Light Transmittance Testing Methods for Packaging Materials

Light transmittance testing is an essential quality control method for evaluating how packaging materials transmit light, which directly affects product protection, shelf life, and consumer visibility. This article provides a comprehensive technical guide to packaging material light transmittance testing methods, covering applicable standards such as ASTM D1003, ISO 13468, and GB/T 2410, sample preparation, test procedures, and practical applications across the food, pharmaceutical, and cosmetic packaging industries.

Technical Overview

Light transmittance, also known as luminous transmittance, is defined as the percentage of incident luminous flux that passes through a packaging material, weighted according to the photopic response of the human eye under standardized illumination conditions such as CIE Standard Illuminant A or D65. For transparent and translucent packaging materials, transmittance is typically measured together with haze, which quantifies the fraction of transmitted light that is scattered at angles greater than 2.5 degrees from the incident beam.

Light transmittance is a critical performance parameter in packaging engineering for two opposing reasons. First, many products require protection from ultraviolet and visible light because photochemical reactions can degrade nutrients, flavors, active pharmaceutical ingredients, and cosmetic formulations. Second, transparent packaging enhances shelf appeal by allowing consumers to see the product. Accurate transmittance measurement allows packaging designers to balance light protection against product visibility, and it enables manufacturers to verify material consistency during incoming inspection and production control.

• Primary measurand: total luminous transmittance (T), expressed as a percentage
• Secondary measurand: haze, expressed as a percentage of diffusely transmitted light
• Typical spectral range: 380–780 nm for luminous transmittance; 200–800 nm for full protection assessment
• Reference standards: ASTM D1003, ISO 13468, ISO 14782, GB/T 2410, JIS K 7361-1, and JIS K 7136

Testing Samples

Flexible Plastic Films and Laminates

Single-layer films such as BOPP, PET, PE, CPP, PA, and PVC, as well as multi-layer laminates including PET/AL/PE, PET/VMPET/PE, and paper-plastic composites, are the most frequently tested sample types. Films should be cut into flat specimens, typically 50 mm × 50 mm or sized to match the instrument aperture, and must be free of wrinkles, folds, and surface contamination. For laminated structures, the complete laminate is tested in its as-supplied condition because interlayer reflections and adhesive haze influence the result.

Rigid Plastic Sheets, Blister Films, and Thermoformed Containers

Rigid samples include PET, PP, PS, PC, and PVC sheets, blister packaging films, and coupons cut from the walls of thermoformed trays, bottles, and tubes. Curved containers should be sampled by cutting flat sections from the sidewall at defined locations, since curvature can redirect the transmitted beam and distort readings. Edges should be trimmed cleanly to avoid scattering artifacts at the specimen port.

Glass and Other Specialty Packaging Materials

Flint (clear) glass, amber glass, and coated glass samples are tested as flat coupons or as sections cut from molded or tubular containers. Amber glass is of particular interest for pharmaceutical packaging, where restricted transmittance in the 290–450 nm range protects photosensitive drug products. Metallic foils and fully opaque materials may also be characterized to confirm negligible transmittance.

Unless otherwise specified, specimens should be conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618, and at least three to five replicate specimens should be tested to obtain a representative average.

Testing Items

A typical transmittance evaluation program for packaging materials includes the following test items, each addressing a distinct aspect of optical performance:

• Total luminous transmittance: the overall percentage of visible light passing through the material
• Haze: the percentage of wide-angle scattered light, which affects clarity and see-through quality
• UV transmittance: transmission in the UV-A (315–400 nm) and UV-B (280–315 nm) bands, relevant to photo-protection
• Spectral transmittance curve: wavelength-by-wavelength transmittance across 200–800 nm for full characterization
• Transmittance at specified wavelengths: single-point values required by product specifications or pharmacopoeial-style acceptance criteria
• Comparative protection ratios: transmittance before and after aging, sterilization, or surface treatment

Testing Methods

Method 1: Hazemeter Method (ASTM D1003 Method A, ISO 14782, GB/T 2410)

The hazemeter uses a calibrated light source, a photometric sphere, and a photopic detector to measure both luminous transmittance and haze in a single operation. The instrument is first zeroed with a light trap, then standardized to 100% with an empty beam. The specimen is placed at the transmission port, and total transmitted flux is recorded. A light trap is then inserted to exclude the unscattered beam, allowing the diffuse (scattered) component to be measured. Haze is calculated as the ratio of diffuse transmittance to total transmittance, multiplied by 100. This method is fast and well suited to routine production control of transparent films and sheets.

Method 2: Spectrophotometer Method with Integrating Sphere (ASTM D1003 Method B, ISO 13468-1/-2)

A UV-Visible spectrophotometer equipped with an integrating sphere collects both regular and diffuse transmitted light, providing accurate total transmittance even for slightly hazy or textured materials. A spectral scan is performed across the visible range, and the photopically weighted transmittance is calculated by integrating the spectral transmittance against the CIE standard illuminant and the photopic luminosity function. This method offers the highest accuracy and full traceability, and it is preferred for certification testing at accredited third-party laboratories.

Method 3: UV-Visible Spectral Scan for Light Protection Assessment

For photosensitive products such as vitamins, edible oils, dairy products, and light-sensitive pharmaceuticals, a full spectral scan from approximately 290 nm to 800 nm is performed. The resulting transmittance profile identifies the wavelength bands in which the packaging blocks or transmits radiation. Acceptance criteria are typically expressed as maximum allowable transmittance in defined bands, for example restricted transmission between 290 nm and 450 nm for protective pharmaceutical containers. Comparing clear and protective material variants quantifies the protective benefit of tints, UV absorbers, and barrier coatings.

General Test Procedure

• Step 1: Condition specimens at standard laboratory atmosphere (23 °C ± 2 °C, 50% ± 10% RH)
• Step 2: Clean specimen surfaces with lint-free wipes and inspect for scratches, fingerprints, and dust
• Step 3: Warm up and calibrate the instrument using certified reference standards
• Step 4: Mount the specimen flat against the transmission port, avoiding wrinkles and air gaps
• Step 5: Record total luminous transmittance and, where required, haze or spectral data
• Step 6: Repeat for all replicate specimens and report the average, standard deviation, test standard, instrument geometry, and specimen thickness

Applications

Transmittance testing supports quality assurance and product development across all major packaging sectors:

• Food packaging: verifying light barriers in milk bottles, edible oil containers, and beer packaging to prevent riboflavin loss, lipid oxidation, and off-flavor formation
• Pharmaceutical packaging: qualifying amber glass, protective films, and blister materials for light-sensitive tablets, capsules, and injectable products
• Cosmetic packaging: evaluating protection of vitamin-rich formulations, essential oils, and photosensitive actives in bottles, tubes, and jars
• Consumer and industrial packaging: confirming clarity of display windows, shrink films, and retail blister packs
• Material development: benchmarking new resins, masterbatches, UV absorbers, coatings, and recycled-content materials against optical specifications
• Shelf-life studies: tracking transmittance changes after accelerated aging, sterilization, or exposure to light and humidity

FAQ

What is the difference between light transmittance and haze?

Light transmittance measures how much light passes through the material, while haze measures how much of that transmitted light is scattered away from the direct beam. A material can have high transmittance but poor see-through quality if haze is elevated, which is common with textured or microvoided films.

Which test standard should be selected for packaging films?

ASTM D1003 and ISO 13468 are the most widely used international references for total luminous transmittance, while GB/T 2410 is the corresponding Chinese national standard and JIS K 7361-1 applies in Japan. The choice depends on the target market, customer specification, and regulatory context; results from equivalent methods generally agree well when instrument geometry and calibration are matched.

What sample size and condition are required?

Most instruments accept flat specimens of approximately 50 mm × 50 mm or 100 mm × 100 mm. Specimens must be clean, flat, and representative of production material, and they should be conditioned at 23 °C and 50% relative humidity before measurement.

Can curved containers such as bottles be tested directly?

Direct testing of whole curved containers is not recommended with standard instruments because curvature distorts the light path. Flat coupons are cut from defined locations on the container wall and tested instead, with sampling positions documented in the report.

What transmittance level is required for light-sensitive products?

There is no universal limit; requirements are product-specific. Clear PET films and sheets often exceed 85–90% transmittance, whereas protective amber containers are typically specified to remain below defined limits in the 290–450 nm band. Manufacturers should establish acceptance criteria based on product photostability data.

How often should instruments be calibrated?

Instruments should be calibrated daily before use with certified transmittance and haze reference standards, and full metrological verification should be performed periodically, ideally annually, through an accredited third-party laboratory or the instrument manufacturer's calibration service.