It's not at all necessary - that was probably just the way they felt would be easiest for most of their users to follow (or for people who only have ENVI runtime not ENVI+IDL). You could do what they say to do in excel with IDL for sure.

Thank you very much. But we have to make X axis= Wavelength and Y axis = FWHM? either in exel or in IDL. Please confirm.

The section of the document that you point to tells you how to add FWHM data to the spectral library that you import into ENVI. You don't have to do that, if you don't want to use the FWHM data with your spectral library. If you choose not to, then ENVI will estimate the FWHM (when it needs to use a FWHM) as being equal to the sampling interval (the interval between the band wavelengths). If you *do* want to use the real FWHM of the sensor with your library, instead of letting ENVI estimate it, then you will need to create an ASCII file that has wavelengths as one column, and FWHM as another column. This ASCII file will be imported in the first step of ENVI's Spectral Library Builder, when it asks for the Input Spectral Wavelength information. - Peg