Light Transmittance Testing of Packaging Materials: Experimental Operation Guide and Technical Stand

Light transmittance testing is a fundamental quality control procedure used to evaluate the optical performance of packaging materials such as plastic films, rigid sheets, and transparent containers. This technical guide explains the experimental operation of transmittance measurement in accordance with recognized standards including ASTM D1003, ISO 13468, and GB/T 2410, covering sample preparation, instrument calibration, test procedures, and data evaluation. Understanding the correct laboratory workflow helps manufacturers verify material specifications, optimize barrier performance, and ensure compliance for food, pharmaceutical, and consumer packaging applications.

Technical Overview

Light transmittance, also referred to as luminous transmittance, is defined as the percentage of incident luminous flux that passes through a specimen. It is calculated using the CIE standard illuminant A and the photopic response of the human eye, expressed over the visible wavelength range. For transparent and translucent packaging materials, transmittance is one of the most important optical quality indicators, directly affecting product visibility, shelf presentation, and light-induced degradation of the packaged contents.

The experimental operation is typically performed using a spectrophotometer equipped with an integrating sphere or a dedicated hazemeter. During measurement, a parallel light beam passes through the specimen, and transmitted light is collected by the integrating sphere. Total transmittance and haze can be determined in a single sequence. The measurement is governed by several internationally recognized methods, including ASTM D1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics), ISO 13468-1 (Determination of the total luminous transmittance of transparent materials), ISO 14782 (Determination of haze), and the Chinese national standard GB/T 2410. Because results depend strongly on instrument geometry and specimen preparation, strict adherence to the selected standard is essential for reproducible and comparable data.

Testing Samples

Flexible Plastic Films

Flexible films represent the largest category of transmittance test samples. These include single-layer films such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyamide (PA), as well as multi-layer composite laminates used for food pouches and barrier packaging. Typical specimen thickness ranges from approximately 20 micrometers to 200 micrometers. Films should be tested in their as-received state, since surface texture, anti-fog coatings, and printing layers all influence the measured optical values.

Rigid Sheets and Thermoformed Packaging

Rigid transparent sheets made from PET, PVC, polystyrene (PS), polycarbonate (PC), or polymethyl methacrylate (PMMA) are commonly tested for thermoformed trays, blister packs, and clamshell packaging. Specimens are cut to fully cover the instrument aperture, and the thickness of each specimen must be recorded because transmittance decreases predictably with increasing material thickness. For blister foil and forming films used in pharmaceutical packaging, both visible and ultraviolet transmittance data may be required.

Containers and Special Optical Packaging

For bottles, jars, and other three-dimensional containers, the test is generally performed on flat sections cut from the container wall or on representative plaques produced from the same resin and processing conditions. Glass packaging and surface-coated materials such as metallized or silicon-oxide coated films can also be evaluated, although coated specimens require careful orientation and documentation of the coated versus uncoated side facing the light source.

Testing Items

The core optical parameters determined during the experimental operation include the following:

• Total luminous transmittance (Tt): the ratio of total transmitted light to incident light within the visible spectrum, expressed as a percentage.
• Haze: the percentage of transmitted light that is scattered by more than a specified angle from the direction of the incident beam, indicating clarity loss.
• Spectral transmittance curve: wavelength-dependent transmittance scanned typically from 200 nm to 800 nm, useful for evaluating UV-blocking performance.
• Ultraviolet transmittance: light transmission in the UV-A and UV-B ranges, critical for protecting photosensitive contents such as fats, vitamins, pharmaceuticals, and dyes.
• Clarity and see-through quality: qualitative or instrumental assessment of image distortion through the material, complementary to haze measurement.

Testing Methods

Specimen Preparation and Conditioning

Specimens must be clean, flat, and free from wrinkles, scratches, fingerprints, and embedded dust particles, as surface contamination can significantly alter scattering results. A minimum of three specimens, and preferably five, should be prepared from different locations across the sample roll or sheet to account for material inhomogeneity. Each specimen is cut to a size that fully covers the integrating sphere aperture, typically at least 10 mm larger than the aperture diameter. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618, unless otherwise specified by the applicable product standard.

Instrument Calibration and Verification

Before measurement, the spectrophotometer or hazemeter must be warmed up according to the manufacturer's instructions, usually for at least 15 to 30 minutes. Baseline calibration is performed in four steps: measuring the incident flux with an empty aperture, verifying the zero reading with a light trap or opaque shutter, confirming the 100 % transmittance reference with the aperture open, and checking the instrument against certified reference standards traceable to national metrology institutes. Calibration records should be maintained as part of the laboratory quality system.

Measurement Procedure

The specimen is mounted flat against the integrating sphere aperture, taking care to avoid stretching flexible films or trapping air bubbles between the specimen and the port. The operator records the total transmittance reading, after which the haze measurement sequence captures the diffuse and regular transmitted components according to the four-flux method defined in ASTM D1003. Each specimen is measured at least three times, rotating or repositioning the specimen between readings, and the average value is calculated. For spectral scans, the instrument records transmittance at defined wavelength intervals across the selected range, and the luminous transmittance is integrated using the CIE photopic weighting function.

Data Processing and Calculation

Total luminous transmittance is calculated as the ratio of the luminous flux measured with the specimen in place to the luminous flux measured with the empty aperture, multiplied by 100. Haze is calculated as the ratio of diffuse transmittance to total transmittance, multiplied by 100. The final test report should include the arithmetic mean of all specimens, the standard deviation, specimen thickness, the number of layers measured, the testing standard applied, instrument model and geometry, and the conditioning environment. Any deviation from the standard procedure, such as testing through printed or metallized areas, must be explicitly documented.

Quality Control Points

• Verify that the illuminant and photometric response settings match the requirements of the selected standard before starting the sequence.
• Handle specimens only by the edges and wear lint-free gloves to prevent surface contamination.
• Confirm that film specimens remain flat without tension, since elongation can reduce thickness and artificially increase transmittance.
• Re-check the 100 % baseline periodically during extended test sequences to compensate for lamp drift.
• Retain calibrated reference standards and plot control charts to monitor long-term instrument performance.

Applications

Transmittance testing of packaging materials supports quality assurance and product development across many industries. In food packaging, the data is used to select films that minimize light-induced oxidation of oils, dairy products, beverages, and cured meats, thereby extending shelf life. In pharmaceutical packaging, transmittance measurements in combination with photostability studies aligned with ICH Q1B guidance help verify that blister films and bottles adequately protect photosensitive active ingredients. UV-blocking films for agricultural and horticultural produce packaging, transparent window films for folding cartons, cosmetic containers, and display-grade PET sheets for retail blister packs all rely on transmittance and haze specifications to guarantee visual presentation and functional protection. Accredited third-party laboratories also use these measurements for incoming material inspection, supplier qualification, and regulatory compliance documentation.

FAQ

What is the difference between light transmittance and haze?

Light transmittance quantifies the total amount of visible light passing through the specimen, while haze quantifies the portion of transmitted light that is scattered away from the incident direction. A material can exhibit high transmittance yet high haze, meaning it transmits plenty of light but produces a cloudy, poorly defined see-through image.

Which standard should be selected for testing?

ASTM D1003 and GB/T 2410 cover both transmittance and haze using the integrating sphere method and are widely applied to plastic packaging materials. ISO 13468-1 addresses total luminous transmittance, and ISO 14782 addresses haze. The choice depends on customer specification, target market, and contractual requirements, and the same standard should be used consistently when comparing datasets.

Does specimen thickness affect the result?

Yes. Transmittance decreases and haze generally increases as thickness increases because of bulk absorption and scattering. Thickness must always be reported together with optical results, and comparisons between materials are valid only at equivalent thickness and layer structure.

Can colored, printed, or translucent materials be tested?

Yes. The method applies to any transparent or translucent material, including colored films and printed packaging. For printed samples, the test location must be defined, since measurements taken over ink coverage will naturally yield much lower transmittance than measurements over unprinted areas.

How often should the instrument be calibrated?

Baseline verification is performed at the start of each test session, while full calibration against certified reference standards is typically performed at scheduled intervals defined in the laboratory quality manual, along with intermediate verification using working standards to ensure continued accuracy.