The study was performed at the Lawrence Berkeley National Laboratory (LBNL) windows testbed in Berkeley California. The location is 37.4°N latitude. The maximum solar altitude ranges from approximately 29° to 76°, with a yearly average of about 53°. The testbed rooms have their windows mounted due south. The azimuthal angle at sunrise varies from approximately 60° from due south in the winter to 120° in summer. The analysis was restricted to the period from 6:00-18:00 standard time. Sunrise ranges from 4:42 to 7:17 (solar time), so the sun was not visible during part of the winter study periods.