Light transmittance testing of packaging materials to the Chinese national standard GB/T 2410-2008 quantifies the percentage of visible light that passes through plastic films, sheets, and containers, and is a critical quality index for products requiring display visibility or light protection. This technical article explains the testing principle, applicable sample types, instrumentation, step-by-step measurement procedure, calculation formulas, and typical industrial applications, providing a practical reference for manufacturers, suppliers, and accredited third-party laboratories.
Technical Overview
Light transmittance (luminous transmittance) is defined as the ratio of the luminous flux transmitted through a specimen to the luminous flux incident upon it, expressed as a percentage. For transparent and translucent packaging materials, this parameter directly determines product visibility on the shelf and the degree of light protection provided to photosensitive contents such as food, pharmaceuticals, and cosmetics.
In China, the determination of light transmittance for transparent plastics is governed by GB/T 2410-2008, "Determination of Light Transmittance and Haze of Transparent Plastics." This national standard is technically aligned with ASTM D1003 and ISO 13468-1, ensuring that test results are comparable across domestic and international supply chains. The method is applicable to transparent plastic films, sheets, plates, and comparable packaging components.
- Measurement geometry: integrating sphere (d/0 or 0/d) with a photopic detector response
- Light source: standard illuminant A (tungsten, 2856 K) or illuminant C as specified
- Spectral range: visible spectrum from approximately 380 nm to 780 nm, weighted photopically
- Simultaneously measured parameters: total luminous transmittance and haze
- Result expression: percentage (%) averaged over a minimum of three specimens
Haze, defined as the percentage of transmitted light that is scattered more than 2.5 degrees from the incident beam axis, is typically determined together with transmittance because the two properties jointly characterize the optical clarity of packaging materials.
Testing Samples
Flexible Plastic Films
Single-layer and multilayer films such as PE, CPP, BOPP, PET, PA, EVOH-containing laminates, and biodegradable PLA films used for pouches, bags, wrapping films, and lidding materials. Specimens are cut flat without folds, wrinkles, or creases, since any deformation distorts the light path.
Rigid Sheets and Blister Materials
Transparent and translucent sheets including APET, PVC, PP, PS, and PVDC-coated blister films used in thermoformed packaging, clamshells, and pharmaceutical blister packs. Sheet thickness must be uniform and reported together with the transmittance value.
Rigid Containers and Bottles
Transparent bottles, jars, vials, and tubs manufactured from PET, PP, PE, or glass. Curved containers are sectioned to obtain flat test pieces from the wall, avoiding zones with high curvature, embossing, or gating marks.
Paper-Based and Composite Packaging
Translucent paper, glassine, label facestocks, and paper-plastic composites can be evaluated for transmittance when required; for highly opaque paper and paperboard, opacity testing per GB/T 1543 is generally more appropriate.
Testing Items
- Total luminous transmittance (Tt): the overall percentage of incident visible light passing through the specimen
- Diffuse transmittance (Td): the scattered component of transmitted light
- Haze: the ratio of diffuse transmittance to total transmittance, expressed as a percentage
- Spectral transmittance scan: optional wavelength-by-wavelength scanning (for example 200-800 nm) for evaluating UV-blocking or amber light-protection performance
- Transmittance at specified wavelengths: for example 450 nm or 550 nm, when product specifications define single-wavelength acceptance limits
Pharmaceutical packaging for photosensitive drug products is frequently assessed against national pharmaceutical packaging material standards (YBB standards), which set maximum transmittance limits in the wavelength band of approximately 290 nm to 450 nm.
Testing Methods
Instrumentation and Principle
Two instrument types are recognized for the determination. Method A uses a dedicated hazemeter (luminous transmittance and haze meter) consisting of a light source, collimating optics, an integrating sphere, and a photopically corrected detector. Method B uses a UV-Vis spectrophotometer equipped with an integrating sphere accessory, which records transmittance across the visible spectrum and computes the photopically weighted average.
Specimen Preparation and Conditioning
At least three specimens are cut to the size required by the instrument sample port, commonly 50 mm x 50 mm. Specimens must be free of dust, fingerprints, scratches, and static attraction of particles. Prior to testing, specimens are conditioned in the standard laboratory atmosphere specified in GB/T 2918, namely 23 ± 2 °C and 50 ± 10 % relative humidity, until equilibrium is reached.
Measurement Procedure
- Warm up and calibrate the instrument using the certified reference standards for 0 % and 100 % transmittance
- Measure the incident luminous flux T1 without a specimen in the optical path
- Mount the specimen flush against the entrance port of the integrating sphere and record the total transmitted flux T2
- For haze, install the light trap and record the instrumental scatter reading T3 without a specimen, then the combined scatter reading T4 with the specimen in place
- Repeat the measurement for each specimen and at each required position or orientation
Calculation and Expression of Results
Total luminous transmittance is calculated as Tt = (T2 / T1) x 100 %. Haze is calculated as Haze = ((T4 / T2) - (T3 / T1)) x 100 %, where the T3/T1 term corrects for the intrinsic scatter of the instrument itself. The final result is reported as the arithmetic mean of all valid specimens, rounded to 0.1 %, together with specimen thickness, material designation, and the conditioning atmosphere.
Key Factors Affecting Accuracy
Surface contamination, thickness variation, poor flatness, specimen orientation in anisotropic films, and improper zero calibration are the most common sources of error. Transparent but highly textured or embossed films should be noted as such, since surface texture converts regular transmittance into diffuse transmittance and elevates the measured haze value.
Applications
Transmittance data determined to GB/T 2410 support material qualification, incoming inspection, and regulatory submission across multiple sectors:
- Food packaging: verifying display clarity of barrier films while controlling light-induced oxidation of fats, vitamins, and natural colorants
- Pharmaceutical packaging: qualifying transparent and amber blister films, bottles, and vials for photosensitive formulations, where transmittance limits in the UV and short-wavelength visible region are mandated
- Agricultural and horticultural films: confirming sufficient visible light transmission for greenhouse covering films and mulch films
- Cosmetics and personal care: balancing shelf appeal against photostability of fragrances, oils, and active ingredients
- Electronics and industrial packaging: ensuring antistatic and protective films meet optical clarity requirements for inspected components
- Research and development: comparing resin formulations, masterbatch loadings, coating chemistries, and process parameters during material development
FAQ
What is the difference between light transmittance and haze?
Light transmittance measures how much visible light passes through the specimen, while haze measures how much of that transmitted light is scattered. A material can exhibit high transmittance yet high haze, appearing cloudy rather than optically clear.
Is GB/T 2410 equivalent to ASTM D1003 or ISO 13468?
The three standards are technically harmonized in principle, optical geometry, and calculation basis. GB/T 2410-2008 corresponds closely to ASTM D1003 Method A and ISO 13468-1, so results are generally comparable, although small deviations may arise from illuminant and instrument differences, which should be stated in the test report.
Can colored, opaque, or very thick materials be tested?
Colored and light-filtering materials can be tested, and the resulting low transmittance values are themselves the quality index. Fully opaque materials return a transmittance near 0 %, and very thick or highly scattering specimens may exceed the linear range of a hazemeter, in which case a spectrophotometric method is preferred.
How many specimens are required and what is a typical turnaround?
The standard requires a minimum of three specimens, and five or more are recommended for anisotropic films. Routine testing by an accredited third-party laboratory is usually completed within three to five working days after sample receipt and conditioning.
Are there regulatory acceptance limits for transmittance?
Yes, for pharmaceutical and photosensitive product packaging, national pharmacopoeia requirements and YBB pharmaceutical packaging standards define maximum transmittance limits across specified wavelength bands. For general food and consumer packaging, limits are typically set by the brand owner or product specification rather than by mandatory regulation.