PET Film Light Transmittance Testing: Standards, Methods, and Technical Guide

Matte and Anti-Glare PET Film

Surface-textured films designed to diffuse light and reduce glare. These films intentionally exhibit higher haze while maintaining acceptable total transmittance, so both parameters must be reported.

Functional Coated PET Film

Films carrying functional layers such as ITO transparent conductive coatings, anti-reflection (AR) stacks, blocking layers, or adhesive layers. AR-coated films may show transmittance above 94%, while ITO coatings introduce spectral selectivity that requires full-spectrum measurement.

White and Opaque PET Film

Pigment-filled or voided PET films used in photovoltaic backsheets and labels. Total transmittance for these materials is typically below 30%, and the test method must account for strong light scattering.

Testing Items

A comprehensive optical characterization of PET film generally includes the following measurement items:

• Total luminous transmittance (Tt): the ratio of total transmitted luminous flux to incident luminous flux over the visible spectrum (380–780 nm), weighted to the CIE standard illuminant D65 and photopic observer function.
• Diffuse transmittance (Td): the portion of transmitted light scattered at angles greater than 2.5 degrees from the incident beam axis.
• Haze: calculated as the ratio of diffuse transmittance to total transmittance, expressed as a percentage.
• Spectral transmittance curve: transmittance as a function of wavelength across the ultraviolet, visible, and near-infrared regions, useful for identifying absorption from additives, UV absorbers, or coatings.
• Parallel transmittance and clarity: narrow-angle scattering measurement relevant for display applications where image sharpness is critical.

Testing Methods

Transmittance testing of PET film is conducted according to internationally recognized standards, primarily ASTM D1003 and ISO 13468. Both standards define the geometry of illuminating and receiving optics, reference calibration procedures, and calculation formulas.

ASTM D1003 Method A — Haze Meter

A calibrated haze meter (hazemeter) illuminates the specimen with a parallel light beam and uses an integrating sphere to collect total and diffuse transmitted light. The unilluminated sphere is first measured without the specimen to establish the reference. This method is fast, highly repeatable, and widely used for production quality control.

ASTM D1003 Method B — Spectrophotometer

A UV-Visible spectrophotometer equipped with an integrating sphere accessory measures spectral transmittance at each wavelength, and the total luminous transmittance is calculated by spectral weighting. This method provides additional spectral information and is preferred when coatings or additives introduce wavelength-dependent behavior.

ISO 13468-1 and ISO 13468-2

The ISO standard specifies total luminous transmittance measurement using either a single-beam integrating sphere arrangement (Part 1) or a dual-beam arrangement (Part 2) for improved accuracy. GB/T 2410 is the corresponding Chinese national standard with similar technical requirements.

Sample Preparation and Conditioning

• Condition specimens at 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours prior to testing.
• Cut at least three specimens from different regions of the film roll, free of wrinkles, creases, scratches, fingerprints, and dust particles.
• Specimen size should fully cover the instrument aperture; typical specimens are 50 mm × 50 mm squares.
• Clean gently with lint-free wipes and dry filtered air if needed; avoid solvents that may swell or craze coated surfaces.
• Report the film thickness, coating configuration, and manufacturer lot information with each result.

Instrument Calibration and Measurement Procedure

Calibrate the haze meter using the standard reference tiles and zero the system with an empty light path. Place the specimen flat against the transmission port without stretching or trapping air bubbles, take a minimum of three readings per specimen, and rotate the specimen 90 degrees to check for directional anisotropy. Report the arithmetic mean of all valid measurements along with the measurement uncertainty.

Applications

Transmittance test data for PET film are used across many industries and product development stages:

• Display industry: qualification of PET substrates and protective films for LCD modules, OLED encapsulation, and touch panels, where high transmittance above 90% and low haze below 1.5% are commonly specified.
• Photovoltaic industry: verification of PET-based backsheets and front encapsulation films, ensuring adequate light transmission to the solar cells and long-term UV stability.
• Electronics manufacturing: incoming inspection of release films, carrier films, and insulating films used in flexible printed circuits and component assembly.
• Optical films: development and production control of anti-glare, anti-reflective, and diffuser films built on PET substrates.
• Packaging industry: evaluation of transparent packaging films where product visibility and shelf presentation are important.
• Research and development: comparison of new resin formulations, stretching processes, and coating chemistries during film development.

FAQ

What is the typical transmittance of PET film?

Standard transparent PET film measures approximately 88% to 92% total luminous transmittance. Optical-grade films can reach 90% to 93%, and films with anti-reflection coatings may exceed 94%.

Which standards govern PET film transmittance testing?

The primary standards are ASTM D1003, ISO 13468-1, ISO 13468-2, and GB/T 2410. All define the illuminant, receiver geometry, and calculation method for total transmittance and haze.

What is the difference between transmittance and haze?

Transmittance measures the total percentage of light passing through the film, while haze measures the percentage of that transmitted light that is scattered by more than 2.5 degrees. A film can have high transmittance yet unacceptable haze if its surface causes significant scattering.

Does film thickness affect transmittance results?

Yes. Thicker films generally show slightly lower transmittance due to increased bulk absorption and scattering. Transmittance specifications should always state the nominal thickness at which the value applies.

How many specimens should be tested?

A minimum of three specimens sampled from different locations of the film roll is recommended, with multiple readings per specimen, to account for thickness variation and coating uniformity across the web.

What factors cause measurement error?

Common error sources include dust and fingerprints on the specimen, wrinkles or trapped air at the measurement port, insufficient instrument warm-up, missing calibration, and exposure of UV-sensitive films to strong illumination before measurement.