mm or matched to the sample holder), cleaned with lint-free wipes and isopropanol or another specified solvent, and conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a defined period, typically 40 hours per ISO 291 or ASTM D618, before the baseline measurement. Surface marks, bubbles, and inclusions are documented because they influence local transmittance readings.
Baseline and Post-Treatment Measurement
At least three positions on each specimen are measured, and replicate specimens, typically three to five, are tested to ensure statistical validity. After the baseline transmittance T0 is recorded, matching specimens are subjected to the agreed treatment or aging protocol, for example xenon-arc exposure per ISO 4892-2 or ASTM G155, UV fluorescent exposure per ISO 4892-3 or ASTM G154, or damp-heat aging per IEC 61215. The specimens are then reconditioned and re-measured under identical geometry to obtain the post-treatment transmittance T1.
Calculation and Reporting
The transmittance change rate is calculated as ΔT = (T0 − T1) / T0 × 100%. The test report documents the instrument and measurement geometry, illuminant and observer conditions, all aging or treatment parameters, individual and mean transmittance values before and after treatment, the calculated change rate, and photographic records of visible defects such as yellowing, crazing, or delamination. Acceptance limits depend on the product specification: safety glazing standards commonly require the post-irradiation to pre-irradiation transmittance ratio to remain at or above 0.95, while photovoltaic encapsulant specifications frequently limit transmittance loss to no more than 2 % after UV or damp-heat aging.
Applications
- Automotive: windshields, side windows, lamp lenses, and light covers requiring stable light transmission after prolonged solar exposure
- Photovoltaics: qualification and incoming-material inspection of encapsulant films and covers, where transmittance loss directly reduces module power output
- Architecture: durability verification of coated glazing, curtain-wall panels, and polycarbonate roofing used in building envelopes
- Consumer electronics: hard-coated and anti-glare cover windows, touch panels, and display stack films evaluated after abrasion and chemical resistance tests
- Lighting and optics: LED lenses, light guides, and diffusers checked for yellowing after thermal and photothermal aging
- Packaging and medical: transparent containers and sterilization packaging assessed for clarity retention after sterilization or shelf-life aging
- Research and quality control: comparative evaluation of stabilizers, UV absorbers, and coating formulations during material development
FAQ
What sample size and quantity are required?
Standard specimens of 50 mm × 50 mm are recommended for benchtop spectrophotometers. A minimum of three replicate specimens for the baseline measurement and three matching specimens for each aging or treatment condition is advised to obtain a reliable change-rate value. Thin films can be mounted flat between clean quartz plates, provided air bubbles and wrinkles are eliminated.
What is the difference between total transmittance and regular transmittance?
Total luminous transmittance includes both directly transmitted and diffusely scattered light and is measured with an integrating sphere. Regular transmittance counts only light transmitted within a narrow angular range around the incident beam. A large difference between the two values indicates haze, which is why haze is frequently reported alongside the transmittance change rate.
Which aging conditions are most commonly combined with this test?
Typical pairings include xenon-arc weathering (ISO 4892-2, ASTM G155), UV fluorescent weathering (ISO 4892-3, ASTM G154), damp-heat exposure at 85 °C / 85 % RH per IEC 61215, thermal cycling, and Taber abrasion, for which ASTM D1044 evaluates haze change on the same abraded track. The selection depends on the failure mode most relevant to the intended application.
What causes transmittance to decrease during aging?
Photo-oxidation of the polymer matrix, depletion or migration of UV absorbers and antioxidants, hydrolysis, crystallization changes, micro-void formation, surface erosion, and coating delamination all increase light absorption or scattering. Measuring the change rate at intermediate exposure intervals helps identify the degradation onset and its underlying mechanism.
How long does the testing take?
The transmittance measurement itself takes only minutes per specimen. Total turnaround is governed by the duration of the pre-treatment or aging program, since UV or xenon exposures may range from several hundred to a few thousand hours, so project schedules should be planned around the aging duration specified in the applicable standard.