Experimental analysis for Madrid, Spain, of a simple graphic daylight calculation method based on the CIE standard overcast sky

A simple graphical method proposed by the CIE (Commission Internationale de lʹEclairage) to estimate the daylight factor for overcast skies, at an early stage of daylighting design, at a reference point located at 60 cm from the rear wall along the centre line of a room having a window in the front wall, has been tested at Madrid, Spain, using a scale model (1:10) simulating a school room. Using the predicted daylight factor and estimated values of the external illuminance available on a horizontal surface within a specified percentage of daytime working hours, both proposed by the CIE, values for the internal illuminances at the reference point have been estimated. The predicted daylight factor and the estimated external illuminance values are far from the measured ones, and thus the method cannot be recommended for Madrid.