Precise Determination of Light Transmittance in Packaging Materials

Light transmittance is a critical optical quality parameter for transparent and translucent packaging materials, directly influencing product visibility, shelf appeal, and the light protection of sensitive contents such as foods, vitamins, and pharmaceuticals. This technical article explains how precise transmittance measurement is performed on packaging films, sheets, and containers in accordance with recognized standards such as ASTM D1003, ISO 13468, and GB/T 2410, covering instrumentation, sample preparation, test procedures, reporting requirements, and typical industrial applications.

Technical Overview

Light transmittance quantifies the fraction of incident luminous flux that passes through a packaging material. Total luminous transmittance (Tt) is defined as the ratio of transmitted light to incident light, expressed as a percentage, measured using the photopic response of the human eye under a standardized light source. For packaging applications, the property governs two competing requirements: on one hand, high clarity is desired so consumers can inspect the product; on the other hand, reduced transmittance in the ultraviolet and visible ranges protects photosensitive contents from degradation.

Transmittance measurement must be distinguished from related optical quantities. Regular (specular) transmittance describes light passing through without directional change, while diffuse transmittance describes light scattered by the material. Haze is the percentage of transmitted light that deviates by more than 2.5 degrees (per ASTM D1003) or 4 degrees (per ISO 14782) from the incident beam. A material may therefore show high total transmittance but unacceptable haze, appearing cloudy despite transmitting most incident light.

Several internationally recognized methods govern the precise determination of transmittance in packaging materials, including ASTM D1003 (haze and luminous transmittance of transparent plastics), ISO 13468-1 and ISO 13468-2 (total luminous transmittance using single-beam and double-beam instruments), ASTM D1746 (regular transmittance of transparent sheet), ISO 14782 (haze), and the Chinese national standard GB/T 2410, which is technically aligned with ASTM D1003. Test environments, instrument geometry, and photometric calibration are strictly defined in these standards to ensure reproducible, comparable results between laboratories.

Testing Samples

Flexible Films

Single-layer and multilayer flexible films, including LDPE, LLDPE, CPP, OPP, PET, PVC, PVDC-coated films, PLA biopolymer films, and cellophane, are among the most frequently tested packaging formats. Typical specimen thickness ranges from 15 to 120 micrometers. Films must be flat and free of wrinkles, since any curvature at the measurement port introduces measurement error.

Rigid Sheets and Blister Films

Rigid transparent sheets used for thermoformed trays, clamshells, and blister packaging, such as PVC, PVC/PVDC, PETG, amorphous PET, and cyclic olefin copolymer, are tested in thicknesses commonly ranging from 0.15 to 1.0 mm. Surface texture, embossing, and forming-induced orientation can substantially affect both transmittance and haze.

Bottles, Jars, and Containers

For containers, flat specimens are typically cut from the sidewall or base of PET bottles, PP jars, and similar articles. Because curvature focuses or scatters the beam, curved sections must be flattened or measured using dedicated holders, and this deviation is noted in the test report.

Coated, Metallized, and Laminated Structures

Transparent barrier laminates, anti-fog coated films, oxide-coated high-barrier films, and lightly metallized structures can be measured for total transmittance through the complete laminate. Fully opaque metallized foils yield near-zero transmittance and are more appropriately evaluated by reflectance or opacity methods.

Paper-Based and Translucent Materials

Translucent packaging such as glassine, wax-coated paper, and mineral-filled films transmit light predominantly by diffuse mechanisms. These materials are measured for total transmittance, and their light-diffusing behavior is separately characterized by haze or opacity testing.

Sample Preparation and Conditioning

• Condition specimens at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618.
• Clean both surfaces with a lint-free cloth; never use solvents that could alter, swell, or coat the surface.
• Avoid fingerprints, scratches, dust, and static attraction of particles, all of which artificially reduce transmittance.
• Cut specimens of at least 50 mm × 50 mm to fully cover the instrument aperture, typically 25 to 30 mm.
• Prepare a minimum of three, preferably five, specimens per material sampled from different positions across the web or article.

Testing Items

Depending on the packaging format, regulatory context, and end-use requirement, the following optical properties are routinely determined by accredited third-party laboratories and in-house quality control departments:

• Total luminous transmittance (Tt) over the visible spectrum, photopically weighted.
• Regular (specular) transmittance for direct, non-scattered light.
• Diffuse transmittance, calculated as the difference between total and regular transmittance.
• Haze percentage per ASTM D1003 or ISO 14782.
• Spectral transmittance scanned from 200 to 800 nm for light-protection assessment.
• Ultraviolet blocking rate across the UVA and UVB regions (280 to 400 nm).
• Optional color and tint characterization of the transmitted light using CIE colorimetry.

Testing Methods

Instrumentation

Measurements are performed either with an integrating-sphere UV-Vis spectrophotometer or with a dedicated hazemeter. ASTM D1003 offers Method B for hazemeter-type instruments and Method A for spectrophotometer-based measurement; ISO 13468-1 addresses single-beam geometries while ISO 13468-2 addresses double-beam designs that provide superior stability. The standard measurement condition employs a tungsten light source approximating Standard Illuminant A (correlated color temperature 2856 K) combined with a detector matching the photopic luminosity function, so that reported values correlate with human visual perception.

Calibration and Baseline Correction

Before testing, the instrument photometric zero is set with the beam blocked, and a full-scale baseline is established with the transmission port open or fitted with a certified reference standard traceable to national metrology institutes. Reference transmittance standards in the ranges of approximately 10 %, 30 %, 60 %, and 90 % are used to verify linearity, and calibration records are retained as part of the quality system.

Total Transmittance Procedure

The incident luminous flux (T1) is measured with the port empty, and the total transmitted flux (T2) is measured with the specimen mounted at the transmission port, normal to the beam. Total luminous transmittance is calculated as Tt = 100 × T2 / T1. Each specimen is measured at least three times, repositioning between readings, and the mean of all specimens is reported together with the range.

Haze and Regular Transmittance Procedure

For haze determination, two additional readings are taken: T3, the flux scattered by the instrument itself with no specimen present and the exit port closed, and T4, the flux with the specimen in place and the exit port closed. Haze is computed from these four quantities per the formula in ASTM D1003 or ISO 14782, with instrumental scatter correction applied. Regular transmittance of smooth transparent sheet is measured with a collimated beam geometry at normal incidence in accordance with ASTM D1746, using a limited-acceptance-angle detector.

Spectral Scanning for Light Protection Studies

For photosensitive products, specimens are scanned across the ultraviolet and visible regions, typically from 200 to 800 nm at 1 to 5 nm intervals. The photopic transmittance is integrated from the spectrum, and the UV blocking percentage is calculated over the 280 to 400 nm band. This allows packaging engineers to compare clear, tinted, amber, and UV-absorbing structures quantitatively.

Reporting Requirements

• Material identification, nominal and measured thickness, and structure of the specimen.
• Test standard and method applied, including instrument type and sphere geometry.
• Conditioning atmosphere and duration.
• Individual values, mean, and range for total transmittance, and haze where required.
• Any deviations, such as curved specimen mounting or anisotropic films.

Applications

• Food packaging: verifying the clarity of window films, lidding films, and thermoformed trays so that products remain visually inspectable at the point of sale.
• Pharmaceutical packaging: qualifying clear, amber, and UV-blocking containers, blister films, and infusion bags against light-induced degradation of active ingredients and excipients.
• Cosmetics and personal care: balancing premium glass-like clarity with protection of light-sensitive formulations such as essential oils and vitamins.
• Barrier film development: quantifying the optical penalty of coatings, tie layers, and metallization in high-barrier laminate design.
• Incoming inspection and supplier qualification: setting numerical acceptance limits for transmittance and haze in procurement specifications.
• Shelf-life and photostability studies: correlating measured spectral transmittance with product stability data to justify packaging choices.
• Regulatory and customer compliance: generating test reports from accredited third-party laboratories for market submission and buyer audits.

FAQ

What is the difference between transmittance and haze?

Transmittance measures how much light passes through the material, while haze measures how much of the transmitted light is scattered away from the incident direction. A film can transmit 90 % of light yet appear hazy if a large share of that light is diffused. Both properties are needed to fully describe visual quality, and both are obtained in a single ASTM D1003 test sequence.

Which standard should be selected for testing?

ASTM D1003 and GB/T 2410 are the most widely applied methods for packaging plastics and are technically harmonized; ISO 13468 is preferred when international alignment with European customers is required; ASTM D1746 is used when only regular, non-scattered transmittance is relevant. The choice should be agreed between supplier and buyer and stated in the specification.

Is transmittance corrected for specimen thickness?

No. Standards report transmittance as measured at the actual specimen thickness. Because transmittance decreases roughly exponentially with thickness, the measured thickness must always accompany the result, and comparisons between materials are only valid at equivalent thickness.

Can metallized or fully opaque packaging be tested for transmittance?

Such materials typically transmit less than 1 % of light, which is at or below the practical detection limit for routine transmittance testing. Light-interaction properties are then better characterized by opacity or reflectance measurements, for example ISO 2471 for paper and board, or by full spectral reflectance analysis.

How are bottles and other curved containers handled?

Flat panels are cut from the container wall and tested as sheet specimens. Where the design intent requires testing the intact article, specialized holders minimize beam deviation, and the report must clearly state that curvature influences the result.

What precision can be expected from the measurement?

Within a single laboratory using a well-maintained, calibrated instrument, repeatability of total transmittance on homogeneous film is typically within ±0.5 %. Between laboratories, reproducibility is influenced by instrument geometry, calibration standards, and specimen handling, which is why comparative testing is best performed by accredited third-party laboratories with traceable calibration and documented procedures.