An Empirical Formula for the Computation of the Indirect Component of Daylight Factor

An empirical formula for the computation of daylight factor has been developed from formulae proposed by Pleijel, Dresler and Arndt. The formula, like those from which it was derived, assumes that light is inter-reflected in rooms in accordance with the theory of the integrating sphere.

Light entering a room through a window is considered in two parts, (a) that received directly from the sky or by reflection from obstructions above the horizon, and (b) light received by reflection from the ground. In determining the first reflected flux in the room, the sky and obstruction flux referred to under (a) above is modified by the average reflection factor of the floor and certain lower parts of the walls. The ground flux referred to under (b) above is modified by the average reflection factor of the ceiling and by certain upper parts of the walls. These two components of first reflected flux are then added and the theory of the integrating sphere applied.

This formula lends itself easily to simple forms of calculation such as a nomogram or slide-rule type of calculator. It is shown to give results approximating closely to measured values. It is therefore put forward as a tool for use by architects and lighting engineers for the calculation of daylight factor where great precision is not required.