I have already pointed out that we may also consider the efficiency of a light in relation to the sensibility of our own eye. That is, we take into account not only the energy distribution in the spectrum of the light but also the fact that different wave-lengths of an equal energy spectrum affect our eye very differently. As the normal light-adapted eye is most sensitive to yellow green of λ = 0.565µ, monochromatic light of this wave-length will appear much brighter than monochromatic light of any other wave-length with the same energy. Monochromatic
light of λ = 0.565µ will then be the theoretically most efficient possible, when we consider the energy radiated in relation to the sensitivity of our eye. This is the usual method of determining the luminous efficiency of artificial
lights and is obtained from a knowledge of the radiated energy and the visual sensibility. Reduced luminous efficiency = light (radiated energy × visual sensibility) or luminosity ÷ total radiated energy.
Fig. 12.—Visibility curves of various investigators obtained by different methods (after Hyde, Forsyth and Cady).
Fig. 13.—Luminous efficiency of the 4-watt carbon glow lamp, shaded area ÷ total area (after Ives and Coblentz).