In order to obtain a "seamless" mosaic, radiance (and preferably reflectance) may need to be calculated before performing the mosaic, because gain changes may occur in one or more of the scenes. When two scenes have different gain states, they have different dynamic ranges, and there will be a shift in Digital Number (DN) values from one scene to the next. This can be determined by reviewing the included metadata file for each scene.
Some imaging software packages have a utility that can automatically create the radiance image, based on the gain/bias in the header file. If this is not available, a manual "band math function" may be necessary.
Details about conversion to radiance and converting radiance to reflectance are provided in Sections 11.3.1 and 11.3.2 of the Landsat Data Users Handbook.