PET film haze testing quantifies the light-scattering behavior of transparent polyethylene terephthalate films, a decisive quality parameter for optical, electronic, and packaging applications. This technical guide explains the definition and calculation of haze, the applicable international standards such as ASTM D1003 and ISO 14782, sample preparation, measurement procedures, and typical haze specifications for different PET film grades.
Technical Overview
Polyethylene terephthalate (PET) film is a biaxially oriented polyester film widely used in packaging, electronics, displays, photovoltaics, and industrial laminates because of its excellent mechanical strength, dimensional stability, chemical resistance, and optical transparency. In applications where visual clarity or controlled light diffusion is required, haze is one of the most important optical quality indicators.
Haze is defined as the percentage of transmitted light that is scattered by the specimen away from the incident beam by more than a specified angle, typically 2.5 degrees under ASTM D1003 or 4 degrees under ISO 14782. Haze is calculated using luminous transmittance values obtained with an integrating sphere:
- Total luminous transmittance: Tt = T2 / T1, where T1 is the incident light flux and T2 is the total transmitted light flux.
- Diffuse transmittance: Td = (T4 − T3) / T1, where T3 is the instrumental scatter and T4 is the specimen plus instrumental scatter measured with a light trap.
- Haze (%) = Td / Tt × 100 = (T4 − T3) / T2 × 100.
For PET film, haze originates from several sources: surface roughness created by film-forming and stretching processes, bulk scattering from crystalline domains, inorganic anti-blocking and matting particles such as silica or calcium carbonate, thickness variations, and internal defects or gels. Because haze often correlates with raw material quality, processing conditions, and thickness, it serves as a sensitive process-control and quality-control parameter in PET film manufacturing.
Testing Samples
Optical Grade PET Film
High-purity, smooth-surface films with very low additive content, typically exhibiting haze below 1.5% to 2%. These samples are used as base films for polarizers, touch panels, optical adhesives, and protective films for displays, where maximum transparency is required.
General Purpose and Packaging PET Film
Standard biaxially oriented films containing small amounts of slip and anti-blocking additives, with typical haze values between 2% and 6%. These samples are commonly tested for food packaging, labels, laminating, and thermal lamination applications.
Matte and Anti-Glare PET Film
Films containing engineered silica matting agents that produce a controlled rough surface. Haze is deliberately specified within a range, commonly 15% to 60%, to achieve anti-glare or writable surfaces while maintaining acceptable see-through quality.
Light Diffusion and Light Management Films
Diffuser films for backlight units and lighting applications require very high haze, often above 85%, combined with high total transmittance. Testing verifies that light is scattered uniformly without excessive optical loss.
Functional and Coated PET Films
Heat-stabilized films, hard-coated films, anti-static films, photovoltaic backsheets, and release-liner films. Coatings, primers, and filler particles can significantly alter both transmittance and haze, so finished structures are tested in addition to the base film.
Testing Items
A comprehensive PET film optical evaluation typically includes the following measured and derived parameters:
- Total luminous transmittance, Tt (%): the ratio of total transmitted light to incident light under CIE Standard Illuminant A or D65 and photopic observer response.
- Diffuse transmittance, Td (%): the scattered component of transmitted light collected beyond the specified angular limit.
- Haze (%): the ratio of diffuse transmittance to total transmittance, the primary reported result.
- Clarity or see-through quality: narrow-angle scattering that affects the distinctness of imaged objects, important for display and window applications and measured separately from wide-angle haze.
- Optional optical parameters: yellowness index, whiteness, and CIE color coordinates when color neutrality is part of the specification.
- Thickness dependence: haze measured at defined thickness or normalized to allow comparison between different film gauges.
Testing Methods
Applicable Standards
- ASTM D1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics): Method A uses a dedicated hazemeter and Method B uses a spectrophotometer with an integrating sphere; scattered light is defined beyond an average angle of 2.5 degrees.
- ISO 14782 (Plastics — Determination of haze for transparent materials): defines haze as wide-angle scattering beyond 4 degrees, excluding narrow-angle scatter that contributes to clarity loss but not haze.
- GB/T 2410: the Chinese national standard for light transmittance and haze of transparent plastics, broadly harmonized with ASTM D1003 and widely referenced in the domestic PET film industry.
- JIS K 7136 and JIS K 7361: Japanese standards corresponding to ISO 14782 for haze and total luminous transmittance respectively.
Instrumentation
Testing is performed with either a transmission hazemeter equipped with an integrating sphere and a light trap, or a UV-Visible spectrophotometer fitted with an integrating sphere and haze calculation software. The instrument must be capable of the four-flux measurement sequence (incident, total transmitted, instrument scatter, and specimen scatter) and calibrated with certified reference standards traceable to national metrology institutes.
Sample Preparation and Conditioning
- Condition specimens at 23 ± 2 °C and 50 ± 10% relative humidity for at least 40 hours in accordance with ASTM D618 unless otherwise specified.
- Cut specimens large enough to fully cover the transmission port, typically 50 mm × 50 mm or circular discs of equivalent diameter.
- Clean both surfaces with a lint-free cloth or clean air to remove dust, fingerprints, and electrostatically adhered particles, which can artificially inflate haze readings.
- Avoid wrinkles, creases, scratches, and air bubbles between layers; when testing multi-layer structures, define whether single layers or the complete laminate is measured.
- Prepare a minimum of three to five specimens from different positions across the web to represent overall film uniformity.
Measurement Procedure
- Warm up the instrument, then perform zero and standard calibrations using certified transmittance and haze reference tiles.
- Measure the incident light flux T1 with an empty port and no specimen in the beam path.
- Place the specimen against the transmission port and measure total transmitted light flux T2.
- Insert the light trap and measure instrumental scatter T3 without the specimen, then specimen scatter T4 with the specimen in place.
- Calculate Tt, Td, and haze; repeat for every specimen and report the arithmetic mean, standard deviation, and the test standard used.
- Record film identification, thickness, grade, surface orientation, and conditioning details together with the results for full traceability.
Result Interpretation
Reported haze values must always be interpreted together with total transmittance and the governing standard, since the 2.5-degree and 4-degree collection geometries can yield slightly different values for the same specimen. Elevated haze combined with low transmittance usually indicates excessive scattering loss from fillers, gels, or crystallinity, whereas high haze with high transmittance is the intended behavior of diffuser-type films. When measured haze exceeds specification, manufacturers typically investigate raw-material purity, additive dispersion, casting and stretching conditions, line speed, and surface handling.
Applications
- Display and optics: qualification of base films for polarizers, retardation films, hard coats, anti-reflection stacks, and protective films where haze must remain below approximately 1% to 2%.
- Photovoltaics: incoming inspection of PET-based backsheets, where transmittance and haze affect module aesthetics and long-term light exposure behavior.
- Flexible electronics and insulation: verification of optical uniformity of films used in flexible printed circuits, capacitor films, and electrical insulation layers.
- Food, medical, and general packaging: routine quality control ensuring product visibility, print clarity, and consistent visual appearance on high-speed packaging lines.
- Lighting and backlight units: certification of diffuser films that require haze above 85% with minimal transmittance loss for edge-lit and direct-lit panels.
- Release liners, labels, and graphic media: confirmation of controlled haze for anti-glare, writable, and printable surfaces.
- Research and process development: correlation of haze with crystallization kinetics, orientation ratios, additive loading, and coextruded layer design during film development.
FAQ
What haze value is acceptable for PET film? Acceptable haze depends entirely on the application. Optical-grade films for displays typically require haze below 1.5% to 2%, general packaging films commonly fall between 2% and 6%, matte films are specified between roughly 15% and 60%, and diffuser films intentionally exceed 85% haze.
How does thickness affect haze? Haze generally increases with film thickness because scattering events accumulate over a longer optical path and because thicker films may contain more crystalline or particulate volume. Specifications should therefore always state the nominal thickness at which haze is measured, or normalize results when comparing different gauges.
Can a film have both high haze and high transmittance? Yes. Haze describes the angular distribution of transmitted light, not the amount of light lost. Well-designed diffuser films scatter light efficiently while absorbing very little, achieving haze above 85% with total transmittance above 85% to 90%.
Which standard should be used, ASTM D1003 or ISO 14782? Both are internationally recognized, but they define the scattered-light collection geometry differently, approximately 2.5 degrees for ASTM D1003 and 4 degrees for ISO 14782. Choose the standard specified by the customer or product datasheet and state it clearly in the test report, because values from the two methods are not directly interchangeable.
What is the difference between haze and clarity? Haze quantifies wide-angle scattering that causes a milky or cloudy appearance and reduces contrast, while clarity quantifies narrow-angle scattering that blurs the distinctness of objects seen through the film. A film can be low in haze yet poor in clarity, so display-critical applications often specify both parameters.
How many specimens should be tested? Best practice, supported by the standards, is to test at least three to five specimens taken from different areas of the web and report the average and variability. Single-point measurements can be misleading because additive dispersion and surface roughness vary across the film width and length.
Why did the measured haze come out higher than expected? The most common causes are surface contamination such as dust or fingerprints, electrostatically attracted particles, creases or wrinkles at the measurement port, inadequate conditioning, instrument calibration drift, or measurement of multiple film layers instead of a single layer. Cleaning the specimen and verifying calibration typically resolve most anomalies.