We present the polarization maps of three selected Bok globules CB56 , CB60 and CB69 constructed using a V-band data from a CCD imaging polarimeter .
The aim of this work is to measure the optical polarization ( p _ { v } ) of background field stars in order to determine the polarization efficiency , p _ { v } / A _ { v } .
We find that the local magnetic field of the cloud CB56 is almost aligned with the galactic field , but not in CB60 and CB69 .
A trend of decreasing polarization efficiency with increasing extinction ( A _ { v } ) is observed : it can be well represented by a power law , p _ { v } / A _ { v } \propto A _ { v } ^ { - \alpha } , where \alpha = -0.56 \pm 0.36 , -0.59 \pm 0.51 and -0.52 \pm 0.49 for CB56 , CB60 and CB69 respectively .
This indicates that the linear polarization of the starlight due to aligned dust grains in these clouds is produced more efficiently in low extinction regions , compared with high obscured lines of sight .