SRI testing: Can membrane products be tested without substrate?

We are often requested to test the solar reflectance index (SRI) of membrane products, particularly liquid applied membrane products.

Unlike paints, membrane products can be free-standing. A membrane product sample can be prepared either as a standalone membrane layer or with a substrate. The question from many customers is whether the SRI of membrane products can be tested without substrate.

Two examples

To answer this question, two examples are illustrated below:

The sample on the left is translucent (with large transmission). For such samples, a substrate is required.

The sample on the right is opaque (with zero transmission). For such samples, a substrate is not required.

The theory

For a material, we have the following relationship:

Solar transmittance + solar reflectance + solar absorptance = 1

In SRI calculations, the solar transmittance is assumed 0 (solar transmittance = 0). The solar reflectance is directly measured by the instrument. Therefore, we use the following relationship to calculate the solar absorptance:

Solar absorptance = 1 – solar reflectance

However, if the material is not opaque (solar transmittance ≠ 0, for example, the left example), the equation above is not valid.

For such translucent samples, the calculated solar absorptance is higher than the actual solar absorptance, as the solar transmittance is counted as part of the solar absorptance, and the resultant SRI is therefore lower.

In order to eliminate this error, translucent samples shall not be used for SRI testing. If a membrane product is translucent, it shall be applied onto a suitable substrate for testing.

The practice

To determine if a membrane sample is translucent, a simple method is to check if the flashlight from a phone can transmit through the sample, as shown in the examples above.