Determination of Efficiency of the Vertical Specularly Reflecting Cylindrical Light Shaft for Different Types of Firmaments

Article is devoted to determination of efficiency of the vertical specularly reflecting cylindrical light shaft for various types of the firmaments standardized by CIE (International Commission on Illumination). Efficiency of the light shaft was calculated as the relation of the output luminous flux (exiting through the lower base of the shaft) to the entering luminous flux (entering through the upper base of the shaft). The output luminous flux consists of the luminous flux created by direct light (gets on base of the shaft directly from the sky) and the luminous flux created by light which is repeatedly reflected from its inner side surface. For the 4th and 15th types of firmaments surfaces of dependence of the total efficiency (formed by the direct and reflected light) of the light shaft from solar time and the index of the shaft and also – graphs of dependence of total efficiency of the light shaft on the index of the shaft are given. For the 4th types of firmament the maximum values correspond to solar noon (from 29.3% to 96.3%), and the minimum values are at sunrise and sunset (from 28.7% to 95.3%). For the 15th types of firmament the maximum values correspond to solar noon (from 15.5% to 95.8%), and the minimum values are at sunrise and sunset (from 13% to 91.5%). The results showed that the solar time has almost no effect on the efficiency of the light shaft, which allows us to average the results in a certain way. As the light shaft index increases, that is, as the ratio of the radius to the shaft height increases, the efficiency value asymptotically approaches one hundred percent, which is physically correct. Knowing the radius, height, index of the shaft and the specular reflection coefficient of its inner surface, it is possible to predict natural lighting under the shaft and use energy resources more rationally.