Coating Haze Value Determination: Testing Methods, Standards, and Applications

Coating haze value determination is a key optical quality test that quantifies light scattering in transparent coatings and the milky cloudiness visible on high-gloss coated surfaces. This technical guide explains the measurement principles, applicable international standards, sample requirements, instrumentation, and typical industrial applications of haze testing for coated materials.

Technical Overview

Haze is an optical property that quantifies the degree to which a material scatters light. In the context of coatings, haze value determination measures how a coated surface or a transparent coated film redistributes light away from its regular path, either in transmission or in reflection. A high haze value generally indicates a cloudy, milky, or low-clarity appearance, while a low haze value indicates excellent optical purity.

Two distinct phenomena are covered by coating haze measurement, and they must not be confused:

• Transmission haze: the percentage of light passing through a transparent coating or coated film that is diffusely scattered by more than a defined angle (typically 2.5 degrees) from the incident beam. It is the standard metric for clear coatings on films, glass, and plastic substrates.
• Reflection haze: the milky halo that appears around bright reflections on high-gloss opaque coatings, caused by microscopic surface texture and near-surface scattering. It is evaluated at a specular geometry of 20 degrees with additional narrow-angle detectors.

Both quantities are dimensionless. Transmission haze is reported as a percentage of total transmitted light, while reflection haze is reported in hazemeter units on a logarithmic-type scale defined by the corresponding test method. Haze measurement is widely used for incoming material inspection, product qualification, process control, and failure analysis, and it can be performed either in-house or by accredited third-party laboratories.

Testing Samples

Transparent Coated Films and Free Films

Flexible substrates such as PET, BOPP, PE, and PVC films carrying transparent functional coatings (hard coats, anti-fog coatings, barrier coatings, adhesives) are the most common transmission haze samples. Free films, meaning cast or peeled coating films without a substrate, may also be tested when the coating itself must be evaluated in isolation.

Coated Glass and Rigid Transparent Panels

Coated glass, PMMA, and polycarbonate panels used in architectural glazing, display windows, lighting covers, and instrument lenses are tested to verify that the coating does not degrade the optical clarity of the base material. The haze of the coated panel is typically compared against an uncoated reference panel.

High-Gloss Coated Panels

Opaque substrates such as steel panels, aluminum panels, automotive body panels, plastic trim parts, and coated paperboard finished with high-gloss paints, clearcoats, varnishes, or UV-cured coatings are evaluated for reflection haze. These samples must be flat, clean, and free of visible defects such as orange peel, dust inclusions, or fingerprints, because such flaws dominate the measured signal.

Laboratory Drawdowns and Spray-Out Panels

When the haze performance of a liquid coating formulation must be assessed, laboratory drawdowns are prepared on standardized charts or glass plates at controlled film thickness, or spray-out panels are produced under representative application and curing conditions. Film thickness must be recorded, since transmission haze increases with optical path length.

Testing Items

Depending on the sample type and the selected test method, the following items are typically reported in a coating haze determination:

• Transmission haze, expressed as a percentage of total luminous transmittance.
• Total luminous transmittance of the specimen, measured with a spectrophotometer or hazemeter.
• Regular (specular) transmittance and diffuse transmittance, which allow full calculation and verification of the haze ratio.
• Reflection haze of high-gloss coatings, expressed in hazemeter units.
• Specular gloss at 20 degrees and 60 degrees, frequently measured together with reflection haze to give a complete appearance profile.
• Haze difference between coated and uncoated reference specimens, useful for isolating the coating contribution from the substrate contribution.

Testing Methods

Transmission Haze: ASTM D1003, ISO 14782, GB/T 2410, JIS K 7361

Transparent coatings and coated films are measured with a integrating-sphere hazemeter or a spectrophotometer in transmission mode. A collimated beam illuminates the specimen, total luminous transmittance is determined first, and then the regularly transmitted beam is excluded with a light trap so that only diffusely scattered light is collected. Haze is calculated as the ratio of diffuse transmittance to total transmittance, multiplied by 100. The methods use Standard Illuminant C or D65 with photopic detector response, and only scattered light deviating more than 2.5 degrees from the incident direction is counted as haze.

Reflection Haze: ASTM E430, ISO 13803

High-gloss opaque coatings are evaluated with a dedicated haze-gloss instrument. The light source is set at a 20-degree specular geometry, and additional detectors are placed at small angular offsets on either side of the specular direction to capture narrow-angle scattered light that surrounds the mirror image. The measured scatter intensity is processed into reflection haze units. Because reflection haze is meaningful only on smooth, high-gloss surfaces, the method is normally applied to coatings with a 20-degree gloss above approximately 70 gloss units.

General Test Procedure

Regardless of the selected method, a consistent workflow is followed:

• Condition the specimens at 23 ± 2 °C and 50 ± 5% relative humidity, or as specified by the governing standard and the client agreement.
• Clean the specimen surfaces with lint-free wipes and, where appropriate, a suitable solvent that does not attack the coating; handle specimens by the edges only.
• Calibrate the instrument with certified reference standards, including a haze reference and a zero-reference, before each measurement session.
• Measure at least three to five positions on each specimen, avoiding marks, scratches, and coating defects, and report the arithmetic mean together with the standard deviation.
• Measure an uncoated substrate reference under identical conditions and, where required, correct the coated value to isolate the haze contribution of the coating layer.
• Report the test standard, specimen description, film thickness, substrate, conditioning, instrument geometry, and individual and mean results in the test report.

Applications

Coating haze value determination supports quality control, specification compliance, and product development across many industries:

• Automotive coatings: reflection haze control of clearcoats, refinish systems, and plastic trim to ensure deep, brilliant images on body panels.
• Optics and displays: transmission haze qualification of hard coats, anti-reflection coatings, and protective layers on lenses, touch panels, and display windows.
• Flexible packaging: haze measurement of heat-seal, barrier, slip, and lacquer coatings on transparent films where shelf presentation depends on clarity.
• Architectural and appliance glass: verification that low-emissivity, decorative, and protective coatings preserve visual transmittance and clarity.
• Wood and furniture finishes: appearance grading of high-gloss lacquers, UV coatings, and piano-finish systems.
• Research and failure analysis: troubleshooting cloudy or blotchy coatings by correlating haze with pigment dispersion, film thickness, cure conditions, and surface micro-roughness.

FAQ

What does the haze value of a coating actually represent?

For transparent coatings, the haze value is the percentage of transmitted light that is scattered away from the direct beam, so a value of 1.5% means that 1.5% of all transmitted light travels in scattered directions. For high-gloss opaque coatings, the reflection haze value quantifies the milky halo surrounding specular reflections, with lower values indicating sharper, cleaner reflected images.

How is haze different from gloss and clarity?

Gloss measures the intensity of light reflected at the specular angle, haze measures scattered light near or around the specular or transmitted direction, and clarity describes the ability to see fine detail through a material. A coating can simultaneously show high gloss yet elevated haze, which is precisely why dedicated haze instruments and standards exist.

Which standard should be selected for my product?

Choose ASTM D1003, ISO 14782, GB/T 2410, or JIS K 7361 for transparent coatings and coated films, and ASTM E430 or ISO 13803 for high-gloss opaque coatings. The correct choice depends on whether the quality characteristic of interest is see-through clarity or reflected-image quality, and the specifying document or customer drawing normally names the governing method.

What are typical acceptance levels for haze?

Acceptance limits are product-specific. High-performance optical coatings on smooth films often require transmission haze below 0.5% to 1%, general packaging coatings commonly allow up to about 3% to 5%, and reflection haze limits for premium automotive clearcoats are set as low single-digit to low tens of hazemeter units depending on the OEM specification.

What causes excessive haze in coatings?

Frequent root causes include incomplete pigment and additive dispersion, flocculation, micro-foam or pinholes, poor leveling, substrate texture telegraphing through the film, refractive-index mismatch between binder and particles, contamination by dust, moisture blush during cure, overbake, and film thickness above the specified range. Corrective actions target dispersion quality, filtration, application environment, and cure window optimization.

Can coating haze testing be outsourced?

Yes. Accredited third-party laboratories routinely perform haze determination to ISO, ASTM, EN, GB, and JIS methods, provide traceable calibration, and issue test reports suitable for supplier qualification, customer submission, and certification purposes. Submitting flat, clean, defect-free specimens of adequate size together with substrate references ensures the most reliable results.