Examples of a just noticeable difference, or JND, include the detection of change in the volume of ambient sound, the luminosity of a light in a room, or the weight of a handheld object. The difference threshold is demonstrated at the moment a change in the nature of such stimuli is detected.
For example, an individual is not likely to notice a slight, gradual increase in the volume of music if the change in volume remains below the threshold of detection. At a certain point, however, the individual notices that the volume of the music has increased. The volume at which the increase was noticed demonstrates the concept of just-noticeable difference. In other words, the threshold of detection has been surpassed, and the individual is now able to perceive that a change in volume has occurred. Similarly, a gradual change in the brightness of a light in a room or in the weight of two similar objects held in each hand remains unnoticed until the change surpasses the threshold at which a difference in luminosity or weight is perceived.
Just noticeable difference, referred to formally as Weber's Law, is a concept in psychology based on the findings of Ernst Heinrich Weber, a forerunner in the field of experimental psychology and perception.