Light transmittance evaluation is a critical quality control method for packaging materials, quantifying how much visible and ultraviolet radiation passes through films, sheets, and containers that directly affect product shelf life, photostability, and visual presentation. This technical article outlines the applicable international standards, sample requirements, testing items, and laboratory procedures used by accredited third-party laboratories to measure total luminous transmittance, haze, and UV-blocking performance of packaging materials.
Technical Overview
Light transmittance is defined as the ratio of luminous flux passing through a specimen to the luminous flux incident upon it, expressed as a percentage. For packaging materials, this optical property governs two essential functions: protecting light-sensitive contents such as fats, vitamins, pharmaceuticals, and pigments from photodegradation, and preserving the transparent or translucent appearance expected by consumers at the point of sale. A complete optical evaluation distinguishes between total luminous transmittance, regular (specular) transmittance, diffuse transmittance, haze, and clarity, because two materials with identical total transmittance can present very different see-through quality depending on how the transmitted light is scattered.
The principal standards governing this evaluation include ASTM D1003 for haze and luminous transmittance of transparent plastics, ISO 13468-1 and ISO 13468-2 for total luminous transmittance using single-beam and double-beam instruments respectively, ISO 14782 for haze determination, GB/T 2410 as the Chinese national equivalent covering both transmittance and haze, and JIS K 7361-1 and JIS K 7136 as the Japanese counterparts. All of these methods rely on integrating-sphere optics that collect both directly transmitted and scattered light under a defined light source and photopic response approximating human vision.
- Total luminous transmittance (Tt): overall light-passing capability in the visible spectrum (380–780 nm)
- Haze: percentage of transmitted light scattered at wide angles, causing a milky or cloudy appearance
- Clarity: see-through resolution affected by narrow-angle scatter, evaluated per methods such as ASTM D1746 and ISO 21996
- UV transmittance: blocking performance in the ultraviolet region relevant to product protection
Testing Samples
Flexible Plastic Films
Single-layer and multilayer films such as BOPP, CPP, PE, PET, PVC, EVOH, PA, and PLA laminates are the most frequently tested sample category. Specimens should be at least the size required by the instrument port (commonly 50 mm × 50 mm), free from wrinkles, creases, surface contamination, and trapped air bubbles between layers. At least three to five specimens taken from different positions across the web should be measured to account for thickness and additive uniformity variations, and results are reported as the mean value with its range.
Rigid Sheets and Container Sections
Blister films (PVC, PVDC-coated PVC, PETG), thermoforming sheets, and flat sections cut from bottles, jars, or preforms can be evaluated provided a representative flat area is obtained. Curved container walls must be cut and flattened carefully without introducing stress birefringence or surface scratches, and the measured thickness must be recorded because transmittance and haze both increase with material thickness. Glass containers are tested on cut flat panels using the same optical geometry.
Sample Preparation and Conditioning
All specimens must be cleaned with lint-free wipes and, if necessary, a suitable solvent that does not swell or etch the polymer. Unless otherwise specified, conditioning is performed at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or the equivalent conditioning clauses of the test standards. Protective release films and slip sheets should be removed immediately before measurement.
Testing Items
Depending on the product specification and regulatory context, a transmittance evaluation program for packaging materials typically includes the following measurable items:
- Total luminous transmittance, expressed as a percentage of incident visible light
- Regular transmittance and diffuse transmittance, obtained by separating direct and scattered components
- Haze percentage, calculated as the ratio of diffuse transmittance to total transmittance
- Clarity or see-through quality for high-transparency films used in display and window applications
- Spectral transmittance curves scanned from 200 nm to 800 nm for full optical characterization
- Ultraviolet transmittance and UV-blocking rate, commonly reported for specific bands such as UVA (315–400 nm) and UVB (280–315 nm)
- Light-protective performance against defined wavelength thresholds for photosensitive products
Testing Methods
Hazemeter Method (ASTM D1003 and ISO 14782)
The hazemeter uses a collimated beam from a standardized light source, typically Illuminant A simulating a tungsten lamp, directed through the specimen into an integrating sphere fitted with a light trap. Four successive readings are taken: incident light without the specimen, total transmitted light with the specimen, sphere scatter without the specimen, and combined instrument-and-specimen scatter. Total transmittance is computed as the ratio of total transmitted to incident light, while haze is obtained after correcting the diffuse component for instrumental scatter. Wide-angle scattering, generally beyond the small acceptance cone defined by the standard, is what constitutes haze.
Spectrophotometric Method (ISO 13468)
ISO 13468-1 describes a single-beam procedure and ISO 13468-2 a double-beam procedure using a spectrophotometer equipped with an integrating sphere. The specimen is scanned across the visible spectrum, and the transmittance values are weighted by the spectral distribution of a standard daylight source combined with the photopic luminosity function of the human eye to yield total luminous transmittance. The double-beam configuration compensates in real time for source drift and is preferred for high-precision work or dark, low-transmittance materials.
UV Transmittance Measurement
For packaging intended to shield photosensitive contents, specimens are scanned in the ultraviolet region using a UV-visible spectrophotometer with an integrating sphere attachment. The results quantify how efficiently UV absorbers, pigments, or metallized layers block damaging wavelengths. When a specific protection threshold is required, transmittance is evaluated at or below the stated wavelength limit, for example below 450 nm for certain amber pharmaceutical containers.
Data Reduction and Reporting
A compliant test report identifies the standard applied, the light source and observer conditions, specimen thickness, number of specimens, mean values, and standard deviation. Because results from Illuminant A and daylight-illuminant configurations differ slightly, the illumination condition must always be stated to allow valid comparison between laboratories.
Applications
Transmittance evaluation supports material selection, incoming inspection, and failure analysis across the packaging industry. Typical application scenarios include:
- Food and beverage packaging: verifying that clear films and bottles deliver both retail transparency and adequate protection against photo-oxidation of lipids, riboflavin and vitamin degradation, and lightstruck flavor formation
- Pharmaceutical packaging: demonstrating the light-protective capability of blister films, amber bottles, and laminated sachets for photosensitive drug products stored under defined light-exposure conditions
- Agricultural films: confirming greenhouse cover films transmit sufficient photosynthetically active radiation in the 400–700 nm band while scattering light for even canopy illumination
- Consumer goods and cosmetics: balancing decorative tinting with content visibility, and validating anti-fog and anti-reflective coatings
- Electronics packaging: qualifying protective films for displays and optical components where total transmittance above 90 % and haze below 2 % are common acceptance targets
- Supplier qualification and quality control: benchmarking film lots, verifying UV-absorber masterbatch dosing, and investigating field complaints of discoloration or product degradation
FAQ
What is the difference between light transmittance and haze?
Light transmittance measures how much light passes through the material in total, while haze measures the portion of that transmitted light that is scattered at wide angles. A film can have 90 % transmittance yet appear cloudy if its haze is high, so both parameters must be evaluated together for transparent packaging.
Are ASTM D1003, ISO 13468, and GB/T 2410 interchangeable?
They are technically aligned but not strictly interchangeable. ASTM D1003 and GB/T 2410 combine transmittance and haze in one method, whereas ISO 13468 and ISO 14782 address them separately. The standards also specify different light source conditions, so laboratories must report which standard and illuminant were used and should not compare results across methods without confirming equivalency.
How does specimen thickness affect the results?
Both total transmittance generally decrease and haze generally increase as thickness increases, because absorption and scattering accumulate over the optical path. Specimens must therefore be tested at the nominal thickness of the finished package, and thickness must accompany every reported result.
Can finished bottles, blisters, and formed containers be tested?
Yes. Flat, representative sections are cut from the formed article, provided the cutting process does not introduce scratches, stress, or deformation that would alter the optical reading. Preform sections may also be used when they represent the final container wall.
Why do two laboratories sometimes report slightly different values?
Differences arise from illuminant choice, sphere geometry, specimen conditioning, and surface handling. Engaging accredited third-party laboratories that operate calibrated integrating-sphere instruments under documented standard procedures minimizes inter-laboratory variation and ensures legally and commercially defensible data.