Haze testing is an essential optical quality control procedure for biaxially oriented plastic films such as BOPP, BOPET, and BOPA, where light-scattering behavior determines transparency, shelf appeal, and suitability for optical applications. This article explains the definition and origins of haze, the governing international standards—ASTM D1003, ISO 14782, GB/T 2410, and JIS K 7136—specimen preparation, measurement procedures, and the industries that depend on accurate haze data.
Technical Overview
Biaxially oriented (BO) films are produced by stretching a cast polymer web sequentially or simultaneously in both the machine direction (MD) and the transverse direction (TD). This orientation aligns the polymer chains and refines the crystalline structure, delivering outstanding tensile strength, stiffness, dimensional stability, and barrier performance. Major products in this family include biaxially oriented polypropylene (BOPP), biaxially oriented polyethylene terephthalate (BOPET or polyester film), biaxially oriented polyamide (BOPA), biaxially oriented polystyrene (BOPS), and emerging biaxially oriented polylactic acid (BOPLA) films.
Haze is defined as the percentage of transmitted light that is scattered from its incident path by more than a specified angle—2.5 degrees in ASTM D1003—so that it no longer contributes to clear image formation. In biaxially oriented films, haze originates from four principal sources: surface micro-roughness generated during stretching and annealing, bulk scattering at crystalline structures, optical defects such as gels and fisheyes, and dispersed functional additives such as antiblock and slip agents. Because haze directly affects the see-through quality of packaging and the visual performance of display components, it is a routine release specification in film manufacturing and incoming inspection.
Haze is measured photoelectrically with an integrating-sphere hazemeter or spectrophotometer using a defined illuminant (typically D65 or C) and a 2-degree standard observer. Results are expressed as a dimensionless percentage, where lower values indicate better clarity. Typical commercial specifications range from below 1.0 percent for optical-grade BOPET to roughly 2–3 percent for standard packaging BOPP, with the acceptable limit depending on film thickness and end use.
Testing Samples
BOPP Films
BOPP is the highest-volume biaxially oriented film worldwide, used in flexible packaging, labels, adhesive tape, and capacitor dielectrics. Test specimens commonly range from 12 to 40 micrometers in thickness. Both plain and co-extruded heat-sealable grades should be evaluated, because sealing layers and cavitated white grades measurably increase haze. Thickness must always be recorded, as haze rises with gauge.
BOPET Films
BOPET films span roughly 12 to 250 micrometers and serve graphics, lamination, photovoltaic backsheets, and optical applications. Optical-grade BOPET specifications frequently demand haze below 1.0 percent at a reference thickness, so instrument precision and specimen cleanliness are critical. Surface-treated and hard-coated variants should be tested in their as-supplied condition.
BOPA Films
Biaxially oriented nylon films used for pouches, lidding, and retort structures are hygroscopic. Moisture absorption changes the optical response of polyamide, so conditioning at a standard atmosphere before measurement is particularly important for reproducible BOPA haze data.
Specialty and Coated Films
This group includes BOPLA compostable films, BOPS, and films carrying acrylic hard coats, antifog layers, or barrier coatings. Metallized, pigmented, or white opaque films are not suitable for transmission haze testing; reflection-based appearance properties such as gloss or reflection haze should be specified for these products instead.
Testing Items
A complete optical characterization of biaxially oriented film typically includes the following measurable quantities:
- Total luminous transmittance (Tt) — the percentage of incident light transmitted through the specimen in all directions.
- Diffuse luminous transmittance (Td) — the scattered portion of the transmitted light.
- Haze — calculated as Td/Tt × 100, quantifying the milky or cloudy appearance of the film.
- Regular (specular) transmittance — the unscattered transmitted component, closely related to image clarity.
- Optional transmission color (CIE L*a*b*) and see-through clarity evaluations for premium optical grades.
Testing Methods
Reference Standards
- ASTM D1003 — Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics (Method A: hazemeter; Method B: spectrophotometer).
- ISO 14782 — Determination of haze for homogeneous translucent materials.
- ISO 13468-1 — Determination of the total luminous transmittance of 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 standards corresponding to the ISO methods for total transmittance and haze.
- ISO 291 / ASTM D618 — standard atmospheres for conditioning specimens prior to testing.
Specimen Preparation and Conditioning
Specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours. At least three to five specimens are cut from different positions across the web, each large enough to fully cover the instrument transmission port—commonly 50 mm × 50 mm. Specimens must be free of wrinkles, folds, fingerprints, and dust; a lint-free wipe and antistatic treatment are recommended, since static attraction of dust is a major source of spurious readings. Specimen thickness is measured and reported to interpret haze correctly against thickness-dependent specifications.
Method A — Hazemeter Procedure (ASTM D1003)
The integrating-sphere method acquires four successive quantities:
- T1 — incident light reference with the port empty and no light trap.
- T2 — specimen in position with no light trap, giving total transmittance.
- T3 — port empty with an opaque light trap at the exit port, quantifying instrument stray light.
- T4 — specimen in position with the light trap, giving the diffuse transmittance.
Haze (%) = [(T4/T2) − (T3/T1)] × 100, and total luminous transmittance is calculated as Tt = (T2/T1) × 100. Modern automatic hazemeters collect all four quantities from a single specimen insertion and report transmittance and haze simultaneously.
Method B — Spectrophotometer Measurement
Method B uses a UV-Vis spectrophotometer fitted with an integrating sphere to capture the transmittance spectrum across the visible range, which is then weighted by the specified illuminant and observer functions to derive luminous transmittance and haze. It is preferred for tinted or colored films whose photopic response must be integrated wavelength by wavelength. Instruments must be verified before each session using zero checks and calibrated haze reference standards traceable to a national metrology institute. Method A and Method B agree closely for near-clear films but may diverge for hazy or colored specimens, so the standard and method used must always be stated in the test report.
Factors Influencing Test Results
- Thickness — haze increases with gauge as bulk and surface scattering accumulate along the light path.
- Surface texture — matte finishes and cavitated structures dramatically raise haze values.
- Additives — antiblock (silica, calcium carbonate), slip agents, and antifog compounds scatter light depending on particle size and loading.
- Morphology — crystallinity, spherulites, and annealing history affect bulk scattering.
- Environmental effects — dust, static charge, and, for hygroscopic BOPA, absorbed moisture.
- Instrument geometry — differences in sphere design, gloss trap angle, and illuminant between ASTM and ISO configurations can shift results by several tenths of a percent.
Applications
Accurate haze data support quality control, product development, and supplier qualification throughout the biaxially oriented film value chain, including testing performed by accredited third-party laboratories:
- Flexible packaging — window films and laminate webs where product visibility drives shelf appeal.
- Labels and graphic arts — BOPP label facestock and overlamination films requiring consistent clarity.
- Optical and display — protective films, release liners, and base films for functional optical stacks demanding ultra-low haze.
- Photovoltaics — PET-based backsheets where transmittance and haze influence module performance.
- Capacitor films — BOPP dielectric grades where optical quality reflects cleanliness and crystalline uniformity.
- Adhesive tapes — BOPP and PET tape backings where transparency is a selling point.
- Process optimization — evaluating resin formulation, stretching ratios, and annealing parameters during R&D.
FAQ
What is the difference between haze, transmittance, and clarity?
Transmittance quantifies how much light passes through the film, haze quantifies how much of the transmitted light is scattered and causes a milky appearance, and clarity describes the sharpness of images viewed through the film. A film can show high transmittance yet high haze, so the three properties are measured and specified independently.
Which standard should be selected for testing?
ASTM D1003 and GB/T 2410 are the most widely applied in global packaging supply chains, ISO 14782 is preferred in ISO-based international markets, and JIS K 7136 and JIS K 7361 apply to Japanese customers. The methods are technically comparable for clear films, but the specific standard must be stated in the report because small systematic deviations arise from geometric differences.
Can metallized or opaque biaxially oriented films be haze-tested?
No. Haze is a transmission property and requires the specimen to transmit light. For metallized, pigmented, or white cavitated films, reflection-based appearance tests such as gloss (ASTM D2457 / ISO 2813) or reflection haze (ASTM E430) are appropriate alternatives.
How does film thickness affect the haze value?
Haze generally increases with thickness because both bulk and surface scattering accumulate over the light path. Specifications are therefore always tied to a nominal thickness, and results from films of different gauges should not be compared directly.
What sample size and how many specimens are required?
Specimens must fully cover the instrument transmission port—typically 50 mm × 50 mm. A minimum of three to five specimens taken from different positions across the web is recommended, with the mean, range, and standard deviation reported to capture cross-direction variation typical of tenter-frame processes.
How often should the hazemeter be calibrated?
A zero check and verification against traceable haze reference standards should be performed daily, with full calibration following the instrument manufacturer's schedule and the quality management requirements, such as those of ISO/IEC 17025, when operating within accredited laboratory systems.