ASTM E903-20 Standard Test Method for Solar Absorption, Reflectance, and Transmittance of Materials Using Integrating Spheres

The standard "ASTM E903-20 Standard test method for solar absorption, reflectance, and transmittance of materials using integrating spheres" published by the American Society for Testing and Materials (ASTM) describes a test method for determining the measurement of spectral absorption, reflectance, and transmittance of materials using spectrophotometers equipped with integrating spheres.

This standard specifies methods for calculating solar weighted properties from measured spectral values. This test method is applicable to materials having both specular and diffuse optical properties. This test method is applicable to materials having optical coatings applied with special consideration for the effect on the textures of the material under test.

Transmissive sheet materials that are non-homogeneous, textured, patterned or corrugated require special considerations regarding the applicability of this test method. In this regard, the “ASTM E1084-86 Standard test method for Solar transmittance of sheet materials using sunlight (terrestrial)” standard is considered more appropriate for determining the bulk optical properties of textured or non-homogeneous materials. For homogeneous materials, this test method is preferred to the test method described in ASTM E1084-86(2023).

This test method refers to applications using standard reference solar spectral distributions, but can also be implemented using alternative selected spectra as long as the source and details of the solar spectral distribution and weighting are reported.

For the purposes of this standard, the following terms and definitions apply, in addition to the definitions given in ASTM E772-15:

An integrating sphere is an optical device used to collect the reflected or transmitted flux from a sample into a hemisphere or to provide isotropic irradiation of a sample from a complete hemisphere. It consists of an approximately spherical cavity with apertures to accept and detect the flux and usually with additional apertures in which the sample and reference samples are placed.
The near-normal hemisphere indicates that the radiation is directed close to the sample surface and the flux from the surface or medium is concentrated over an entire hemisphere for detection.
Photovoltaic solar refers to an optical property and refers to the weighted average of the spectral property using the number of photons per second per unit area derived from a standard solar irradiance distribution as the weighting function.
It is a surface that has a smooth, flat and horizontal surface, without roughness or protrusions, and is free from surface inequalities or irregularities.
Specular indicates that the flux leaves a surface or medium by an angle numerically equal to the angle of incidence, lying in the same plane as the incident ray and the perpendicular, but on the opposite side of the perpendicular to the surface. Diffuse was historically used to refer to a collection of hemispheres (including the specular component). This usage has been deprecated in favor of the more precise term hemisphere.
The nature of a textured, uneven surface. Having some degree of roughness, unevenness, or ridges.

Measurements of transmittance (or reflectance) near the spectral normal hemisphere are made with an integrating sphere spectrophotometer in the spectral range 300 to 2500 nm. Solar transmittance, reflectance or absorbance are obtained by direct calculation of appropriate convolution integrals or by calculating a weighted average with a standard or selected solar spectral irradiance as a weighting function with the weighted or selected ordinate method.

Solar energy absorption, reflection and transmittance are important to the performance of all solar energy systems, from passive building systems to central receiver power systems. This test method provides a means of determining these values under constant conditions that represent an average that would be encountered during the operation of a system in the temperate zone.

Solar energy absorption, reflection, and transmittance are important for thermal control of spacecraft and solar energy of extraterrestrial systems. This test method also provides a means of determining these values for extraterrestrial conditions.

This test method is designed to provide reproducible data suitable for comparing results between laboratories or by the same laboratory at different times and for comparing data obtained on different materials.

This test method has proven practical for smooth materials with both specular and diffuse optical properties. Textured, non-homogeneous, patterned, or grooved materials require special attention. Surface roughness can arise from physical or chemical processes such as pressing, rolling, abrasion, or the deposition of film or chemical layers onto materials, resulting in textured surfaces.

The magnitude of surface roughness in the spectrophotometer and its components (beam sizes, sphere apertures, sample holder configuration) can significantly affect the accuracy of measurements made using this test method. Even if the repeatability or sensitivity of textured material measurements is good, including repeated measurements in various locations or orientations of the sample, the different specifications of different spectrophotometers in different laboratories can lead to significant differences in measurement results.

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