The following procedure was conducted to determine the photopic luminous efficiency function: First, light of constant intensity was emitted and its frequency was varied until the lightness perceived by the observer was found to be maximal. This occurred at a frequency of about 540 THz, corresponding to wavelength λm = 555 nm. The wavelength was then set to another λ and the power was readjusted until the lightness was judged to be the same as at λm. V(λ) could thus be computed as the ratio of the radiated power at λm and λ, respectively.
Of course, this experiment has been conducted by many observers and the resulting average was used to define the so called CIE standard eye which is an optical sensor with sensitivity corresponding to the function V(λ).