The question asked in the absolute threshold method of psychophysics is, how much stimulus energy is required to elicit the response, "yes I saw something."? When dealing with color this question can take on one of two forms. 1. How much stimulus energy is required to just detect color? 2. How much stimulus energy is required to just detect color correctly?
When measuring the photopic spectral sensitivity by the absolute threshold method it is critical that the stimulus fall within the rod free area of the fovea. Because one is dealing with absolute threshold the stimuli necessarily have to be presented in a dark field. A typical method for assuring proper fixation is to present the stimulus in the center of an area bounded by four fixation lights located extrafoveally.
As it turns out, an absolute threshold spectral sensitivity function is nearly the same as a brightness matching function, but rather different than the CIE modified photopic luminous efficiency function which is based mostly on flicker photometry.