Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
lighten [ˈlaɪtn] vi (become less dark) светле́ть (посветле́ть perf)
vt (make less heavy) облегча́ть (облегчи́ть perf)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: