The Spectral Irradiance Monitor (SIM) is a newly designed spectrometer that provides the first long-duration solar spectral irradiance measurements in the visible and near infrared (Vis/NIR). The wavelength coverage is primarily from 300 to 2400 nm, with an additional channel to cover the 200-300 nm ultraviolet spectral region to overlap with the SOLSTICE, another instrument on-board the SORCE satellite. Understanding the wavelength-dependent variability throughout SIM’s wavelength range is of primary importance for long-term climate change studies on Earth. SIM is a single optical element Fèry prism spectrometer; only one optical element is needed to focus and disperse the light onto a series of detectors in the spectrometer’s focal plane. In this focal plane, four photodiode detectors and an electrical substitution radiometer (ESR) are used to detect solar radiation. SIM contains two completely independent and identical (mirror-image) spectrometers to provide redundancy and self-calibration capability.