Light transmittance testing measures how much visible and ultraviolet light passes through packaging materials, a critical parameter for protecting photosensitive foods, pharmaceuticals, and cosmetics from photodegradation. This guide explains representative sampling procedures, applicable international standards such as ASTM D1003, ISO 13468, and GB/T 2410, laboratory test methods, and typical applications of transmittance testing in packaging quality control.
Technical Overview
Light transmittance, expressed as a percentage, is the ratio of luminous flux that passes through a specimen to the luminous flux incident upon it. For transparent and translucent packaging materials such as films, sheets, bottles, and blister structures, transmittance determines how effectively the package shields its contents from ultraviolet (UV) and visible radiation that can trigger lipid oxidation, vitamin degradation, color fading, and loss of active ingredient potency. Sampling-based transmittance inspection is therefore an essential element of incoming material control, routine production monitoring, and compliance verification for packaging manufacturers and brand owners.
Because transmittance depends on resin type, thickness, additives such as UV absorbers and colorants, and surface finish, results are only meaningful when specimens are sampled representatively from the production lot and tested under standardized illumination and geometry conditions. Accredited third-party laboratories perform these tests in accordance with recognized international and national standards, producing results that are directly comparable across the supply chain.
Testing Samples
A statistically valid sampling plan is the foundation of reliable transmittance testing. Specimens are typically drawn from defined points across the width and length of a production roll or from multiple containers representing the lot, since thickness gauge variation and additive dispersion can vary across the web. Specimens must be clean, free of creases, scratches, fingerprints, and embedded dust, and each specimen is identified with its machine-direction orientation. Conditioning is performed at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618 before measurement.
Flexible Plastic Films
Monolayer and multilayer films such as BOPP, CPP, PE, PET, PVC, and PLA are cut into flat specimens of at least 50 mm × 50 mm to fully cover the instrument aperture. Nominal thickness is recorded for each specimen because transmittance correlates strongly with optical path length.
Rigid Sheets and Thermoformed Blister Materials
PVC, PVDC-coated PVC, PETG, and PP sheets used for thermoforming and blister packaging are tested as flat sheets prior to forming. Where the formed cavity wall thickness differs significantly from the sheet, specimens may be sectioned from the formed part to evaluate the as-used condition.
Container Walls and Bottle Sections
For bottles, jars, and tubes, flat specimens are die-cut from the container sidewall and gently flattened. Curvature must be removed without stretching the material, and the specimen location and orientation relative to the mold parting line are documented.
Composite and Metallized Laminates
Multilayer laminates are tested as complete structures in the delaminated-free, as-supplied state. Highly metallized or aluminum foil-containing structures are evaluated to confirm near-zero transmittance and verify the light barrier function of the finished laminate.
Testing Items
Depending on the product protection requirement and the referenced standard, the following optical characteristics are commonly measured:
- Total luminous transmittance (Tt): the percentage of normally incident light transmitted through the specimen, weighted for the specified standard illuminant and the photopic response of the human eye.
- Haze: the percentage of transmitted light that is scattered by more than 2.5 degrees from the incident beam, indicating wide-angle scattering caused by surface roughness or internal inclusions.
- Regular (specular) transmittance and diffuse transmittance components.
- Spectral transmittance curve across the wavelength range of 200 nm to 800 nm.
- UV transmittance or UV blocking rate in the UVA band (315–400 nm) and UVB band (280–315 nm).
- Clarity, describing the fidelity of seeing a target through the film at small scattering angles.
Testing Methods
Method A: Hazemeter with Integrating Sphere
The most widely applied procedure follows ASTM D1003 Method A, GB/T 2410, and JIS K 7361-1. A collimated light beam passes through the specimen mounted against an integrating sphere, and four successive readings (incident, total transmitted, scattered, and instrument stray light) are used to compute both total transmittance and haze. ASTM D1003 specifies CIE Illuminant A, while ISO 13468 and GB/T 2410 specify standard illuminant D65, so the illuminant must be stated in the report to ensure comparability.
Method B: Spectrophotometric Method
Per ASTM D1003 Method B and ISO 13468-2, a double-beam UV-Vis spectrophotometer fitted with an integrating sphere scans the specimen wavelength by wavelength. This method yields the full spectral transmittance curve and is essential when specific UV bands must be quantified, for example when verifying the performance of UV-absorbing masterbatches or light-resistant packaging claims.
Data Processing and Reporting
A minimum of three valid specimens per sample is measured, and the arithmetic mean, range, and standard deviation are reported. The test report includes the applicable standard, instrument type and illuminant, specimen thickness, conditioning environment, individual and mean values, and photographs or notes on any visible defects. Outliers caused by surface contamination or handling marks are re-measured on replacement specimens drawn from the same sample.
Applications
Transmittance testing supports product development, supplier qualification, and regulatory compliance across multiple packaging sectors:
- Food packaging: verification of light barriers for edible oils, dairy products, dried foods, and beverages susceptible to photodegradation and rancidity.
- Pharmaceutical packaging: qualification of light-resistant containers and blister films for photosensitive active ingredients.
- Cosmetic packaging: protection of formulations containing vitamins, essential oils, and natural extracts from UV-induced discoloration.
- Agricultural films: characterization of light transmission for greenhouse covering films that influence crop growth.
- Electronics and display packaging: control of optical clarity and anti-glare properties of protective films and window materials.
- Manufacturing quality control: batch release testing, incoming inspection of purchased films, and evaluation of process changes such as new additives or gauge adjustments.
FAQ
How many specimens are required per lot?
Most standards require a minimum of three valid specimens per sample; for lot release decisions, five to ten specimens drawn from different positions across the production roll or from different containers are recommended to account for web variation.
What is the difference between total transmittance and haze?
Total transmittance measures how much light passes through the material regardless of direction, while haze measures the portion of transmitted light scattered at wide angles. A film can have high transmittance yet high haze, appearing cloudy rather than optically clear.
Which standard should be selected?
ASTM D1003 and GB/T 2410 are equivalent approaches covering both transmittance and haze with a hazemeter; ISO 13468 addresses total luminous transmittance and ISO 14782 addresses haze; JIS K 7361-1 and JIS K 7136 are the corresponding Japanese procedures. The choice usually follows the market or customer specification governing the product.
Can opaque packaging be tested?
Fully opaque structures such as aluminum foil laminates register effectively zero transmittance. Testing is still performed to document the light barrier, using the same geometry and confirming that no pinholes or thin spots allow measurable light passage.
What factors can influence the results?
Specimen thickness, surface scratches, dust and fingerprints, residual curvature, insufficient conditioning, and mismatched illuminant settings are the most common sources of measurement error, which is why standardized preparation and calibration against certified reference materials are mandatory.
How are curved containers sampled?
Flat specimens are die-cut from defined zones of the sidewall and gently flattened without stretching. Alternatively, whole-container light exposure tests may be agreed upon when cutting is impractical, but the cut-specimen method remains the standard quantitative approach.