  We solve the integral equation describing the propagation of light in an
isothermal plane-parallel atmosphere of optical thickness $\tau^*$, adopting a
uniform thermalization parameter $\epsilon$. The solution given by the ALI
method, widely used in the field of stellar atmospheres modelling, is compared
to the exact solution. Graphs are given that illustrate the accuracy of the ALI
solution as a function of the parameters $\epsilon$, $\tau^*$ and optical depth
variable $\tau$.
