Light transmittance is a key optical performance parameter for packaging materials, directly affecting product visibility, shelf presentation, and protection of light-sensitive contents. This article systematically introduces the testing principles, applicable standards, sample requirements, instrumentation, and operating procedures for measuring the light transmittance of packaging materials, referencing international and national standards such as ISO 13468, ASTM D1003, and GB/T 2410.
Technical Overview
Light transmittance, also referred to as luminous transmittance, is defined as the ratio of luminous flux transmitted through a material to the luminous flux incident upon it, expressed as a percentage. For packaging materials, this parameter quantifies how much visible light passes through films, sheets, or container walls under standardized illumination conditions, typically using CIE Standard Illuminant A and the CIE 1931 standard photopic observer.
Transmittance measurement is closely related to two other optical quantities: regular transmittance, which counts only light transmitted without scattering, and haze, which characterizes wide-angle light scattering within a material. Together, these parameters determine whether a package appears clear, hazy, translucent, or opaque.
The significance of transmittance testing in packaging includes:
- Verifying protection of light-sensitive contents such as dairy products, edible oils, beer, vitamins, and pharmaceuticals from photodegradation
- Controlling product visibility and shelf presentation for transparent and windowed packaging
- Evaluating UV-blocking and light-shielding performance of functional films and laminates
- Supporting incoming material inspection, batch release, and consistency control in production
- Providing objective data for regulatory compliance and customer specifications
Testing Samples
Flexible Films and Laminates
Single-layer and multilayer films such as PE, PP, PET, PA, EVOH barrier films, and aluminum-free transparent laminates used for food, cosmetics, and medical device packaging. Specimens are typically cut into flat sections of an appropriate size for the instrument aperture.
Rigid Sheets and Thermoformed Components
Transparent or translucent sheets including PET, PVC, PS, PP, and PC materials used for blister packs, clamshells, trays, and rigid boxes. Thickness must be reported because transmittance decreases as thickness increases.
Hollow Containers and Bottles
Pharmaceutical bottles, oral liquid containers, dropper bottles, and beverage bottles. Curved containers are usually tested by cutting flat sections from the bottle wall, or by using specially designed fixtures that present the container wall to the light beam.
Other Packaging Materials
Paper-plastic composites, transparent window films on cartons, labels, overwrap films, and light-shielding or photosensitive packaging materials requiring demonstration of low transmittance.
Testing Items
Depending on the product specification and applicable standard, the following optical items can be measured:
- Total luminous transmittance (Tt): overall visible light transmission performance
- Regular transmittance (Tr): transmission of undeviated light, indicating optical clarity
- Haze: percentage of transmitted light scattered by more than 2.5 degrees, indicating milkiness or cloudiness
- Spectral transmittance: transmittance as a function of wavelength across the UV-Vis range, typically 200 nm to 800 nm
- UV transmittance at specified wavelengths: for example, transmittance between 290 nm and 450 nm for evaluating the light-protective capability of pharmaceutical containers
Testing Methods
Method A: Spectrophotometer Method
A UV-Visible spectrophotometer is used to measure transmittance either at specified wavelengths or by full spectral scanning. A blank reference (usually air or a standard white reference) is first measured to establish the baseline, then the flat specimen is placed perpendicular to the light beam in the sample compartment. This method is specified in standards such as GB/T 2410 and ASTM D1003 (Procedure A) and is suitable for precise spectral characterization.
Method B: Integrating Sphere Hazemeter Method
A hazemeter equipped with an integrating sphere simultaneously measures total transmittance and diffuse transmittance. The difference between the two yields regular transmittance, and haze is calculated as the ratio of diffuse to total transmittance. This approach follows ASTM D1003 (Procedure B), ISO 13468-2, and ISO 14782, and is the preferred method for slightly hazy or textured packaging materials where light scattering is significant.
Pharmaceutical Light-Protection Testing
For light-resistant pharmaceutical packaging, spectral transmittance is scanned from 290 nm to 450 nm. Common acceptance criteria require that transmittance at any wavelength within this range not exceed a defined limit, such as 10%, in accordance with pharmacopoeial requirements for containers intended to protect contents from light. Testing is typically performed by accredited third-party laboratories or in-house pharmaceutical quality laboratories.
Calculation and Expression of Results
Total luminous transmittance is calculated as T = It / I0 × 100%, where I0 is the incident luminous flux and It is the transmitted luminous flux. Haze is calculated as H = Td / Tt × 100%, where Td is diffuse transmittance. Multiple specimens, commonly three to five per sample, are measured and the arithmetic mean is reported along with specimen thickness, illuminant condition, and the standard used.
Applications
Light transmittance testing is widely applied across the packaging and materials industries:
- Food and beverage packaging: evaluating light-shielding of films and bottles for dairy, beer, juice, and edible oils to prevent nutrient loss and off-flavor formation
- Pharmaceutical packaging: verifying light-resistant performance of tablets, capsules, and oral liquid containers against photodegradation requirements
- Agricultural packaging films: characterizing light transmission of greenhouse and mulch films for crop growth optimization
- Consumer and electronics packaging: confirming the optical clarity of transparent windows, blister packs, and display packaging
- Research and development: comparing formulation, thickness, and processing effects on optical properties of new packaging materials
- Quality control: incoming inspection of packaging raw materials and batch-to-batch consistency verification during production
FAQ
What is the difference between transmittance and haze?
Transmittance measures the total amount of light passing through a material, while haze measures the proportion of transmitted light that is scattered at wide angles. A material can have high transmittance yet high haze, appearing milky rather than optically clear.
Which standard should be selected for testing?
ASTM D1003 and ISO 13468 are commonly used internationally for luminous transmittance of transparent plastics, while GB/T 2410 is widely applied in China and is technically consistent in principle. Pharmaceutical light-protection testing should follow the relevant pharmacopoeial transmittance limits. The choice depends on customer requirements, target market, and regulatory context.
Can curved containers such as bottles be tested directly?
Generally, flat sections are cut from the container wall and tested as flat specimens to avoid beam deviation caused by curvature. Where cutting is not permitted, dedicated fixtures or specialized optical systems may be used to evaluate whole containers.
How does specimen thickness affect the results?
Transmittance decreases as material thickness increases due to absorption and scattering. Specimen thickness must always be measured and reported together with the transmittance value to ensure comparability between different materials or batches.
What specimen preparation is required before testing?
Specimens should be clean, free of creases, scratches, fingerprints, and surface contamination, and should be conditioned at standard laboratory temperature and humidity, typically 23 °C and 50% relative humidity, before measurement.