In the field of photometry, an inverse square law is often used in which the illuminance value is inversely proportional to the square of the photometric distance. It is well known that this is a rule that assumes that the light source is a point light source. In this research, it is shown by model simulation that the inverse square law cannot be applied with high accuracy depending on the distance and the size of the light-receiving area even in the case of a point light source. Also, when checked experimentally, the experimental results agree well with the simulation.
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