The most effective way of scanning a negative or film transparency is by shining a light through it. Ordinary document scanners do not do this as documents cannot (obviously) be scanned in this way. A scanner designed for the scanning of film media possibly as well as ordinary dicuments (like mine) will have in its software package the facility to "invert" a scanned image to provide a positive from a negative image and, if required, vice versa.
P.P.