PI Film Haze Measurement: Principles, Standards, and Testing Procedures

PI film haze measurement quantifies the light-scattering behavior of polyimide films, a critical optical quality indicator for flexible displays, flexible printed circuits, and photovoltaic applications. This technical article explains the definition of haze, applicable international standards such as ASTM D1003 and ISO 14782, sample requirements, step-by-step testing procedures, and typical acceptance criteria. Whether you are performing incoming inspection of coverlay materials or characterizing optical-grade colorless PI film in R&D, this guide will help you obtain accurate and reproducible haze data.

Technical Overview

Polyimide (PI) film is a high-performance engineering polymer film valued for its exceptional thermal stability, mechanical strength, chemical resistance, and dielectric properties. It is widely used as a base material in flexible printed circuits (FPC), flexible OLED display substrates, chip-on-film (COF) packages, photovoltaic modules, and aerospace insulation systems. As PI films increasingly penetrate optical and display-related applications, the quantitative evaluation of their optical clarity has become an essential quality control task.

Haze is defined as the percentage of transmitted light that is scattered by more than 2.5 degrees from the direction of the incident beam. It is calculated as the ratio of diffuse transmittance (Td) to total luminous transmittance (Tt): Haze (%) = (Td / Tt) × 100. A low haze value indicates that light passing through the film remains highly directional, which is essential for image sharpness, contrast, and readability in display and optical applications.

For PI films, haze originates from both surface and bulk contributions, and it is strongly influenced by the following factors:

• Surface roughness introduced during casting, stretching, imidization, or surface treatment processes
• Bulk scattering caused by crystallinity, molecular aggregation, or inorganic filler particles
• Film thickness, since a longer optical path increases volumetric scattering
• Inclusions, gels, unmelted resin, and contamination introduced during synthesis or coating
• Thermal history, such as high-temperature curing of thermosetting PI films, which can induce microcrystallization or surface texture changes

Typical haze values range from below 0.5 percent for premium optical-grade colorless PI film used in flexible OLED displays, up to 1 to 3 percent for conventional amber insulation-grade films, depending on thickness and manufacturing process.

Testing Samples

Haze determination is applicable to any PI film product that transmits a sufficient amount of visible light. The most common sample categories submitted for haze testing are described below.

Optical-Grade Colorless PI Film

Colorless transparent PI film, typically based on alicyclic or fluorinated diamine chemistries, is used for flexible OLED substrates, cover windows, and touch panel sensors. Nominal thicknesses range from 12.5 to 75 micrometers. These films carry the strictest haze requirements, commonly below 0.5 to 1.0 percent, because any scattering degrades display contrast and outdoor readability.

Conventional Amber PI Film

Standard PI film synthesized from PMDA/ODA chemistry, commercially exemplified by Kapton-type products, exhibits a characteristic amber or brown coloration. It is primarily used for FPC substrates and electrical insulation in thicknesses from 7.5 to 125 micrometers. Haze testing of these films focuses on batch consistency and detection of processing defects such as gels, fisheyes, and surface texture abnormalities.

PI Coverlay and Coated Composite Films

Coverlay consists of PI film laminated with epoxy or acrylic adhesive layers, and it is used to protect copper circuit patterns on flexible circuit boards. Haze is measured on the finished composite structure after adhesive lamination, since the adhesive layer and its interface with the base film contribute additional light scattering. Hard-coated, anti-glare, and anti-reflection coated PI films are tested in a similar manner to evaluate the optical effect of the functional coating.

Thermosetting PI Film for Chip-on-Film Applications

Thermosetting PI film with built-in adhesive properties is used for COF substrates in display driver packaging. Because these films undergo high-temperature bonding cycles, haze is often measured both before and after simulated thermal processing to verify that optical clarity remains stable through the manufacturing sequence.

Testing Items

A complete optical characterization of PI film using an integrating sphere instrument typically delivers the following measurands:

• Haze (%): the ratio of wide-angle scattered transmittance to total transmittance, the primary reported value
• Total luminous transmittance, Tt (%): the total visible light passing through the specimen relative to incident light
• Diffuse transmittance, Td (%): the portion of transmitted light scattered more than 2.5 degrees from the specular direction
• Parallel (specular) transmittance (%): the undeviated component, calculated as Tt minus Td
• Clarity (optional): narrow-angle scattering resolution measured with dedicated optics, relevant for see-through quality assessment

Reporting haze together with total transmittance is strongly recommended, because the two parameters respond independently to film chemistry, thickness, and surface condition, and both are required in most material specifications for optical films.

Testing Methods

Reference Standards

PI film haze determination is performed in accordance with internationally recognized plastics transparency test methods, principally the following:

• ASTM D1003: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, Method A (hazemeter) or Method B (spectrophotometer)
• ISO 14782: Plastics, Determination of Haze for Transparent Materials
• GB/T 2410: Chinese national standard for light transmittance and haze of transparent plastics, technically aligned with ASTM D1003
• JIS K 7136: Japanese industrial standard for the determination of haze of transparent plastics

The standards share the same physical definition of haze based on the 2.5-degree scattering angle but differ slightly in illuminant and observer geometry. ASTM D1003 traditionally specifies illuminant C with a 2-degree observer, while ISO 14782 specifies D65. Results are generally comparable when measurement geometry is matched.

Instrumentation

Testing is conducted with an integrating sphere haze meter equipped with a photopically corrected detector, such as a BYK haze-gard, a WGT-S transmittance-haze analyzer, or an equivalent device. Key instrument parameters include a measurement aperture of 15 to 25 millimeters, stable tungsten or LED illumination, a light trap or ring sensor array that excludes light scattered within 2.5 degrees of the specular beam for the diffuse reading, and traceable calibration standards for zero and span verification. The instrument must be zeroed and standardized against certified haze and transmittance reference tiles before each measurement session.

Sample Preparation and Conditioning

Proper specimen preparation is critical because haze is extremely sensitive to surface contamination and handling artifacts. The following practices are applied:

• Cut at least three, preferably five, specimens of 50 mm × 50 mm or larger, fully covering the instrument aperture
• Sample from defined positions across the web width and along the machine direction to capture spatial uniformity
• Remove dust and fingerprints with a lint-free cloth and oil-free compressed air; do not use solvents unless validated
• Condition specimens at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618
• Mark and consistently orient the measured surface side, particularly for films with asymmetric surface treatment

Measurement Procedure

Following ASTM D1003 Method A, the measurement sequence is as follows:

• Step 1: Measure the incident light flux T1 with the integrating sphere empty to establish the baseline
• Step 2: Place the specimen at the transmission port and measure the total transmitted flux T2
• Step 3: Engage the light trap without a specimen and measure the instrument scattered flux T3
• Step 4: Place the specimen at the port with the light trap engaged and measure the combined specimen and instrument scattered flux T4
• Step 5: Compute total transmittance Tt = (T2 / T1) × 100 and diffuse transmittance Td = [(T4 − T3 × T2 / T1) / T1] × 100
• Step 6: Calculate haze as Haze (%) = (Td / Tt) × 100

Modern haze meters execute this sequence automatically, apply the surface reflection corrections defined in the standard, and display Tt, Td, and haze simultaneously. Each specimen should be measured at a minimum of three positions, and the arithmetic mean and standard deviation of all specimens are reported.

Data Processing and Reporting

A compliant test report shall state the test standard applied, instrument model and aperture size, illuminant and observer geometry, nominal and measured film thickness, specimen conditioning environment, measured surface side, individual and mean haze values with standard deviation, and total transmittance data. Measurement uncertainty is typically within ±0.1 percentage point for low-haze optical films when the instrument is properly calibrated.

Applications

Haze determination of PI film supports quality assurance and materials development across multiple high-technology industries:

• Flexible OLED displays: qualification of colorless PI substrates and hybrid cover windows where haze below 1 percent is mandatory for image fidelity and ambient-light contrast
• Flexible printed circuits: incoming inspection of base films and coverlay laminates for FPC manufacturing, where haze correlates with surface quality and adhesive coating uniformity
• Chip-on-film and display driver packaging: verification that thermosetting PI films retain optical clarity after high-temperature bonding processes
• Photovoltaics and concentrated solar power: evaluation of PI-based front films and fluoropolymer-PI composite backsheets where high transmittance and controlled diffusion influence module efficiency
• Touch sensors and optoelectronics: control of coating and ITO sputtering processes on PI carrier films
• Aerospace and specialty optics: specification of PI laminates used in transparent thermal control and lightweight optical elements

In R&D laboratories, haze data combined with transmittance, yellow index, and surface roughness measurements is routinely used to correlate film formulation, imidization conditions, and stretching parameters with final optical performance.

FAQ

What is the difference between haze and light transmittance?

Transmittance quantifies how much visible light passes through the film regardless of direction, while haze quantifies how much of that transmitted light is scattered more than 2.5 degrees away from the incident direction. A film can have high transmittance yet high haze, appearing cloudy rather than optically clear.

Which standard should be selected for PI film haze testing?

ASTM D1003 is the most widely cited method and is accepted globally for qualification and inspection purposes. ISO 14782 and JIS K 7136 are common in corresponding regional markets, and GB/T 2410 is frequently specified in Chinese domestic supply chains. The choice is usually dictated by the customer specification or the end market.

How does film thickness affect haze results?

Haze generally increases with thickness because the optical path length and the probability of bulk scattering events both increase. Meaningful comparisons between materials should therefore always be made at equal nominal thickness, and thickness must be reported together with haze data.

Can black or opaque PI film be tested for haze?

No. Haze measurement requires sufficient light transmission through the specimen. Black PI films used in OLED modules and camera brackets are instead evaluated using color, gloss, and reflectance measurements.

What haze level is required for flexible display grade PI film?

Leading specifications for colorless PI substrate and cover window films typically require haze of 1.0 percent or below, with premium grades achieving 0.5 percent or lower, combined with total transmittance of 88 to 90 percent or higher across the visible spectrum.

Why is there haze variation across the film web?

Thickness variation, non-uniform stretching, localized surface texture from casting belts or imidization ovens, and uneven crystallinity can all create spatial haze differences. Sampling at multiple positions across the width and length of the web, as specified in the sampling protocol, reveals this variability.

Why did the haze of my colorless PI film increase after high-temperature processing?

Post-treatment haze growth is commonly attributed to partial crystallization, molecular aggregation, shrinkage-induced surface roughening, or contamination from oven atmospheres. Measuring haze before and after simulated process thermal exposure is the recommended method for screening this risk during material selection.