Psychophysics is the quantitative investigation of the relationship between physical stimuli and the sensations and perceptions they produce. This approach is used to analyze perceptual processes by varying certain properties of a stimulus, and observing changes in the person’s subjective experience of the stimulus.
The term psychophysics was coined in 1860 when Gustav Theodor Fechner published his book: Elemente der Psychophysik.

Fechner, G. T. (1860). “Elemente der Psychophysik” (Elements of Psychophysics).