1D: Two disk model

As the thick disk is considered a separate vertical component from the thick disk, the vertical exponential function is expanded and broken into two similar functions to describe the thin and the thick disk separately, doubling the amount of fitting parameters to four, and creating the new fitting function

$$I (z)\ =\ I_n e^{-z/z_n} + I_k e^{-z/z_k}\ ,$$ (6)

where $I_0$ and $z_0$ have now been split into a thin disk central intensity $I_n$ and scaleheight $z_n$ and a thick disk central intensity $I_k$ and scaleheight $z_k$.