The term would be psychophysics
Psychophysics is a science branch that investigates the relationship between physical stimuli (in this case light stimuli) and the sensations and perceptions they produce (like perception of color). It represents the combination of psychological events (psycho-) and physical events (-physics) with the purpose of finding their quantitative relation. Measuring the stimulus parameters (such as amplitude and wavelength of light stimuli) it is possible to determine the minimum perceptible stimulus or the threshold stimulus.
