Spatial distribution of the continuum intensity normalised to the averaged continuum intensity over a quiet-Sun area located at the top of the observed FOV, I_c. The panels represent the recorded intensity in channels 1 (left panel) and 2 (right panel) of GRIS. The long-slit scan was performed from the left to the right side of the image. The vertical axis corresponds to the north-south direction of the Sun. The coloured symbols highlight regions of interest studied in Section 3. At the same time, the contours tentatively underline the location of the umbra (defined as regions with I_c∼0.6) and penumbra (areas where I_c∼0.8). The arrow in the left panel points towards the disc centre.