Glazed apertures in buildings are generally utilized for the controlled admission of both light and solar radiant heat energy into the structure. Other devices may also be used to reflect light and solar radiant heat into a building.

The bulk of the solar radiant energy entering a building in this manner possesses wavelengths that lie from 300 to 2500 nm (3000 to 25 000 E). Only the portion from 380 to 760 nm (3800 to 7600 E) is visible radiation, however. In daylighting applications, it is therefore important to distinguish the radiant (solar radiant energy) transmittance or reflectance of these materials from their luminous (light) transmittance or reflectance.

For comparisons of the energy and illumination performances of building fenestration systems it is important that the calculation or measurement, or both, of solar radiant and luminous transmittance and reflectance of materials used in fenestration systems use the same incident solar spectral distribution.

Solar luminous transmittance and reflectance are important properties in describing the performance of components of solar illumination systems including windows, clerestories, skylights, shading and reflecting devices, and other passive fenestrations that permit the passage of daylight as well as solar radiant heat energy into buildings.

This test method is useful for determining the solar luminous transmittance and reflectance of optically inhomogeneous sheet materials and diffusely reflecting materials used in natural lighting systems that are used alone or in conjunction with passive or active solar heating systems, or both. This test method provides a means of measuring solar luminous transmittance under fixed conditions of incidence and viewing. This test method has been found practical for both transparent and translucent materials as well as for those with transmittances reduced by reflective coatings. This test method is particularly applicable to the measurement of luminous transmittance of inhomogeneous, fiber reinforced, patterned, corrugated, or otherwise optically inhomogeneous materials when the transmittance is averaged over an area that is large in comparison to the inhomogeneities.