this report in order to adequately specify the experiment procedure. In no case does such identification imply recommendations or endorsement by the National Research Council, nor does it imply that the product or material identified is the best available for the purpose. 13 Figure 4: Data Acquisition in Stairwells The picture on the left shows the configuration used in the stairwells (the stairwell depicted was not one measured in this study). The illuminance meters are mounted on a pillar such that the plane of the detectors is 1 m above the stair tread. The operator manually directs the software and data acquisition hardware mounted on the laptop computer to poll the illuminance meter, and store the measured values on the computer hard disk. Once the polling operation has been completed, the operator moves the apparatus to the next measurement location, and repeats the operation. The picture on the right shows the configuration used to collect measurements on the floors. The operator pushes the device along the centreline of the measurement path. As the calibrated wheel (circumference 1 m) turns, the data acquisition system is directed to poll the illuminance meters and store the resulting value in the computer once every 25 cm of travel. For the stairwells, a total of 12 readings were collected between each floor as follows: three readings were collected at the door landing and the mid-landing, and one each at the top, middle and last stair of each staircase. The data acquisition system software was configured so each reading was polled by the operator manually once the apparatus was correctly placed. All stairwells were windowless, so no daylight supplemented the electric lighting. On Floors 9, 10 and 11, the operator pushed the measuring system along the centreli..