Context : Recently discovered scattered light from molecular cloud cores in the wavelength range 3-5 \mu m ( called ” coreshine ” ) seems to indicate the presence of grains with sizes above 0.5 \mu m .

Aims : We aim to analyze 3.6 and 4.5 \mu m coreshine from molecular cloud cores to probe the largest grains in the size distribution .

Methods : We analyzed dedicated deep Cycle 9 Spitzer IRAC observations in the 3.6 and 4.5 \mu m bands for a sample of 10 low-mass cores .
We used a new modeling approach based on a combination of ratios of the two background- and foreground-subtracted surface brightnesses and observed limits of the optical depth .
The dust grains were modeled as ice-coated silicate and carbonaceous spheres .
We discuss the impact of local radiation fields with a spectral slope differing from what is seen in the DIRBE allsky maps .

Results : For the cores L260 , ecc806 , L1262 , L1517A , L1512 , and L1544 , the model reproduces the data with maximum grain sizes around 0.9 , 0.5 , 0.65 , 1.5 , 0.6 , and ¿ 1.5 \mu m , respectively .
The maximum coreshine intensities of L1506C , L1439 , and L1498 in the individual bands require smaller maximum grain sizes than derived from the observed distribution of band ratios .
Additional isotropic local radiation fields with a spectral shape differing from the DIRBE map shape do not remove this discrepancy .
In the case of Rho Oph 9 , we were unable to reliably disentangle the coreshine emission from background variations and the strong local PAH emission .

Conclusions : Considering surface brightness ratios in the 3.6 and 4.5 \mu m bands across a molecular cloud core is an effective method of disentangling the complex interplay of structure and opacities when used in combination with observed limits of the optical depth .