Light Transmittance Determination of PET High-Temperature Film: Standards, Methods, and Applications

Light transmittance determination for PET high-temperature film is a critical optical quality control test that quantifies the percentage of visible light passing through heat-resistant polyester films used in displays, flexible electronics, and photovoltaic modules. This technical article explains the applicable testing standards, sample preparation requirements, instrumentation, and step-by-step measurement procedures for accurately evaluating total luminous transmittance and haze in PET high-temperature films, helping manufacturers maintain consistent optical performance under demanding thermal conditions.

Technical Overview

PET (polyethylene terephthalate) high-temperature film is a biaxially oriented polyester film engineered to retain dimensional stability, mechanical strength, and optical clarity at elevated operating temperatures, typically ranging from 120°C to 180°C, with certain specialty grades performing reliably up to 200°C or higher. Because this material is widely specified in optically sensitive applications such as display modules, flexible printed circuits, and photovoltaic backsheets, its light transmittance is a decisive quality parameter.

Light transmittance determination measures the ratio of luminous flux transmitted through the film to the luminous flux incident upon it, expressed as a percentage. The measurement is normally performed over the visible spectrum (380–780 nm) using the CIE D65 standard illuminant with either a 2° or 10° standard observer. For a comprehensive optical evaluation, transmittance testing is frequently paired with haze measurement, which captures wide-angle light scattering that causes a milky or cloudy appearance even when total transmittance remains high.

Accurate transmittance data serves three essential purposes: verifying incoming material quality against specification limits, monitoring batch-to-batch consistency during production, and confirming that optical properties are preserved after thermal exposure or processing such as coating, laminating, and heat aging.

Testing Samples

Standard Clear High-Temperature PET Film

Uncoated, transparent biaxially oriented PET film in common thicknesses from 12 μm to 250 μm represents the baseline sample type. These specimens establish the reference optical performance of the base polymer and are used for routine release testing and certification.

Coated and Functionalized Films

High-temperature PET films frequently carry surface treatments such as hard coats, release coatings (silicone), anti-static layers, or transparent conductive oxides. Because coatings alter both absorption and scattering behavior, coated samples must be tested in their finished state to reflect real-world optical performance.

Thermally Aged Specimens

To evaluate optical stability, samples are exposed to controlled heat aging, for example 150°C or 180°C for 100 to 1000 hours in a forced-air oven, and then retested. Comparing transmittance before and after aging reveals whether the film yellows, crystallizes excessively, or degrades under sustained thermal load.

Matte, Hazy, and Tinted Grades

Diffuser films, matte release films, and colored high-temperature films scatter or absorb light intentionally. These samples require full integration of haze and diffuse transmittance measurements rather than simple total transmittance alone.

Testing Items

A complete optical characterization of PET high-temperature film typically includes the following test items:

• Total luminous transmittance (Tt): the overall percentage of visible light passing through the specimen, including directly transmitted and diffusely scattered components.
• Diffuse transmittance (Td): the percentage of transmitted light that is scattered at angles wider than 2.5° from the incident beam.
• Haze: the ratio of diffuse transmittance to total transmittance, quantifying cloudiness or milkiness.
• Spectral transmittance curve: transmittance as a function of wavelength across the UV, visible, and near-infrared ranges (typically 200–1100 nm), useful for identifying absorption bands and UV-blocking performance.
• Transmittance retention after thermal aging: the percentage of original transmittance retained following heat exposure, a key durability indicator.
• UV transmittance and UV blocking rate: critical for photovoltaic and outdoor applications where UV screening protects underlying layers.

Testing Methods

Reference Standards

Transmittance determination is conducted in accordance with internationally recognized methods, most commonly ASTM D1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics), ISO 13468-1 (Plastics — Determination of the total luminous transmittance of transparent materials), and GB/T 2410 (Determination of light transmittance and haze of transparent plastics). JIS K 7361-1 is an equivalent Japanese standard. Method A of ASTM D1003 employs a hazemeter, while Method B employs a spectrophotometer with an integrating sphere.

Instrumentation

The primary instrument is either a dedicated hazemeter or a UV-Vis spectrophotometer equipped with a 150 mm integrating sphere. The integrating sphere collects both direct and scattered transmitted light, enabling accurate total transmittance measurement. Regular calibration is performed using a blank air reference and certified transmittance standards traceable to national metrology institutes.

Sample Preparation and Conditioning

Specimens are cut to a minimum size of approximately 50 mm × 50 mm, large enough to fully cover the integrating sphere port. At least three specimens per sample are tested, taken from different positions across the web width to capture spatial uniformity. Specimens must be free of fingerprints, dust, scratches, wrinkles, and processing residue; cleaning is performed with lint-free wipes and reagent-grade isopropyl alcohol where appropriate. Prior to testing, samples are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618.

Measurement Procedure

• Step 1: Warm up and calibrate the instrument, setting the unobstructed beam (air reference) as 100% transmittance and verifying with a certified standard.
• Step 2: Measure the baseline incident luminous flux T1 with no specimen in place.
• Step 3: Mount the specimen flush against the integrating sphere entrance port and record the total transmitted flux T2.
• Step 4: For haze measurement, insert the light trap to exclude the regular (specular) transmitted component and record T3 with the specimen in place and T4 without the specimen.
• Step 5: Calculate total transmittance as Tt = (T2 / T1) × 100%, and diffuse transmittance as Td = [(T3 + T4 − T2) / T1] × 100%.
• Step 6: Calculate haze as Haze = (Td / Tt) × 100%. For spectral measurements, repeat the scan across the full wavelength range and report the photopically weighted visible average.
• Step 7: Repeat for all specimens and report the arithmetic mean, standard deviation, film thickness, conditioning state, and the specific standard used.

Quality Control Considerations

Measurement uncertainty is influenced by sphere geometry, beam alignment, specimen flatness, and surface contamination. Interlaboratory reproducibility is typically within ±0.5% transmittance for clear films when specimens and instruments are properly controlled. Laboratories should participate in periodic round-robin comparisons and maintain uncertainty budgets in accordance with ISO/IEC 17025 accreditation requirements.

Applications

Transmittance determination directly supports quality assurance across the major end uses of PET high-temperature film:

• Flexible printed circuits: coverlay and base films for FPCs require high clarity to enable optical inspection of circuit patterns after high-temperature lamination processes.
• Display and touch panels: protective films, carrier films, and release liners used in OLED and LCD manufacturing must meet tight transmittance and haze specifications, often above 90% transmittance with haze below 1.5%.
• Photovoltaic modules: backsheets and inner films must sustain high transmittance and UV stability for 25-year service lifetimes, making transmittance retention after damp-heat and thermal-cycling aging a certification requirement.
• Electronics manufacturing: high-temperature release films used in chip packaging and multilayer lamination rely on verified optical uniformity as an indicator of coating quality.
• Electrical insulation: transparent insulation films in motors, transformers, and battery components are validated for optical clarity alongside dielectric performance.
• Thermal printing and labels: facestock and liner films must maintain clarity and dimensional stability through heated print heads and converters.

FAQ

What is the typical transmittance value of PET high-temperature film?

Clear biaxially oriented PET high-temperature film generally exhibits total luminous transmittance between 88% and 92% for thicknesses of 25 μm to 125 μm. Specialty optical grades with purified resins and optimized surface finish can exceed 92%, while matte or filled grades fall lower depending on the scattering design.

Does film thickness affect transmittance results?

Yes, but the effect is modest for clear films because PET absorbs very little visible light; thicker films typically show only a slight decrease in transmittance. However, haze increases more noticeably with thickness in films containing scattering particles or rough surfaces, so thickness must always be reported alongside optical data.

What is the difference between transmittance and haze?

Transmittance measures the total amount of light that passes through the film regardless of direction, while haze measures only the portion of transmitted light scattered at wide angles. A film can have high transmittance yet unacceptable haze, which degrades image sharpness in display applications.

How does thermal aging influence transmittance?

Prolonged exposure to elevated temperatures can cause yellowing, increased crystallinity, and coating oxidation, all of which reduce visible transmittance and raise haze. Retesting transmittance after standardized heat aging, such as 1000 hours at 150°C, quantifies the optical durability of the film.

Which standard should be selected for testing?

ASTM D1003 and ISO 13468-1 are the most widely accepted methods for international trade, while GB/T 2410 is commonly specified in the Chinese market. All three methods yield comparable total transmittance values when instrument geometry and illuminant settings are harmonized, so the choice typically follows the specification cited by the customer or end user.

How many specimens are required for a valid test?

A minimum of three specimens is standard practice, tested at multiple locations across the film web. For referee or certification testing, five or more specimens are recommended to improve statistical confidence in the reported mean and standard deviation.