Light Transmittance Testing of Packaging Materials: Standard Experimental Procedures and Methods

Light transmittance testing is a critical quality control procedure for evaluating the optical performance of transparent and translucent packaging materials, including plastic films, sheets, bottles, and glass containers. This article provides a comprehensive guide to the experimental steps for measuring light transmittance in accordance with recognized standards such as ASTM D1003, ISO 13468, GB/T 2410, and pharmacopoeial requirements. Understanding these procedures helps manufacturers ensure product protection, regulatory compliance, and consistent material quality.

Technical Overview

Light transmittance is defined as the percentage of incident luminous flux that passes through a material, expressed as the ratio of transmitted light to the light striking the specimen surface. For packaging materials, this optical property directly determines how effectively the package shields its contents from photodegradation caused by ultraviolet (UV) and visible light exposure.

Packaging materials such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyvinyl chloride (PVC), cyclic olefin copolymer (COC), and glass are routinely evaluated for their light transmittance characteristics. Photosensitive products, including certain pharmaceuticals, vitamins, beverages, oils, and cosmetics, require packaging with carefully controlled light transmission to prevent chemical decomposition, color change, and loss of potency during storage.

The most widely adopted test standards for this purpose include ASTM D1003 (Haze and Luminous Transmittance of Transparent Plastics), ISO 13468-1 (Determination of the Total Luminous Transmittance of Transparent Materials), GB/T 2410 (Transparent Plastics — Light Transmittance and Haze), and pharmacopoeial light transmission tests for containers. These methods rely on either an integrating-sphere hazemeter or a UV-Visible spectrophotometer equipped with an integrating sphere accessory.

Testing Samples

Flexible Plastic Films and Laminates

Flexible films used for pouches, blister lidding, and overwrap materials are typically tested as flat specimens. Samples should be free from wrinkles, creases, bubbles, and surface contamination, as these defects scatter light and distort the measurement. Multilayer laminates should be tested in their final composite structure to reflect real-world performance.

Rigid Plastic Sheets and Containers

Rigid materials such as PET sheets, PVC sheets used in thermoformed blisters, and acrylic panels require specimens cut to the dimensions specified by the instrument aperture. For finished containers such as bottles and jars, sections may be cut from the container wall and flattened, or the container may be tested whole using specialized holders, provided the wall geometry is accounted for in the analysis.

Glass Packaging

Amber, brown, and clear glass containers used for pharmaceuticals and beverages are evaluated for their spectral transmittance across both UV and visible ranges. Glass specimens must be thoroughly cleaned and dried, and the measured values are often normalized to a reference thickness to enable comparison against specification limits.

Sample Quantity and Dimensions

• Prepare a minimum of three to five replicate specimens for statistical validity.
• Cut flat specimens to approximately 50 mm × 50 mm, or as required by the instrument aperture (commonly 25 mm to 100 mm openings).
• Measure and record the thickness of each specimen at multiple points, as transmittance decreases predictably with increasing thickness.
• Mark specimens to identify the machine direction and the surface intended to face the light source, particularly for coated or asymmetric materials.

Testing Items

Depending on the material type and end-use application, the following optical parameters are commonly measured:

• Total luminous transmittance (Tt): The total percentage of visible light passing through the specimen, including both regular (specular) and diffuse transmission, weighted against 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 axis, indicating the degree of light diffusion caused by surface roughness or internal structures.
• UV transmittance: The transmittance at specific wavelengths in the 290 nm to 400 nm range, critical for assessing the UV protection capability of packaging for photosensitive products.
• Spectral transmittance curve: A full scan, typically from 290 nm to 800 nm, showing transmittance as a function of wavelength, used for detailed material characterization and compliance verification against wavelength-specific limits.

Testing Methods

Principle of Measurement

The specimen is placed in the path of a collimated light beam, and the transmitted luminous flux is collected by an integrating sphere. Total transmittance is determined by comparing the sphere response with and without the specimen in place. To quantify haze, a light trap is inserted to exclude the regularly transmitted component, leaving only diffusely scattered light. Calibration against air or a reference standard establishes the 100 percent baseline before measurements begin.

Apparatus and Reference Standards

• Integrating-sphere hazemeter conforming to ASTM D1003 Procedure A, or a UV-Vis spectrophotometer with an integrating sphere accessory conforming to ASTM D1003 Procedure B or ISO 13468.
• Calibrated reference standards for transmittance and haze traceable to national metrology institutes.
• A calibrated micrometer or thickness gauge for specimen thickness measurement.
• A conditioned laboratory environment maintained at 23 °C ± 2 °C and 50 % ± 10 % relative humidity.

Step-by-Step Experimental Procedure

The following generalized procedure applies to both the hazemeter and spectrophotometer methods. Always consult the governing standard for the specific material and market before testing.

• Step 1 — Specimen preparation: Cut the required number of specimens to the specified dimensions using a sharp blade or die cutter to avoid edge deformation. Handle specimens only by the edges and clean both surfaces with lint-free wipes and, if necessary, a suitable solvent such as ethanol, allowing full evaporation before testing.
• Step 2 — Conditioning: Condition the specimens in a controlled environment at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a minimum of 40 hours, in accordance with ASTM D618 or the equivalent conditioning clause of the applicable standard. This step stabilizes the material and ensures reproducible optical readings.
• Step 3 — Instrument warm-up and calibration: Switch on the instrument and allow it to warm up for the manufacturer-recommended period, typically at least 30 minutes. Perform zero calibration with the beam blocked or the standard black body in place, then set the 100 percent baseline using the empty specimen port or the certified reference standard. Verify calibration using an intermediate transmittance reference before proceeding.
• Step 4 — Specimen mounting: Mount the specimen flush against the integrating sphere entrance port with the marked surface facing the light source. Ensure the specimen fully covers the aperture without wrinkles, bends, or protrusion into the sphere, which would alter the optical path.
• Step 5 — Total transmittance measurement (Tt): Record the total luminous transmittance reading, which captures both regularly transmitted and diffusely scattered light. For spectral measurements, scan the designated wavelength range at defined intervals, for example 290 nm to 450 nm in 20 nm steps for pharmaceutical container evaluation.
• Step 6 — Diffuse transmittance measurement (Td): For haze determination, position the light trap to absorb the regular transmission component and record the diffuse transmittance reading, applying the instrument-specific corrections for multiple reflections within the sphere.
• Step 7 — Replicate measurements: Remove and re-mount each specimen at least once to check mounting repeatability, and measure every replicate specimen prepared for the sample set. Record all individual readings.
• Step 8 — Cleaning between readings: If fingerprints or dust are observed between measurements, re-clean the specimen surfaces and allow any solvent to evaporate completely before retesting.

Calculation and Data Processing

Total luminous transmittance is calculated as the ratio of the luminous flux transmitted through the specimen to the incident luminous flux, multiplied by 100 percent. Haze is calculated as the diffuse transmittance divided by the total transmittance, multiplied by 100 percent. For spectrophotometric data, the transmittance values across the visible spectrum are weighted using the CIE photopic luminosity function, often referenced to standard illuminant D65 or A, to derive a single total luminous transmittance value.

Report the arithmetic mean, standard deviation, and range for all replicate specimens. The final test report should include the sample identification, nominal thickness, conditioning details, the standard and instrument used, calibration reference details, individual and mean results, and any deviations from the standard method. For pharmaceutical packaging, results should be compared directly against the applicable wavelength-by-wavelength transmittance limits, often as a function of wall thickness.

Applications

Light transmittance testing supports a wide range of industries and regulatory scenarios:

• Pharmaceutical packaging: Verifying that blister films, bottles, and vials meet pharmacopoeial light transmission limits that protect light-sensitive drug products from photolysis, in support of regulatory submissions and stability programs.
• Food and beverage packaging: Evaluating the UV and visible light barrier properties of bottles, films, and pouches for products such as edible oils, dairy items, beer, and juices that are susceptible to light-induced oxidation and rancidity.
• Cosmetics packaging: Confirming that containers preserve the color, fragrance, and active ingredients of formulations exposed to retail and in-use lighting conditions.
• Agricultural films: Characterizing the light transmission of greenhouse films to optimize photosynthetically active radiation delivery to crops while managing diffused light distribution.
• Material development and quality control: Comparing formulations, coating technologies, colorants, and processing conditions during research and development, and monitoring production consistency through routine incoming and outgoing inspection.
• Compliance verification: Generating technical data for accredited third-party laboratories, regulatory filings, and customer specifications where optical performance claims must be substantiated.

FAQ

What is the difference between transmittance and haze?

Transmittance measures the total amount of light passing through a material, while haze measures only the portion of transmitted light that is scattered away from the straight-through direction. A material can exhibit high transmittance yet high haze, such as opalescent or diffuser films, which transmit plenty of light but scatter it widely.

Why must specimens be conditioned before testing?

Temperature and humidity affect the dimensions, surface condition, and internal structure of polymer materials, all of which influence light scattering and absorption. Conditioning at 23 °C ± 2 °C and 50 % ± 5 % relative humidity stabilizes the specimens so that measurements are reproducible within and between laboratories.

How does specimen thickness affect the results?

Transmittance decreases as thickness increases because absorption and scattering accumulate along the optical path. For this reason, thickness must always be reported alongside transmittance data, and comparison between materials is valid only at equivalent thicknesses or after normalization to a reference thickness.

Which standard should be selected for pharmaceutical packaging?

Pharmaceutical containers are typically evaluated using the light transmission test described in the relevant pharmacopoeia, which specifies wavelength-specific limits between approximately 290 nm and 450 nm. Plastics intended for general applications are more commonly tested to ASTM D1003, ISO 13468, or GB/T 2410. The choice depends on the target market, product type, and regulatory requirements.

What are common sources of measurement error?

Frequent error sources include fingerprints and dust on specimen surfaces, specimens that do not fully cover the aperture, inadequate instrument warm-up or expired calibration, testing materials with curvature without proper flattening, and failure to exclude ambient stray light. Careful specimen handling, verified calibration, and trained operators minimize these risks.

How often should the instrument be calibrated?

Baseline and zero calibrations should be performed at the start of each testing session, with verification against certified reference standards at regular intervals as defined by the laboratory quality management system. Full metrological calibration and maintenance should follow the instrument manufacturer's schedule and the requirements of the laboratory's accreditation scope.