The "mass emission coefficient" j ν is equal to the radiance per unit volume of a small volume element divided by its mass (since, as for the mass absorption coefficient, the emission is proportional to the emitting mass) and has units of power⋅solid angle ⋅frequency ⋅density. Like the mass absorption coefficient, it too is a property of the material itself. The change in a light beam as it traverses a small distance d s will then be d I ν ν = j ν ν ρ ρ d s