ASTM E892-87 Standard Tables for Terrestrial Solar Spectral Irradiance at 37 Air Mass for a 1,5 Degree Sloping Surface

The tables in the standard "ASTM E892-87 Standard Tables for Terrestrial Solar Spectral Irradiance at 37 Air Mass for a Surface Inclined at 1,5 Degrees" published by the American Society for Testing and Materials (ASTM) define an air mass 37 solar spectral irradiance distribution for use in all solar applications where a standard terrestrial spectral irradiance is required for the portion of the solar radiation, diffuse and direct, falling on a surface inclined at 1,5 degrees facing the sun.

These tables are modeled data produced using the zero air mass solar spectrum based on the revised extraterrestrial spectrum of Neckel and Labs, the BRITE Monte Carlo radiative transfer code, and the 1962 US standard atmosphere including rural aerosol. Further details are included in the standard appendix (Appendix Xl).

The air mass zero spectrum used to construct the terrestrial spectrum is a revised and expanded Neckel and Labs spectrum. The Neckel and Labs spectra have been revised using more recent limb obscuration data to convert from luminance to radiance. Comparisons with calibrated sun photometer data indicate that this new extraterrestrial spectrum is the best currently available.

For the purposes of this standard, the following definitions apply:

• Air mass is the ratio of the mass of the atmosphere in the path of the observer to the mass that would be present if the observer were at sea level, at standard barometric pressure, and the sun were directly overhead. Air mass varies with the solar zenith angle and local barometric pressure, which vary with altitude. The solar zenith angle of 62 degrees or less and the local atmospheric pressure is the standard atmospheric pressure.
• Solar irradiance refers to the downward scattered solar flux received on a horizontal surface from a 6x-steradian (hemispherical) solid angle, excluding a conical solid angle with a plane angle of approximately 2 degrees centered on the disk of the sun.
• Solar radiation refers to the solar flux coming directly from the solid angle of the solar disk and onto a surface perpendicular to the axis of that solid angle. In conventional instruments the acceptance cone includes a plane angle of about 6 degrees.

The absorption, reflection and transmittance of terrestrial solar energy are important factors in solar thermal system performance, photovoltaic system performance, materials studies, biomass studies and solar simulation activities. For each of the optical properties, the initial measurements are normally a function of wavelength, which requires that the spectral distribution of the solar flux be known before the solar weighted property can be calculated. A single standard solar spectral irradiance distribution is desired to compare the performance of competitive products.

These tables provide a standard spectral irradiance distribution convenient for use in determining the relative performance of solar thermal, photovoltaic, and other systems, components, and materials where direct plus diffuse irradiance components are desired.

Table 1 given in the standard tabulates the fraction of solar spectral radiation falling on a surface inclined at 37 degrees from the horizontal, between 0,305 and 4,045 µm. The sun is at AM 1,5. The first column in the table gives the wavelength in micrometers, the second column gives the integral range in micrometers, the third column gives the direct radiation, the fourth column gives the integrated solar radiation in the wavelength range 0,3 µm to Δ, and the fifth column gives the fraction of direct normal solar radiation in the wavelength range 0 to Δ. Below 0,3 µm the amount of radiation reaching the earth's surface is negligible.

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