LEWIS (L, M-band Echelle Wide coverage Intermediate-resolution Spectrometer) is an infrared spectrograph designed primarily for spectroscopy in the 2.0-5.0 mu m) region. It is capable of covering the whole L band (2.8-4.2 mu m) or M band (4.5-5.0 mu m) in one exposure with a resolving power over 1250 by using both a prism and a grating to produce a cross-dispersed echelle-type spectrogram onto a Santa Barbara Research Center (SBRC) 256x256 InSb array. The grating is characterized by a large groove spacing of 125 mu m and is utilized at very high orders (25th-37th order in the L band). The achieved throughput of the spectrograph including the quantum efficiency of the detector is about 20%.