Light Transmittance and Light Transmission Testing for Packaging Materials

Light transmittance and light transmission testing quantifies how much visible and ultraviolet light passes through packaging materials such as plastic films, sheets, bottles, and laminates. Accurate measurement of total transmittance, regular transmittance, haze, and UV transmission is essential for product protection, shelf-life performance, and regulatory compliance. This technical article explains the principles, standards, sample requirements, test methods, and applications of optical transmission testing for packaging materials.

Technical Overview

Light transmittance, also referred to as luminous transmittance, is the ratio of the luminous flux transmitted through a specimen to the luminous flux incident upon it, expressed as a percentage. It is one of the most important optical quality indicators for transparent and translucent packaging materials, because it determines both how well consumers can see the packaged product and how much light energy reaches the product itself. Excessive light exposure accelerates photochemical degradation reactions in foods, beverages, pharmaceuticals, and cosmetics, causing nutrient loss, color fading, rancidity, and loss of potency.

In packaging science, two related quantities are typically distinguished. Total luminous transmittance includes all transmitted light, both the portion that passes directly through the material (regular or specular transmission) and the portion that is scattered by the material (diffuse transmission). Haze is defined as the percentage of transmitted light that deviates from the incident beam direction by more than 2.5 degrees, and it quantifies the milkiness or cloudiness of a material. For light-sensitive products, wavelength-specific transmission in the ultraviolet range (280–400 nm) is also evaluated, since UV radiation is the most energetic driver of photodegradation.

Testing is performed using hazemeters and ultraviolet-visible spectrophotometers equipped with integrating spheres, in accordance with internationally recognized methods published by ISO, ASTM, GB, JIS, and pharmacopoeial standards organizations. Accredited third-party laboratories routinely conduct these tests to support material qualification, quality control, and regulatory submissions.

Testing Samples

Flexible Films and Laminates

Single-layer and multilayer flexible films are the most commonly tested packaging samples. These include polyethylene (PE), polypropylene (PP), cast and oriented polyester (PET), polyamide (PA), polyvinyl chloride (PVC), cellophane, and high-barrier laminates containing aluminum oxide or silicon oxide coatings. Specimens must be flat, wrinkle-free, and free of dust, fingerprints, and surface scratches. Standard practice is to cut specimens slightly larger than the instrument aperture, typically 50 mm × 50 mm squares or circular discs.

Rigid Sheets and Thermoformed Articles

Rigid and semi-rigid sheets made from PET, PVC, polystyrene (PS), polycarbonate (PC), polymethyl methacrylate (PMMA), and polyethylene terephthalate glycol (PETG) are used for thermoformed trays, blister packs, and display windows. Sheet thickness strongly influences transmittance, so the nominal thickness of each specimen must be recorded to the nearest 0.01 mm. Specimens cut from finished thermoformed containers should be taken from flat regions to avoid optical distortion caused by curvature.

Bottles, Vials, and Containers

For three-dimensional containers such as bottles, vials, ampoules, and jars, transmittance is measured on sections cut from the container wall or on the container itself using specialized fixtures. Pharmaceutical glass and plastic containers are evaluated over the wavelength range of 290 to 450 nm, because this spectral region is responsible for the degradation of many active pharmaceutical ingredients.

Sample Conditioning

All specimens should be conditioned prior to testing in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity, in accordance with ISO 291 or ASTM D618, for a minimum of 40 hours unless otherwise specified. Consistent conditioning improves measurement reproducibility, particularly for hygroscopic materials such as polyamide films.

Testing Items

Depending on the material type and end-use application, the following optical parameters can be measured and reported:

• Total luminous transmittance (Tt): the percentage of incident visible light that passes through the specimen in all directions, weighted by the photopic response of the human eye under a specified illuminant such as D65 and a 2° or 10° standard observer.
• Regular (specular) transmittance: the percentage of light transmitted without deviation from the incident beam direction, which governs see-through clarity.
• Diffuse transmittance: the difference between total and regular transmittance, representing scattered light.
• Haze: the percentage of transmitted light scattered more than 2.5 degrees from the incident direction, evaluated per ISO 14782, ASTM D1003, and GB/T 2410.
• Ultraviolet transmittance: transmission values in the UV-A (315–400 nm) and UV-B (280–315 nm) bands, critical for assessing the light-protection performance of packaging for photosensitive products.
• Spectral transmittance curve: a full wavelength scan, typically from 200 to 800 nm, characterizing the optical behavior of the material across the ultraviolet and visible spectrum.
• Light transmission of pharmaceutical containers: percentage transmission measured at specified wavelengths between 290 and 450 nm as a function of container wall thickness.

Testing Methods

Hazemeter Method (ASTM D1003, Method A)

A hazemeter uses a collimated light beam, a calibrated photometer with photopic response, and an integrating sphere with a light trap. Four successive measurements—incident light, total transmitted light, scattered light, and instrument baseline—yield both total luminous transmittance and haze in a single operation. This method is fast, highly repeatable, and widely used for routine quality control of films and sheets.

Spectrophotometer with Integrating Sphere (ISO 13468-2, ASTM D1003, Method B)

A double-beam UV-visible spectrophotometer fitted with an integrating sphere measures total transmittance by collecting both regular and diffuse transmitted light. The spectral data are integrated over the visible range using the CIE standard illuminant D65 and the CIE 2° or 10° standard colorimetric observer to obtain luminous transmittance. This approach provides excellent accuracy for low-transmission and high-scattering specimens and allows regular transmittance and haze to be derived through additional measurements with the sphere's specular exclusion port opened.

UV-Visible Spectrophotometric Scan

For light-protection assessment, a wavelength scan from 290 to 450 nm (or 200 to 800 nm) is performed on a standard spectrophotometer. The resulting spectrum reveals absorption cutoffs of UV absorbers, screening effectiveness of opaque or metallized layers, and transmission limits at critical wavelengths. This method directly supports packaging validation for products with defined photosensitivity requirements.

Pharmaceutical Container Light Transmission Test

Pharmacopoeial methods, including the United States Pharmacopeia general chapter on container quality, specify maximum allowable light transmission for plastic and glass containers as a function of wall thickness across the 290–450 nm range. Specimens are prepared as sections of the container wall, and the measured transmission curve is compared against defined limits to verify suitability for light-sensitive formulations, consistent with photostability testing principles described in ICH Q1B.

Key Standards Reference

• ISO 13468-1 / ISO 13468-2: Determination of the total luminous transmittance of transparent plastics, using single-beam and double-beam instruments.
• ISO 14782: Determination of haze for transparent materials.
• ASTM D1003: Haze and luminous transmittance of transparent plastics.
• ASTM D1746: Transparency of plastic sheeting (regular transmittance).
• GB/T 2410: Determination of light transmittance and haze of transparent plastics (Chinese national standard).
• JIS K 7361-1 and JIS K 7136: Japanese standards for total luminous transmittance and haze of plastics.

Applications

Light transmittance and transmission testing supports a broad range of packaging development and compliance activities across multiple industries:

• Food and beverage packaging: evaluating the light barrier of films for fatty foods, cooking oils, beer, wine, dairy products, and vitamin-fortified products, where light exposure causes rancidity, off-flavors, and riboflavin or vitamin C degradation.
• Pharmaceutical packaging: verifying the light-protective performance of blister foils, amber and opaque containers, and primary packaging for photolabile drugs in accordance with pharmacopoeial requirements and ICH Q1B photostability studies.
• Cosmetics packaging: confirming UV protection levels for formulations containing sensitive active ingredients, fragrances, and natural extracts.
• Agricultural films: characterizing light transmission and haze of greenhouse films, mulch films, and silage films to optimize crop light exposure and growing conditions.
• Consumer and industrial packaging: quality control of transparent windows in cartons, thermoformed trays, and display packaging where product visibility is a marketing requirement.
• Material research and supplier qualification: benchmarking optical properties of new resin grades, coatings, masterbatches, and recycled materials against specified acceptance limits.

FAQ

What is the difference between light transmittance and haze?

Light transmittance measures the total amount of light passing through a material, while haze measures only the scattered portion of that transmitted light. Two films can have identical transmittance values but very different haze values; the low-haze film appears clear, whereas the high-haze film appears milky and obscures the view of the packaged product.

What sample size and preparation are required?

Specimens are typically 50 mm × 50 mm flat pieces or circular discs large enough to fully cover the instrument aperture. They must be clean, flat, and free of creases, surface defects, and embedded contaminants. Thickness should be measured and reported because transmittance decreases predictably as thickness increases.

Which test method should be selected?

For routine quality control of clear films and sheets, the hazemeter method per ASTM D1003 or GB/T 2410 is efficient and repeatable. For accurate results on tinted, coated, or high-scattering materials, the spectrophotometric method per ISO 13468-2 is preferred. For pharmaceutical containers and light-protection studies, a wavelength-specific transmission measurement over 290–450 nm is required.

What factors influence measurement accuracy?

Key influences include surface cleanliness, specimen flatness, thickness uniformity, ambient conditioning, instrument calibration against reference standards, and correct configuration of the integrating sphere. Repeatability checks on replicate specimens are recommended, and testing should be performed by accredited third-party laboratories when results are used for certification or regulatory purposes.

What are typical acceptance criteria for common materials?

Optical requirements depend entirely on the application. Clear PET films often specify total transmittance of 88–92% with haze below 2%, whereas light-protective or metallized laminates may require visible transmittance below 1% and near-zero UV transmission. Acceptance limits should always be defined based on the photosensitivity of the packaged product and the intended shelf life.