The absorption coefficient α is the fractional change in the intensity of the light beam as it travels the distance d s, and has units of length. It is composed of two parts, the decrease due to absorption and the increase due to stimulated emission. Stimulated emission is emission by the material body which is caused by and is proportional to the incoming radiation. It is included in the absorption term because, like absorption, it is proportional to the intensity of the incoming radiation. Since the amount of absorption will generally vary linearly as the density ρ of the material, we may define a "mass absorption coefficient" κ ν = ⁠ α / ρ ⁠ which is a property of the material itself. The change in intensity of a light beam due to absorption as it traverses a small distance d s will then be d I ν ν = − − κ κ ν ν ρ ρ I ν ν d s