Light transmittance testing quantifies the amount of visible and ultraviolet radiation that passes through transparent and translucent packaging materials, providing essential data for product protection, shelf presentation, and regulatory compliance. This technical article explains the measurement principles, applicable standards, sample types, instrumentation, and test procedures used in the optical characterization of packaging materials, with reference to ASTM D1003, ISO 13468, ISO 14782, and GB/T 2410. It is intended for packaging engineers, quality-control professionals, and material developers who require accurate and reproducible optical performance data.
Technical Overview
Light transmittance, formally known as total luminous transmittance (Tt), is defined as the ratio of the luminous flux transmitted through a specimen to the luminous flux incident upon it, expressed as a percentage. The measurement is spectrally weighted according to the CIE 1931 standard photopic observer and a specified standard illuminant (typically CIE Illuminant A for hazemeter measurements or D65 for colorimetric applications), ensuring that reported values correlate with human visual perception under normal viewing conditions.
For packaging materials, optical performance fulfills two distinct and often competing functions: presentation and protection. High transmittance and low haze allow consumers to see the product clearly, which is essential for shelf appeal, while deliberate light blocking—particularly in the ultraviolet region between 200 nm and 400 nm—protects light-sensitive contents such as edible oils, dairy products, beer, vitamins, and photolabile pharmaceutical actives from photodegradation, nutrient loss, discoloration, and off-flavor formation.
A complete optical characterization of a packaging material typically evaluates the following interrelated parameters:
- Total luminous transmittance (Tt), the overall percentage of visible light passing through the specimen
- Diffuse transmittance (Td), the portion of transmitted light scattered from its original direction
- Haze, the percentage of transmitted light that is scattered more than 2.5 degrees, causing a milky or cloudy appearance
- Clarity, describing narrow-angle scattering that affects see-through image definition
- Ultraviolet transmittance and UV blocking rate over the 200 nm to 400 nm range
- Full spectral transmittance curve across the UV and visible regions
- Color coordinates (CIE L*a*b*) and yellowness index, which quantify material coloration and aging
Testing Samples
Flexible Plastic Films and Laminates
This category includes mono-layer and multi-layer films of polyethylene (PE), polypropylene (CPP and BOPP), polyethylene terephthalate (PET), polyamide (PA), polyvinyl chloride (PVC), ethylene-vinyl alcohol (EVOH) barrier structures, polylactic acid (PLA) biodegradable films, and shrink and lidding films. Typical thicknesses range from approximately 15 micrometers to 200 micrometers. Films must be tested in the state in which they are used, including any printing, coating, or metallization, since these layers substantially alter optical behavior.
Rigid Plastic Sheets and Thermoformed Containers
Rigid samples include APET, RPET, PETG, PP, HIPS, polycarbonate, and PMMA sheets used for thermoformed trays, clamshells, and transparent windows. These materials are generally thicker than flexible films, commonly 200 micrometers to several millimeters, and their optical properties are strongly influenced by processing parameters such as cooling rate, orientation, and recycled-content fraction.
Pharmaceutical Blister and Container Materials
Pharmaceutical packaging samples include PVC and PVDC-coated blister films, fluoropolymer laminate base films, cold-formable aluminum structures, plastic bottles, vials, and ampoules. These materials are frequently evaluated against pharmacopoeial light-transmission requirements to justify the suitability of the container for light-sensitive drug products.
Glass Packaging
Soda-lime and borosilicate glass containers, including colored amber and green glass, are tested to verify their light-attenuation performance. Flat sections are cut from the container wall for laboratory measurement because the container geometry does not permit direct whole-container measurement in standard instruments.
Testing Items
Depending on the product requirements and the governing specification, one or more of the following optical properties are determined:
- Total luminous transmittance (Tt, in percent), measured per ASTM D1003, ISO 13468-1, ISO 13468-2, or GB/T 2410
- Haze (in percent), measured per ASTM D1003, ISO 14782, or GB/T 2410
- Regular (specular) transmittance, measured per ASTM D1746 for flat transparent sheet
- UV-A transmittance (315 nm to 400 nm), UV-B transmittance (280 nm to 315 nm), and integrated UV blocking rate over 200 nm to 400 nm
- Transmittance at specified wavelengths of concern, for example 310 nm or 350 nm, relevant to photostability assessment
- Spectral transmittance curves across 200 nm to 800 nm for barrier-performance evaluation
- Color coordinates and yellowness index computed per ASTM E313 and ASTM E308
- Comparative optical properties before and after accelerated weathering or photostability exposure
Testing Methods
Applicable Standards
The following internationally recognized standards govern the determination of optical properties of packaging materials:
- ASTM D1003 — Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, providing Method A (hazemeter) and Method B (spectrophotometer)
- ISO 13468-1 and ISO 13468-2 — Plastics: Determination of the total luminous transmittance of transparent materials, using single-beam and double-beam procedures respectively
- ISO 14782 — Plastics: Determination of haze for transparent materials
- ASTM D1746 — Standard Test Method for Regular Transmittance of Transparent Sheet
- GB/T 2410 — Chinese national standard for the determination of light transmittance and haze of transparent plastics, technically aligned with the corresponding ISO methods
- JIS K 7361-1 and JIS K 7136 — Japanese industrial standards equivalent to ISO 13468-1 and ISO 14782
- Pharmacopoeial light-transmission chapters, such as USP General Chapter 671, applied to containers intended for light-sensitive preparations
Instrumentation
Two principal instrument types are used. A dedicated hazemeter employs an integrating sphere, a tungsten-halogen light source filtered to CIE Illuminant A, and a detector matched to the photopic response of the human eye. It sequentially measures four flux values: the incident luminous flux (T1), the total transmitted flux with the specimen in place (T2), the instrument scatter with the specimen removed and the sphere exit port open (T3), and the combined transmitted and scattered flux with the specimen in place and the exit port open (T4).
A UV-Vis spectrophotometer, preferably a double-beam instrument optionally equipped with an integrating sphere coated with barium sulfate or PTFE, performs spectral scans from 200 nm to 800 nm. This configuration separates total transmittance from diffuse transmittance and enables wavelength-specific UV blocking analysis that a broadband luminous measurement cannot provide.
Specimen Preparation and Conditioning
Representative specimens, typically 50 mm by 50 mm or of sufficient size to fully cover the instrument port, are cut from different positions across the material web to capture spatial variability. At least three, and preferably five, specimens per material are tested and averaged. Key preparation requirements include:
- Conditioning at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618, unless the governing specification states otherwise
- Exclusion of specimens exhibiting wrinkles, scratches, gate marks, fingerprints, or embedded contamination
- Gentle cleaning with a lint-free non-abrasive cloth, avoiding solvents that could swell, etch, or stress-craze the surface
- Measurement and reporting of specimen thickness at the test location, since transmittance decreases approximately exponentially with thickness according to the Beer–Lambert relationship
- Cutting of flat panels from the walls of bottles and containers, with the sampling location and orientation documented
Measurement Procedure and Calculation
The instrument is first zeroed using a light trap and full-scale calibrated with an open beam or a traceable transmittance reference filter. Verification against certified transmittance standards is performed at regular intervals. After calibration, the four flux measurements are acquired for each specimen, and the results are calculated as follows:
Total luminous transmittance: Tt = (T2 / T1) × 100 %. Haze (corrected for instrument scatter) = [(T4 / T2) − (T3 / T1)] × 100 %. For spectrophotometric determinations, the total luminous transmittance is obtained by integrating the spectral transmittance weighted by the illuminant spectral distribution and the photopic observer function over 380 nm to 780 nm.
The test report should identify the standard applied, the instrument and measurement geometry, the illuminant and observer, the specimen thickness and conditioning history, individual and mean values for transmittance and haze, and any deviations from the standard method. Accredited third-party laboratories typically achieve repeatability within approximately 0.5 % transmittance units on homogeneous transparent materials, provided that specimen preparation and conditioning are tightly controlled.
Applications
Light transmittance and optical property determination supports packaging development, quality control, and regulatory compliance across a wide range of industries:
- Food and beverage packaging: verification of UV barriers in PET beer bottles and dairy containers, shelf-life protection for edible oils and juices, and clarity validation for transparent windows in thermoformed trays
- Pharmaceutical packaging: qualification of blister films, bottles, and vials against pharmacopoeial light-transmission limits, and support of photostability studies conducted in accordance with ICH Q1B
- Cosmetics packaging: assessment of transparent serum bottles and ampoules where both product visibility and protection of active ingredients from light-induced degradation are required
- Agricultural films: evaluation of greenhouse films where the balance between total transmittance and haze governs photosynthetically active radiation delivery and light diffusion to the crop canopy
- Electronics and display packaging: confirmation of optical clarity and low haze for protective films used on screens, lenses, and optical components
- Quality control and R&D: incoming material inspection, monitoring of process variables such as film thickness, additive dispersion, and quench conditions, and benchmarking of recyclate content such as RPET against virgin resin performance
FAQ
What is the difference between light transmittance and haze?
Light transmittance measures the total percentage of incident visible light that passes through the specimen, regardless of direction. Haze measures only the fraction of transmitted light that is scattered by more than 2.5 degrees from the original beam path. A material can therefore exhibit high transmittance together with high haze; it transmits ample light but appears cloudy and obscures the view of the packaged product.
Which standard should be selected for testing?
ASTM D1003 and GB/T 2410 are the most widely applied methods for combined transmittance and haze measurement of plastic packaging, while ISO 13468 and ISO 14782 are preferred when international alignment is required. The choice should follow the governing product specification, the target market region, and any regulatory or customer-mandated requirement. Pharmacopoeial methods apply specifically to pharmaceutical containers.
How does specimen thickness influence the results?
Transmittance decreases approximately exponentially with increasing thickness, and haze generally increases as thickness grows due to additional bulk and surface scattering. Because of this, optical results are only comparable between specimens of identical nominal thickness, and ISO 13468 requires that thickness be reported together with the transmittance value.
Can bottles and curved containers be tested directly?
Standard instruments require flat specimens, so flat panels are sectioned from the container wall at documented locations. For whole-container light protection assessment, spectral transmittance data from sectioned panels can be combined with wall thickness distribution measurements to model the total light dose reaching the product.
Which wavelengths are evaluated for UV protection performance?
The full UV region from 200 nm to 400 nm is scanned, with UV-B (280 nm to 315 nm) and UV-A (315 nm to 400 nm) bands reported separately. Many specifications also require the transmittance value at a single critical wavelength, such as 310 nm or 350 nm, because these wavelengths are strongly associated with photochemical degradation of foods, vitamins, and drug substances.
How should the results be reported for compliance purposes?
The report should state the applied standard, instrument type, illuminant and observer conditions, specimen description and thickness, conditioning parameters, individual and mean transmittance and haze values, and the testing laboratory scope. Results issued by accredited third-party laboratories in accordance with ISO/IEC 17025 provide the highest level of acceptance for regulatory submissions and customer qualification.