Psychophysics attempts to understand the relationship between a physical stimulus and the psychological impression it creates or how the physical world influences the mind. The connection between perception and psychophysics is that perception is one of the constructs examined in the psychological part of the equation. Psychophysics uses quantitative measurements to analyze the relationship between the sensations and perceptions caused by stimuli.
The relationship between perception and psychophysics is an important one in that what a person perceives to be so is not always indicative of the stimulus. Stimuli possess different properties which affect whether a person is aware of them or not and dictate their identification. The degree of difference between stimuli will affect whether they are distinguishable or not and to what magnitude a stimulus needs to reach before judgment of similarity and difference can be made.
There are three methods used to measure perception in psychophysics. They are magnitude estimation, matching and detection or discrimination. In magnitude estimation, the subject is required to rate a stimulus on how bright or loud it is on a scale. Matching requires the subject to find the stimuli which are similar in look, sound or pitch. In detection, the subject is asked to discriminate between small differences in intensity or whether a light was flashed or a sound played. Two important terms used in perception and psychophysics are the "absolute threshold" and "difference threshold." Absolute threshold refers to the smallest detectable amount of stimulus energy, and the difference threshold, or just noticeable difference, refers to the smallest detectable difference between two stimuli.
One of the most important precepts in perception and psychophysics is Weber's Law which states that the difference threshold between two stimuli is proportional to the baseline or starting intensity. That is, if a small increase in intensity is applied to a small parameter, then that increase can be detected. However, if that same small increase is applied to a larger parameter, it will no longer be detectable. For example, someone holding a small weight will notice the addition of another small weight, whereas someone holding a heavy weight will not notice the addition of a light weight.
Studies in perception and psychophysics are of great use to fields such as ergonomics and human-computer interaction. The design of products such as home appliances, software and flight interfaces refer to the results of psychophysics experiments. Other fields which have benefited from psychophysics include neurology, psychology and ophthalmology.