Derivation, Background and Use of the 'Scissors Curve'

The expression "scissors curve" was facetiously applied to the graph where two straight lines crossed each other at an apparent "fulcrum" point; drawing loops at the left-hand ends of these lines emphasized the resemblance to a pair of scissors. This expression caught on and has been used widely to describe the graph of limiting brightnesses of luminaires for office and school lighting. The two lines represent the limiting conditions between luminaires of uniform brightness (horizontal line) and luminaires having a nonuniform brightness distribution (slant line). For instance, luminaires that have a luminous cylinder enclosing the lamps (which would appear uniformly bright at all angles) would be allowed as high as 250 footlamberts brightness for the usual range of sizes of offices and schoolrooms. The slant line represents the maximum nonuniformity in brightness that would be allowed at the various angles for lighting systems in the same range of room sizes. In other words, each line represents the limiting brightness line for the same degree of protection from discomfort glare. Between these two straight lines there can be any number of straight lines drawn through the "fulcrum" point to represent various conditions of nonuniformity as one departs from the uniform condition. Since both lines represent equal effect and there can be any number of other straight lines that rotate about the "fulcrum" point, one can see that the little triangle at the left-hand side equals the large triangle on the right-hand side in relative comfort effect.