Devices, systems, and methods for measuring tissue oxygen saturation are disclosed. An illustrative spectrometer for interfacing an optical sensor with a display unit includes a number of measurement radiation sources, a number of radiation source fibers each optically coupled to one of the measurement radiation sources, a reflected radiation fiber optically coupled to the optical sensor, a measurement radiation output fiber including an image fiber, and a radiation mixing bar intermediate the radiation source fibers and the measurement radiation output fiber.