Determination of Light Transmittance of Optical Plastics: Testing Standards, Methods, and Applicatio

Light transmittance is one of the most critical performance indicators for optical plastics, directly determining their suitability for lenses, display components, automotive lighting, and medical devices. This article provides a comprehensive technical overview of optical plastic light transmittance determination, covering internationally recognized standards such as ASTM D1003, ISO 13468, ISO 14782, and GB/T 2410, along with sample requirements, instrumentation, and typical applications. Whether you are performing incoming material inspection, product qualification, or research and development, this guide explains how accredited third-party laboratories evaluate the optical quality of transparent polymer materials.

Technical Overview

Light transmittance, also referred to as luminous transmittance, is defined as the ratio of transmitted luminous flux to incident luminous flux, expressed as a percentage. For optical plastics—the family of transparent polymer materials engineered for demanding optical applications—transmittance is the single most important quantitative indicator of clarity and optical quality. Optical-grade polymethyl methacrylate (PMMA) typically achieves a total luminous transmittance of 92–93%, while optical-grade polycarbonate (PC) reaches 88–91% at comparable thicknesses.

Because transmitted light consists of both an undeviated beam and scattered components, laboratories distinguish between total transmittance, regular (specular) transmittance, and diffuse transmittance. Haze, defined as the percentage of transmitted light that is scattered more than 2.5 degrees from the direction of the incident beam, is usually measured together with transmittance to provide a complete picture of optical performance. Transmittance determination is governed by well-established standards, including ASTM D1003, ISO 13468-1, ISO 13468-2, ISO 14782, GB/T 2410, JIS K 7361-1, and JIS K 7136.

Key Optical Parameters

• Total luminous transmittance (Tt): the percentage of all transmitted light, including both regular and diffuse components, weighted according to the photopic response of the human eye over the visible spectrum (380–780 nm).
• Regular (specular) transmittance: the portion of light that passes through the specimen without deviation, which determines image-forming clarity.
• Diffuse transmittance: the portion of transmitted light scattered by the material, calculated as total transmittance minus regular transmittance.
• Haze: wide-angle scattering expressed as a percentage of total transmitted light, a key indicator of clarity loss in transparent plastics.
• Spectral transmittance: the transmittance measured as a function of wavelength across the ultraviolet (UV), visible, and near-infrared (NIR) regions.
• UV cutoff wavelength: the wavelength at which transmittance reaches a defined threshold, critical for UV-blocking and blue-light-filtering products.

Testing Samples

Common Optical Plastic Materials

• PMMA (polymethyl methacrylate, acrylic): the benchmark optical plastic with approximately 92% transmittance and excellent weatherability.
• PC (polycarbonate): high impact resistance with 88–90% transmittance, widely used in automotive lighting and safety glazing.
• PS (polystyrene) and SAN (styrene-acrylonitrile): low-cost transparent resins with high transmittance and good processability.
• COP/COC (cyclic olefin polymer/copolymer): low birefringence and low water absorption, ideal for precision camera lenses and microfluidics.
• CR-39 / ADC (allyl diglycol carbonate): the classic thermoset material for ophthalmic lenses.
• TAC (triacetyl cellulose): the standard transparent film for polarizer substrates in displays.
• PSU, PES, and PEI (polysulfone and polyetherimide): heat-resistant transparent engineering plastics for medical and aerospace applications.
• Optical-grade PET, transparent polyamides, and optical silicone gels: specialty materials used in films, LED encapsulation, and flexible optics.

Sample Forms

• Injection-molded flat plaques and compression-molded sheets.
• Extruded or cast films and sheets of various thicknesses.
• Finished or semi-finished optical lenses, including spherical, aspheric, and diffractive elements.
• Light guide plates, diffuser plates, and microstructured optical components.
• Hard-coated, anti-reflective (AR), or anti-fog coated transparent parts.
• Laboratory cuvettes, microfluidic chips, medical consumables, and 3D-printed optical prototypes.

Sample Requirements and Conditioning

For standard transmittance and haze measurements, specimens should provide a flat, uniform test area of at least 50 mm × 50 mm where possible, although many instruments accept smaller apertures down to approximately 20 mm × 20 mm. The test thickness is typically defined by the product specification, commonly 0.5–3 mm, and must be measured and reported to 0.01 mm because transmittance decreases predictably with increasing thickness according to the Beer–Lambert law. Both surfaces must be free of scratches, fingerprints, dust, and release agents; cleaning with a lint-free wipe and isopropyl alcohol is recommended when necessary. Prior to testing, specimens should be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours in accordance with ISO 291 or GB/T 2918 to ensure stable and comparable results.

Testing Items

Accredited third-party laboratories typically offer the following transmittance-related test items for optical plastics:

• Total luminous transmittance over the visible spectrum (380–780 nm), photopically weighted.
• Regular (specular) transmittance and diffuse transmittance.
• Haze in accordance with ASTM D1003, ISO 14782, GB/T 2410, or JIS K 7136.
• Full spectral transmittance curves from 200 nm to 1100 nm (UV-Vis-NIR).
• UV transmittance and UV-blocking rate, for example transmittance at 365 nm or verification of UV400 performance.
• Blue-light transmittance in the 415–455 nm band for eyewear and display protection products.
• Yellowness index and transmitted color coordinates per ASTM E313 and ISO 11664.
• Transmittance retention after aging tests (UV weathering, damp heat, thermal cycling) for reliability assessment.

Testing Methods

Integrating-Sphere Spectrophotometry

The integrating-sphere method is the primary technique for determining total luminous transmittance and haze. A collimated light beam passes through the specimen, which is placed against the entrance port of an integrating sphere—typically 150 mm in diameter for haze measurement per ISO 14782. The photometric response is weighted using a defined CIE standard illuminant (A, C, or D65) and the CIE 1931 (2-degree) standard color-matching functions. Four sequential measurements are taken: the incident flux (T1), total transmitted flux with the specimen in place (T2), instrumental scatter without the specimen (T3), and instrumental plus specimen scatter (T4). Total transmittance is calculated as T2/T1, while haze is obtained by dividing the corrected diffuse transmittance by the total transmittance and multiplying by 100. This approach underpins ASTM D1003 (Method B), ISO 13468-1 (single-beam compensation), ISO 13468-2 (double-beam method), and GB/T 2410.

Hazemeter Method

ASTM D1003 (Method A) describes the use of a dedicated hazemeter, an instrument with fixed 0-degree illumination and diffuse viewing geometry equipped with a light trap for excluding the regular beam when measuring scattered light. Hazemeters provide rapid, repeatable results and are widely used for routine quality control of transparent sheets, films, and molded parts, although results may differ slightly from spectrophotometric methods and should not be interchanged without correlation studies.

Spectral Scanning Method

A double-beam UV-Vis-NIR spectrophotometer is used to record the transmittance spectrum of the specimen, typically from 200 nm to 1100 nm with a scan interval of 1–5 nm and an appropriate spectral bandwidth. An air baseline serves as the 100% reference. Spectral scans are essential for characterizing UV cutoff behavior, evaluating anti-reflective and hard coatings, and supporting optical design simulations that require wavelength-dependent transmittance data.

General Test Procedure

• Instrument warm-up, baseline acquisition, and verification using calibrated transmittance reference standards traceable to national metrology institutes.
• Visual inspection and thickness measurement of the conditioned specimen at multiple points.
• Mounting of the specimen perpendicular to the optical axis, with the measurement side facing the source as specified.
• Acquisition of the four readings (T1 through T4) or spectral scan, repeated at three or more positions across the specimen.
• Calculation of total transmittance, diffuse transmittance, regular transmittance, and haze; reporting of mean values, thickness, standard designation, illuminant/observer condition, and measurement uncertainty.

Applicable Standards Comparison

• ASTM D1003: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics (United States).
• ISO 13468-1 / ISO 13468-2: Determination of the total luminous transmittance of transparent materials (international).
• ISO 14782: Determination of haze for transparent materials (international).
• GB/T 2410: Chinese national standard for the determination of transmittance and haze of transparent plastics, technically aligned with the international methods.
• JIS K 7361-1 and JIS K 7136: Japanese standards for total luminous transmittance and haze of plastics.

Applications

Imaging and Consumer Optics

Transmittance determination is essential for camera module lenses, smartphone cover lenses, AR/VR optical elements, and ophthalmic lenses, where every percentage point of transmittance affects image brightness, display efficiency, and user experience.

Display and Touch Electronics

Light guide plates, diffuser plates and films, TAC polarizer substrates, and protective cover windows all rely on certified transmittance and haze values to guarantee brightness, uniformity, and visual clarity in LCD, OLED, and touch panel assemblies.

Automotive Lighting and Glazing

Polycarbonate headlamp lenses, tail lamp covers, instrument cluster windows, panoramic roof glazing, and LiDAR or sensor windows must demonstrate high transmittance combined with long-term weathering resistance, making transmittance testing a core element of automotive material qualification.

LED and General Lighting

PMMA and PC primary lenses, light pipes, lamp covers, and diffuser housings are validated through transmittance and haze measurements to balance light extraction efficiency against glare control.

Medical and Life Science Devices

Cuvettes, microfluidic chips, IV administration components, diagnostic consumables, and sterilizable polysulfone or polyetherimide parts require verified optical performance for accurate photometric detection and regulatory compliance.

Photovoltaics and Agriculture

Transparent encapsulation films for solar modules and greenhouse covering films are evaluated by spectral transmittance to maximize energy conversion efficiency or optimize plant-growth lighting.

Quality Control and Data Interpretation

Typical acceptance criteria for optical-grade materials include total luminous transmittance of at least 92% for PMMA, at least 91% for COP/COC, and at least 88% for polycarbonate, with haze below 1.0% for optical sheets and below 0.5% for premium ophthalmic applications. Because measured transmittance depends strongly on thickness, surface finish, and measurement geometry, comparisons between datasets are only valid when these parameters are consistent.

Factors Affecting Transmittance Results

• Specimen thickness: thicker sections absorb and scatter more light, reducing transmittance logarithmically.
• Surface roughness and mold finish: poor polishing or tooling marks increase scatter and haze.
• Internal stress and birefringence: uneven injection molding induces optical anisotropy and localized clarity loss.
• Additives and colorants: UV absorbers, dyes, and impact modifiers reduce visible transmittance by design or as a side effect.
• Moisture absorption and contamination: water uptake, gels, and black specks degrade optical uniformity.
• Aging and yellowing: UV exposure and thermal aging progressively lower transmittance and raise yellowness index.

The Value of Accredited Third-Party Testing

Partnering with ISO/IEC 17025-accredited third-party laboratories ensures that transmittance data are generated with calibrated, traceable instrumentation under controlled environmental conditions, providing impartial evidence for supplier qualification, product certification, regulatory submission, and international trade documentation.

FAQ

What is a good light transmittance value for optical plastics?

Optical-grade PMMA reaches approximately 92–93%, COP/COC around 91–92%, and polycarbonate 88–91% at thicknesses of 2–3 mm. Values above 90% at these thicknesses are generally considered excellent, while haze below 1% is expected for optical-grade materials.

What is the difference between transmittance and haze?

Transmittance quantifies how much light passes through the material, whereas haze quantifies how much of the transmitted light is scattered away from the incident direction (beyond 2.5 degrees). A material can have high transmittance yet high haze, transmitting plenty of light but producing blurred or milky images.

Which standard should I choose: ASTM D1003 or ISO 13468?

Both are internationally accepted. ASTM D1003 is widely requested in North American supply chains and covers haze as well as transmittance, while ISO 13468 and ISO 14782 are preferred in Europe and Asia. GB/T 2410 and JIS K 7361/JIS K 7136 serve the Chinese and Japanese markets respectively. The choice typically follows the specification agreed with the end customer.

What sample size and thickness are required?

A flat, defect-free area of at least 50 mm × 50 mm is ideal, though smaller apertures of about 20 mm × 20 mm can be accommodated. Test thickness is defined by the product specification, commonly 0.5–3 mm, and must always be reported together with the result.

Can finished lenses with curved surfaces be tested?

Yes, curved lenses can be measured using appropriate fixturing and small apertures, but results depend on geometry and are best used for comparative purposes. For standard-compliant reporting, flat coupons injection-molded under identical conditions are strongly recommended.

Can UV-blocking and blue-light-filtering performance be measured?

Yes. Spectral scanning from 200 nm to 1100 nm provides complete transmittance curves, allowing determination of UV cutoff wavelengths, transmittance at 365 nm, UV400 compliance, and blue-light transmittance in the 415–455 nm band for eyewear and display products.

How should specimens be prepared before testing?

Specimens should be cleaned with isopropyl alcohol using lint-free wipes, handled by the edges to avoid fingerprints, and conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours prior to measurement.