There are many materials or optical systems that are not ideally measured using current ASTM or other test methods. The transmittance or reflectance of various window treatments, patterned or corrugated materials, and full-size tempered glazings are but a few examples of optical systems that are difficult if not impossible to measure using present test methods. In most cases, the problems encountered are: (1) inability to illuminate a representative section of the material, (2) the receptor is not capable of measuring all of the transmitted or reflected flux, or (3) size restrictions associated with conventional measurement techniques. In order to determine the optical properties of large and/or complex systems, a unique 8-foot diameter integrating sphere has been designed and constructed. Designated the LMPI sphere, it has a 12-inch diameter aperture, and is capable of making absolute solar transmittance, reflectance, and absorptance measurements from a 0° to 60° angle of incidence. The sphere uses natural sunlight as its source illuminant, and incorporates a novel solar siderostat to focus sunlight into the sphere. A detailed description of the sphere and its optical system, along with a complete mathematical analysis of the energetics of the sphere, is presented. Solar transmittance, re-flectance, and absorptance measurements of large specimens at various angles of incidence are compared to both the pyranometric method (ASTM Standard E 424, Method B) and the spectrophotometric method (Method A). In general, transmittance and reflectance measurements of isotropic surfaces using the LMPI sphere gave results that agreed to within 0.01 units of the measurements obtained utilizing DSET's multiple-integrating sphere (MIS) spectro-photometer. For nonisotropic, textured, or corrugated specimens, the superiority of the LMPI sphere measurement vs. either Method A or Method B of ASTM Standard E 424 is demonstrated.