Light transmittance testing is a fundamental optical measurement for diffuser plates used in LED panel lights, LCD backlight units, and luminaires, determining how efficiently the material delivers usable light while achieving uniform illumination. This technical guide explains the principles of diffuser plate transmittance measurement, the governing international standards such as ASTM D1003, ISO 13468, and GB/T 2410, specimen preparation requirements, laboratory test procedures, and practical guidance for interpreting results. Engineers, quality managers, and procurement specialists can use this information to specify, verify, and optimize the optical performance of diffuser plate materials.
Technical Overview
A diffuser plate is an optical component designed to scatter and homogenize light, converting concentrated sources such as LEDs or light guide panel outputs into a uniform luminous surface. Diffuser plates are commonly manufactured from PMMA (polymethyl methacethacrylate), PC (polycarbonate), PS (polystyrene), or MS (methyl methacrylate styrene) copolymers, and achieve their scattering behavior through embedded micro-particles, volumetric phase separation, or micro-textured surface structures.
Light transmittance, expressed as total luminous transmittance (Tt), is defined as the ratio of luminous flux transmitted through the specimen to the luminous flux incident upon it, expressed as a percentage. For diffuser plates, transmittance must be evaluated together with haze, because the two parameters are inherently coupled: increasing the diffusion effect generally shifts light from the direct (specular) beam into the wide-angle scattered component. A well-designed diffuser plate maximizes total transmittance while maintaining sufficiently high haze to eliminate visible hot spots and light-source imaging.
Transmittance determination is typically performed using integrating sphere photometry or spectrophotometry in accordance with internationally recognized standards, and is routinely carried out by manufacturers, optical design teams, and accredited third-party laboratories for quality control, incoming inspection, and product certification purposes.
Testing Samples
PMMA (Acrylic) Diffuser Plates
PMMA is the most widely used substrate for LED panel light diffusers due to its high optical clarity, excellent UV weatherability, and low cost. Clear PMMA transmits approximately 92% of visible light, while commercial diffuser grades with particle loading typically achieve total transmittance values between 80% and 90%, with haze levels ranging from 70% to 95% depending on the formulation. Common thicknesses are 1.5 mm to 6 mm.
Polycarbonate (PC) Diffuser Plates
PC diffuser plates offer superior impact resistance and higher heat tolerance, making them suitable for luminaires subjected to mechanical stress or elevated operating temperatures, including IP-rated fixtures and vandal-resistant lighting. Typical total transmittance values fall between 80% and 88% for diffusion grades.
PS and MS Diffuser Plates
Polystyrene and methyl methacrylate styrene copolymers provide economical alternatives for light boxes, signage, and backlight modules where extreme weatherability is not required. Their optical performance is verified with the same test methods used for other substrate families.
Diffuser Films and Coated Plates
Thin diffuser films used in LCD backlight units, as well as plates with anti-glare or hard-coat surface treatments, can also be characterized for total transmittance and haze, provided the specimen is mounted flat and fully covers the measurement aperture.
Specimens should be flat, free of scratches, fingerprints, dust, and molding defects, and should be larger than the instrument aperture (typically 50 mm × 50 mm or circular disks of 50 mm to 100 mm diameter). Prior to testing, samples are conditioned per ISO 291 or ASTM D618 at 23 ± 2 °C and 50 ± 5% relative humidity, and a minimum of three specimens per material is recommended for statistical validity.
Testing Items
- Total luminous transmittance (Tt): the percentage of incident visible light transmitted through the specimen in all directions, weighted for the photopic response of the human eye.
- Haze: the percentage of transmitted light that is scattered more than 2.5 degrees from the incident beam direction, quantifying the diffusion effect.
- Parallel (specular) transmittance: the unscattered component that passes directly through the plate, relevant for evaluating residual imaging of the light source.
- Diffuse transmittance: the difference between total and parallel transmittance, representing the redirected light responsible for uniformity.
- Spectral transmittance distribution: wavelength-dependent transmission data across the visible range (380 nm to 780 nm), useful for color-critical display and lighting applications.
Testing Methods
Integrating Sphere Hazemeter Method (ASTM D1003 / GB/T 2410)
The most common approach uses a hazemeter equipped with an integrating sphere and a standard illuminant D65 light source. Four sequential readings are taken: the incident flux (T1), total transmitted flux with the specimen in place (T2), instrument-scattered flux with a light trap and without the specimen (T3), and combined instrument plus specimen scattering with the trap (T4). Total transmittance is calculated as Tt = (T2 / T1) × 100, and haze is calculated as [(T4 − T3) / T2] × 100. This method simultaneously yields both key optical parameters in a single measurement.
Spectrophotometric Method (ISO 13468-1 / ISO 13468-2)
ISO 13468 specifies the determination of total luminous transmittance of transparent and translucent plastics using either single-beam (Part 1) or double-beam (Part 2) ultraviolet-visible spectrophotometers fitted with integrating spheres. Spectral transmittance is scanned across the visible wavelength range and mathematically integrated using the D65 illuminant spectrum and the photopic luminosity function V(λ) to obtain the luminous transmittance value. This approach is preferred when full spectral data or color analysis is also required.
Haze Determination (ISO 14782 / JIS K 7361)
ISO 14782 and the Japanese standard JIS K 7361-1 define procedures for haze and luminous transmittance measurement with close harmonization to ASTM D1003, enabling consistent comparison of results across regional supply chains. GB/T 2410 serves as the equivalent reference for the Chinese market.
Practical Measurement Considerations
Instrument calibration against air or certified reference standards, verification of aperture alignment, and careful specimen cleaning are essential for reproducibility. Test report variables such as substrate thickness, surface structure, measurement geometry, and ambient conditioning must be recorded, since transmittance decreases measurably with increasing plate thickness and particle loading. Repeatability of ±0.5% or better is typically achievable with well-maintained integrating sphere instrumentation.
Applications
- LED panel lights and linear fixtures: verifying that diffuser plates deliver the required brightness, uniformity, and absence of LED imaging.
- LCD backlight units: qualifying diffuser films and plates in television, monitor, notebook, and automotive display manufacturing.
- Advertising light boxes and signage: balancing luminous efficiency with visual uniformity over large illuminated areas.
- Optical design and material selection: comparing substrate formulations, particle systems, and micro-structure geometries during product development.
- Incoming quality control and supplier qualification: confirming that delivered materials conform to purchase specifications.
- Compliance verification: providing objective optical data to accredited third-party laboratories for product certifications and buyer audits.
FAQ
Q: What is the typical transmittance of a diffuser plate?
A: Values depend on material and design. Clear PMMA reaches approximately 92%, while practical diffuser plates for LED panel lights generally exhibit total transmittance between 80% and 90%, combined with haze values of 70% to 95%. Higher haze formulations usually trade a few points of transmittance for improved uniformity.
Q: Why are transmittance and haze reported together?
A: The two parameters describe complementary aspects of optical performance. A plate with high transmittance but low haze transmits light efficiently but reveals the light sources behind it, while a plate with very high haze but low transmittance appears uniform yet wastes optical power. Lighting design requires an optimized balance of both.
Q: Which standards govern the measurement?
A: The primary references are ASTM D1003, ISO 13468-1, ISO 13468-2, ISO 14782, GB/T 2410, and JIS K 7361-1, all of which specify integrating sphere based measurement of luminous transmittance and haze for plastic materials.
Q: How does specimen thickness affect the result?
A: Transmittance decreases as thickness increases because scattering and absorption events accumulate over a longer optical path. Comparisons between materials are only valid when thickness is identical or when results are normalized with thickness clearly reported.
Q: Can textured or micro-structured diffuser plates be tested accurately?
A: Yes. As long as the specimen is mounted flat, fully covers the instrument aperture, and is oriented consistently, integrating sphere methods capture both the direct and scattered components. For strongly asymmetric structures, testing both orientations is recommended and should be noted in the report.
Q: How many specimens should be tested?
A: A minimum of three specimens per batch is standard practice, with mean values and standard deviation reported, so that material variability and measurement uncertainty can be properly assessed.
