Automotive coating haze measurement quantifies the milky, low-contrast veil that can degrade the appearance of high-gloss vehicle paint finishes. Using 20-degree reflectometer geometry in accordance with ISO 13803 and ASTM E430, accredited third-party laboratories evaluate reflection haze to support quality control, defect diagnosis, and coating development. This article explains the measurement principle, sample requirements, test procedures, and typical applications of haze testing for automotive coatings.
Technical Overview
Reflection haze, often simply called haze, is an optical phenomenon observed on high-gloss surfaces. It appears as a milky or cloudy halo surrounding the sharply reflected image of a light source, which reduces the contrast between the specular highlight and the surrounding area. On dark, high-gloss automotive finishes viewed under strong directional light such as sunlight or showroom lighting, haze is particularly visible and is perceived as a loss of depth and clarity.
Haze measurement is a critical complement to conventional gloss testing. Two panels may show identical 20-degree gloss values, yet one may appear noticeably milky because a larger fraction of the reflected light is scattered just outside the specular direction. Haze therefore captures an appearance attribute that gloss readings alone cannot detect.
For automotive coatings, haze originates from a variety of sources, including:
- Incomplete pigment dispersion or pigment flocculation in basecoats and tinted clearcoats
- Micro-roughness caused by poor leveling, solvent pop, or micro-bubbles
- Overspray, dry spray, or airborne contamination deposited in the wet film
- Incompatibility of additives such as waxes, slip agents, or light stabilizers
- Telegraphing of substrate or primer texture through the coating layers
- Polishing marks, fine scratches, and etch or degradation resulting from weathering
Because haze is fundamentally governed by near-specular light scatter, it is measured with a reflectometer operating at a 20-degree geometry equipped with a dedicated haze detector, as standardized in ISO 13803 and ASTM E430.
Testing Samples
OEM Coated Test Panels
Standard laboratory panels consist of rigid substrates such as cold-rolled steel, aluminum, or glass coated with the complete automotive paint system, typically electrocoat, primer surfacer, basecoat, and clearcoat. Panels are spray-applied and cured according to the paint supplier's specification, then conditioned in a controlled environment before measurement. Each panel must provide a flat, defect-free test area comfortably larger than the instrument aperture.
Production Body Panels and Plastic Parts
Finished components such as hoods, fenders, doors, bumpers, mirror housings, and trim parts can be measured directly. The test location must be flat and free of curvature within the measurement aperture, since surface curvature distorts both the specular and scattered light paths and produces misleading results.
Laboratory Films and Exposure Samples
Drawn-down or cast films on glass, artificial weathering panels exposed to xenon-arc or fluorescent UV light, outdoor exposure racks, and refinished or repaired areas from body shops are also routinely evaluated to track changes in haze over time or after specific treatments.
Testing Items
The primary measured quantity is the reflection haze value determined at 20-degree geometry. In practice, a complete appearance characterization typically includes the companion parameters listed below:
- Reflection haze at 20 degrees, the principal test item
- Specular gloss at 20 degrees, usually acquired simultaneously by the same instrument
- Gloss at 60 and 85 degrees in accordance with ISO 2813, ASTM D523, or GB/T 9754 where required
- Distinctness of image (DOI) and orange peel structure, evaluated with appropriate appearance instruments to build a full surface profile
- Statistical evaluation of mean value, range, and standard deviation across multiple measurement positions
- Delta haze before and after exposure, polishing, or processing steps in durability and process studies
Testing Methods
Haze measurement follows the internationally recognized methods described in ISO 13803 (Paints and varnishes — Determination of haze on paints at 20 degrees) and ASTM E430 (Standard Test Methods for Measurement of Gloss, Haze, and Luminous Reflectance of High-Gloss Surfaces by Goniophotometry). Related appearance methods include ISO 2813, ASTM D523, DIN 67530, and GB/T 9754 for specular gloss.
The measurement principle is as follows: a collimated light beam illuminates the coated surface at an incidence angle of 20 degrees. A specular receptor measures the intensity of the directly reflected beam, which determines gloss. A second, annular haze receptor located immediately outside the specular reflection cone collects the light scattered within a few degrees of the specular direction. The haze result is derived from the ratio of this near-angle scattered light to the reflected light, and may be expressed as a logarithmic haze index on a scale in which lower values indicate a clearer, higher-contrast reflection.
A typical test procedure conducted by accredited third-party laboratories includes the following steps:
- Instrument calibration and verification using certified high-gloss black glass reference standards and haze reference standards traceable to national metrology institutes
- Conditioning of samples in a controlled atmosphere, commonly 23 ± 2 degrees Celsius and 50 ± 5 percent relative humidity
- Gentle cleaning of the test surface with a lint-free cloth to remove dust without altering the surface
- Placement of the instrument flush against the flat test area, shielding the measurement from stray ambient light
- Measurement at a minimum of three positions, rotating the instrument orientation where practical to detect directional effects such as polishing marks
- Recording of mean haze value, range, companion gloss values, instrument geometry, and calibration standard identification in the test report
Key influence factors that must be controlled include surface curvature, orange peel texture, dirt and fingerprints, external light leakage at the instrument interface, and instrument tilt or uneven contact pressure.
Applications
Haze measurement is applied throughout the automotive coatings value chain for purposes such as:
- Appearance quality control on OEM coating lines and clearcoat release testing
- Incoming inspection of paint batches and raw materials at vehicle plants and paint manufacturers
- Formulation development of clearcoats, basecoats, additives, and pigment dispersions
- Optimization of milling, dispersion, and application process parameters
- Weathering and durability studies tracking haze development after xenon-arc, fluorescent UV, humidity, or outdoor exposure
- Root-cause analysis of customer appearance complaints and field failures
- Validation of refinish materials and procedures in body shop audits
- Benchmarking of competitive coating systems and supplier qualification programs
FAQ
What exactly is haze on an automotive paint surface?
Haze is the milky, low-contrast halo produced when a small portion of the reflected light is scattered just outside the mirror-like specular reflection. It reduces the sharpness and contrast of reflected images, giving the finish a cloudy or dull appearance despite normal gloss readings.
Which standards govern haze measurement for coatings?
The principal methods are ISO 13803 for the determination of haze on paints at 20 degrees and ASTM E430 for goniophotometric measurement of gloss, haze, and luminous reflectance of high-gloss surfaces. Companion gloss methods include ISO 2813, ASTM D523, DIN 67530, and GB/T 9754.
How is haze different from gloss?
Gloss measures the intensity of light reflected at the specular angle, while haze measures the light scattered in a narrow zone immediately surrounding that specular reflection. A coating can exhibit high gloss yet pronounced haze, so both parameters are needed for a complete appearance assessment.
What haze level is acceptable for an automotive clearcoat?
Lower haze values always indicate a clearer, higher-contrast finish. Acceptance limits are set individually by vehicle manufacturers and paint suppliers according to the color, gloss level, and positioning of the coating system, so results should always be compared against the relevant specification.
Can curved parts such as bumpers be tested?
Yes, provided the measurement is taken on a region that is flat across the entire instrument aperture. On strongly curved areas, measurements should be taken with appropriate support fixtures, or samples should be prepared as flat laboratory panels to ensure reliable data.
How often should the instrument be verified?
Best practice is to verify the instrument against certified gloss and haze reference standards at the start of each measurement session and after any change of location or temperature, with full calibration performed at intervals defined by the laboratory quality system.
Does haze relate to orange peel?
They are related but distinct attributes. Orange peel describes longer-wavelength surface waviness perceived as a textured reflection pattern, whereas haze arises from fine near-specular light scatter. A complete appearance evaluation of automotive coatings typically combines haze, gloss, distinctness of image, and orange peel measurements.