PET Plastic Light Transmittance Test: Principles, Methods, and Standards

PET plastic light transmittance testing is a critical optical quality control procedure used to evaluate the transparency of polyethylene terephthalate materials in packaging, films, and optical applications. This test measures the percentage of incident light that passes through a PET specimen according to recognized standards such as ASTM D1003 and ISO 13468, providing manufacturers with essential data for product consistency and regulatory compliance. Understanding the testing principles, sample requirements, and influencing factors helps engineers and quality professionals optimize PET material formulations and processing conditions.

Technical Overview

Polyethylene terephthalate (PET) is a thermoplastic polyester widely valued for its excellent optical clarity, mechanical strength, barrier properties, and recyclability. Because PET is frequently used in applications where visual transparency is essential—such as beverage bottles, food containers, optical films, and protective windows—light transmittance is one of the most important optical performance indicators for this material.

Light transmittance (also called luminous transmittance or total transmittance, Tt) is defined as the ratio of the luminous flux transmitted through a specimen to the luminous flux incident upon it, expressed as a percentage. For amorphous PET, transmittance values typically exceed 88–92% in the visible spectrum, rivaling glass in many packaging applications. However, transmittance can be significantly reduced by crystallinity, thickness, colorants, additives, surface roughness, and processing defects.

A PET plastic light transmittance test quantifies this property under standardized illumination and detection conditions, usually with a standardized light source (CIE source A or D65) and a photodetector matched to the photopic response of the human eye. The test is routinely performed in quality control laboratories, third-party testing institutions, and research facilities to verify conformance with material specifications, customer requirements, and regulatory standards.

Testing Samples

PET Films and Sheets

Biaxially oriented PET (BOPET) films and extruded PET sheets are among the most commonly tested sample types. Film specimens are typically cut into squares or circles sized to fit the instrument aperture, commonly 50 mm × 50 mm. Film thickness may range from a few micrometers to several hundred micrometers, and thickness must be reported alongside transmittance because the two parameters are strongly correlated.

PET Bottle Walls and Preforms

For beverage and liquid food packaging, transmittance is measured on sections cut from bottle sidewalls, shoulders, or base regions. Because bottle walls are curved, small flat coupons are die-cut from representative locations and flattened for measurement. Preform specimens may also be tested to evaluate resin quality before stretch-blow molding.

Injection-Molded and Compressed Plaques

When evaluating resin grades, masterbatch formulations, or recycled PET (rPET) content effects, laboratory plaques are prepared by injection molding or compression molding under controlled temperature and cooling conditions. Standardized plaque dimensions and mold-polish surface finishes eliminate surface-texture variables and allow direct material-to-material comparison.

Sample Preparation Requirements

• Specimen surfaces must be clean, free of dust, fingerprints, oil, and scratches that scatter light.
• At least three to five specimens per sample should be tested, with results averaged.
• Specimens must be conditioned at standard laboratory atmosphere, typically 23 °C ± 2 °C and 50% ± 10% relative humidity, for a minimum of 40 hours prior to testing.
• Specimen thickness must be measured at multiple points and reported with the test result.
• Curved or wrinkled specimens should be flattened or replaced, as geometry distortions alter the light path.

Testing Items

A comprehensive PET optical test program may include several related parameters, each characterizing a different aspect of light interaction with the material:

• Total luminous transmittance (Tt): The percentage of incident visible light that passes through the specimen in all directions, including both regular (specular) and diffuse transmission.
• Regular transmittance (Tr): The percentage of light transmitted without deviation from the original optical path, measured with a small-angle receiver.
• Diffuse transmittance (Td): The difference between total and regular transmittance, representing light scattered by the specimen.
• Haze: The percentage of transmitted light that is scattered more than 2.5 degrees from the incident beam axis; haze quantifies the milky or cloudy appearance of PET.
• Clarity (see-through quality): The ability to resolve fine detail through the specimen, affected by small-angle forward scattering.
• Yellowness index and color coordinates: Often measured simultaneously to detect resin degradation or oxidation during processing.

Testing Methods

Primary Test Standards

• ASTM D1003: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics; the most widely referenced method in North America, using Method A (hazemeter) or Method B (spectrophotometer).
• ISO 13468-1 / 13468-2: Determination of the total luminous transmittance of transparent materials, using an integrating sphere with either a single-beam (Part 1) or double-beam (Part 2) configuration.
• GB/T 2410: The Chinese national standard for determining light transmittance and haze of transparent plastics, technically aligned with ASTM D1003.
• JIS K 7361-1 and JIS K 7136: Japanese standards covering total luminous transmittance and haze of plastics, respectively.

Test Apparatus

The two principal instrument types are the integrating-sphere hazemeter and the UV-Vis spectrophotometer equipped with a sphere attachment. A hazemeter illuminates the specimen with a parallel light beam, typically from a CIE source A illuminant, and collects transmitted light with an integrating sphere. A spectrophotometer scans the visible wavelength range (typically 380–780 nm) and calculates transmittance values weighted by the photopic luminosity function. Instruments must be calibrated with reference standards and verified with air or blank references before measurement.

Typical Test Procedure

• Calibrate and zero the instrument according to the manufacturer's instructions and the applicable standard.
• Measure and record specimen thickness at several locations.
• Place the specimen flush against the entrance port of the integrating sphere, ensuring full beam coverage and no gaps.
• Record total transmittance, and if haze is required, repeat the measurement with a light trap (zone ring) to obtain diffuse-only values for haze calculation.
• Test all replicate specimens and report the mean, standard deviation, specimen thickness, test standard, instrument model, and environmental conditions.

Key Factors Influencing Results

• Crystallinity: Spherulites and crystalline regions scatter light, sharply reducing transmittance and increasing haze; annealed or slow-cooled PET shows markedly lower transparency than quenched amorphous PET.
• Thickness: Transmittance decreases as optical path length increases, following an approximately exponential (Beer–Lambert-type) relationship.
• Resin quality and moisture: Hydrolytic degradation during processing creates acetaldehyde byproducts and discoloration that reduce optical performance.
• Additives and colorants: Tinting agents, UV absorbers, slip agents, and reheat additives modify both transmittance and spectral distribution.
• Surface condition: Tooling marks, micro-scratches, and matte surface finishes increase diffuse scattering and haze.
• Recycled content: rPET streams containing contamination or prior thermal history may exhibit reduced clarity compared with virgin resin.

Applications

Light transmittance testing supports a broad range of PET industry applications:

• Beverage bottle quality control: Verifying that bottle walls meet brand-owner clarity specifications for water, carbonated soft drinks, and juices, where consumer perception of product freshness is linked to package transparency.
• Food and pharmaceutical packaging: Confirming visible-light transmission levels for products sensitive to photodegradation, and validating the effectiveness of UV-blocking PET formulations.
• Optical and display films: Qualifying BOPET films used in touch panels, LCD components, protective cover films, and lamination substrates where haze and transmittance tolerances are tight.
• Material development: Comparing new resin grades, nucleating agents, clarifiers, and bio-based or recycled formulations during research and development.
• Incoming material inspection: Screening resin lots and films at goods-receiving checkpoints to prevent off-specification material from entering production.
• Certification and compliance: Providing documented optical test data for customer specifications, third-party certification, and export documentation.

FAQ

What is a good light transmittance value for PET plastic?

High-quality amorphous PET typically exhibits total luminous transmittance of 88% to 92% for typical bottle wall and film thicknesses. BOPET optical films often exceed 90% with haze below 1–3%. Specific acceptance criteria depend on the application, thickness, and customer specification.

How does PET thickness affect transmittance test results?

Thicker specimens reduce transmittance due to increased bulk absorption and scattering along the longer optical path. For this reason, test standards require reporting specimen thickness, and comparisons between materials are only valid at equivalent thicknesses.

Can a PET bottle be tested without cutting it?

Standard transmittance measurements require flat specimens at the instrument aperture, so representative coupons are cut from bottle walls. Some laboratories use specialized fixtures for whole-container screening, but regulatory and specification testing is performed on cut flat specimens per the applicable standard.

What is the difference between transmittance and haze?

Transmittance measures how much light passes through the specimen regardless of direction, while haze measures only the portion of transmitted light scattered at angles greater than 2.5 degrees. A PET part can have high transmittance but unacceptable haze if the material scatters light significantly.

Which standard should be selected for testing?

ASTM D1003 is the most common choice in North America, ISO 13468 is preferred in many international markets, and GB/T 2410 is widely used in China. The choice is usually dictated by customer specifications, the destination market, or contractual requirements; results from these standards are generally comparable when specimen preparation and instrument configuration are consistent.

Why did my PET sample become cloudy and fail the transmittance test?

Common causes include excessive crystallinity from slow cooling or high mold temperatures, hydrolytic degradation from inadequately dried resin, contamination or foreign particles in recycled content, excessive regrind ratio, poor melt homogenization, and surface defects from worn tooling. Investigating drying conditions, melt temperature, cooling rate, and material purity usually identifies the root cause.