抄録
An intermediate-equation has been obtained by analysing a minute plane source located on a picture plane of the “via-perspective method” by a polar coordinate system. When adopting a calculating system to convert the “intermediate-equation” to an orthogonal coordinate system, practically usable formulae may be led from a primary formula based on the inverse-square law and cosine law. This paper was prepared to analyse the above-mentioned formulae for a horizontal illuminance due to flat surface source with an arbitrary shape by means of the new calculating system, after classifying the states of flat surface sources into horizontal, inclined and vertical positions. This calculating systems as well as analyses of various types of spacial illuminance as discussed in the following paragraphs are also applicable to cubic source of light.
