mmm... thank you very much in effect.. too many vegetation, and I don't know exatly what kind of vegetation is present in order to choose a correct endmember I can first mask my data using the vegetation index and after trying to perfome some band ratios what is your opinion? antonio

sorry for lack of punctuation mark....

Hi, I've the same problem, and I think that you could do a NDVI. After try to find out for each scene the NDVI value that can discriminate vegetation from rocks. Use this value to build a mask: for exemple NDVI>0.5 = vegetation (the value depends from season and type of vegetation), NDVI The ratios don't change if you do the ratio from masked image or from original image (each one in reflectance value). I found that lichens have low NDVI ratios, similar to rocks. So if you have a cliff with lichens the spectral signatures are more influenced  from lichens than rocks. I'm performing some script that can be helpfull. Contact me, I'm a PhD student in Padova.

You might try the Vegetation Suppression tool available from the Spectral menu in ENVI.