Determination of Light Transmittance of Plastic Films: Standards, Methods, and Applications

Light transmittance is one of the most important optical performance indicators for plastic films, directly affecting crop growth in greenhouses, product visibility in packaging, and conversion efficiency in photovoltaic modules. This article presents a comprehensive technical overview of plastic film light transmittance determination, covering the relevant international and national standards such as ASTM D1003, ISO 13468, and GB/T 2410, typical sample types, instrumental test methods, and practical quality control considerations. Whether you are a film manufacturer, quality engineer, or materials researcher, this guide will help you understand how accredited third-party laboratories evaluate the optical properties of plastic films.

Technical Overview

Light transmittance, also referred to as luminous transmittance, is defined as the percentage of incident luminous flux that passes through a plastic film specimen relative to the total luminous flux striking its surface. It is a dimensionless optical parameter expressed as a percentage and is measured under standardized illumination and detection conditions, typically using a CIE standard illuminant A or D65 combined with a photopic (human-eye-weighted) detector response.

For transparent and translucent plastic films, three related quantities are commonly evaluated:

• Total luminous transmittance (Tt): the ratio of all transmitted light, including both regularly transmitted and diffusely scattered light, to the incident light.
• Regular (specular) transmittance: the portion of transmitted light that passes through the film without deviation from the original optical path.
• Diffuse transmittance: the portion of transmitted light that is scattered by the material, typically at angles greater than 2.5 degrees from the incident beam. The ratio of diffuse transmittance to total transmittance is reported as haze.

Accurate determination of light transmittance is essential across many industries. In protected agriculture, the photosynthetically active radiation (PAR, approximately 400–700 nm) transmitted through greenhouse films governs crop yield and quality. In food and consumer packaging, high transmittance and low haze ensure product visibility. In photovoltaic encapsulation and optical display films, even small losses in transmittance can significantly reduce module efficiency or display brightness. Light transmittance is therefore a routine incoming inspection, outgoing inspection, and product certification parameter tested by accredited third-party laboratories.

Testing Samples

Agricultural and Greenhouse Films

Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), ethylene-vinyl acetate (EVA), and polyolefin (PO) greenhouse and mulch films are tested to verify sufficient light transmission for crop production. Light-diffusing greenhouse films are additionally evaluated for haze, which redistributes incident sunlight and reduces shadow zones within the growing area.

Food and Consumer Packaging Films

Polyethylene (PE), polypropylene (PP, including cast and biaxially oriented CPP/OPP), polyethylene terephthalate (PET), and polyvinyl chloride (PVC) packaging films are tested for clarity and transparency. High total transmittance combined with low haze allows consumers to clearly see packaged products.

Photovoltaic Encapsulant Films

EVA and POE encapsulant films used in crystalline silicon and thin-film photovoltaic modules are tested for high transmittance, typically above 90%, often together with UV cut-off wavelength and yellowing index evaluation, because encapsulant transmittance directly influences module power output.

Optical and Display Functional Films

PET, TAC, polycarbonate (PC), and PMMA films used in liquid crystal displays, touch panels, protective covers, and lighting diffusers are tested for both total transmittance and haze to ensure optical uniformity and image clarity.

Industrial, Construction, and Specialty Films

Vapor barrier films, window glazing films, self-adhesive protective films, anti-fog films, biodegradable films, and metallized or coated functional films are also subject to transmittance determination, either for performance verification or for comparison before and after weathering, aging, or surface treatment.

Testing Items

Depending on the product specification and end use, the following optical parameters can be determined for plastic film samples:

• Total luminous transmittance (Tt), expressed as a percentage
• Regular (specular) transmittance
• Diffuse transmittance
• Haze (transmission haze), often co-determined in the same measurement
• Spectral transmittance distribution across the visible wavelength range, where required for optical-grade films

Typical acceptance criteria vary by application: high-clarity packaging films generally require total transmittance above 90% with low haze, greenhouse films commonly specify total transmittance of 85–92% or higher, and photovoltaic encapsulant films are usually required to achieve transmittance above 90–91%.

Testing Methods

Light transmittance of plastic films is determined using either a dedicated hazemeter or a UV-Vis spectrophotometer equipped with an integrating sphere. The principal standards include:

• ASTM D1003: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, using illuminant A; Method A employs a hazemeter and Method B employs a spectrophotometer.
• ISO 13468-1 / ISO 13468-2: Determination of the total luminous transmittance of transparent materials using a single-beam instrument (Part 1) or a double-beam instrument (Part 2), using standard illuminant D65.
• GB/T 2410: Chinese national standard for the determination of transmittance and haze of transparent plastics, technically aligned with the hazemeter principle of ASTM D1003.
• ISO 14782: Determination of haze for transparent materials, applicable when haze is co-reported with transmittance.
• ASTM D1746: Standard Test Method for the regular transmittance of transparent plastic sheeting, applicable when only undeviated transmitted light is of interest.

Instrument Principle and Measurement Procedure

A collimated light beam is directed at the specimen mounted at or near the entrance port of an integrating sphere. The sphere collects both regularly transmitted and diffusely scattered light, and a photometric detector with a spectral response matching the photopic luminosity function measures the transmitted luminous flux. A typical hazemeter procedure according to ASTM D1003 involves the following steps:

• Perform zero and full-scale calibration using the unobstructed beam and certified reference standards.
• Measure the incident luminous flux (T1) without the specimen.
• Place the specimen against the integrating sphere entrance port and measure the total transmitted flux (T2).
• Position a light trap to exclude the regular beam and measure instrument scattering (T3) and combined specimen-plus-instrument scattering (T4).
• Calculate total transmittance as T2/T1, diffuse transmittance as (T4 − T3)/T1, and haze as diffuse transmittance divided by total transmittance, all multiplied by 100.

Specimen Preparation and Conditioning

Specimens shall be flat, clean, and free from wrinkles, scratches, fingerprints, bubbles, and surface contamination, as any of these defects will artificially increase light scattering. Typical specimen size is at least 50 mm × 50 mm or sufficient to fully cover the instrument aperture. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 or ASTM D618 for a minimum of 40 hours. At least three to five specimens are tested from different positions across the film web, and the arithmetic mean and range of results are reported.

Report Content

A complete test report should identify the test standard, instrument type, illuminant and detector geometry, specimen description including nominal thickness and conditioning, individual and mean transmittance values (and haze, if determined), and the test date and laboratory environment. Results from different methods or illuminants are not directly interchangeable and should be compared with caution.

Applications

Determination of plastic film light transmittance supports a wide range of engineering and commercial purposes:

• Agricultural production: verifying greenhouse, tunnel, and mulch film light transmission to match crop light requirements and seasonal growing conditions.
• Packaging development: evaluating the clarity of packaging films and the optical impact of additives, masterbatches, coatings, and printing layers.
• Photovoltaic manufacturing: incoming inspection of encapsulant films and verification of transmittance retention after damp-heat or UV aging tests.
• Display and optics: quality control of optical-grade films used in displays, lighting, and imaging systems.
• Material research: comparing transmittance of new resin formulations, biodegradable materials, and nano-filled or functionalized films.
• Compliance and certification: providing objective optical performance data in product data sheets, tenders, export documentation, and regulatory submissions through accredited third-party laboratories.

FAQ

What is the difference between light transmittance and haze?

Light transmittance measures how much light passes through the film in total, while haze measures how much of the transmitted light is scattered away from the regular beam. A film can have high transmittance yet high haze, which produces a milky, non-transparent appearance; conversely, a high-clarity film exhibits both high transmittance and low haze.

Which standard should be selected for testing?

The choice depends on the destination market and product specification. ASTM D1003 and GB/T 2410 use illuminant A and are widely used for agricultural and packaging films, while ISO 13468-2 with illuminant D65 is common for optical-grade materials and international trade. Because the illuminant and instrument geometry differ, results obtained by different standards should not be compared directly.

Does film thickness affect transmittance results?

Yes. Transmittance generally decreases as film thickness increases due to bulk absorption and surface reflection. Specimens of different nominal thicknesses should not be compared without normalization, and the reported thickness should always accompany the transmittance value.

Can colored, printed, or embossed films be tested?

Yes, provided the specimen is flat and fully covers the measurement aperture. Colored or printed films will show lower luminous transmittance due to spectral absorption, and heavily embossed or textured films will show elevated diffuse transmittance and haze because surface structures scatter light. In such cases, spectral transmittance measurement may provide more informative characterization.

How many specimens are required, and how are results expressed?

Normally three to five specimens taken from representative positions of the film are tested, and the results are expressed as the arithmetic mean total luminous transmittance in percentage, together with haze where applicable. Any specimen showing visible defects or contamination should be excluded and documented.

How does sample handling influence accuracy?

Fingerprints, dust, creases, and static-attracted particles can significantly reduce measured transmittance and increase haze. Specimens should be handled with clean gloves or tweezers, kept flat, and cleaned only with methods that do not alter the surface, such as gentle dry cleaning with lint-free wipes, before conditioning and testing.