Plastic light transmittance testing is an essential optical experiment that quantifies how much visible light passes through transparent and translucent plastic materials such as acrylic (PMMA), polycarbonate (PC), and polymer films. Using internationally recognized methods such as ASTM D1003, ISO 13468, and GB/T 2410, accredited third-party laboratories measure total luminous transmittance and haze to verify material quality for packaging, automotive, lighting, and construction applications. This article explains the testing principles, sample requirements, experimental procedures, calculation methods, and practical applications of light transmittance testing for plastics.
Technical Overview
Light transmittance, also known as luminous transmittance, is the ratio of the luminous flux transmitted through a plastic specimen to the luminous flux incident upon it, expressed as a percentage. Because transparent plastics are used wherever visibility, display, or light management matters, transmittance, together with haze, is one of the most frequently specified optical properties in material data sheets, purchase agreements, and incoming inspection procedures.
The experiment is typically performed with a hazemeter or an integrating-sphere spectrophotometer. The instrument captures both the regularly transmitted beam, which passes through the specimen without deviation, and the diffusely scattered light that deviates from the incident beam by more than 2.5 degrees. Total luminous transmittance is calculated from these readings, while haze is defined as the percentage of transmitted light scattered beyond this angle. Testing is conducted under photopic viewing conditions using CIE Standard Illuminant A, so the reported value corresponds to the response of the human eye across the visible spectrum from 380 to 780 nanometers.
The key optical concepts involved in this experiment are summarized below:
- Total luminous transmittance (Tt): the overall percentage of incident visible light that passes through the specimen.
- Regular (specular) transmittance: light transmitted without scattering, responsible for sharp image formation.
- Diffuse transmittance: light scattered within the material, responsible for a milky or cloudy appearance.
- Haze: the fraction of transmitted light deviating by more than 2.5 degrees, expressed as a percentage of total transmittance.
Testing Samples
Rigid Transparent Plastics
Sheets and injection-molded plaques of polymethyl methacrylate (PMMA, acrylic), polycarbonate (PC), polystyrene (PS), rigid polyvinyl chloride (PVC), cyclic olefin copolymer (COC), and transparent ABS grades are tested to verify optical-grade quality and to evaluate the effects of additives, colorants, fillers, and processing conditions on light transmission.
Flexible Polymer Films
Thin films of polyethylene terephthalate (PET), biaxially oriented polypropylene (BOPP), polyethylene (PE), cast polypropylene (CPP), and multilayer laminates used in flexible packaging, protective surfaces, agricultural covers, and optical displays are evaluated for clarity and haze against agreed specifications.
Specimen Preparation and Conditioning
- Specimen size: typically flat pieces of 50 mm × 50 mm, or discs large enough to completely cover the instrument transmission port.
- Surface condition: specimens must be free of scratches, fingerprints, dust, dents, and warpage; handle only with lint-free gloves or tweezers.
- Thickness: absorption and scattering accumulate along the optical path, so only specimens of the same nominal thickness should be compared, and thickness must always be reported with the results.
- Conditioning: condition specimens at 23 ± 2 °C and 50 ± 10 % relative humidity in accordance with ISO 291 or ASTM D618 before measurement, unless otherwise specified.
- Number of specimens: test at least three to five specimens per sample and report the arithmetic average.
Testing Items
The core measured parameters of the light transmittance experiment are:
- Total luminous transmittance in the visible range (380–780 nm) under CIE Standard Illuminant A.
- Haze, characterizing wide-angle light scattering.
Optional extended items frequently combined with the same experiment include:
- Full spectral transmittance curves from 200 to 800 nm using a UV-Vis spectrophotometer.
- Ultraviolet transmittance for UV-blocking films, sheets, and protective windows.
- Near-infrared and solar transmittance for heat-insulating films and glazing products.
- Clarity, or see-through quality, particularly relevant for retail packaging films.
Testing Methods
Applicable Standards
- ASTM D1003: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, covering Method A (hazemeter) and Method B (spectrophotometer).
- ISO 13468-1: Determination of the total luminous transmittance of transparent materials using a single-beam instrument.
- ISO 13468-2: Determination of total luminous transmittance using a double-beam instrument.
- ISO 14782: Determination of haze for transparent materials.
- GB/T 2410: Chinese national standard for light transmittance and haze of transparent plastics, technically aligned with ASTM D1003.
- JIS K 7361-1 and JIS K 7136: Japanese industrial standards corresponding to ISO 13468-1 and ISO 14782 respectively.
Hazemeter Method (ASTM D1003, Method A)
The instrument consists of a stable light source matching the spectral distribution of CIE Illuminant A, a collimated optical beam, an integrating sphere with a photopic detector, and a removable light trap at the exit port. Four sequential readings are taken during the experiment:
- T1: incident luminous flux, no specimen, light trap removed.
- T2: total transmitted flux, specimen in place, light trap removed.
- T3: instrument baseline scatter, no specimen, light trap in position.
- T4: diffuse transmitted flux, specimen in place, light trap in position.
The results are calculated as follows: total transmittance Tt = (T2 / T1) × 100 %, and haze = [(T4 / T2) − (T3 / T1)] × 100 %. A well-maintained instrument typically achieves repeatability within ±0.5 % for transmittance on flat, clean specimens.
Spectrophotometer Method (ASTM D1003, Method B / ISO 13468)
A UV-Vis spectrophotometer equipped with an integrating sphere records the spectral transmittance of the specimen across the visible band. Photopic transmittance is obtained by weighting the spectral data with the luminous efficiency function of the human eye and the Illuminant A source distribution. This approach is preferred when a full spectral curve, ultraviolet blocking data, or colored and tinted specimens are involved.
Experimental Procedure
- Calibrate and zero the instrument, then verify performance using a calibrated reference specimen.
- Measure and record the thickness of each conditioned specimen at several points.
- Clean the specimen surfaces with a lint-free cloth and clean compressed air if required, and inspect them visually for defects.
- Mount the specimen flush against the transmission port without bending, wrinkling, or trapping air bubbles, which is especially critical for thin films.
- Acquire the four readings T1 through T4 according to the selected standard, or record the full spectral scan.
- Repeat for all specimens and report the average total transmittance and haze, together with specimen thickness, the standard applied, and the instrument configuration.
Major factors influencing the results include specimen thickness, surface roughness and texture, internal bubbles or gels, moisture content, molecular orientation in films, and ambient temperature during measurement. Consistent control of these variables is essential for reproducible data.
Applications
Light transmittance test data support material selection, product design, quality control, and regulatory compliance across many industries:
- Packaging: transparency of BOPP, PET, and PE films for retail display, and haze control in matte, antifog, and light-diffusing films.
- Automotive: polycarbonate headlamp lenses, panoramic glazing, sunroof panels, and instrument cluster covers.
- Lighting and display: PMMA and PC light diffusers, light guide plates, LED tube covers, and touch panel windows.
- Construction and agriculture: multiwall polycarbonate roofing sheets, greenhouse films, window films, and skylight glazing.
- Medical and consumer products: transparent containers, blister packs, syringe barrels, and appliance windows.
- Research and quality control: formulation screening, additive optimization, incoming material inspection, and batch-to-batch consistency verification.
FAQ
What is the difference between transmittance and haze?
Transmittance quantifies how much visible light passes through a specimen in total, while haze quantifies how much of that transmitted light is scattered at wide angles. A frosted diffuser can show high transmittance with high haze, whereas optical-grade acrylic shows high transmittance with very low haze.
Why is thickness important when reporting results?
Both absorption and scattering accumulate along the optical path, so a thicker specimen usually exhibits lower transmittance and higher haze. Results are only comparable when specimens of equal nominal thickness are tested, which is why thickness must always accompany the reported values.
Which standard should be selected for testing?
ASTM D1003 and GB/T 2410 are widely used in American and Chinese markets respectively, while ISO 13468 and ISO 14782 dominate international specifications. The choice depends on the target market and customer requirements; results are generally comparable when the same illuminant, photopic detector, and 2.5-degree scattering geometry are applied.
Can colored, translucent, or textured materials be tested?
Yes. Colored and tinted materials are best measured with the spectrophotometer method to capture the full spectral curve, and textured or embossed sheets can be measured for total transmittance, although very high haze values may exceed the intended scope of the standard for transparent materials.
What are typical transmittance values of common transparent plastics?
- PMMA (acrylic): approximately 92–93 %.
- Amorphous polycarbonate: approximately 87–91 %.
- General-purpose polystyrene: approximately 88–92 %.
- Clear rigid PVC: approximately 80–88 %.
- Thin-gauge PET film: approximately 88–90 %.
Actual values vary with resin grade, thickness, additive package, and surface finish, so testing the specific material at its final thickness is always recommended. For reliable and traceable results, testing should be carried out by accredited third-party laboratories using calibrated instruments and validated procedures.
