To maintain the spectral integrity, you would have to make sure that both the multispectral and pan images are in the same spectral space before you use them together in a pan sharpening. So, it sounds like you will need to correct your pan image to be in radiance or reflectance (whichever your corrected multispectral image is - probably reflectance). Or if your multispectral and pan images were collected by the same sensor, then you could do pan sharpening with the raw data, and then apply an atmospheric correction to the result of that. I hope that helps. Peg

Thanks for your reply, Peg. My multispectral and monospetral image are from a same sensor, at the beginning I thought fusion them first then apply atmospheric correction, but usually "Correction has to be done before fusion, as sensor characteristic slightly gets modified after fusion". I did this and found the histogram of band 4 was quite different from reflectance image without fusion. It might be caused by a strip in orignal band 4. My multispectral image has been corrected to reflectance by FLAASH, I tried to do the same correction for monospectral imge, but FLAASH does not support single band image for further correction. Now I am analysing data without fusion. Thanks! Rong