The lumen method provides a means of computing the number of light fixtures required to illuminate a rectangular indoor space to a given target illuminance, or to compute the resulting illuminance given a specific number of fixtures. It computes average, maintained illuminance provided the room is rectangular and the ratio of the length of the long wall to the short wall in no greater than 1.6:1, and that the space will be illuminated with only one model of uniformly distributed luminiares. This method has been employed by lighting engineers since before the publication of the Illuminating Engineering Society's first edition of the Lighting Handbook in 1947. Although there are sophisticated, CAD-based design tools available today, this method - using this calculator app - can quickly answer the common question: "How many fixtures will I need to light my workshop, office, classroom, etc.
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