ON THE ERROR OF ZONAL CAVITY CALCULATIONS FOR INDIRECT LIGHTING.

Two independent techniques are shown to produce identical average illuminance values, both differing from the value obtained using current zonal cavity coefficients of utilization. Previous measurement comparisons support the validity of the new technique. Both of these facts provide strong evidence that current zonal cavity coefficients of utilization underestimate the average illuminance produced by indirect lighting systems. It is suggested that this modified procedure be used to determine coefficients of utilization for luminaires that have significant upward directed components of flux. A table of such values will have an additional variable; the ceiling cavity ratio.