Solar absorptance and thermal emittance of transparent/translucent materials

We had a post on the sample requirements of translucent membrane products for SRI testing before: SRI testing: Can membrane products be tested without substrate? Essentially, if a transparent/translucent material is to be laid on top of an opaque substrate, it is required to test the transparent/translucent material together with the substrate.

There are also scenarios that transparent/translucent materials are with standalone installations and without substrates, .e.g. canopies made of fabric materials. In such cases, it is sometimes necessary to determine the solar absorptance and thermal emittance of transparent/translucent materials.

Solar absorptance of transparent/translucent materials

According to ASTM E903, the solar absorptance of a transparent/translucent material can be calculated as:

Solar absorptance = 1 – solar transmittance – solar reflectance

The solar transmittance and solar reflectance can be measured with a UV/VIS/NIR spectrophotometer.

Thermal emittance of transparent/translucent materials

It is out of the scope of ASTM C1371 to measure the thermal emittance of a transparent/translucent material. A manufacturer’s technical note is available to measure the IR transmittance and thermal emittance of transparent/translucent materials, with a portable emissometer.

SRI of transparent/translucent materials

It is out of the scope of ASTM E1980 to determine the solar reflectance index (SRI) of transparent/translucent materials. As suggested by its title, ASTM E1980 is for “horizontal and low-slope opaque surfaces”.