Dependence of the scattering coefficient, Q_sca, on the wavelength as derived from modelling of the echo. The values are normalized to Q_sca at the effective wavelength of the F814W filter. Asterisks: results of the models fitting the total flux, STmag, (upper rows in Table 2). Open circles: results of the models fitting the mean surface brightness, S_b, (middle rows in Table 2). Filled circles: results of the models fitting the mean surface brightness derived from the brightest pixels filling f_c = 0.5, S_B   0.5, (bottom rows in Table 2). The horizontal error bars represent the widths of the photometric bands. Open and filled symbols are shifted by  ± 0.05   μm in the abscissa axis for clarity. Curves: relations expected from modelling the interstellar extinction curve of Weingartner & Draine (2001) for R_V = 3.1 (dotted) and R_V = 5.5 (dashed).