ABSTRACT
This chapter reports the results of one of the most enduring projects ever undertaken at the National Bureau of Standards (NBS), research into the physical description of human vision. The principal result of this work was the "visibility curve," a quantified model of how well a typical person can see the different wavelengths (colors) of light. This model function, essentially unchanged, underlies all physical measurements of photometric quantities and their interpretation in photometric units of measure. Light is a form of radiant energy, with a power that can be measured in watts, but the connection between this physical description and the visual result in the human eye was not well established. K. S. Gibson and E. P. T. Tyndall carefully compared their own results with those of their predecessors and proposed a mean visibility curve based upon the accumulated data from more than 200 different observers.
