Dust temperature map of CB 68 (color scale), overlaid with contours of the hydrogen column density (white: 10²¹ and 10²² cm^-2, black: 0.5 – masked by yellow N_out contour – 2, 4, 6, 8, and 20 × 10²¹ cm^-2). Yellow contours mark N_out (Eq. (3)) and N_1′ (Eq. (10)). The dotted yellow ellipse marks the N_1′ approximation described in Sect. 5.3. The asterisk marks the position of the protostar and the reference position listed in Table 5.