A bandcenter is the center of a spectral band.

A bandstrength is the relative strength of a spectral band.

First of all, there is no "spectral" or specter or ghost in the story, and she used the term "speckled band" because that is what she thought she saw.

Firstly spectral lines means the spectrum the band of 7 colors. The spectral lines are formed because when the light passes through tough medium it passes through the medium but the white light gets disintegrated into seven colors . The band includes the colors violet, indigo, blue, green ,yellow orange , red.

That's the spectral band that scientists have dubbed "visible light".