Observed NIR surface brightness I_ν as the function of optical depth derived with NICER method in J, H, and K bands. WFCAM data for TMC-1N are shown with a 2D histogram, the colour scale corresponding to the density of points. The black line shows the fitted Eq. (2) (using instead of column density N). For comparison, Corona Australis data from Juvela et al. (2008) are shown with black dots and the fitted function with a red line. The black dashed vertical line shows the upper limit used in the fitting of TMC-1N data in all frames and of Corona Australis data in left and middle frames. In the right frame (K band), the red dashed vertical line shows the upper limit used in the fitting of Corona Australis data, as in this case higher upper limit was needed to make a better fit. The obtained parameter values are marked in the figures with black for TMC-1N and red for Corona Australis.