Light Transmittance Test Procedure for Packaging Materials

This technical article defines a standardized light transmittance test procedure for packaging materials, covering specimen preparation, instrumentation, measurement steps, and calculation methods in accordance with ISO 13468, ASTM D1003, and GB/T 2410. Reliable transmittance data is essential for quality control of transparent and translucent films, sheets, and rigid containers used in the food, pharmaceutical, and consumer goods industries.

Technical Overview

Light transmittance, formally known as total luminous transmittance, is the ratio of luminous flux transmitted through a packaging specimen to the luminous flux incident upon it, expressed as a percentage. It is one of the most fundamental optical performance indicators for transparent and translucent packaging materials, determining both product visibility on the shelf and the degree to which the package protects its contents from photodegradation.

For packaging applications, transmittance serves two opposing design objectives. High transmittance is required for display-oriented packaging where consumers must inspect the product, such as clear PET bottles, blister films, and window films on cartons. Low transmittance, particularly in the ultraviolet and visible regions, is required for light-sensitive products such as certain pharmaceuticals, vitamins, beers, and oils, where actinic radiation accelerates degradation of active ingredients, colorants, and flavors.

Transmittance is typically evaluated together with haze, the percentage of transmitted light that is scattered by more than 2.5 degrees from the direction of the incident beam. The principal reference standards governing this test procedure include ISO 13468-1 and ISO 13468-2 for total luminous transmittance of transparent plastics, ASTM D1003 for haze and luminous transmittance of transparent plastics, GB/T 2410 for the Chinese national method, and JIS K 7361-1 for the corresponding Japanese procedure. Measurements are performed using CIE Standard Illuminant D65 with the 10-degree standard observer under ISO and GB methods, or Illuminant A with the 2-degree observer under ASTM D1003.

Testing Samples

Flexible Plastic Films

Single-layer and multilayer films of PE, PP, PET, PVC, PLA, and cellophane are the most frequently tested sample category. Films should be flat, free of wrinkles, folds, and surface contamination, and cut to a size sufficient to cover the instrument aperture, typically at least 50 mm by 50 mm. Because thin films may exhibit thickness-dependent scattering, specimen thickness must be measured and reported at multiple points.

Rigid Sheets and Thermoformed Containers

Rigid sheets used for blister packaging, thermoformed trays, and transparent boxes are tested in flat plaque form. Curved container walls, such as those of bottles and jars, should be sectioned and flattened or cut from regions of uniform curvature, since surface curvature redirects transmitted light and can bias both transmittance and haze readings.

Glass Packaging

Amber, green, flint, and molded glass containers for pharmaceutical and beverage applications are evaluated by cutting flat panels from the container body or by using specially molded flat glass plates of identical composition. For pharmacopoeial light-protection verification, spectral transmittance is additionally measured across the 290 to 450 nm range.

Coated, Metallized, and Composite Laminates

Transparent barrier coatings such as silicon oxide and aluminum oxide layers, as well as metallized films with partial optical density, are tested in finished laminate form. The orientation of the specimen relative to the light source must be recorded, since coated and coextruded structures can be optically asymmetric.

Testing Items

• Total luminous transmittance (Tt): the overall percentage of visible light passing through the specimen, measured over the 380 to 780 nm photopic range.
• Haze (H): the percentage of transmitted light scattered wider than 2.5 degrees, quantifying loss of see-through clarity.
• Spectral transmittance: wavelength-by-wavelength transmittance data across the ultraviolet, visible, and near-infrared regions, used for light-protection and barrier performance assessment.
• Ultraviolet transmittance: transmittance in the UVA and UVB bands, critical for packaging of photosensitive drugs, dairy products, and beer.
• Transmittance after conditioning or aging: retained transmittance following heat, humidity, or accelerated weathering exposure to evaluate optical durability.

Testing Methods

Apparatus

The test requires a spectrophotometer or a dedicated hazemeter equipped with an integrating sphere, a collimated light beam, a photometric detector with a photopic response, and a standardized aperture of approximately 25 mm diameter. The instrument must be verified before use using calibrated reference standards traceable to national metrology institutes, including a fully transmitting reference and a haze reference.

Specimen Preparation and Conditioning

Specimens shall be clean, dry, and representative of production material. Unless otherwise specified, specimens are conditioned at 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618. Surface dust, fingerprints, and anti-block agents must be removed with lint-free wipes and, where appropriate, a suitable solvent that does not attack or swell the material. A minimum of five specimens per sample is recommended, and the thickness of each specimen is recorded to the nearest 0.001 mm.

Measurement Procedure

• Warm up and zero-calibrate the instrument, then verify calibration with the certified transmittance and haze reference standards.
• Measure the incident luminous flux T1 with no specimen and the unscattered reference reading T3 to establish the baseline.
• Mount the first specimen flat against the input aperture without stretching, touching the optical zone, or trapping air bubbles.
• Record the total transmitted luminous flux T2 and the diffuse transmitted luminous flux T4 for each specimen.
• Repeat the measurement for all specimens, rotating flat, isotropic materials by 90 degrees where relevant, and re-check calibration at least every two hours of continuous operation.
• Record ambient temperature and relative humidity, instrument model, illuminant, observer angle, specimen thickness, and the standard applied.

Calculation and Expression of Results

Total luminous transmittance is calculated as Tt = (T2 divided by T1) multiplied by 100 percent. Haze is calculated as H = ((T4 divided by T2) minus (T3 divided by T1)) multiplied by 100 percent, where the T3 over T1 term corrects for the intrinsic scatter of the unobstructed optical system. Results for all specimens are reported individually together with the arithmetic mean, standard deviation, and coefficient of variation. When measurements are performed by accredited third-party laboratories, test reports must identify the applied standard, specimen geometry, conditioning history, and instrument calibration status to ensure full traceability.

Applications

Food and Beverage Packaging

Transmittance data supports material selection for clear bottles, films, and rigid containers where product visibility is a marketing requirement, and conversely for UV-blocking designs protecting beverages, edible oils, and dairy products from light-induced oxidation and nutrient loss.

Pharmaceutical Packaging

Transmittance testing verifies compliance with light-protection requirements for primary packaging of photosensitive drug products, complementing photostability studies conducted under ICH Q1B exposure conditions. Amber glass and opaque laminate performance is documented through full spectral scans in the actinic range.

Agricultural and Industrial Films

Greenhouse films, mulch films, and silage films are specified by total transmittance and haze, which govern photosynthetically active radiation delivery and condensation behavior, directly affecting crop yield and film formulation development.

Consumer Goods and Electronics

Window films for cartons, clamshell packaging, display windows, and photovoltaic module encapsulation films rely on transmittance and haze specifications to balance presentation, protection, and conversion efficiency.

FAQ

What is the difference between transmittance and haze?

Transmittance measures how much light passes through the material regardless of direction, while haze measures the portion of transmitted light that is scattered away from the incident beam by more than 2.5 degrees. A material can show high transmittance yet high haze, meaning it lets light through but scatters it, blurring the view of the contents.

Which standard should be selected for testing?

ISO 13468 and GB/T 2410 use Illuminant D65 with the 10-degree observer, whereas ASTM D1003 uses Illuminant A with the 2-degree observer, so results from different standards are not directly interchangeable. The standard should be chosen based on the destination market and the specification invoked by the customer or regulatory authority.

How many specimens are required for a valid test?

A minimum of five specimens per material is recommended for statistical reliability, and additional specimens are advisable for anisotropic or variable-thickness films. Each specimen is measured at least once, with the mean and standard deviation reported.

Can colored, translucent, or opaque materials be tested?

Yes. Colored and translucent materials are fully measurable for total transmittance and spectral distribution, although haze becomes less meaningful at very low transmittance levels. Opaque materials return a transmittance of essentially zero percent, which itself documents effective light-blocking performance.

What repeatability should be expected?

With a well-calibrated instrument and properly conditioned specimens, the repeatability of total luminous transmittance within a single laboratory is typically within 0.5 percent absolute, and reproducibility between laboratories is normally within 2 percent absolute for homogeneous transparent materials.

How are curved bottles and containers handled?

Curved containers are tested by cutting representative flat panels from the sidewall or by measuring molded flat plates of the same composition and wall thickness. The sampling location must be documented because wall thickness variations in blow-molded containers cause measurable transmittance differences.