LEOI-94 is manually operated monochromator that utilizes a dial for wavelength selection. Both the entrance and exit slits are straight with width continuously adjustable from 0 to 2mm with a reading resolution of 0.01mm. The beam passes through entrance slit S1 (S1 is on the focal plane of reflectance collimation mirror), then reflected by mirror M2. The parallel light shoots to grating G. Mirror M3 form the image of diffraction light comes from the grating on S2.