One embodiment provides an optical spectrometer. The optical spectrometer can include a lens-and-filter system configured to collect light scattered from a sample, a spot converter configured to convert a substantially circular beam outputted from the lens-and-filter system into a substantially rectangular beam, and a slit comprising a rectangular aperture to allow a predetermined portion of the substantially rectangular beam to enter the rectangular aperture while blocking noise. The slit can further include at least one microelectromechanical systems (MEMS)-based movable structure configured to adjust a width of the rectangular aperture.