Haze value determination is the key optical test used to quantify the clarity and light-scattering behavior of acrylic (PMMA) materials, directly determining their suitability for lighting, display, and optical applications. This article explains the testing principles, applicable standards such as ASTM D1003, ISO 14782, GB/T 2410, and JIS K 7361, sample requirements, calculation methods, and typical acceptance criteria for acrylic sheets, films, and molded parts.
Technical Overview
Haze, also known as turbidity, is defined as the percentage of transmitted light that deviates from the incident beam by forward scattering when passing through a transparent specimen. Under internationally recognized test methods, only light scattered more than 2.5 degrees from the normal direction of incidence is counted toward haze. The haze value is calculated as the ratio of diffuse transmittance to total luminous transmittance, expressed as a percentage.
Acrylic (polymethyl methacrylate, PMMA) is one of the most widely used transparent plastics because of its exceptional optical purity, with visible light transmittance typically above 92 percent. Haze value determination is therefore a critical quality control index for acrylic products. Low haze values (typically below 1–2 percent) are required for optical-grade and glazing applications, while deliberately high haze values (often 30–90 percent) are engineered into light-diffusing acrylic used in LED luminaires and panel lights.
Haze determination for acrylic materials is generally performed according to the following reference standards:
- ASTM D1003 – Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics
- ISO 14782 – Plastics: Determination of haze for transparent materials
- ISO 13468-1 – Determination of the total luminous transmittance of transparent materials using an integrating sphere
- GB/T 2410 – Chinese national standard for determination of light transmittance and haze of transparent plastics
- JIS K 7361 and JIS K 7136 – Japanese standards for luminous transmittance and haze of plastics
Testing Samples
Cast Acrylic Sheets
Cast PMMA sheets are produced by cell casting between glass plates and generally exhibit extremely low haze, often below 0.5 percent for optical grades. Specimens should be cut to at least 50 mm × 50 mm so that they fully cover the measuring aperture of the hazemeter. Both surfaces must be free of scratches, fingerprints, dust, and protective-film residue, as surface contamination directly inflates measured haze.
Extruded Acrylic Sheets
Extruded acrylic is manufactured through continuous melt extrusion and may show slightly higher and more thickness-dependent haze due to die-line texture, melt inclusions, and molecular orientation. Because haze increases with optical path length, the specimen thickness must always be recorded and compared materials should be evaluated at equal thickness.
Light-Diffusing Acrylic
Diffuser-grade acrylic contains micro-particles or surface micro-structures intentionally designed to scatter light. Haze values for these materials commonly range from 40 to 95 percent, and the test is used to verify diffusion uniformity and to match transmittance–haze balance targets for luminaire design.
Acrylic Films, Tubing, and Molded Parts
Thin PMMA films, tubes, and injection-molded optical components such as lenses, covers, and display windows can also be evaluated. Curved or flexible specimens must be held flat without stress during measurement; where flattening is impossible, only flat sections or specially prepared plaques molded under identical conditions should be tested. For each material, a minimum of three to five specimens is recommended to obtain a representative average.
Testing Items
A complete optical characterization of acrylic materials typically includes the following measurable parameters:
- Total luminous transmittance (Tt) – the ratio of total transmitted light to incident light under standard illuminant D65 or A
- Diffuse transmittance (Td) – the portion of transmitted light scattered beyond 2.5 degrees from the incident direction
- Haze value – calculated as (Td / Tt) × 100, expressed as a percentage
- Parallel (regular) transmittance – derived as Tt minus Td, indicating the undeviated light fraction relevant to see-through clarity
- Thickness dependence of haze – optional evaluation of haze growth per millimeter for design purposes
Testing Methods
Hazemeter Method (ASTM D1003 Method A)
The most common approach uses a integrating-sphere hazemeter. A collimated beam from a standard light source passes through the specimen mounted against the sphere entrance port. Four successive readings are taken: incident light (T1), total light transmitted by the specimen (T2), instrument-scattered light without the specimen (T3), and instrument-plus-specimen scattered light (T4). From these readings, total transmittance equals T2/T1 × 100, and haze equals (T4 − T3)/T2 × 100. Modern instruments automate the full sequence and provide photometrically standardized results.
Spectrophotometer Method (ASTM D1003 Method B)
Method B employs a UV-Visible spectrophotometer fitted with an integrating sphere accessory, scanning the visible wavelength range (typically 380–780 nm) with illumination at the specimen. This method allows spectral analysis of haze and transmittance, useful when wavelength-dependent scattering must be characterized, for example in colored or coated acrylic.
ISO 14782 / JIS K 7136 and GB/T 2410 Procedures
The ISO and GB methods follow the same integrating-sphere principle with harmonized 2.5-degree scattering geometry and CIE standard illuminant conditions. GB/T 2410 additionally specifies compensation of surface reflection effects, which improves agreement between laboratories when comparing high-clarity specimens. Results obtained under different standards are generally comparable, but the exact standard used must always be stated in the test report.
Sample Preparation and Conditioning
Specimens should be conditioned at 23 ± 2 °C and 50 ± 5 percent relative humidity for at least 40 hours in accordance with ISO 291 or ASTM D618 before measurement. Surfaces must be cleaned with lint-free wipes and, if necessary, optical-grade isopropyl alcohol, then allowed to dry completely. Specimens must be positioned perpendicular to the beam, fully covering the aperture, with no bending stress that could induce birefringence-related artifacts.
Instrument Calibration and Quality Control
Prior to testing, the hazemeter must be zeroed and calibrated using certified reference standards for haze and transmittance traceable to national metrology institutes. Repeat measurements at different positions on each specimen help reveal local defects, surface texture variations, or non-uniform dispersion of diffusing agents.
Applications
Haze value determination for acrylic supports quality control and product development across many industries:
- LED lighting – verification of diffuser panels and light guide plates to achieve uniform illumination while minimizing light loss
- Display and electronics – acceptance testing of PMMA display windows, touch-panel covers, and transparent housings requiring haze below 1–2 percent
- Automotive – evaluation of headlamp lenses, tail-lamp covers, and instrument panel glazing for regulatory clarity and appearance requirements
- Architecture and construction – specification of skylights, glazing, canopies, and transparent noise barriers balancing daylighting with privacy
- Signage and retail – quality control of illuminated sign faces and acrylic lettering ensuring even brightness
- Medical and laboratory – optical windows, cuvettes, and transparent equipment enclosures where visual inspection clarity is essential
- Furniture and sanitary ware – glossy acrylic surfaces where haze measurement detects processing defects and surface degradation
FAQ
What is the difference between haze and light transmittance?
Light transmittance measures how much light passes through the specimen regardless of direction, while haze measures the fraction of that transmitted light that has been scattered sideways by more than 2.5 degrees. A material can have high transmittance yet high haze (a milky diffuser) or high transmittance and low haze (clear glazing), so both parameters must be reported together.
What haze value is acceptable for clear acrylic?
Optical-grade cast acrylic typically shows haze below 0.5–1 percent. General-purpose clear acrylic sheets are commonly specified at haze below 2 percent. Diffuser acrylic is intentionally specified at much higher values, often 40–95 percent depending on the luminaire design.
What causes high haze in acrylic?
Common causes include surface scratches and texture, dust or fingerprints, embedded particles and gels, micro-voids from processing, moisture absorption, residual internal stress, and aging phenomena such as UV-induced micro-cracking or chemical crazing.
Does specimen thickness affect the haze result?
Yes. Because scattering occurs along the light path, haze generally increases with thickness for a given material. Test reports must always state specimen thickness, and comparative evaluations should use materials of identical nominal thickness.
How many specimens should be tested?
Most standards and sound laboratory practice call for at least three specimens per sample, with results averaged. For non-uniform or diffusing materials, five or more measurements at different locations provide a more reliable mean value.
Can coated or textured acrylic be tested?
Anti-glare, hard-coated, or patterned acrylic can be tested, but the measured haze will include the contribution of the surface structure as well as the bulk material. The report should clearly identify surface treatments so that results are interpreted correctly against the intended specification.
