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ISO 26723:2020 — Plastics — Determination of Total Luminous Transmittance and Reflectance
A comprehensive guide to measuring optical properties of plastic materials using integrating sphere spectrophotometry
ISO 26723:2020, published by the International Organization for Standardization (ISO), provides a standardized method for determining the total luminous transmittance and total luminous reflectance of plastic materials. This standard extends beyond the scope of ISO 13468-1 by covering not only clear and transparent plastics but also translucent and opaque materials such as opal sheets, films, and moulded plaques. The method employs an integrating sphere optical system to capture scattered light, making it suitable for a wide range of plastic products used in lighting, display, packaging, and automotive industries.
For materials with near-zero absorption (e.g., PMMA milky white sheets), the sum of total luminous transmittance and total luminous reflectance approaches 100%, providing a valuable internal consistency check during measurement.
1. Principle and Apparatus Design
The measurement principle is based on collecting both directly transmitted and scattered light using an integrating sphere. When a collimated light beam passes through or reflects off a specimen, the integrating sphere captures all forward-scattered or reflected luminous flux. The ratio of transmitted (or reflected) flux to incident flux defines the total luminous transmittance or total luminous reflectance.
The apparatus consists of a stabilized light source with photopic correction (matching the CIE standard luminous efficiency function V(lambda) under illuminant D65), an integrating sphere with precisely positioned ports, and a photometer with appropriate baffling. Key design specifications include:
| Parameter | Requirement | Engineering Significance |
|---|---|---|
| Total port area | Less than or equal to 4.0% of sphere internal surface | Minimizes light loss through ports, preserving sphere integration accuracy |
| Sphere diameter | Greater than or equal to 150 mm | Ensures sufficient path length for complete light diffusion and scatter collection |
| Beam collimation | Less than or equal to 0.05 rad (3 degrees) | Prevents stray light and ensures accurate incident flux definition |
| Beam diameter at port | 50 to 60 percent of specimen diameter | Prevents edge losses while maintaining adequate signal-to-noise ratio |
| Detector linearity | Within plus or minus 1% of incident flux | Ensures radiometric accuracy across the measurement range |
The photometer must be fitted with baffles to prevent direct illumination from the specimen. Without proper baffling, the detector would receive unscattered light, causing systematic errors in both transmittance and reflectance measurements.
2. Specimen Preparation and Measurement Procedure
Specimens can be prepared from film, sheet, injection-moulded articles, or compression-moulded plaques. The standard defines two specimen configurations:
Without specimen holder: For specimens thinner than 10% of the port diameter or with haze lower than 30%. The specimen diameter should be 1.7 to 2.0 times the port diameter. This configuration is suitable for clear films and low-haze sheets where edge scattering is minimal.
With specimen holder: For thicker or higher-haze specimens. The holder has a metallic lustre inner surface that redirects edge-scattered light back into the integrating sphere, ensuring complete flux collection. This is critical for materials like opaque white diffusers or thick translucent panels used in LED lighting applications.
The measurement procedure involves four steps: incident flux measurement, total transmittance measurement, total reflectance measurement, and Fresnel correction for refractive index effects.
Modern digital spectrophotometers can automate the entire four-step sequence in under 30 seconds per specimen, with real-time correction for sphere drift and temperature effects. This makes ISO 26723 suitable for production-line quality control, not just laboratory certification.
3. Engineering Design Insights and Precision Considerations
The precision of ISO 26723 measurements depends critically on three factors: sphere coating stability, specimen positioning repeatability, and spectral match to the V(lambda) function.
Sphere coating: Barium sulfate (BaSO4) and polytetrafluoroethylene (PTFE) are the recommended white reference materials. These offer high diffuse reflectance (greater than 96%) across the visible spectrum. However, BaSO4 is hygroscopic and can degrade in humid environments. PTFE-based coatings offer better long-term stability but are more expensive to apply.
Specimen positioning: The standard requires that the centre of the light flux be precisely aligned with the centre of port a. Misalignment of even 0.5 mm can introduce errors of 1 to 2% in transmittance values for high-haze materials. For quality control applications, automated specimen handling with vision-guided alignment is recommended.
| Material Type | Typical Transmittance (%) | Typical Reflectance (%) | Common Application |
|---|---|---|---|
| Clear PMMA (acrylic) | 92 to 93 | 4 to 5 | Light guides, displays |
| Milky PC (polycarbonate) | 55 to 75 | 18 to 28 | LED diffusers, signage |
| White PP (polypropylene) | 10 to 30 | 65 to 80 | Reflectors, housings |
| PET film (clear) | 88 to 90 | 5 to 7 | Packaging, optical films |
| Opal polystyrene | 40 to 60 | 30 to 45 | Lighting panels |
Fluorescent and chromatic-colour plastics are explicitly excluded from ISO 26723 because the measurement method assumes no wavelength conversion. Testing fluorescent materials with this method produces meaningless transmittance values due to the Stokes shift emission contaminating the transmitted flux reading.
Frequently Asked Questions
Q1: What is the difference between ISO 26723 and ISO 13468-1?
ISO 13468-1 covers only planar transparent and substantially colourless plastics. ISO 26723 extends the method to translucent and opaque materials by using an integrating sphere that captures scattered light, and by defining both transmittance and reflectance measurements.
ISO 13468-1 covers only planar transparent and substantially colourless plastics. ISO 26723 extends the method to translucent and opaque materials by using an integrating sphere that captures scattered light, and by defining both transmittance and reflectance measurements.
Q2: Can ISO 26723 be used for textured or patterned plastic surfaces?
Yes, but the specimen should be as flat as possible. Textured surfaces increase scattering, and the specimen holder is recommended to collect edge-scattered light. Multiple readings at different orientations may be needed for patterned materials.
Yes, but the specimen should be as flat as possible. Textured surfaces increase scattering, and the specimen holder is recommended to collect edge-scattered light. Multiple readings at different orientations may be needed for patterned materials.
Q3: What is the recommended calibration frequency for the integrating sphere system?
The standard recommends verifying the system with certified reference standards at least annually. For high-volume quality control, daily verification with a stable reference material is good practice.
The standard recommends verifying the system with certified reference standards at least annually. For high-volume quality control, daily verification with a stable reference material is good practice.
Q4: How does specimen thickness affect the measurement?
For transparent materials, thickness has minimal effect on transmittance. For translucent materials, increasing thickness typically reduces transmittance and increases reflectance due to increased internal scattering.
For transparent materials, thickness has minimal effect on transmittance. For translucent materials, increasing thickness typically reduces transmittance and increases reflectance due to increased internal scattering.