Agricultural Film Light Transmittance and Haze Testing: Standards, Methods, and Technical Requiremen

Agricultural film light transmittance and haze testing evaluates the optical performance of greenhouse films, mulch films, and other agricultural plastics that determine how sunlight reaches crops. Accurate measurement of total luminous transmittance and haze, performed in accordance with standards such as ASTM D1003, ISO 14782, and GB/T 2410, helps film manufacturers, greenhouse operators, and accredited third-party laboratories verify product quality and confirm compliance with specifications. This article explains the principles, sample types, test items, procedures, and applications of transmittance and haze inspection for agricultural films.

Technical Overview

Agricultural films, including greenhouse covering films, mulch films, and tunnel films, serve as the optical interface between solar radiation and the crop growing environment. Their light transmission characteristics directly influence photosynthetically active radiation (PAR, 400–700 nm) availability, canopy light distribution, greenhouse temperature, and ultimately crop yield and quality. Two parameters are central to the optical evaluation of these films: total luminous transmittance and haze.

Total luminous transmittance (Tt) is defined as the ratio of luminous flux transmitted through the specimen to the incident luminous flux, expressed as a percentage. Haze (H) is defined as the percentage of transmitted light that deviates from the incident beam direction by more than 2.5 degrees due to forward scattering. Scattering originates from surface roughness, internal additives, pigments, crystalline structures, and micro-voids within the film.

For agricultural applications, the two parameters must be considered together. High transmittance maximizes light input, which is critical in low-radiation climates and winter production. Moderate haze produces a diffuse-light film that scatters sunlight, eliminates sharp shadows, and distributes light more evenly to lower leaves within dense canopies. Excessive haze, however, reduces direct beam clarity and can lower effective light penetration in certain crops. Condensation droplets, dust accumulation, polymer aging under UV exposure, and anti-drip or anti-fog surface treatments all significantly affect measured values, making standardized laboratory testing essential for objective evaluation.

Testing Samples

Greenhouse Covering Films

Typical samples include single-layer and multi-layer coextruded films made of low-density polyethylene (LDPE/LLDPE), ethylene-vinyl acetate (EVA), polyolefin (PO) compounds, and PVC. Film thicknesses commonly range from 60 to 200 micrometers for seasonal films and above 200 micrometers for long-life multi-season films. Functional grades containing infrared thermal additives, UV stabilizers, anti-fog coatings, or light-diffusing masterbatches are frequently submitted for optical verification against declared specifications.

Mulch Films

Transparent polyethylene mulch films are tested for high transmittance because they are used in soil solarization, where maximizing solar heating of the soil is the objective. Black, silver-black, and colored mulch films may also be characterized to document their optical profiles, although their transmittance values are inherently very low by design.

Other Agricultural Film Products

Low-tunnel films, floating covers, rain-shelter films, and silage films are tested when optical performance is part of the product specification. Samples are typically supplied as flat sheet cuttings, free of creases, weld lines, and printed areas unless those features are specifically under evaluation.

Testing Items

The core optical test items for agricultural films include:

• Total luminous transmittance (Tt): the overall percentage of visible light passing through the specimen.
• Haze (H): the percentage of transmitted light scattered more than 2.5 degrees from the incident beam axis.
• Regular (specular) transmittance: the portion of transmitted light that passes without deviation, used to characterize clear films.
• Diffuse transmittance: derived as the difference between total and regular transmittance, used to characterize diffusing films.
• Optional spectral measurements: transmittance distribution across the 400–700 nm PAR band and UV blocking rate below 380 nm for UV-stabilized or insect-exclusion grades.

Testing Methods

The internationally recognized test methods are ASTM D1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics), ISO 14782 (Plastics — Determination of Haze for Transparent Materials), and GB/T 2410 (Determination of Light Transmittance and Haze of Transparent Plastics). These methods are technically harmonized and employ either a dedicated hazemeter with an integrating sphere (Method A) or a spectrophotometer equipped with an integrating sphere accessory (Method B), using standard illuminant D65 or C with the 2-degree standard observer.

The classical four-step measurement sequence defined by ASTM D1003 records four luminous flux readings: T1 for the incident beam with no specimen, T2 for total light transmitted by the specimen, T3 for light scattered by the instrument itself, and T4 for light scattered by both instrument and specimen. The results are calculated as total transmittance Tt = (T2 / T1) × 100 and haze H = (T4/T2 − T3/T1) × 100, both expressed in percent.

Specimen preparation and conditioning follow ISO 291 or ASTM D618, typically at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 40 hours prior to testing. Specimens must be clean, flat, and free of fingerprints, dust, scratches, and embedded wrinkles, and should be handled with gloves. A minimum of three specimens cut from different positions across the film width is recommended, with multiple measurement points on each specimen, and the reported result is the arithmetic average. Product-level requirements for agricultural films are referenced in standards such as GB/T 4455 for agricultural polyethylene films and EN 13206 for covering thermoplastic films, which classify films by light transmittance and haze performance.

Applications

Transmittance and haze testing of agricultural films supports multiple stages of the product lifecycle:

• Factory quality control: routine verification of production consistency for thickness, additive dispersion, and masterbatch uniformity.
• Incoming inspection: greenhouse builders and growers verifying that delivered films meet declared optical grades before installation.
• Product certification and compliance: demonstrating conformity with national and European product standards that specify optical classification levels.
• Research and development: evaluating new diffusing grades, anti-fog coatings, UV-blocking formulations, and coextruded layer structures.
• Durability assessment: measuring retention of transmittance and haze after natural or accelerated weathering exposure, for example per ISO 4892 or ASTM G155, to predict long-term optical service life.
• Dispute resolution: independent data from accredited third-party laboratories for trade disputes or supplier qualification.

FAQ

What is the difference between light transmittance and haze?

Transmittance measures how much visible light passes through the film regardless of direction, while haze measures how much of that transmitted light is scattered away from the straight-through path by more than 2.5 degrees. A film can have high transmittance and high haze (a diffusing film) or high transmittance and low haze (a clear film), so the two properties are independent and must both be specified.

What haze level is appropriate for a greenhouse film?

It depends on climate and crop strategy. Clear films with haze below approximately 20 percent maximize direct light and suit low-radiation regions and winter production. Diffusing films with haze values of roughly 30 to 70 percent improve light uniformity inside the canopy and reduce scorching and heat stress in high-radiation climates. Product standards such as EN 13206 define the classification limits to be applied.

How do condensation and surface contamination affect results?

Water droplets, dust, fingerprints, and surface scratches scatter light and can substantially inflate haze readings and distort transmittance. Specimens must be conditioned, cleaned according to the material specification, and handled carefully, and laboratories must document surface condition as part of the test report.

Which standard should be selected: ASTM D1003, ISO 14782, or GB/T 2410?

The three methods are technically aligned, so the choice usually follows the market or the product specification governing the transaction: ASTM D1003 is common in North American supply chains, ISO 14782 in international trade, and GB/T 2410 in the Chinese market. When a dispute is possible, the standard should be agreed between buyer and seller before testing.

What sample size and quantity are required?

Specimen dimensions of approximately 50 mm × 50 mm, or discs matching the instrument aperture, are typical, and at least three specimens sampled from different locations across the film width are required to represent material variability. Additional specimens are recommended for wide films, coextruded structures, or products with localized functional coatings.