Printed Film Light Transmittance Testing: Standards, Methods, and Applications

Printed film light transmittance testing measures the percentage of visible light that passes through printed plastic films, delivering critical optical performance data for displays, packaging, automotive, and architectural products. This technical guide covers the key optical parameters, applicable standards such as ASTM D1003, ISO 13468, and GB/T 2410, sample requirements, test procedures, and typical industry applications for printed film transmittance measurement.

Technical Overview

Light transmittance (luminous transmittance) is defined as the ratio of luminous flux transmitted through a specimen to the luminous flux incident upon it, expressed as a percentage over the visible wavelength range of 380 nm to 780 nm. Printed films, including PET, PC, BOPP, PE, and PVC substrates carrying graphic inks, functional coatings, or metallized layers, exhibit reduced and spatially non-uniform light transmission because ink coverage, layer thickness, and pattern geometry all attenuate and scatter light. Accurate quantification of transmittance is therefore essential for verifying print quality, predicting end-product optical performance, and confirming compliance with customer specifications.

For transparent and translucent printed films, testing typically evaluates several interrelated optical properties:

• Total luminous transmittance (Tt): the overall percentage of visible light passing through the specimen.
• Diffuse transmittance (Td): the portion of transmitted light scattered beyond a defined angle, typically 4 degrees or more in wide-angle scattering instruments.
• Haze: the ratio of diffuse transmittance to total transmittance, quantifying cloudiness or milkiness.
• Spectral transmittance distribution across 380 to 780 nm, weighted by the photopic response of the human eye under a standard illuminant such as CIE illuminant A or D65.
• Optical density (OD) for heavily printed or near-opaque regions, calculated as the negative base-10 logarithm of transmittance.

Commonly referenced standards include ASTM D1003, ISO 13468-1, ISO 13468-2, ISO 14782, GB/T 2410, JIS K 7361, and JIS K 7136. Test reports issued by accredited third-party laboratories are widely accepted for supplier qualification, incoming inspection, design validation, and product certification purposes.

Testing Samples

Transmittance testing can be performed on virtually any flexible or rigid printed film, provided the specimen is flat, clean, and free of wrinkles or curvature that could deflect the measurement beam. Typical sample categories submitted for testing include the following.

Display and Touch Panel Films

Decorative printed PET films, bezel masks, window layers, ITO-coated conductive films, and diffusion or backlight components used in LCD, OLED, and touch modules. Transmittance in these materials directly influences display brightness, contrast, readability, and power consumption, making tight tolerance control necessary.

Packaging and Label Films

Printed BOPP, PE, CPP, PET, and multilayer laminate films used for flexible packaging, shrink sleeves, and self-adhesive labels. Characterized transmittance values support shelf presentation, product visibility assessment, and light or UV barrier performance evaluation.

Automotive and Architectural Window Films

Tinted laminates, printed dot-matrix gradient bands, and coated glazing films. Because many national regulations impose minimum visible light transmittance (VLT) requirements on vehicle and building glazing, precise and traceable measurement is mandatory for market access.

Graphic, Decorative, and Functional Films

Advertising films, light guide panels, membrane switch overlays, printed polycarbonate control panels, and printed electronics substrates, where optical uniformity and defined transmission windows must be verified across production batches.

General specimen requirements for reliable results are as follows:

• Specimen dimensions of approximately 50 mm by 50 mm, or at minimum large enough to fully cover the instrument aperture, which commonly ranges from 10 mm to 25 mm in diameter.
• Clean surfaces free of dust, fingerprints, scratches, adhesive residue, and processing oils.
• Conditioning at 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours prior to testing, in accordance with ISO 291 or ASTM D618.
• A minimum of three specimens per material or print condition, with additional specimens when printed-pattern variability is expected.

Testing Items

Standard test items and reported parameters for printed film transmittance evaluation include the following:

• Total luminous transmittance, Tt, expressed as a percentage, which is the primary reported result.
• Diffuse transmittance, Td, expressed as a percentage.
• Haze, expressed as a percentage and calculated as the ratio of diffuse to total transmittance.
• Full spectral transmittance curve across the 380 to 780 nm visible range.
• Transmittance at specified wavelengths, for example 550 nm, which is a common reference point for display applications.
• Optical density for dark printed areas and opaque ink zones.
• Uniformity assessment comparing printed and unprinted regions, or mapping multiple positions across a patterned film.
• Optional color-related parameters such as yellowness index and CIE chromaticity coordinates.

Acceptance criteria are product-specific. Clear unprinted PET base films typically achieve total transmittance above 88 to 92 percent, while printed areas may be specified anywhere from a few percent for dark ink coverage to values approaching the base film for light or partial coverage.

Testing Methods

Method A: Hazemeter Measurement

This approach follows ASTM D1003 Method A, GB/T 2410, JIS K 7361, and JIS K 7136. The instrument uses a light source approximating CIE illuminant A, a photopic detector, and an integrating sphere with a standardized light trap. Four sequential readings are taken: the incident flux without a specimen (T1), the total transmitted flux with the specimen (T2), the scattered flux with the specimen and trap engaged (T3), and the instrumental scatter without the specimen (T4). Total transmittance is calculated as Tt = (T2/T1) × 100, diffuse transmittance as Td = ((T3 − T4)/T1) × 100, and haze as (Td/Tt) × 100. The method is fast, highly repeatable, and well suited to routine quality control of transparent and lightly translucent printed films.

Method B: Spectrophotometer with Integrating Sphere

This approach follows ASTM D1003 Method B and ISO 13468-2. A UV-Vis spectrophotometer fitted with an integrating sphere scans the specimen across the visible spectrum and computes photopically weighted luminous transmittance under a specified illuminant and observer condition, such as D65/2° or A/2°. The method yields a complete spectral transmittance curve and is particularly valuable for films printed with colored inks, where transmission varies strongly with wavelength. ISO 13468-1 describes the corresponding single-beam procedure, while ISO 14782 governs haze measurement of transparent materials.

Measurement Considerations for Printed Films

• Define measurement positions in advance, distinguishing solid print areas, unprinted areas, gradient regions, and patterned zones.
• For patterned films, either measure a representative aperture-averaged region or record multiple point measurements and report positional statistics.
• Avoid measurement locations containing wrinkles, bubbles, delamination, ink streaks, or surface contamination, as these distort both transmittance and haze results.
• Report the illuminant, observer geometry, aperture size, instrument type, number of specimens, and measurement positions to ensure reproducibility.
• Verify instrument calibration before each session using certified reference standards traceable to national metrology institutes.

Test Procedure Summary

• Condition the specimens to the standard laboratory atmosphere.
• Calibrate and verify the hazemeter or spectrophotometer against reference standards.
• Mount each specimen perpendicular to the light beam and record total transmittance, diffuse transmittance, and haze, or the full spectral scan.
• Repeat measurements for all specimens and designated positions on printed patterns.
• Calculate mean values and standard deviations, and issue a test report identifying the standard applied, instrument conditions, and specimen details.

Applications

Transmittance test data for printed films support decision-making across the full product lifecycle, from material development to field failure analysis. Representative applications include:

• Display manufacturing, where bezel, window, and decorative film transmittance directly determines module brightness, contrast ratio, and energy efficiency.
• Touch panel and membrane switch production, where overlay films must balance graphic legibility with sufficient backlight transmission.
• Automotive glazing and window film compliance, where visible light transmittance results are used to demonstrate conformity with regulatory limits.
• Architectural window films, supporting daylighting design, solar heat gain evaluation, and energy performance claims.
• Flexible packaging development, where measured transmittance and haze inform light barrier design, product protection, and shelf appearance.
• Research and development of ink formulations, print coverage optimization, coating thickness, and curing parameters.
• Incoming material inspection, supplier qualification, batch release testing, and root-cause analysis of optical defects, with results typically obtained from accredited third-party laboratories.

FAQ

What is the difference between transmittance and haze?

Transmittance measures the total percentage of incident visible light that passes through the film, while haze measures the percentage of that transmitted light that has been scattered, causing a cloudy or milky appearance. A film can have high transmittance and high haze, or high transmittance and low haze; the two parameters describe different optical qualities and are reported together under ASTM D1003 and related standards.

Which standard should be specified for printed film transmittance testing?

ASTM D1003 and GB/T 2410 are the most widely specified methods for luminous transmittance and haze of transparent plastics and films. ISO 13468-1 and ISO 13468-2 cover total luminous transmittance determination, ISO 14782 covers haze, and JIS K 7361 and JIS K 7136 are the corresponding Japanese standards. The choice depends on the end market and customer specification, and the standard applied should always be stated in the test report.

Can films with printed patterns be tested accurately?

Yes. Printed areas can be measured individually by positioning the measurement aperture over solid ink regions, unprinted regions, or gradient zones. For fine patterns that cannot be resolved by a single aperture, an aperture-averaged value or a multi-point map can be reported. Defining the measurement positions before testing is essential for meaningful, repeatable results.

What sample size and quantity are required?

A specimen of approximately 50 mm by 50 mm is generally sufficient, as long as it fully covers the instrument aperture. A minimum of three specimens per material or print condition is recommended so that average values and variability can be reported.

How long does printed film transmittance testing take?

Instrument measurement itself takes only minutes per specimen. Including conditioning, calibration, multiple specimens, and report preparation, routine testing is typically completed within a few working days at accredited third-party laboratories, with expedited service available when required.

What transmittance values are considered acceptable?

There is no universal pass or fail threshold; acceptance limits are set by product design and applicable regulations. Clear base films commonly exceed 88 to 92 percent transmittance, automotive window films must satisfy specified VLT minimums, and printed display films are approved against customer-defined windows for each print zone. Comparing results against the agreed specification is the correct basis for judgment.