In the list of L and T dwarfs at 'dwarfarchives.org' (http://ldwarf.ipac.caltech.edu/archive/version5/viewlist.php?table=ltdwarf&format=html") there are two columns named 'spectral_type_opt' and 'spectral_type_ir'. I guess this comes from the type of instrument used in the measurements? If I'm correct, the Spectral Type indicates the surface temperature of stars. If so why would the optical and infrared spectra give two different measurements? Which one is more accurate? Since the visual spectra of L and T dwarfs are very faint, would the 'infrared spectral type' be more accurate?