Scale length is used to describe quantities that decline exponentially, such as the surface brightness of spiral galaxies. In this case it is defined as the radius at which the brightness of the galaxy has fallen off by a factor of e (~2.71828) from the centre. The brightness follows a functional form:

b=b₀e^-r/r₀

where b₀ is the brightness in the centre of the galaxy and r₀ is the scale length.