In the present paper, a new daylight performance metric, Daylight Illuminance Ratio (DIR) has been developed for an atrium space to take into account the direct and diffuse components (Sun and sky). The daylight performance on each inner surface of a square atrium has been investigated and validated with hourly measured experimental data under clear sky conditions. There is good agreement between theoretical and experimental values, with a correlation coefficient (r) of 0.92-0.96 and a root mean square percentage error (e) of 8.53-10.21%. Results show that the interior surface of the west and north walls receive more daylight in June (summer) and October (winter), respectively. The presented metric can be used to predict vertical illuminance at a given point on the atrium walls.