ASTM E424 Standard Test Methods for Solar Energy Transmittance and Reflectance (Terrestrial) of Sheet Materials 

• Methods: There are two methods described in ASTM E424. Method A uses a spectrophotometer to measure both transmittance and reflectance. Method B applies only to transmittance measurements using a pyranometer in an enclosure with the sun as the energy source. Method B is particularly suitable for the measurement of transmittance of inhomogeneous, patterned, or corrugated materials since the transmittance is averaged over a large area.
• Material: The approaches have been found to work for both clear and translucent materials, as well as those with highly reflective coatings that reduce transmittance. Because the transmittance is averaged across a vast area, Method B is particularly well suited to measuring the transmittance of inhomogeneous, patterned, or corrugated materials.

• Method A (Spectrophotometer)

Over the spectral range of 350 to 2500 nm, an integrating sphere spectrophotometer is used to measure spectral transmittance or reflectance versus a magnesium oxide standard. Normal-diffuse or diffuse-normal illumination and viewing mode are required. Integrating over a conventional solar energy distribution curve using weighted or selected ordinates for the appropriate solar-energy distribution yields the solar energy transmitted or reflected. 

• Method B (Pyranometer)

Using the sun as the source and a pyranometer as the detector, the specimen is used as the cover of an enclosure, with the specimen plane perpendicular to the incident radiation; transmittance is determined as the ratio of transmitted energy to incident energy.