Optical depth of the absorption profile of CO 1-0 calculated by MADCUBA (black continuous line) for Δv = 10 km s⁻¹, log N = 17 cm⁻², and T_bg = 2.73 K. From the values measured in the synthetic spectra and using Eq. (32), the approximate optical depth is calculated (black dashed line). The ratio between the two optical depth estimates is shown in red. Top and bottom panels are equal but for the continuum temperatures of 10 and 100 K, respectively. Vertical dotted line represent the temperature for which the line completely absorbs the continuum emission (i.e. T_L = J(T_c)), and beyond which the CMB continuum is absorbed.