Recycled Material Light Transmittance Testing: Standards, Methods and Applications

Light transmittance testing measures the percentage of visible light that passes through recycled plastic materials such as rPET, rPC and rPMMA, making it one of the most important indicators of optical quality for recyclates. By testing to recognized standards such as ASTM D1003, ISO 13468 and GB/T 2410, recyclers and converters can verify that recycled resins meet the transparency requirements of packaging, lighting, optical and automotive applications. This article explains the technical principle, sample requirements, test items, methods and typical applications of light transmittance testing for recycled materials.

Technical Overview

Light transmittance, also referred to as total luminous transmittance (Tt), is defined as the ratio of the luminous flux transmitted through a specimen to the luminous flux incident upon it, expressed as a percentage. The measurement is performed with a light source approximating CIE Standard Illuminant A or D65 in combination with a photopic response detector, so the result reflects human visual perception rather than a purely physical energy ratio. It is almost always reported together with haze, which quantifies the wide-angle scattering that causes a milky or cloudy appearance.

Recycled plastics undergo multiple heat histories, mechanical shear and possible contamination from labels, inks, adhesives, masterbatch residues and foreign polymers. These factors introduce yellowing, gels, black specks, unmelted particles and micro-voids, all of which reduce transmittance and increase haze. Light transmittance testing therefore acts as a sensitive, quantitative screen for the overall purity and reprocessing consistency of recycled resins, and it is frequently used to benchmark recyclate performance against virgin material or against a defined retention rate, for example "transmittance greater than 90% of the virgin resin value".

The most commonly referenced standards for this testing include:

• ASTM D1003 – Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics (Method A: hazemeter; Method B: spectrophotometer)
• ISO 13468-1 and ISO 13468-2 – Determination of the total luminous transmittance of transparent materials, using single-beam and double-beam instruments respectively
• ISO 14782 – Determination of haze of transparent materials
• GB/T 2410 – Chinese national standard for the determination of light transmittance and haze of transparent plastics, technically aligned with ASTM D1003
• JIS K 7361-1 and JIS K 7136 – Japanese standards corresponding to the ISO transmittance and haze methods

Testing Samples

Light transmittance can be evaluated on recycled materials in almost any transparent or translucent form. Typical sample categories submitted to accredited third-party laboratories are listed below.

Recycled PET (rPET)

Bottle-grade flake, washed and dried pellet, extruded or calendered film, biaxially oriented sheet, and injection stretch-blow molded preforms or bottles. rPET is the most frequently tested recyclate because packaging specifications usually require transmittance above 85–90% at a defined wall thickness.

Recycled Polycarbonate (rPC)

Reground optical-grade sheet, pellets and molded plaques used for glazing, roofing sheets, skylights and lighting covers, where high transparency and low haze are critical performance criteria.

Recycled PMMA (rPMMA)

Reprocessed acrylic from signage, display panels and light guides, evaluated for transmittance retention after multiple reprocessing cycles.

Recycled PS, SAN and Other Transparent Resins

Crystal polystyrene, SAN and selected transparent ABS grades recycled from packaging or optical parts, typically tested as compression-molded or injection-molded plaques.

Recycled PE and PP Films

Post-industrial and post-consumer polyolefin films for bags, liners and flexible packaging, where haze and clarity determine downstream visual quality.

Specimen Requirements

Because transmittance results depend strongly on geometry and surface quality, the following conditions should be observed:

• Specimens must have flat, plane-parallel surfaces, free from scratches, bubbles, sink marks, embedded dust and fingerprints.
• Recommended specimen size is approximately 50 mm × 50 mm (or a circular disc of at least 25 mm diameter) to fully cover the instrument aperture.
• Pellets, flakes and granules cannot be measured directly; they must first be converted into plaques or films of defined thickness, typically 1.0 mm or 2.0 mm, by compression molding or injection molding under controlled temperature and pressure.
• At least three to five specimens per material should be tested and averaged, with the thickness of each specimen measured and reported.
• Condition specimens at 23 °C ± 2 °C and 50% ± 10% relative humidity for at least 40 hours prior to testing, unless otherwise specified.

Testing Items

Depending on the product specification and end-use requirement, the following optical parameters can be determined in a single measurement sequence:

• Total luminous transmittance (Tt) – the overall percentage of incident light transmitted through the specimen.
• Haze – the percentage of transmitted light that is scattered by more than 2.5° from the incident beam direction, indicating cloudiness.
• Parallel (regular) transmittance – the unscattered, directly transmitted component, relevant for image clarity.
• Diffuse transmittance – the scattered component, relevant for light-diffusing applications such as LED covers.
• Supplementary color evaluations – yellowness index and CIE color coordinates (e.g., per ASTM E313), often requested alongside transmittance to characterize the typical yellow shift of recycled resins.
• Comparative analysis – transmittance and haze of the recyclate compared with the corresponding virgin resin, or across successive reprocessing cycles, expressed as a percentage retention.

Testing Methods

Hazemeter Method (ASTM D1003 Method A / GB/T 2410)

The specimen is placed at the entrance port of an integrating sphere illuminated by a collimated beam. Four sequential readings are taken: the incident luminous flux (T1), the total transmitted flux with the specimen at the sphere port and a light trap open (T2), the instrument scatter with no specimen and the trap closed (T3), and the total scatter with the specimen in place and the trap closed (T4). This method provides rapid, direct readings and is the industry default for routine quality control of recycled films and plaques.

Spectrophotometer Method (ASTM D1003 Method B / ISO 13468)

A double-beam or scanning spectrophotometer fitted with an integrating sphere measures the spectral transmittance over the visible wavelength range, and the photometric value is calculated by weighting with the CIE photopic function and the selected standard illuminant. This approach offers higher accuracy, better reproducibility between laboratories, and full spectral information that is useful when diagnosing the cause of discoloration in recyclates.

Calculation Principle

Total luminous transmittance is calculated as Tt = (T2 / T1) × 100%. Haze is calculated as H = [(T4 / T2) − (T3 / T1)] × 100%, where T3 corrects for the inherent scatter of the instrument itself. Regular and diffuse transmittance values are derived from the corresponding optical configurations of the sphere.

Test Report

A complete report from an accredited third-party laboratory should identify the test standard and method used, the illuminant and observer condition, specimen description and thickness, individual and mean transmittance and haze values, conditioning details, and the identification of the sampling lot, so that results remain traceable and comparable between shipments.

Applications

Light transmittance testing of recycled materials supports decision-making across the entire plastics value chain:

• Recycler quality control – verifying washing, sorting and melt-filtration performance, and releasing recyclate lots against optical specifications.
• Food and beverage packaging – confirming that rPET used in bottles, thermoformed clamshells and jars retains the transparency demanded by brand owners.
• Construction and glazing – qualifying recycled polycarbonate and PMMA sheets for roofing, skylights and greenhouse panels against transmittance and haze limits.
• Lighting and LED industry – evaluating recycled diffuser materials where a controlled balance of transmittance and haze determines light output and glare control.
• Automotive sector – accepting recycled-content lamp covers, interior trim lenses and glazing components according to OEM optical specifications.
• Consumer electronics – approving recycled transparent resins for housings, display windows and appliance panels.
• Sustainability verification – substantiating recyclate-content claims by demonstrating that performance parameters remain within agreed tolerances relative to virgin resin.
• Research and development – screening compatibilizers, stabilizers, optical brighteners and advanced decontamination processes by their effect on transmittance recovery.

FAQ

What is the difference between light transmittance and haze?

Transmittance measures how much light passes through the specimen in total, while haze measures how much of that transmitted light is scattered away from the direct beam. A material can have high transmittance but high haze, meaning it lets light through yet appears cloudy and obscures the view of objects behind it.

Why is the transmittance of recycled material usually lower than that of virgin resin?

Thermo-oxidative degradation during repeated processing causes yellowing, while residual contaminants, foreign polymers, pigments, gels and micro-voids scatter light. Advanced sorting, deep cleaning, melt filtration and the addition of optical stabilizers can recover most of the lost transmittance.

Can pellets or flakes be tested directly?

No. Light transmittance is defined for solid, flat specimens, so pellets, flakes and regrind must first be molded into plaques or pressed into films of controlled thickness. The preparation conditions should be documented because they influence the measured values.

Does specimen thickness affect the result?

Yes. Transmittance generally decreases and haze increases with thickness due to bulk absorption and scattering. Results are only comparable at the same thickness, which is why the thickness must always accompany the reported values.

Which standard should be selected?

ASTM D1003 and GB/T 2410 are the most widely used for routine control, while ISO 13468 is preferred when international comparability is required. The choice should match the customer specification, the destination market and, where applicable, the relevant regulatory or OEM requirement.

How precise is the test and what is the typical turnaround time?

With calibrated reference standards and properly conditioned specimens, repeatability within approximately ±0.5% transmittance and ±0.5% haze is achievable. Most accredited third-party laboratories can complete standard testing within three to five working days after sample receipt, excluding any plaque-molding preparation that may be required.

Which other optical tests are commonly combined with transmittance testing?

Yellowness index and color coordinates, gloss measurement, and visual inspection for black specks and gels are frequently requested together, because together they give a complete picture of the optical quality of a recycled material.