Light transmittance determination of acrylic glass (PMMA) is a critical optical performance test that quantifies how efficiently this transparent polymer transmits visible light, typically achieving values of 92% or higher. This article provides a comprehensive technical guide to PMMA light transmittance testing, covering applicable international standards such as ASTM D1003, ISO 13468, GB/T 2410, and JIS K 7361, sample preparation requirements, instrumentation, and typical applications across architectural, optical, and lighting industries.
Technical Overview
Acrylic glass, chemically known as polymethyl methacrylate (PMMA) and commonly referred to as organic glass or plexiglass, is one of the most widely used transparent thermoplastic materials in optical and structural applications. Light transmittance determination is the fundamental optical test used to characterize how much visible light passes through the material, expressed as the ratio of transmitted luminous flux to incident luminous flux over the visible spectrum (380 nm to 780 nm).
PMMA exhibits exceptional optical clarity due to its amorphous molecular structure, low level of internal scattering centers, and refractive index of approximately 1.49. High-quality clear acrylic transmits up to 92–93% of incident visible light, which is comparable to or better than most common silicate glasses. The small percentage of light loss is primarily attributed to Fresnel reflection losses at the two air–polymer interfaces, which together account for approximately 7–8% of the total incident light.
Light transmittance determination distinguishes between several optical quantities: total luminous transmittance (Tt), which includes both directly transmitted and diffusely transmitted light; regular (specular) transmittance (Tr), which measures only light transmitted within a narrow angular range of the incident beam; diffuse transmittance (Td), the difference between total and regular transmittance; and haze, defined as the percentage of transmitted light that deviates by more than 2.5 degrees from the incident beam direction. Accurate determination of these parameters is essential for material qualification, quality control, product certification, and compliance verification with procurement specifications.
Testing Samples
Cast Acrylic Sheets
Cast PMMA sheets are produced by cell casting between glass plates and generally offer superior optical quality, surface flatness, and lower haze. Typical test specimens are flat panels ranging from 1 mm to 25 mm in thickness, cut to the size required by the spectrophotometer sample port, commonly 50 mm × 50 mm or circular discs of 45–50 mm diameter.
Extruded Acrylic Sheets
Extruded PMMA sheets are manufactured by continuous melt extrusion and may exhibit slightly higher haze or thickness variation. Specimens should be taken from representative positions across the sheet width to account for process-related optical variation, and surface protection films must be completely removed before testing.
Oriented (Stretched) Acrylic Sheets
Biaxially stretched or oriented PMMA, used in impact-resistant glazing, may show stress birefringence and angular scattering. Specimens require careful handling to avoid additional surface scratches, and orientation direction should be documented relative to the incident beam geometry.
PMMA Films and Thin Gauges
Thin acrylic films below 1 mm must be mounted flat without wrinkles, using a holder that does not obstruct the optical beam. Multiple film layers should never be stacked unless a laminated structure is explicitly the object of the test, as interfacial reflections would falsify the result.
Molded Optical Components and Tubes
Injection-molded lenses, light guides, lamp covers, and extruded acrylic tubes or rods can be tested by preparing flat sections from the finished part or by using fixtures that present a representative flat area at the beam path, provided curvature effects are accounted for.
Testing Items
The principal optical quantities determined in a comprehensive light transmittance evaluation of acrylic glass include the following:
- Total luminous transmittance (Tt): the percentage of incident visible light transmitted through the specimen in all forward directions, weighted by the CIE photopic luminosity function.
- Regular (specular) transmittance (Tr): the percentage of light transmitted without deviation from the incident direction, characterizing image-forming clarity.
- Diffuse transmittance (Td): the percentage of transmitted light scattered from the incident beam direction, calculated as Tt minus Tr.
- Haze: the percentage of transmitted light scattered more than 2.5 degrees from the incident beam, an index of milky appearance and scattering defects.
- Spectral transmittance curve: transmittance as a function of wavelength across the visible and near-infrared range, useful for colored, tinted, or IR-blocking acrylic grades.
- Clarity and see-through quality: relevant for applications requiring sharp image transmission through the material.
Testing Methods
Light transmittance determination of PMMA is performed using spectrophotometers or hazemeters equipped with integrating spheres, operated under the conditions specified in recognized international and national standards. Specimens are conditioned prior to testing, typically at 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 40 hours, and must be free of dust, fingerprints, scratches, and anti-static residues.
ASTM D1003 (Haze and Luminous Transmittance of Transparent Plastics)
ASTM D1003 is the most widely referenced method in North America. Method A uses a hazemeter with a unidirectional illuminating beam and an integrating sphere with a 2.5-degree trap for measuring wide-angle scattering; Method B uses a spectrophotometer with an integrating sphere to obtain the luminous transmittance by spectral integration. The standard covers total transmittance, diffuse transmittance, and haze, and requires reporting of specimen thickness because transmittance decreases measurably with thickness.
ISO 13468-1 and ISO 13468-2 (Total Luminous Transmittance of Transparent Materials)
ISO 13468-1 specifies the determination of total luminous transmittance using a single-beam method with an integrating sphere, while ISO 13468-2 specifies a double-beam instrument method. Both methods use either standard illuminant D65 or A with the CIE 1931 standard colorimetric observer (2-degree field of view) and integrate the spectral transmittance over the 380–780 nm range. These standards are preferred for international supply chain specifications and CE-related declarations.
ISO 14782 (Determination of Haze)
For PMMA grades where scattering must be quantified separately, ISO 14782 defines the haze determination complementary to ISO 13468, using an integrating sphere with a light trap and defined tolerance on beam geometry.
GB/T 2410 (Chinese National Standard)
GB/T 2410 specifies the determination of light transmittance and haze of transparent plastics using a spectrophotometer or hazemeter with integrating sphere, and is technically aligned with ASTM D1003 and JIS K 7361. It is the mandatory reference method for products placed on the Chinese market and for supplier acceptance testing against Chinese national standards.
JIS K 7361-1 and JIS K 7136
The Japanese industrial standards JIS K 7361-1 (total luminous transmittance) and JIS K 7136 (haze) mirror the corresponding ISO methods and are applied for products destined for the Japanese market.
Measurement Procedure and Calculation
In a typical integrating sphere procedure, the instrument first records the full-scale incident light flux with an empty sample port (T1), then the total transmittance with the specimen at the port (T2), then the diffuse component with the specimen moved behind the sphere and the light trap open (T3), and finally the dark reference (T4). Total luminous transmittance is calculated as Tt = T2/T1, diffuse transmittance as Td = (T3 − T4)/T1, regular transmittance as Tr = Tt − Td, and haze = Td/Tt × 100%. Instruments are calibrated and verified using traceable reference standards, with air as the 100% baseline and a calibrated light trap or black standard as the zero line. At least three specimens per material are tested, and results are reported with specimen thickness, conditioning state, standard illuminant, observer angle, and the specific method used.
Typical acceptance values for optical-grade clear PMMA are total luminous transmittance of 92% or greater (for thin gauges) and haze below 1–2%, while diffuser and light-guiding grades are specified with deliberately high diffuse fractions rather than low haze.
Applications
Light transmittance determination supports a broad range of acrylic glass applications across multiple industries:
- Architectural glazing: skylights, canopies, domes, and transparent noise barriers where daylight transmission must meet design and energy performance targets.
- Lighting engineering: lamp covers, LED diffusers, light guides, and lens covers where the balance of regular and diffuse transmittance controls illumination efficiency and glare.
- Display and instrument windows: protective covers for screens, gauges, showcases, and museum cases requiring high clarity and low haze.
- Transportation: aircraft cabin windows, marine glazing, vehicle lamp housings, and windshields for specialty vehicles, where optical quality and weathering resistance are jointly specified.
- Signage and retail: illuminated sign faces and display panels where uniform light transmission determines visual quality.
- Optical components: precision lenses, prisms, and optical filters where spectral transmittance curves govern system performance.
- Medical and laboratory equipment: transparent barriers, cuvettes, incubator doors, and protective shields requiring documented optical properties for regulatory files.
FAQ
What is the typical light transmittance of acrylic glass?
Optical-grade clear PMMA transmits approximately 92–93% of visible light for common thicknesses between 2 mm and 10 mm. The value decreases slightly as thickness increases because of bulk absorption, and increases marginally for very thin films. The remaining 7–8% is almost entirely Fresnel reflection at the two surfaces.
How does acrylic compare with glass in light transmittance?
Clear acrylic matches or slightly exceeds standard clear window glass (approximately 90–92%) in visible transmittance, while offering half the weight and higher impact resistance. Low-iron glass can reach 91–92%, comparable to PMMA, but at higher weight and lower formability.
Does specimen thickness affect the test result?
Yes. Transmittance determination is thickness dependent because absorption and internal scattering accumulate along the optical path. Standards require reporting the specimen thickness, and comparisons between materials are valid only at equal thickness.
What is the difference between transmittance and haze?
Transmittance quantifies how much light passes through the specimen regardless of direction, while haze quantifies the portion of transmitted light that is scattered away from the incident direction by more than 2.5 degrees. A material can have high transmittance yet high haze, giving a milky appearance instead of clear see-through quality.
Which standard should be specified for procurement documents?
The choice depends on the target market and product sector: ASTM D1003 is common in North America, ISO 13468-2 and ISO 14782 for international specifications, GB/T 2410 for the Chinese market, and JIS K 7361-1/JIS K 7136 for Japan. Independent verification can be obtained from accredited third-party laboratories with optical testing capability covering these standards.
How are colored or tinted acrylic grades evaluated?
Tinted grades are evaluated by full spectral transmittance curves in addition to photometric total transmittance, and often supplemented by colorimetric values (CIE L*a*b* and chromaticity coordinates) calculated from the same spectral data under the specified illuminant and observer.