PC Material Light Transmittance Testing: Standards, Methods, and Technical Specifications

Polycarbonate (PC) light transmittance testing is a critical quality control process that measures how efficiently visible light passes through PC materials, which directly determines their suitability for optical, lighting, and glazing applications. This technical guide covers internationally recognized testing standards such as ASTM D1003 and ISO 13468, sample preparation requirements, instrumentation, and typical transmittance values for transparent polycarbonate products.

Technical Overview

Polycarbonate (PC) is an amorphous engineering thermoplastic renowned for its exceptional combination of optical clarity, impact resistance, and thermal stability. Unmodified, optically graded PC typically achieves a total luminous transmittance of 87% to 91% for standard thicknesses, approaching the transparency of optical glass while offering dramatically better toughness and lighter weight. Because transmittance directly governs product performance in lighting, display, automotive, and optical applications, accurate and repeatable measurement is essential for material qualification, incoming inspection, and production consistency.

Transmittance in PC materials is influenced by multiple material and processing variables, including molecular weight distribution, residual monomer content, additive packages such as UV absorbers and release agents, mold or die surface finish, internal stress, crystallinity (in rare cases), and part thickness. Light transmittance testing quantifies these effects by measuring the ratio of transmitted luminous flux to incident luminous flux under standardized illumination and detection geometry, usually with Standard Illuminant D65 and the CIE photopic observer function.

Testing Samples

Injection Molded Plaques and Panels

Flat plaques with defined thicknesses (commonly 2 mm and 3 mm) are molded under controlled temperature and pressure conditions. The plaque surface must be free of flow lines, sink marks, weld lines, and gate stress whitening, as these defects scatter light and distort haze readings.

Extruded Sheets and Films

Commercial PC glazing sheets, corrugated profiles, and films are sampled from representative production zones. Corrugated or textured surfaces require special handling because the test geometry assumes parallel-plane specimens; textured samples are often measured on the flat region or milled to parallel faces with documented surface treatment.

Finished Optical Components

Lenses, light guides, lamp covers, and windows may be tested in finished form. Curved or complex-geometry parts cannot be measured directly on standard integrating-sphere instruments; in such cases, companion flat plaques molded under identical conditions serve as proxies, or specialized fixtures are used.

Sample Conditioning

Prior to testing, specimens should be conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618. Specimens must be cleaned with lint-free wipes and isopropyl alcohol, and handled with gloves to prevent fingerprints, which measurably increase haze.

Testing Items

A comprehensive PC transmittance evaluation typically includes the following measurable parameters:

• Total luminous transmittance (Tt): the percentage of incident visible light that passes through the specimen in all directions, integrated over the visible spectrum.
• Regular (specular) transmittance: the percentage of light transmitted within a narrow angular range without scattering, measured with a small-aperture or unscattered-light-excluded geometry.
• Diffuse transmittance: the scattered component, calculated as the difference between total and regular transmittance.
• Haze: the percentage of transmitted light that deviates more than 2.5 degrees from the incident beam direction; optical-grade PC typically exhibits haze below 1%, while light-diffusing grades are engineered to haze values above 50%.
• Spectral transmittance curve: the full transmittance profile versus wavelength, commonly scanned from 380 nm to 780 nm, or extended into the UV region to characterize UV-blocking additives.
• Yellowness index and color coordinates: frequently reported alongside transmittance to document thermal or photochemical degradation after aging tests.

Testing Methods

PC light transmittance testing follows internationally harmonized standards. The most widely applied methods are:

• ASTM D1003, Method A: uses a hazemeter or luminometer with a CIE standard light source C or D65 and specifies haze and luminous transmittance in a single measurement.
• ASTM D1003, Method B: uses a spectrophotometer equipped with an integrating sphere, offering spectral information and better suitability for tinted or wavelength-selective materials.
• ISO 13468-1: defines total luminous transmittance determination using a single-beam instrument.
• ISO 13468-2: defines the same measurement using a double-beam spectrophotometer, improving baseline stability.
• ISO 14782: specifies the measurement of haze for transparent plastics, applying a 2.5-degree scatter criterion.
• GB/T 2410: the Chinese national standard for transparent plastics, technically aligned with ASTM D1003 and widely referenced for domestic compliance.

The general test procedure consists of calibrating the instrument to a full-scale (100%) reference with the empty sphere aperture, establishing a zero baseline with an opaque light trap, and then placing the conditioned specimen against the input port. The integrating sphere collects both regular and diffuse transmitted light; closing a light trap port excludes the regular component for haze calculation. Measurements are normally taken at a minimum of three positions per specimen, with results averaged and reported to the nearest 0.1%. Instrument geometry, illuminant, observer angle, and specimen thickness must always accompany the reported values, because transmittance and haze both vary with thickness.

Applications

Verified transmittance data are essential across many PC end-use sectors:

• Automotive: headlamp lenses, panoramic roof panels, rear lamp covers, and instrument covers requiring transmittance above 88% with controlled haze and weathering stability.
• LED and general lighting: diffuser plates and lamp housings where engineered haze distributes light uniformly while maintaining high total transmittance for luminous efficiency.
• Optical components: camera lenses, ophthalmic lenses, prisms, and light guides demanding optical-grade clarity with sub-1% haze.
• Construction and architecture: greenhouse panels, skylights, safety glazing, and riot shields where light transmission, impact resistance, and UV stability must be documented together.
• Consumer electronics: display windows, touch panel substrates, and transparent device housings.
• Medical devices: transparent sterile packaging, IV components, and diagnostic consumables requiring validated optical performance.

FAQ

What transmittance value can be expected for standard PC?

Unmodified optical-grade PC with a thickness of 2 mm to 3 mm typically shows a total luminous transmittance of 87% to 91%. Adding UV stabilizers, flame retardants, colorants, or diffusing agents reduces this value, while premium optical grades and polished surfaces can approach 92%.

What is the difference between transmittance and haze?

Transmittance measures how much light passes through the material regardless of direction, whereas haze measures how much of that transmitted light is scattered beyond 2.5 degrees from the incident path. A material can have high transmittance and high haze simultaneously, which is exactly the design goal for LED diffusers.

Which test standard should be selected?

ASTM D1003 is preferred for North American markets and combined transmittance-plus-haze reporting, while ISO 13468 is preferred for European and international supply chains. GB/T 2410 is required for many Chinese regulatory and procurement submissions. Results from these standards correlate well, but they are not automatically interchangeable on certificates without note of method and geometry.

Does thickness affect the test result?

Yes. Transmittance decreases and haze increases with thickness due to bulk scattering and absorption. Comparisons between suppliers or batches are only valid at equal, documented thicknesses, and specifications should always state thickness alongside the transmittance limit.

How can the transmittance of PC products be improved?

Practical measures include using high-purity optical-grade resin, minimizing additive loading, optimizing mold temperature and hold pressure to reduce frozen-in stress, polishing tooling surfaces to a mirror finish, drying resin thoroughly before processing to eliminate moisture-induced defects, and avoiding over-thermal history that causes yellowing.