Overview
EN ISO 13468-2:2021 - "Plastics - Determination of the total luminous transmittance of transparent materials - Part 2: Double‑beam instrument" (CEN/ISO) specifies a laboratory method to measure the total luminous transmittance (in the visible spectrum) of planar, transparent and substantially colourless plastics using a double‑beam scanning spectrophotometer with an integrating sphere. It applies to transparent moulding materials, films and sheets up to 10 mm thick. The standard excludes plastics that contain fluorescent materials.
Key topics and technical requirements
- Measurement principle: Total luminous transmittance is determined using a double‑beam optical system (sample and reference beams) feeding an integrating sphere and photodetector.
- Apparatus components:
- Stabilized light source and monochromator (scanning spectrophotometer).
- Optical system producing two parallel beams of equal wavelength and approximately equal radiant flux.
- Integrating sphere with entrance and reference ports and a photodetector port.
- Instrument performance:
- Accuracy of total luminous transmittance: ±1.0%.
- Repeatability: standard deviation ≤ 0.2%.
- Within‑laboratory reproducibility: should not exceed repeatability by more than a factor of 3 over long intervals.
- Photodetector linearity and sufficiently small spectral bandwidth are required.
- Design constraints:
- Beam divergence ≤ 0.087 rad (5°); beam diameter 0.5–0.8 of port diameter.
- Integrating sphere: total port area ≤ 3% of internal area; internal luminous reflectance ≥ 90% (±3%).
- Entrance and reference ports equal size; specific angular relationships between ports and photodetector required.
- Instrument must be light‑tight; the apparatus should ensure total transmittance reads zero when incident radiant flux is zero.
- Specimen handling:
- Specimens held normal to the sample beam (±2°), close to the sphere entrance to collect scattered light.
- Applicable specimen thickness: generally ≤ 10 mm (thicker specimens measurable only if instrument accommodates them, but results are not comparable to ≤10 mm specimens).
- Limitations:
- Not suitable for plastics containing fluorescent materials.
- For single‑beam alternatives, see ISO 13468‑1.
Applications and users
EN ISO 13468-2:2021 is used for quality control, material specification and optical characterization in:
- Plastics manufacturers and converters (films, sheets, mouldings)
- Test laboratories and certification bodies
- Automotive, glazing, packaging and optical component manufacturers
- R&D teams assessing optical clarity, haze and light transmission properties
This standard supports decisions on material selection, product compliance and comparative optical performance across transparent plastics.
Related standards
- ISO 13468-1 (single‑beam instrument alternative)
- ISO 291 (conditioning and testing atmospheres)
- CIE Publication No. 15 (Colorimetry)
- CIE Publication No. 17 (International lighting vocabulary)
Keywords: EN ISO 13468-2:2021, total luminous transmittance, double‑beam instrument, plastics, transparent materials, integrating sphere, spectrophotometer, visible spectrum.