Aspects of a monochromator are described herein. In one embodiment, the monochromator includes a light source that provides light, a diffraction grating including a first diffraction grating and a second diffraction grating, a grating drive motor that rotates the diffraction grating to provide dispersed wavelengths of light, a detector that detects a portion of the dispersed wavelengths of light, and processing circuitry that controls a grating drive motor to regulate an angular velocity of the grating based on an angular orientation of the diffraction grating. By using a diffraction grating having multiple, different dispersive surfaces, measurements of relatively high precision and quality may be taken throughout a wider spectral range. In another aspect, the processing circuitry controls a sample drive motor to vary an angle of incidence of the dispersed wavelengths of light onto a sample for evaluation.