ASTM C1549

ASTM C1549 – Standard Test Method for Determination of Solar Reflectance Near Ambient Temperature Using a Portable Solar Reflectometer

Description:

Significance and Use

5.1 The temperatures of opaque surfaces exposed to solar radiation are generally higher than the adjacent air temperatures. In the case of roofs or walls enclosing conditioned spaces, increased inward heat flows result. In the case of equipment or storage containers exposed to the sun, increased operating temperatures usually result. The extent to which solar radiation affects surface temperatures depends on the solar reflectance of the exposed surface. A solar reflectance of 1.0 (100 % reflected) would mean no effect on surface temperature while a solar reflectance of 0 (none reflected, all absorbed) would result in the maximum effect. Coatings of specific solar reflectance are used to change the temperature of surfaces exposed to sunlight. Coatings and surface finishes are commonly specified in terms of solar reflectance. The initial (clean) solar reflectance must be maintained during the life of the coating or finish to have the expected thermal performance.

5.2 The test method provides a means for periodic testing of surfaces in the field or in the laboratory. Monitor changes in solar reflectance due to aging and exposure, or both, with this test method.

5.3 This test method is used to measure the solar reflectance of a flat opaque surface. The precision of the average of several measurements is usually governed by the variability of reflectances on the surface being tested.

5.4 Use the solar reflectance that is determined by this method to calculate the solar energy absorbed by an opaque surface as shown in Eq 1.

5.4.1 Combine the absorbed solar energy with conductive, convective and other radiative terms to construct a heat balance around an element or calculate a Solar Reflectance Index such as that discussed in Practice E1980.

Scope

1.1 This test method covers a technique for determining the solar reflectance of flat opaque materials in a laboratory or in the field using a commercial portable solar reflectometer. The purpose of the test method is to provide solar reflectance data required to evaluate temperatures and heat flows across surfaces exposed to solar radiation.

1.2 This test method does not supplant Test Method E903 which measures solar reflectance over the wavelength range 250 nm to 2500 nm using integrating spheres. The portable solar reflectometer is calibrated using specimens of known solar reflectance to determine solar reflectance from measurements at four wavelengths in the solar spectrum: 380 nm, 500 nm, 650 nm, and 1220 nm. This technique is supported by comparison of reflectometer measurements with measurements obtained using Test Method E903. This test method is applicable to specimens of materials having both specular and diffuse optical properties. It is particularly suited to the measurement of the solar reflectance of opaque materials.

1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.