Intensity is the amount of energy in a light or sound wave. It is the physical property that corresponds most closely to the psychological experiences of brightness and loudness.

For light, intensity is related to the amplitude of the electromagnetic wave and is measured in units such as candelas or lumens. For sound, intensity is related to the amplitude of the pressure wave and is measured in decibels. The relationship between physical intensity and perceived magnitude is not linear; Fechner’s and Stevens’ laws describe the compressive functions that map large ranges of physical energy onto more manageable ranges of sensation.

Intensity interacts with other stimulus dimensions. A light of given intensity appears brighter or dimmer depending on wavelength and on adaptation; a sound of given intensity appears louder or softer depending on frequency. The absolute threshold and the difference threshold both vary with intensity level.

Intensity is the physical strength of the stimulus. Brightness and loudness are the experiences the nervous system constructs from it.