Absolute uncertainty of the optical depth as a function of the optical depth itself for three different continuum background sources. The dashed, dotted, and solid line represent continuum sources with a brightness temperature of T_cont = 500 K, 1000 K and 2000 K, respectively. The crosses on each line show the lower limit of the optical depth that we can observe for the given continuum brightness temperature. For larger optical depths, the absorption spectra saturates (see Sect. 4.1 for more detail).