What type of soil properties are exploit by using brightness component of T-cap and Bare soil index: (Rikimaru, A., Roy, P.S., Miyatake, S.,2002.
Tropical forest cover density mapping. Tropical Ecology 43(1): 39-47)
Using the following formula in comparison:
( ( (float(b1)+float(b2)) - (float(b3)+float(b4)) ) / ( (float(b1)+float(b2)) + (float(b3)+float(b4)) ) )
For Landsat Bands: b1 = 5, b2 = 3, b3 = 4, b4 = 1
http://www.tropecol.com/pdf/open/PDF_43_1/43104.pdf