PURPOSE: To obtain the coefficient of utilization in a simple way, by using a graph of orthogonal axes in which the scales of a room area are plotted to an axis in the form of a logarithmic scale and the scales of the coefficient of utilization are plotted to the other axis.

CONSTITUTION: The scales 4 of the room index R are plotted to a horizontal axis in the form of a logarithmic scale, and the scales 2 of the coefficient of utilization U are plotted to a vertical axis in the form of an integer scale respectively. A coefficient of utilization curve (a) is drawn to show the relation between the index R and the coefficient of utilization U with the attachment height (h) set at 2.5m. Here the reflection factor is usually set at the ceiling (60%), the wall (40%) and the floor (20%) respectively. Then the scales 1 of the room area A are plotted to the horizontal axis. The scales 4 are not used usually, and the coefficient of utilization U can be read directly from the area A. In case the height (h) is not set at 2.5m, the (h) is compensated by shifting the horizontal axis by S with an attachment height compensation gauge 5.