We speak of a light source with higher efficacy as being more efficacious . It will take nearly five of the 100 W incandescent lamps to produce as many lumens as three of the fluorescent lamps. Spectral Luminous Efficiency Spectral luminous efficiency is the ratio of radiant flux at wavelength lambda m to that at a given wavelength such that both wavelengths produce equally intense visual sensations. Lambda m is chosen so that the maximum value of this ratio is 1. The spectral luminous efficiency of all wavelengths defines a curve that represents the response of the human visual system to radiant energy.
Symbols are V lambda for photopic vision and V' lambda for scotopic vision.
The function of V lambda is commonly known as the Standard Observer Curve . Therefore, the total surface area of the unit sphere is 4 pi or 12.57 square feet. If the luminous flux of 1 lumen falls on each square foot, the 1 candela uniform point source will produce a total of 12.57 lumens or 4 pi lumens. The above formula makes two assumptions. First, the light source is uniform in its distribution, which means that the luminous intensity vectors radiating from the point source are the same in all directions.