Left: correlation plot N_H i vs. 857 GHz for the entire field. The black solid line is our best fit of Eq. (1)to the data below N_H i = 2 × 10²⁰ cm^-2. Right: residual map at 857 GHz. The residuals are derived by subtracting for each pixel the FIR sky brightness calculated from Eq. (1)using the linear-fit parameters. The black contours denote N_H i as in Fig. 1. The black rectangles mark the locations of two H i clouds (IVC 1 on the left, IVC 2 on the right) which are presented in more detail in Sect. 5.