We present complete near infrared (0.85-2.45 μm), low-resolution (˜100) spectra of a sample of 26 disk L-dwarfs with reliable optical spectral type classification. The observations have been obtained with a prism-based optical element (the Amici device) that provides a complete spectrum of the source on the detector. Our observations show that low-resolution near-infrared spectroscopy can be used to determine the spectral classification of L-dwarfs in a fast but accurate way. We present a library of spectra that can be used as templates for spectral classification of faint dwarfs. We also discuss a set of near infrared spectral indices well correlated with the optical spectral types that can be used to accurately classify L-dwarfs earlier than L6. Our results show that with the VLT/NIRMOS instrument we will be able to extend the study of objects below the deuterium burning limit to all young clusters and associations within ˜1 kpc.