Optical transmittance change testing quantifies how much light a transparent material transmits before and after environmental exposure, abrasion, or accelerated aging, making it essential for validating the durability of glazing, plastic sheets, films, and photovoltaic materials. This article explains the governing standards such as ASTM D1003, ISO 13468, ISO 4892, and IEC 60068, together with typical sample types, test items, measurement methods, acceptance criteria, and applications commonly used by accredited third-party laboratories.
Technical Overview
Optical transmittance change refers to the difference in light-transmitting performance of a transparent or translucent material measured before and after a defined conditioning treatment, such as light aging, heat, humidity, abrasion, or corrosion exposure. The baseline property is typically expressed as total luminous transmittance (Tt), the percentage of normally incident luminous flux that passes through the specimen, calculated over the visible wavelength range (380 nm to 780 nm) using a specified standard illuminant and observer. The change is then reported as an absolute difference in percentage points (delta T = T1 minus T0) or as a relative percentage change.
Because long-term optical stability directly affects product safety, energy efficiency, and visual quality, transmittance change requirements appear in specifications across automotive, architectural, photovoltaic, display, and personal protective equipment sectors. Measurement and conditioning are governed by internationally recognized standards organizations, with the most frequently cited documents including:
- ASTM D1003 and ISO 13468-1/-2 for total luminous transmittance of transparent plastics, with GB/T 2410 and JIS K 7361-1 as corresponding national standards
- ISO 14782 and ASTM D1003 for haze (wide-angle light scattering) measurement
- ISO 9050 and EN 410 for luminous and solar transmittance of glazing in buildings
- ISO 4892-2/-3 and ASTM G155/G154 for xenon-arc and fluorescent-UV accelerated weathering
- IEC 60068-2 series for damp heat, thermal cycling, and dust/sand environmental conditioning
- ASTM D1044 and ISO 9352 for Taber abrasion conditioning of optical surfaces
- ASTM B117 and ISO 9227 for salt spray corrosion exposure of coated optical parts
- IEC 62788-1-4 for optical transmittance measurement of photovoltaic encapsulant materials
Testing Samples
Transparent Plastic Sheets and Films
Common materials include PMMA, polycarbonate, PET, PVC, TPU, and multilayer optical films. Specimens are usually flat coupons of 50 mm by 50 mm or 100 mm by 100 mm, cut from production material at delivery thickness, free of scratches, bubbles, and surface contamination.
Glass and Coated Glazing
Architectural glass, low-emissivity coated glass, laminated safety glass, and automotive glazing are tested to verify that coatings and interlayers retain their optical performance after humidity, thermal cycling, or abrasion conditioning.
Photovoltaic Materials
EVA and POE encapsulant films, front sheets, backsheets, and glass-glass laminate stacks are evaluated per the IEC 62788 measurement procedures to assess discoloration and light transmission loss that would reduce module power output.
Optical and Display Components
Lenses, headlamp covers, instrument windows, touch panel cover glass, and optically clear adhesives are tested after UV exposure or abrasion to confirm sustained clarity and image quality.
Protective Visors and Eye Protors
Face shields, goggle lenses, and welding helmets are assessed for optical class retention after aging in accordance with requirements aligned with EN 166 personal eye protection specifications.
Sample Conditioning Before Testing
Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618 before the baseline transmittance measurement.
Testing Items
A typical transmittance change evaluation program includes the following test items, each performed by comparing post-conditioning values against the initial baseline:
- Total luminous transmittance (Tt) over the visible spectrum using illuminant D65 or A and a 2-degree or 10-degree standard observer
- Spectral transmittance scanning from 200 nm to 2500 nm for solar and UV performance characterization
- Haze measurement for wide-angle scattering before and after conditioning
- Transmittance change after xenon-arc or fluorescent-UV accelerated weathering
- Transmittance change after damp heat exposure (85 °C / 85 % RH) and temperature-humidity cycling
- Transmittance change after Taber, falling sand, or reciprocating abrasion of optical surfaces
- Transmittance change after neutral or acidified salt spray corrosion exposure of coated parts
- Yellowness index per ASTM E313 as a complementary indicator of optical degradation
Testing Methods
Baseline Transmittance Measurement
Specimens are measured with an integrating sphere spectrophotometer in accordance with ASTM D1003 or ISO 13468-2. The instrument records total transmittance, diffuse transmittance, and haze across the visible band, calibrated against certified reference standards traceable to national metrology institutes. A minimum of three specimens per material condition is recommended to ensure statistical validity.
Accelerated Weathering Conditioning
Light aging is performed with a xenon-arc lamp filtered to daylight spectrum per ISO 4892-2 or ASTM G155, or with fluorescent UV lamps per ISO 4892-3 or ASTM G154. Irradiance, black panel temperature, chamber humidity, and water spray cycles are defined by the applicable material specification, with total exposures commonly ranging from several hundred to several thousand hours.
Damp Heat and Thermal Cycling Conditioning
Environmental endurance is evaluated using IEC 60068-2-78 for steady-state damp heat (for example 85 °C and 85 % relative humidity for 1000 hours), IEC 60068-2-14 for thermal cycling between temperature extremes, IEC 60068-2-30 for cyclic humidity, and IEC 60068-2-68 for dust and sand exposure where abrasion by airborne particles is a concern.
Abrasion Conditioning
Surface durability conditioning uses a Taber abraser with CS-10F wheels under a defined load (typically 500 g or 1000 g) for a specified number of cycles per ASTM D1044 or ISO 9352, or falling sand and silicon carbide abrasion per ASTM D968. Transmittance and haze are remeasured in the abraded track after cleaning.
Corrosion Conditioning
Coated optical components such as metallized reflectors and coated glass are exposed to neutral salt spray per ASTM B117 or ISO 9227, followed by gentle cleaning and transmittance remeasurement to quantify coating degradation.
Post-Exposure Measurement and Calculation
After conditioning, specimens are reconditioned to standard laboratory atmosphere and remeasured on the same instrument with identical settings. The transmittance change is calculated as the absolute difference T1 minus T0 in percentage points, or as the relative change given by (T1 minus T0) divided by T0 multiplied by 100 percent. Haze change is reported in the same manner.
Typical Acceptance Criteria
- Absolute transmittance change of no more than 3 percentage points after specified weathering exposure for many glazing and film specifications
- Transmittance retention of at least 90 percent of the initial value after damp heat exposure for photovoltaic encapsulant materials
- Haze increase limited to a defined increment, frequently 3 to 5 percentage points, after Taber abrasion
- No visible blistering, cracking, delamination, or discoloration that would impair the optical function
Exact limits are always defined by the applicable product standard or the contractual specification between manufacturer and purchaser.
Applications
Optical transmittance change testing supports material qualification, quality control, and regulatory compliance across a wide range of industries:
- Automotive: headlamp and signal lamp lenses, panoramic roofs, instrument panel covers, and window glazing after weathering and abrasion
- Architecture: transparent and coated glass, polycarbonate and multiwall sheets, window films evaluated against ISO 9050 and EN 410 optical requirements
- Photovoltaics: encapsulant films, front sheets, and laminates assessed within IEC 61215 module qualification sequences and IEC 62788-1-4 material measurements
- Consumer electronics and displays: cover glass, touch panels, and optically clear adhesive films after UV exposure and reliability testing
- Personal protection: face shields, goggles, and visors verifying sustained optical class after aging in line with EN 166 requirements
- Packaging: transparent sterile barrier films and blister materials checked for clarity retention after sterilization and storage
- Aerospace and rail: cabin windows and transparent structural panels validated after combined environmental exposure
FAQ
What is optical transmittance change?
It is the measured difference in the light transmission of a material, expressed in percentage points or relative percent, between an initial baseline measurement and a repeat measurement after a defined conditioning treatment such as weathering, damp heat, abrasion, or corrosion.
Which standards define how transmittance is measured?
Total luminous transmittance of plastics is measured per ASTM D1003 and ISO 13468-1/-2, with GB/T 2410 and JIS K 7361-1 as the corresponding Chinese and Japanese national standards. Haze is measured per ISO 14782, and glazing products are characterized per ISO 9050 and EN 410.
What equipment is used?
An integrating sphere spectrophotometer operating in the UV-Vis and near-IR range, equipped with calibrated reference standards, is the primary instrument. Conditioning equipment includes xenon-arc and fluorescent-UV weathering chambers, damp heat and thermal cycling chambers, Taber abraders, and salt spray cabinets.
How is the result calculated?
The absolute change is T1 minus T0 in percentage points, and the relative change is (T1 minus T0) divided by T0 multiplied by 100 percent, where T0 is the initial transmittance and T1 is the post-conditioning transmittance measured under identical instrument settings.
What are typical pass criteria?
Specifications commonly limit the absolute transmittance loss to about 3 percentage points or require at least 90 percent retention of the initial value after the specified exposure, together with a bounded haze increase. The exact limits depend on the product standard or purchase specification.
How many specimens are needed?
A minimum of three specimens per condition is standard practice, with additional specimens retained as unexposed controls so that instrument drift and batch variation can be separated from genuine material degradation.
Can transmittance change testing be combined with other evaluations?
Yes. Accredited third-party laboratories frequently combine transmittance change with yellowness index per ASTM E313, colorimetry, visual defect inspection, and adhesion or mechanical testing to provide a complete durability assessment in a single conditioning campaign.
