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Related services Solar reflectance index (SRI)

The concept of Solar Reflectance Index (SRI) is not directly applicable to translucent materials, such as fabrics and membranes. Please refer to this post for a simple way to check if a material is translucent using mobile phone flashlight.

This limitation is explicitly reflected in the title of ASTM E1980:

ASTM E1980: Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces

The scope clearly specifies opaque surfaces only. Therefore, applying ASTM E1980 to calculate the SRI of translucent materials is not technically valid.

This post explains the key challenges involved in determining the SRI of translucent materials and discusses practical workarounds.

The challenges

There are three main challenges when attempting to determine the SRI of translucent materials:

1. Solar energy transmission

Solar radiation interacting with a material follows the relationship:

solar reflectance + solar transmittance + solar absorptance = 1

For opaque materials, solar transmittance = 0, so solar absorptance can be derived directly from solar reflectance.

However, for translucent materials:

• Both solar reflectance and solar transmittance need to be measured
• Solar absorptance can only be determined after accounting for transmission

This adds complexity to the characterization process.

2. Infrared radiation transmission

Thermal emittance is commonly measured using ASTM C1371. However, this method assumes the material is opaque.

For translucent materials:

• Infrared radiation may pass through the sample
• The measured emittance can be influenced by the substrate or background behind the material

As a result, the measurement may not represent the intrinsic property of the material itself.

3. SRI calculation model

The SRI calculation defined in ASTM E1980 is based on a heat balance model that assumes:

• No solar transmission
• Surface-only energy exchange

Because translucent materials allow radiation to pass through:

• The underlying heat transfer model becomes invalid
• The calculated SRI does not accurately represent real thermal behavior

Practical workarounds

Although SRI cannot be directly applied, there are practical approaches to characterize translucent materials:

1. Make the system opaque

The translucent material can be mounted onto an opaque substrate to form a composite system. This prevents solar and infrared radiation transmission
Standard SRI measurement methods can then be applied

Please note that the result represents the SRI of the system, not the standalone material.

2. Test the material by assuming it is opaque

In some cases, translucent materials may be tested using the same procedures as opaque materials, effectively assuming that the material is opaque during the SRI measurement.

This approach can still provide meaningful results when the material has low solar transmittance, such that only a small portion of the incident solar radiation passes through the sample. Under these conditions, the measured SRI may still serve as a useful practical indicator of surface thermal behaviour.

However, the transmitted solar component is not properly accounted for in the measurement. Therefore, results obtained using this approach should be interpreted with caution, particularly for materials with significant solar transmission.

3. Measure optical properties separately

It is still possible to characterize the optical behaviour of translucent materials by measuring solar reflectance, solar transmittance, and solar absorptance separately.

This approach provides a more complete physical understanding of the material. The measured optical properties can then be used in thermal models, other than the SRI model, for further analysis.