Left: histograms of the Hellinger distance (D_H) between FIR or MIR and optical maps, which indicates the proximity of two cirrus maps. Smaller D_H represents a closer local PDF. Histograms with lighter shades represent smaller kernel scales used for the PDF extraction. The PDF distance of FIR relative to optical is systematically lower than that of MIR. The dispersion is also much smaller. Right: mean D_H between FIR (orange) or MIR (green) and optical data as a function of kernel scale. The mean D_H of FIR relative to optical is lower than MIR in all scales. The dashed lines show the asymptotic values at large scales.