Panel A): background-subtracted theoretical surface brightness through the core center at 3.6 (4.5) μm in blue (green), respectively (see Eq. (7)) as a function of the maximum grain size of the dust size distribution for the core L260. The roots of both functions indicate the regions where the core becomes visible in excess of emission for each wavelength (region outside yellow regions). The dashed lines give the observed maximum coreshine value with its error range, and the dots label the crossing points with the theoretical curve. Panel B): resulting theoretical ratio R_theo of the surface brightnesses shown in panel A) as a function of the maximum grain size. The dots from panel A) mark the agreement of the central surface brightnesses with the observed range for each wavelength.