Band Math looks at the pixel values so for a classification image, the classified pixels will typically have a value of 1,2,3...n depending on the number of classes. So those are the values you will use in Band Math. The help discusses the requirements and expected syntax for Band Math: http://www.harrisgeospatial.com/docs/...

There is one term used in remote sensing images captured at different times i.e, relative radiometric calibration. In this preprocessing step, people have used both images to normalize one wrt another image and in ENVI there is one option of radiometric calibration inside the radiometric correction.