Light transmittance evaluation is a fundamental optical performance test for transparent and translucent packaging materials, quantifying how much visible, ultraviolet, and infrared radiation passes through films, sheets, bottles, and blister packs. Accurate measurement of total transmittance, haze, and spectral transmission characteristics enables manufacturers to balance shelf visibility against protection from light-induced degradation of the packaged contents. This technical article explains the testing principles, applicable standards, specimen requirements, instrumentation, and industry applications for packaging material light transmittance evaluation.
Technical Overview
Light transmittance, formally defined as luminous transmittance (Tt), is the ratio of luminous flux transmitted through a specimen to the luminous flux incident upon it, expressed as a percentage and weighted by the photopic response of the human eye (CIE 1931 standard colorimetric observer) under a specified standard illuminant such as A or D65. For packaging materials, this parameter governs two competing requirements: the marketing need for clear product visibility on the shelf, and the protective need to limit photodegradation of sensitive contents such as fats, vitamins, pigments, flavors, and active pharmaceutical ingredients.
Transmittance evaluation is closely coupled with the measurement of haze, which is the percentage of transmitted light that is scattered by more than 2.5 degrees from the direction of the incident beam. A material can exhibit high total transmittance yet high haze, producing a milky or cloudy appearance that obscures the packaged product. Together with spectral transmittance data across the ultraviolet and visible regions, these quantities form a complete optical fingerprint of a packaging material and are routinely verified by accredited third-party laboratories during material qualification, incoming inspection, and regulatory submission.
The principal standards governing these measurements include:
- ASTM D1003 — Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics (Method A: hazemeter; Method B: spectrophotometer)
- ISO 13468-1 / ISO 13468-2 — Plastics: Determination of the total luminous transmittance of transparent materials (illuminants A and D65 respectively)
- ISO 14782 — Plastics: Determination of haze for transparent materials
- ASTM D1746 — Standard Test Method for Regular Transmittance of Transparent Plastics
- GB/T 2410 — Determination of luminous transmittance and haze of transparent plastics
- JIS K 7361 and JIS K 7136 — Japanese industrial equivalents for luminous transmittance and haze
- ISO 2471 and TAPPI T425 — Opacity determinations for paper and paperboard used in packaging
- USP General Chapter <671> — Light transmission requirements for pharmaceutical containers
Testing Samples
Transmittance results are highly sensitive to specimen geometry, surface condition, and preparation technique. The following material categories are commonly submitted for evaluation, and each requires specific handling to ensure repeatable, standards-compliant measurements.
Flexible Films
Single-layer and multilayer polymer films such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and ethylene-vinyl alcohol (EVOH) barrier films are tested as flat, wrinkle-free specimens mounted in card frames. Films should be free of gauge bands, gels, and processing wrinkles. Multiple thickness points are recorded because transmittance decreases with increasing thickness, particularly in filled or pigmented films.
Rigid Sheets and Blister Packaging
Calendered or extruded sheets used for thermoformed trays, clamshells, and blister cavities (PVC, PET, amorphous PET, polystyrene) are cut into flat coupons, typically 50 mm square or another size matching the instrument port. Formed blister pockets should also be tested in their final geometry when the curved pocket wall is the functional light barrier, since drawing during thermoforming locally reduces thickness and increases haze.
Hollow Containers
Bottles, jars, vials, and ampoules cannot be measured directly in most flat-specimen instruments. Sections are carefully cut from the container wall, flattened, or measured with adapted fixtures that present the curved wall to the beam. Pharmaceutical glass and plastic containers may alternatively be assessed against pharmacopoeial light transmission limits across the 290–450 nm range, where photolabile drug substances are most vulnerable.
Coated, Metallized, and Laminated Structures
Transparent barrier coatings such as silicon oxide (SiOx) and aluminum oxide (AlOx), as well as high-clarity metallized films, are tested with the functional coating intact and in the correct orientation. The coated side facing the light source versus the detector can influence measured transmittance and haze, so orientation is documented in the test report.
Paper, Paperboard, and Translucent Substrates
For non-transparent packaging such as folding cartons and glassine windows, opacity and diffuse transmission behavior are characterized instead of regular transmittance, using ISO 2471 (paper backing method) or TAPPI T425 as appropriate.
All specimens should be conditioned prior to testing in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618, unless the governing product standard specifies otherwise. Surfaces must be cleaned with lint-free wipes and suitable solvents that do not attack or swell the polymer, and fingerprints, scratches, dust, and entrapped air bubbles must be avoided because they artificially inflate the haze value.
Testing Items
A comprehensive transmittance evaluation of packaging materials typically encompasses the following measurands:
Total luminous transmittance (Tt) — the overall percentage of incident visible light passing through the specimen, photopically weighted; the headline parameter for transparency grade.
Diffuse transmittance (Td) — the portion of transmitted light scattered away from the geometric propagation direction, calculated as (T4 − T3)/T1 in the four-port measurement scheme.
Haze — wide-angle scattering expressed as the ratio Td/Tt × 100 %, the primary indicator of see-through quality and glossiness of appearance.
Regular (specular) transmittance — undeviated light measured per ASTM D1746, relevant for rigid glazing-like sheets and display window films.
Spectral transmittance curve (200–800 nm) — full wavelength-resolved transmission using UV-Vis spectrophotometry, revealing absorption edges of UV absorbers, pigment cutoffs, and tint characteristics.
UV transmittance and UV blocking rate — integrated transmittance in the UVA (315–400 nm) and UVB (280–315 nm) bands, critical for products susceptible to photo-oxidation.
Visible photostability-relevant transmittance — transmission-weighted comparisons at wavelengths known to drive degradation of specific contents, such as riboflavin-sensitive dairy products and beer.
Opacity (paper and board) — the ratio of reflectance of a single sheet backed by a black body to that backed by a white body, characterizing show-through.
Transmission uniformity — multi-point mapping across the web or surface to detect thickness variation, die lines, and coating defects.
Testing Methods
Hazemeter Method (Integrating Sphere, ASTM D1003 Method A)
A collimated light beam, typically from a tungsten-filament source approximating standard illuminant A, illuminates the specimen held at the entrance port of an integrating sphere. Four sequential readings are taken: T1 (incident light without specimen), T2 (total light transmitted by the specimen), T3 (instrumental scatter with no specimen), and T4 (total scatter from specimen plus instrument with a light trap in place). Total transmittance is computed as Tt = (T2/T1) × 100 %, diffuse transmittance as Td = ((T4 − T3)/T1) × 100 %, and haze as (Td/Tt) × 100 %. This method is the industry workhorse for quality control of films and sheets because of its speed and excellent repeatability.
Spectrophotometer Method (ASTM D1003 Method B / ISO 13468)
A double-beam spectrophotometer equipped with an integrating sphere attachment measures spectral transmittance at defined wavelength intervals, commonly 10 nm or finer, across the visible region. The spectral data are then mathematically weighted with the CIE photopic luminous efficiency function V(λ) and the spectral power distribution of illuminant A (ISO 13468-1) or D65 (ISO 13468-2) to calculate total luminous transmittance. Haze can be derived by repeating the measurement with a white standard versus a light trap at the sphere exit port. The spectrophotometric approach is preferred when color, tint, and wavelength-specific behavior must be characterized simultaneously.
UV-Vis Spectral Scanning for Light Protection Assessment
For products requiring light protection, specimens are scanned from 200 nm to 800 nm against air or a blank reference. The resulting curves quantify UV cutoff wavelengths, blocking efficiency of additives such as benzotriazoles and hindered amine stabilizers, and the transmission characteristics of amber, opal, or pigmented containers. Results are frequently compared against pharmacopoeial limits or product-specific photostability requirements, for example those aligned with ICH Q1B photostability testing of drug substances and products.
Measurement Quality Practices
- Verify instrument performance before use with certified reference standards for transmittance and haze traceable to national metrology institutes.
- Test a minimum of three to five specimens per sample and report individual values together with the mean and standard deviation.
- Document specimen thickness at each measurement point, since transmittance generally decreases and haze may increase with thickness.
- Keep beam geometry, sphere diameter, and port sizes consistent with the cited standard, because geometric differences are a known source of inter-laboratory discrepancy.
- Condition specimens before testing and handle them only by the edges with powder-free gloves to avoid surface contamination.
Applications
Food and Beverage Packaging
Light-induced oxidation is a leading cause of quality loss in milk, butter, edible oils, beer, and juice, where riboflavin, chlorophyll, and porphyrin photosensitizers drive off-flavor formation. Transmittance data guide the selection of full-shield PET beer bottles, UV-blocking capsuled films, and pigment-loaded HDPE dairy containers, and support shelf-life studies correlating light exposure with sensory and nutritional deterioration.
Pharmaceutical and Medical Packaging
Photosensitive formulations require containers that limit transmission below defined thresholds in the actinic region. Transmittance evaluation of clear and amber glass vials, PP syrup bottles, blister foils, and infusion bags is used to demonstrate compliance with pharmacopoeial container requirements and to select protective secondary packaging for photolabile drug products.
Agricultural and Construction Films
Greenhouse covers, mulch films, and silage films are engineered for specific spectral windows: high photosynthetically active radiation (PAR) transmission for crop growth, UV-blocking to control insect behavior and polymer lifespan, and anti-condensation surface treatments. Transmittance curves directly inform film formulation and field performance expectations.
Cosmetics and Consumer Goods
Transparent jars, sachets, and windowed cartons must present the product attractively while protecting colorants, fragrance, and active ingredients from photobleaching. Optical measurements support claims of "UV protection," "crystal clarity," and "frost finish" with objective, reproducible data.
Quality Control, R&D, and Certification
Manufacturers integrate transmittance and haze testing into incoming resin inspection, extrusion line monitoring, and final release testing. Independent verification by accredited third-party laboratories is routinely requested for supplier qualification, customer specifications, export documentation, and dispute resolution, always conducted against the published ISO, ASTM, GB, JIS, or EN methods described above.
FAQ
What is the difference between light transmittance and haze?
Light transmittance measures how much light passes through the specimen regardless of direction, while haze measures only the portion of transmitted light that is scattered more than 2.5 degrees from the incident direction. A material can therefore be highly transmissive yet hazy, appearing milky rather than clear. Both parameters are needed to fully describe optical quality.
Which standard should be used: ASTM D1003 or ISO 13468?
Both are technically equivalent in intent but differ in illuminant convention and measurement details. ASTM D1003 is dominant in North American supply chains, ISO 13468 in Europe and Asia, and GB/T 2410 aligns closely with the ISO approach for the Chinese market. The governing specification of the end customer or regulatory authority should dictate the choice, and the method actually used must be stated in the test report.
Does specimen thickness affect the result?
Yes. Absorption and scattering both scale with optical path length, so thicker specimens generally show lower total transmittance and, for scattering-dominated materials, higher haze. Transmittance values are only comparable between specimens of the same nominal thickness, and thickness must always be reported alongside the optical data.
How are curved containers such as bottles and vials evaluated?
Sections are cut from the container wall and flattened or measured with curved-surface adapters, or the container is assessed against pharmacopoeial light transmission methods designed for hollow containers. The measurement location should be marked, and finished-container measurements are preferable to flat-sheet data whenever the thermoformed or blow-molded geometry is the actual light barrier.
What transmittance and haze values are considered acceptable?
Acceptance limits are application-specific. High-clarity packaging films typically target total transmittance above 90 % with haze below 2 %, whereas light-protective containers may deliberately limit transmission to a few percent in the UV and blue region. Regulatory and pharmacopoeial containers instead reference wavelength-dependent maximum transmission limits. In all cases, the specification should be agreed between supplier and customer and verified using the standards described in this article.