Abstract: Even today lighting design of buildings is too often solely based on task illuminance levels with little consideration of the importance of the light distribution for the appearance and visual appeal of the lit spaces. In respect to no available information or calculation models on illuminance in Iran, equations proposed by Illuminating Engineering Society of North America in 2000 were adopted to calculate illuminance on the south facing vertical surfaces. Generally 315 times measurements were taken at three measuring stations over 15 days at one hour intervals between 12 July and 1 August 2007 from 9:00 a.m. to 3:00 p.m. to confirm calculated data. Mean respective values of measured and calculated illuminance exceed 33.59 and 33.27 KLx. Measured illuminance in each standard time at three stations exhibited a better agreement with calculated illuminance by IESNA equations in correspondent daylight times (related in 1 h later) rather than those calculated for the same standard times. A regression model with a good linear correlation (R2 = 0.806) was developed between measured and calculated values of south facing vertical illuminance. Economic and ergonomic daylight found to be 46.5 and 17.8 KLx, respectively. The maximum Daylight Autonomy for maintaining average internal illuminance of 500Lx, also maximum useful daylight illuminance to achieve internal illuminance in range of 100-2000 Lx found to be more than 55% of working year in Iran.