Light transmittance testing quantifies the percentage of visible light that passes through packaging materials, a critical parameter for protecting light-sensitive products such as foods, pharmaceuticals, cosmetics, and agricultural goods. This article provides a comprehensive technical overview of light transmittance data determination for packaging materials, covering measurement principles, applicable international standards such as ASTM D1003, ISO 13468, and GB/T 2410, sample requirements, instrumentation, and typical industrial applications.
Technical Overview
Light transmittance, also known as luminous transmittance, is defined as the ratio of the luminous flux transmitted through a specimen to the luminous flux incident upon its surface, expressed as a percentage. For transparent and translucent packaging materials, this parameter largely determines how effectively a package shields its contents from photodegradation, discoloration, vitamin degradation, lipid oxidation, and other light-induced quality changes during storage and distribution.
Transmittance measurements are performed under standardized illumination and detection conditions, typically using a light source approximating CIE Standard Illuminant A (a tungsten source with a correlated color temperature of approximately 2856 K) and a detector corrected to match the photopic response of the human eye. Because most packaging materials both transmit and scatter light, the total luminous transmittance is distinguished from the regular (specular) transmittance, and their difference represents the diffuse transmittance, which is directly related to haze. Accurate determination of these optical data requires integrating-sphere optics, traceable calibration standards, controlled specimen preparation, and standardized environmental conditioning.
Testing Samples
Flexible Films and Multilayer Laminates
Single-layer and multilayer plastic films, including polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), polyvinyl chloride (PVC), and high-barrier laminates with aluminum oxide or silicon oxide coatings, are among the most frequently tested packaging formats. Flat, wrinkle-free specimens free of scratches, fingerprints, and processing defects are cut to the dimensions required by the instrument aperture.
Rigid Sheets and Panels
Thermoforming sheets and structural panels made from amorphous polyethylene terephthalate (APET), polystyrene (PS), polycarbonate (PC), and polymethyl methacrylate (PMMA) are tested in flat plaque form. For materials supplied in different thickness grades, each thickness must be evaluated separately because transmittance decreases as the optical path length increases.
Hollow Containers and Formed Parts
Bottles, jars, blister cavities, and thermoformed trays cannot be measured directly in their formed geometry on most standard instruments. Representative flat sections are typically cut from the sidewall or base of the container, or preform plaques produced under equivalent processing conditions are used to represent the final container wall.
Glass and Coated Substrates
Glass containers, UV-blocking coated bottles, and transparent composite structures can also be evaluated using spectrophotometric transmittance methods, provided that flat sections of appropriate size can be obtained and correctly oriented at the measurement port.
All specimens should be conditioned prior to testing in accordance with ISO 291 or ASTM D618, typically at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours, to ensure that moisture content and temperature do not influence the optical readings.
Testing Items
A complete transmittance characterization of a packaging material typically includes the following measurable parameters:
- Total luminous transmittance (Tt): the percentage of incident visible light transmitted through the specimen in all directions, measured with an integrating sphere.
- Regular (specular) transmittance (Tr): the percentage of light transmitted without deviation from the original optical path.
- Diffuse transmittance (Tt − Tr): the portion of transmitted light scattered by the material, calculated as the difference between total and regular transmittance.
- Haze: the percentage of transmitted light that is scattered more than 2.5 degrees from the incident beam axis, determined in accordance with ISO 14782 or ASTM D1003.
- Spectral transmittance curve: the wavelength-resolved transmittance profile across the ultraviolet and visible range, typically 200 nm to 800 nm, used to identify blocking performance in specific wavelength bands.
- UV blocking rate: the calculated average transmittance in the UV-A (315–400 nm) and UV-B (280–315 nm) regions, often a key acceptance criterion for light-protective packaging.
Testing Methods
ASTM D1003 Method
ASTM D1003, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, defines two procedures. Method A uses a dedicated hazemeter with an integrating sphere and a unidirectional illumination, normal-detection geometry; the sphere is first calibrated with a reference standard and the empty port, after which total transmittance is measured with the specimen at the sphere entry port, and diffuse transmittance is measured with the specimen at the exit port using a light trap. Method B permits the use of a spectrophotometer equipped with an integrating sphere, recording the spectral transmittance and mathematically weighting it against the photopic function to obtain luminous transmittance.
ISO 13468 Method
ISO 13468 specifies the determination of total luminous transmittance of transparent plastics using either a single-beam instrument (Part 1) or a double-beam instrument (Part 2). The double-beam configuration continuously compensates for source intensity drift and is therefore preferred when the highest repeatability is required, for example in lot release testing performed by accredited third-party laboratories. Haze as a complementary parameter is measured separately according to ISO 14782.
GB/T 2410 Method
GB/T 2410, the Chinese national standard for the determination of light transmittance and haze of transparent plastics, is technically aligned with ASTM D1003 in terms of optical geometry, illuminant, and calculation. It is widely specified for packaging materials destined for the Chinese domestic market and for quality-control purposes in manufacturing plants located in China.
Instrumentation and Calibration
Regardless of the standard applied, the measuring system comprises a stabilized light source, an integrating sphere with high-reflectance diffuse coating, a photopic detector, and either fixed or scanned wavelength selection. Calibration must be performed using reference standards traceable to national metrology institutes, including a full-beam zero check, a white-light (100%) check, and haze reference plaques. Instrument aperture size should be selected according to specimen dimensions, and at least three, preferably five, replicate specimens are measured with readings taken at multiple positions to account for material non-uniformity.
Test Report
A compliant test report should identify the material and supplier, specimen thickness and conditioning, the test standard and instrument used, individual and mean values of total transmittance, regular transmittance, diffuse transmittance, and haze, spectral transmittance curves where applicable, and the date of test and laboratory accreditation status.
Applications
Food and Beverage Packaging
Light exposure accelerates the degradation of riboflavin, vitamins, natural pigments, and lipids. Transmittance data are used to select bottle walls, films, and lidding materials for dairy products, edible oils, beer, and juices, where excessive light transmission can cause off-flavors and nutrient loss during the declared shelf life.
Pharmaceutical and Nutraceutical Packaging
Pharmacopoeial light transmission limits restrict the transmittance of containers for light-sensitive drug products, and transmittance data support photostability studies and the selection of amber and opaque container systems. Blister films, infusion bottles, and secondary packaging sleeves are all routinely characterized.
Agricultural Films
- Greenhouse films are optimized for high visible light transmittance combined with controlled UV blocking to regulate plant growth and inhibit pest propagation.
- Mulch films and silage films are evaluated for their light-screening performance and their effect on photosynthetically active radiation.
Consumer Electronics and Display Packaging
Protective window films, transparent blister lids, and display-grade substrates require documented transmittance and haze values to guarantee optical clarity, anti-glare behavior, and UV protection for sensitive electronic components.
FAQ
What is the difference between light transmittance and haze?
Light transmittance measures the total quantity of light passing through a material, while haze measures the fraction of that transmitted light which is scattered by more than 2.5 degrees. A material can show high transmittance yet high haze, appearing cloudy rather than optically clear.
Which standard should be selected: ASTM D1003, ISO 13468, or GB/T 2410?
The choice depends on the destination market and contractual requirements. The three standards are technically harmonized in optical geometry and photometric calculation, and accredited third-party laboratories can generally report results under any of them, often with a note confirming equivalence.
What specimen dimensions and how many specimens are required?
Specimens must fully cover the instrument measurement port, commonly 25 mm to 50 mm in diameter, and be flat, clean, and free of defects. A minimum of three specimens, each measured at several positions, is recommended, with the arithmetic mean reported.
Can curved bottles and containers be tested directly?
Most integrating-sphere instruments require flat specimens, so representative flat sections are cut from the container wall, or plaques equivalent to the container material and thickness are molded for measurement. Some specialized accessories allow limited measurement of cylindrical surfaces.
What measurement uncertainty can be expected?
For well-prepared specimens on calibrated equipment, repeatability of transmittance values within approximately 0.5% absolute is typical, while inter-laboratory reproducibility is generally within about 1% to 2% absolute, as documented in the precision statements of the corresponding standards.